6 WATER QUALITY

Introduction

6.1 Stage 1 of HATS, comprising the Stonecutters Island Sewage Treatment Works (SCISTW) and the deep tunnels, was commissioned in late 2001 to bring early improvement to the harbour water quality. The deep tunnels collect sewage from Kwai Chung, Tsing Yi, Tseung Kwan O, parts of eastern Hong Kong Island and all of Kowloon and deliver it to SCISTW for chemically enhanced primary treatment (CEPT). Stage 2 of HATS would be implemented in two phases, namely Stage 2A and Stage 2B. Under Stage 2A, deep tunnels would be built to bring sewage from the northern and western areas of Hong Kong Island to SCISTW and the treatment works would be expanded to meet the demands of both existing and future developments. Stage 2B of HATS involves the provision of secondary treatment at the SCISTW to improve the effluent quality.

6.2 This Project involves the construction and operation of Stage 2A. Construction of the Project (i.e. Stage 2A) is tentatively scheduled to commence in 2009 for commissioning the Stage 2A scheme by 2014.The scope of Stage 2A includes the following:

· Upgrading of existing preliminary treatment works (PTW) at North Point, Wan Chai East, Central, Sandy Bay, Cyberport, Wah Fu, Aberdeen and Ap Lei Chau on Hong Kong Island;

· Extension of the deep tunnel network to collect and transfer sewage from the above mentioned PTWs to SCISTW for treatment and disposal;

· Expansion of the existing CEPT capacity at SCISTW to meet the demands of both existing and future developments; and

· Provision of disinfection to all HATS effluent before discharging into the harbour.

6.3 Based on the technical review of disinfection technologies and option evaluation as well as the technical environmental assessment conducted under the EIA study for “Provision of Disinfection Facilities at Stonecutters Island STW – Investigation (ADF)”, the purchase of sodium hypochlorite solution for chlorination and sodium bisulphite for dechlorination was recommended as the disinfection technology for SCISTW. Based on the current programme, construction of the chlorination plant would commence in April 2008 for commissioning the advance disinfection facilities (ADF) in October 2009. As for the dechlorination plant, construction would commence in September 2008 for completion in September 2009.

6.4 During the ADF stage, use of the existing effluent culvert as the chlorine contact tank is proposed. At Stage 2A, the permanent disinfection facilities of SCISTW (including a new chlorine contact tank) will be available for the HATS effluent to increase the chlorine contact time so as to further improve the effectiveness of the chlorination and minimize the chlorine dosage.

6.5 This Section evaluates the potential water quality impacts that are likely to be generated during the construction and operation of Stage 2A. The water quality impacts of Stage 1, Stage 2A and Stage 2B of the HATS have also been taken into consideration in this water quality impact assessment. Appropriate mitigation measures were identified, where necessary, to mitigate the potential water quality impacts.

Water Sensitive Receivers

6.6 To evaluate the potential water quality impacts from the Project, water sensitive receivers within the North Western, Western Buffer, Victoria Harbour, Eastern Buffer, Junk Bay and Southern Water Control Zones (WCZ) were considered. Major water sensitive receivers identified within the Study Area include:

· Cooling Water Intakes;

· WSD Flushing Water Intakes;

· Fish Culture Zones (FCZ);

· Beaches;

· Sites of Special Scientific Interest (SSSI);

· Marine Parks and Marine Reserves;

· Seagrass Beds;

· Artificial Reefs;

· Corals;

· Chinese White Dolphins; and

· Green Turtle Nesting Grounds

6.7 Figure 6.1 shows the locations of the ecological resources and water sensitive receivers.

Identification of Environmental Impacts

Operational Phase

6.8 During the operational phase, the potential water quality impacts will be mainly related to the treated effluent discharge from the HATS Stage 2A. Key concerns are:

· Effects on marine water quality and sediment quality due to the discharge of disinfected CEPT effluent from SCISTW at different time horizons under normal plant operation.

· Water quality effects of occasional overflow of screened (or untreated) effluent at individual PTW during extreme storm event under normal plant operation.

· Water quality impacts of emergency sewage discharges due to the failure of equipment or power supply or as a result of treatment process failure.

6.9 The water quality parameters considered for the assessment of water quality impact from the HATS Stage 2A effluent include pH, temperature, dissolved oxygen (DO), salinity, suspended solids (SS), biochemical oxygen demand (BOD), nutrients, chlorophyll, E.coli, sedimentation rates, chlorination by-products (CBP) and toxic chemicals such as metals, total residual chlorine (TRC) and unionized ammonia etc.

6.10 A comprehensive review and identification of toxic contaminants of concern (COC) in the HATS effluent and their respective assessment criteria and detailed assessment of the toxic effects of the identified COC were conducted under the human health and marine ecological risk assessment in Section 7 and Section 8.

Construction Phase

6.11 No dredging and filling activity would be anticipated for the tunnel construction. The general construction activities that will be undertaken for the upgrading works will be primarily land-based. Key water quality issues associated with land-based construction would include the impacts from site run-off, sewage from workforce, accidental spillage and discharges of wastewater from various construction activities.

6.12 Based on the preliminary engineering design, the proposed upgrading works at one of the PTWs, Aberdeen PTW, would require seawall re-construction. Seawall re-construction would involve excavation of existing seawall. Fine content in the filling materials in the seawall would be negligible and loss of fill material during seawall excavation is not expected. No dredging would be required for the seawall re-construction works.

6.13 Temporary bypass of sewage effluent via seawall or submarine outfalls of SCISTW and individual PTW would be required during the construction stage. The temporary sewage bypass would cause transient increase of pollution level in the receiving marine water.

Environmental Impacts from the Provision of Disinfection at SCISTW

6.14 Key water quality impacts related to the provision of disinfection facilities at the SCISTW include:

· The reduction of faecal bacteria in the effluent after disinfection.

· The potential generation of low-level total residual chlorine (TRC) and chlorination by-products (CBP) in the effluent due to chlorination of the sewage effluent.

· The potential impact of TRC in the event of dechlorination plant failure.

· The potential impact of faecal pollution in the event of chlorination plant failure.

· The potential minor oxygen depletion impact due to addition of dechlorination chemical.

6.15 Total residual chlorine (TRC) and chlorination by-products (CBP) are major concerns of chlorination. TRC includes free chlorine residuals such as hypochlorous acid (HOCl) and dissolved hypochlorite ion (OCl-) after chlorine is added to water, plus combined chlorine residuals such as chloramines formed by the reaction of free residuals with ammonia present in the sewage. CBP refer to chlorinated organic compounds (or total organic halogen) formed by the reaction of chlorine (mainly free chlorine residuals) with some specific organic compounds such as humic substances, which generally are not present in any large quantity in CEPT effluent. CBP consist of a whole range of halogenated organic compounds, and are generally considered of concern to human health. Examples of CBP formed during chlorination include trihalomethanes (THM) and haloacetic acids (HAA). THM are suspected as being carcinogens and are strictly monitored in drinking water. CBP concentrations may vary in orders of magnitude during different chlorination processes. Typical concentrations of THM and HAA in chlorinated drinking water are usually in the range 1-100 mg/l ⁽[1]⁾. Range of concentrations in chlorinated sewage effluent for specific CBP compounds has been identified under the EIA study for ADF and is also presented in this EIA report for completeness.

6.16 Water quality impacts in relation to the chlorination and dechlorination of the HATS effluent have been quantitatively assessed by mathematical modelling under the EIA study for the ADF. Details of the assessment results are presented in the separate EIA report for ADF. Under the ADF study, water quality model simulations were performed for 30 days (excluding the spin-up time) each under the typical wet and dry seasons for normal operation scenarios and 15 days (excluding the spin-up time) each under the typical wet and dry seasons for emergency situations (due to temporary failure of chlorination or dechlorination plant) in accordance with the EIA Study Brief for ADF.

6.17 The EIA Study Brief for this Project (i.e. Stage 2A) requires that the model simulations shall be performed for at least one complete calendar year under normal operation scenarios. As such, the water quality impacts in relation to the normal operation of the disinfection facilities for HATS have been re-examined under this EIA based on a series of 1-year model simulations incorporating monthly variations in Pearl River discharges, solar radiation, water temperature and wind velocity to confirm the findings of the ADF study.

6.18 According to the EIA Study Brief for this Project, assessment of temporary or emergency discharges as well as the short-term construction phase impacts may be assessed by simulating typical spring-neap cycles (at least 15 days) in the dry and wet seasons. Furthermore, the HATS flow rates adopted in the EIA study for ADF were more conservative as compared to the latest flow projections adopted in this EIA. It is considered that the water quality impacts in relation to the temporary failure of chlorination or dechlorination plant have been fully quantified and assessed under the ADF study.

Environmental Legislation, Policies, Plans, Standards and Criteria

Environmental Impact Assessment Ordinance (EIAO)

6.19 The Technical Memorandum on Environmental Impact Assessment Process (EIAO-TM) is issued by the EPD under Section 16 of the EIAO. It specifies the assessment method and criteria that need to be followed in the EIA. Reference sections in the EIAO-TM provide the details of the assessment criteria and guidelines that are relevant to the water quality impact assessment, including:

· Annex 6 Criteria for Evaluating Water Pollution

· Annex 14 Guidelines for Assessment of Water Pollution

Marine Water Quality Objectives under WPCO

6.20 The Water Pollution Control Ordinance (WPCO) provides the major statutory framework for the protection and control of water quality in Hong Kong. According to the WPCO and its subsidiary legislation, Hong Kong waters are divided into ten Water Control Zones (WCZ). Corresponding statements of Water Quality Objectives (WQO) are stipulated for different water regimes (marine waters, inland waters, bathing beaches subzones, secondary contact recreation subzones and fish culture subzones) in the WCZ based on their beneficial uses. With reference to the EIA Study Brief, the Study Area for this water quality assessment covers the North Western, Western Buffer, Victoria Harbour, Eastern Buffer, Junk Bay and Southern WCZ (see Figure 6.2). Their corresponding WQO are listed in Table 6.1 to Table 6.6 respectively.

Table 6.1 Summary of Water Quality Objectives for North Western WCZ

Parameters	Objectives	Sub-Zone
Offensive odour, tints	Not to be present	Whole zone
Visible foam, oil scum, litter	Not to be present	Whole zone
Dissolved oxygen (DO) within 2 m of the seabed	Not less than 2.0 mg/l for 90% of samples	Marine waters
Depth-averaged DO	Not less than 4.0 mg/l	Tuen Mun (A), Tuen Mun (B) and Tuen Mun (C) subzones, water gathering ground subzones and other inland waters
Depth-averaged DO	Not less than 4.0 mg/l for 90 % sample	Marine waters
pH	To be in the range of 6.5 - 8.5, change due to human activity not to exceed 0.2	Marine waters excepting bathing beach subzones
	To be in the range of 6.5 – 8.5	Tuen Mun (A), Tuen Mun (B) and Tuen Mun (C) subzones and water gathering ground subzones
	To be in the range of 6.0 –9.0	Other inland waters
	To be in the range of 6.0 –9.0 for 95% samples	Bathing beach subzones
Salinity	Change due to human activity not to exceed 10% of ambient	Whole zone
Temperature	Change due to human activity not to exceed 2^oC	Whole zone
Suspended solids (SS)	Not to raise the ambient level by 30% caused by human activity	Marine waters
	Change due to waste discharges not to exceed 20 mg/l of annual median	Tuen Mun (A), Tuen Mun (B) and Tuen Mun (C) subzones and water gathering ground subzones
	Change due to waste discharges not to exceed 25 mg/l of annual median	Inland waters
Unionized ammonia (UIA)	Annual mean not to exceed 0.021 mg(N)/l as unionized form	Whole zone
Nutrients	Shall not cause excessive algal growth	Marine waters
Total inorganic nitrogen (TIN)	Annual mean depth-averaged inorganic nitrogen not to exceed 0.3 mg(N)/l	Castle peak bay subzone
Total inorganic nitrogen (TIN)	Annual mean depth-averaged inorganic nitrogen not to exceed 0.5 mg(N)/l	Marine waters excepting castle peak bay subzone
E.coli	Not exceed 610 per 100 ml, calculated as the geometric mean of all samples collected in one calendar year	Secondary contact recreation subzones
	Should be less than 1 per 100 ml, calculated as the geometric mean of the most recent 5 consecutive samples taken between 7 and 21 days.	Tuen Mun (A) and Tuen Mun (B) subzones and water gathering ground subzones
	Not exceed 1000 per 100 ml, calculated as the geometric mean of the most recent 5 consecutive samples taken between 7 and 21 days	Tuen Mun (C) subzone and other inland waters
	Not exceed 180 per 100 ml, calculated as the geometric mean of all samples collected from March to October inclusive.	Bathing beach subzones
Colour	Change due to waste discharges not to exceed 30 Hazen units	Tuen Mun (A) and Tuen Mun (B) subzones and water gathering ground subzones
Colour	Change due to waste discharges not to exceed 50 Hazen units	Tuen Mun (C) subzone and other inland waters
5-Day biochemical oxygen demand (BOD₅)	Change due to waste discharges not to exceed 3 mg/l	Tuen Mun (A), Tuen Mun (B) and Tuen Mun (C) subzones and water gathering ground subzones
5-Day biochemical oxygen demand (BOD₅)	Change due to waste discharges not to exceed 5 mg/l	Inland waters
Chemical oxygen demand (COD)	Change due to waste discharges not to exceed 15 mg/l	Tuen Mun (A), Tuen Mun (B) and Tuen Mun (C) subzones and water gathering ground subzones
Chemical oxygen demand (COD)	Change due to waste discharges not to exceed 30 mg/l	Inland waters
Toxins	Should not cause a risk to any beneficial uses of the aquatic environment	Whole zone
Toxins	Waste discharge shall not cause the toxins in water significant to produce toxic carcinogenic, mutagenic or teratogenic effects in humans, fish or any other aquatic organisms.	Whole zone
Phenol	Quantities shall not sufficient to produce a specific odour or more than 0.05 mg/l as C₆H₅OH	Bathing beach subzones
Turbidity	Shall not reduce light transmission substantially from the normal level	Bathing beach subzones

Source: Statement of Water Quality Objectives (North Western Water Control Zone).

Table 6.2 Summary of Water Quality Objectives for Western Buffer WCZ

Parameters	Objectives	Sub-Zone
Offensive odour, tints	Not to be present	Whole zone
Visible foam, oil scum, litter	Not to be present	Whole zone
Dissolved oxygen (DO) within 2 m of the seabed	Not less than 2.0 mg/l for 90% of samples	Marine waters
Depth-averaged DO	Not less than 4.0 mg/l for 90% of samples	Marine waters excepting fish culture subzones
	Not less than 5.0 mg/l for 90% of samples	Fish culture subzones
	Not less than 4.0 mg/l	Water gathering ground subzone and other Inland waters
5-Day biochemical oxygen demand (BOD₅)	Change due to waste discharges not to exceed 3 mg/l	Water gathering ground subzones
5-Day biochemical oxygen demand (BOD₅)	Change due to waste discharges not to exceed 5 mg/l	Inland waters
Chemical oxygen demand (COD)	Change due to waste discharges not to exceed 15 mg/l	Water gathering ground subzones
Chemical oxygen demand (COD)	Change due to waste discharges not to exceed 30 mg/l	Inland waters
pH	To be in the range of 6.5 – 8.5, change due to waste discharges not to exceed 0.2	Marine waters
	To be in the range of 6.5 – 8.5	Water gathering ground subzones
	To be in the range of 6.0 – 9.0	Inland waters
Salinity	Change due to waste discharges not to exceed 10% of ambient	Whole zone
Temperature	Change due to waste discharges not to exceed 2 ^oC	Whole zone
Suspended solids (SS)	Not to raise the ambient level by 30% caused by waste discharges and shall not affect aquatic communities	Marine waters
	Change due to waste discharges not to exceed 20 mg/l of annual median	Water gathering ground subzones
	Change due to waste discharges not to exceed 25 mg/l of annual median	Inland waters
Unionized ammonia (UIA)	Annual mean not to exceed 0.021 mg(N)/l as unionized form	Whole zone
Nutrients	Shall not cause excessive algal growth	Marine waters
Total inorganic nitrogen (TIN)	Annual mean depth-averaged inorganic nitrogen not to exceed 0.4 mg(N)/l	Marine waters
Toxic substances	Should not attain such levels as to produce significant toxic effects in humans, fish or any other aquatic organisms	Whole zone
Toxic substances	Waste discharges should not cause a risk to any beneficial use of the aquatic environment	Whole zone
E.coli	Not exceed 610 per 100 ml, calculated as the geometric mean of all samples collected in one calendar year	Secondary contact recreation subzones and fish culture subzones
	Not exceed 180 per 100 ml, calculated as the geometric mean of all samples collected from March to October inclusive in 1 calendar year. Samples should be taken at least 3 times in 1 calendar month at intervals of between 3 and 14 days	Bathing beach subzones
	Less than 1 per 100 ml, calculated as the geometric mean of the most recent 5 consecutive samples taken at intervals of between 7 and 21 days	Water gathering ground subzones
	Not exceed 1000 per 100 ml, calculated as the geometric mean of the most recent 5 consecutive samples taken at intervals of between 7 and 21 days	Inland waters
Colour	Change due to waste discharges not to exceed 30 Hazen units	Water gathering round
Colour	Change due to waste discharges not to exceed 50 Hazen units	Inland waters
Turbidity	Shall not reduce light transmission substantially from the normal level	Bathing beach subzones

Source: Statement of Water Quality Objectives (Western Buffer Water Control Zone).

Table 6.3 Summary of Water Quality Objectives for Victoria Harbour WCZ

Parameters	Objectives	Sub-Zone
Offensive odour, tints	Not to be present	Whole zone
Visible foam, oil scum, litter	Not to be present	Whole zone
E coli	Not to exceed 1000 per 100 mL, calculated as the geometric mean of the most recent 5 consecutive samples taken at intervals between 7 and 21 days	Inland waters
Dissolved oxygen (DO) within 2 m of the seabed	Not less than 2.0 mg/l for 90% of samples	Marine waters
Depth-averaged DO	Not less than 4.0 mg/l for 90% of samples	Marine waters
DO	Not less than 4.0 mg/l	Inland waters
pH	To be in the range of 6.5 - 8.5, change due to human activity not to exceed 0.2	Marine waters
pH	Not to exceed the range of 6.0 - 9.0 due to human activity	Inland waters
Salinity	Change due to human activity not to exceed 10% of ambient	Whole zone
Temperature	Change due to human activity not to exceed 2 ^oC	Whole zone
Suspended solids (SS)	Not to raise the ambient level by 30% caused by human activity	Marine waters
Suspended solids (SS)	Annual median not to exceed 25 mg/l due to human activity	Inland waters
Unionized ammonia (UIA)	Annual mean not to exceed 0.021 mg(N)/l as unionized form	Whole zone
Nutrients	Shall not cause excessive algal growth	Marine waters
Total inorganic nitrogen (TIN)	Annual mean depth-averaged inorganic nitrogen not to exceed 0.4 mg(N)/l	Marine waters
5-Day biochemical oxygen demand (BOD₅)	Not to exceed 5 mg/l	Inland waters
Chemical Oxygen Demand (COD)	Not to exceed 30 mg/l	Inland waters
Toxic substances	Should not attain such levels as to produce significant toxic, carcinogenic, mutagenic or teratogenic effects in humans, fish or any other aquatic organisms.	Whole zone
Toxic substances	Human activity should not cause a risk to any beneficial use of the aquatic environment.	Whole zone

Source: Statement of Water Quality Objectives (Victoria Harbour (Phases One, Two and Three) Water Control Zone).

Table 6.4 Summary of Water Quality Objectives for Eastern Buffer WCZ

Parameters	Objectives	Sub-Zone
Offensive odour, tints	Not to be present	Whole zone
Visible foam, oil scum, litter	Not to be present	Whole zone
Dissolved oxygen (DO) within 2 m of the seabed	Not less than 2.0 mg/l for 90% of samples	Marine waters
Depth-averaged DO	Not less than 4.0 mg/l for 90% of samples	Marine waters excepting fish culture subzones
	Not less than 5.0 mg/l for 90% of samples	Fish culture subzones
	Not less than 4.0 mg/l	Water gathering ground subzone and other Inland waters
5-Bay biochemical oxygen demand (BOD₅)	Change due to waste discharges not to exceed 3 mg/l	Water gathering ground subzones
5-Bay biochemical oxygen demand (BOD₅)	Change due to waste discharges not to exceed 5 mg/l	Inland waters
Chemical oxygen demand (COD)	Change due to waste discharges not to exceed 15 mg/l	Water gathering ground subzones
Chemical oxygen demand (COD)	Change due to waste discharges not to exceed 30 mg/l	Inland waters
pH	To be in the range of 6.5 – 8.5, change due to waste discharges not to exceed 0.2	Marine waters
	To be in the range of 6.5 – 8.5	Water gathering ground subzones
	To be in the range of 6.0 – 9.0	Inland waters
Salinity	Change due to waste discharges not to exceed 10% of ambient	Whole zone
Temperature	Change due to waste discharges not to exceed 2 ^oC	Whole zone
Suspended solids (SS)	Not to raise the ambient level by 30% caused by waste discharges and shall not affect aquatic communities	Marine waters
	Change due to waste discharges not to exceed 20 mg/l of annual median	Water gathering ground subzones
	Change due to waste discharges not to exceed 25 mg/l of annual median	Inland waters
Unionized ammonia (UIA)	Annual mean not to exceed 0.021 mg(N)/l as unionized form	Whole zone
Nutrients	Shall not cause excessive algal growth	Marine waters
Total inorganic nitrogen (TIN)	Annual mean depth-averaged inorganic nitrogen not to exceed 0.4 mg(N)/l	Marine waters
Dangerous substances	Should not attain such levels as to produce significant toxic effects in humans, fish or any other aquatic organisms	Whole zone
Dangerous substances	Waste discharges should not cause a risk to any beneficial use of the aquatic environment	Whole zone
E.coli	Not exceed 610 per 100 ml, calculated as the geometric mean of all samples collected in one calendar year	Fish culture subzones
	Less than 1 per 100 ml, calculated as the geometric mean of the most recent 5 consecutive samples taken at intervals of between 7 and 21 days	Water gathering ground subzones
	Not exceed 1000 per 100 ml, calculated as the geometric mean of the most recent 5 consecutive samples taken at intervals of between 7 and 21 days	Inland waters
Colour	Change due to waste discharges not to exceed 30 Hazen units	Water gathering ground
Colour	Change due to waste discharges not to exceed 50 Hazen units	Inland waters

Source: Statement of Water Quality Objectives (Eastern Buffer Water Control Zone).

Table 6.5 Summary of Water Quality Objectives for Junk Bay WCZ

Parameters	Objectives	Sub-Zone
Offensive odour, tints	Not to be present	Whole zone
Visible foam, oil scum, litter	Not to be present	Whole zone
Dissolved oxygen (DO) within 2 m of the seabed	Not less than 2.0 mg/l for 90% of samples	Marine waters
Depth-averaged DO	Not less than 4.0 mg/l for 90% of samples	Marine waters excepting fish culture subzones
	Not less than 5.0 mg/l for 90% of samples	Fish culture subzones
	Not less than 4.0 mg/l	Inland waters
5-Bay biochemical oxygen demand (BOD₅)	Change due to waste discharges not to exceed 5 mg/l	Inland waters
Chemical oxygen demand (COD)	Change due to waste discharges not to exceed 30 mg/l	Inland waters
pH	To be in the range of 6.5 - 8.5, change due to waste discharges not to exceed 0.2	Marine waters
pH	To be in the range of 6.0 –9.0	Inland waters
Salinity	Change due to waste discharges not to exceed 10% of ambient	Whole zone
Temperature	Change due to waste discharges not to exceed 2 ^oC	Whole zone
Suspended solids (SS)	Not to raise the ambient level by 30% caused by waste discharges and shall not affect aquatic communities	Marine waters
Suspended solids (SS)	Change due to waste discharges not to exceed 25 mg/l of annual median	Inland waters
Unionized ammonia (UIA)	Annual mean not to exceed 0.021 mg(N)/l as unionized form	Whole zone
Nutrients	Shall not cause excessive algal growth	Marine waters
Total inorganic nitrogen (TIN)	Annual mean depth-averaged inorganic nitrogen not to exceed 0.3 mg(N)/l	Marine waters
Dangerous substances	Should not attain such levels as to produce significant toxic effects in humans, fish or any other aquatic organisms	Whole zone
Dangerous substances	Waste discharges should not cause a risk to any beneficial use of the aquatic environment	Whole zone
E.coli	Not exceed 610 per 100 ml, calculated as the geometric mean of all samples collected in one calendar year	Secondary contact recreation subzones and fish culture subzones
E.coli	Not exceed 1000 per 100 ml, calculated as the geometric mean of the most recent 5 consecutive samples taken at intervals of between 7 and 21 days	Inland waters
Colour	Change due to waste discharges not to exceed 50 Hazen units	Inland waters

Source: Statement of Water Quality Objectives (Junk Bay Water Control Zone).

Table 6.6 Summary of Water Quality Objectives for Southern WCZ

Parameters	Objectives	Sub-Zone
Offensive odour, tints	Not to be present	Whole zone
Visible foam, oil scum, litter	Not to be present	Whole zone
Dissolved oxygen (DO) within 2 m of the seabed	Not less than 2.0 mg/l for 90% of samples	Marine waters
Depth-averaged DO	Not less than 4.0 mg/l for 90 % sample	Marine waters excepting fish culture subzones
	Not less than 5.0 mg/l for 90% of samples	Fish culture subzones
	Not less than 4.0 mg/l	Inland waters
pH	To be in the range of 6.5 - 8.5, change due to human activity not to exceed 0.2	Marine waters excepting bathing beach subzones; Mui Wo (A), Mui Wo (B), Mui Wo (C), Mui Wo (E) and Mui Wo (F) subzones
	To be in the range of 6.0 – 9.0	Mui Wo (D) sub-zone and other inland waters.
	To be in the range of 6.0 –9.0 for 95% of samples, change due to human activity not to exceed 0.5	Bathing beach subzones
Salinity	Change due to human activity not to exceed 10% of ambient	Whole zone
Temperature	Change due to human activity not to exceed 2 ^oC	Whole zone
Suspended solids (SS)	Not to raise the ambient level by 30% caused by human activity	Marine waters
	Change due to waste discharges not to exceed 20 mg/l of annual median	Mui Wo (A), Mui Wo (B), Mui Wo (C), Mui Wo (E) and Mui Wo (F) subzones
	Change due to waste discharges not to exceed 25 mg/l of annual median	Mui Wo (D) subzone and other inland waters
Unionized ammonia (UIA)	Annual mean not to exceed 0.021 mg(N)/l as unionized form	Whole zone
Nutrients	Shall not cause excessive algal growth	Marine waters
Total inorganic nitrogen (TIN)	Annual mean depth-averaged inorganic nitrogen not to exceed 0.1 mg(N)/l	Marine waters
E.coli	Not exceed 610 per 100 ml, calculated as the geometric mean of all samples collected in one calendar year	Secondary contact recreation subzones and fish culture subzones
E.coli	Not exceed 180 per 100 ml, calculated as the geometric mean of all samples collected from March to October inclusive in 1 calendar year. Samples should be taken at least 3 times in 1 calendar month at intervals of between 3 and 14 days.	Bathing beach subzones
5-Day biochemical oxygen demand (BOD₅)	Change due to waste discharges not to exceed 5 mg/l	Inland waters
Chemical oxygen demand (COD)	Change due to waste discharges not to exceed 30 mg/l	Inland waters
Dangerous substances	Should not attain such levels as to produce significant toxic effects in humans, fish or any other aquatic organisms	Whole zone
Dangerous substances	Waste discharges should not cause a risk to any beneficial use of the aquatic environment	Whole zone

Source: Statement of Water Quality Objectives (Southern Water Control Zone).

Water Supplies Department (WSD) Water Quality Criteria

6.21 Besides the WQO set under the WPCO, the WSD has specified a set of objectives for water quality at flushing water intakes as listed in Table 6.7.

Table 6.7 WSD Standards at Flushing Water Intakes

Parameter (in mg/l unless otherwise stated)	WSD Target Limit
Colour (Hazen Unit)	< 20
Turbidity (NTU)	< 10
Threshold Odour Number (odour unit)	< 100
Ammoniacal Nitrogen	< 1
Suspended Solids	< 10
Dissolved Oxygen	> 2
Biochemical Oxygen Demand	< 10
Synthetic Detergents	< 5
E.coli (no. per 100 ml)	< 20000

Practice Note

6.22 A practice note for professional persons has been issued by the EPD to provide guidelines for handling and disposal of construction site discharges. The ProPECC PN 1/94 “Construction Site Drainage” provides good practice guidelines for dealing with ten types of discharge from a construction site. These include surface runoff, groundwater, boring and drilling water, bentonite slurry, water for testing and sterilisation of water retaining structures and water pipes, wastewater from building construction, acid cleaning, etching and pickling wastewater, and wastewater from site facilities. Practices given in the ProPECC PN 1/94 should be followed as far as possible during construction to minimize the water quality impact due to construction site drainage.

Technical Memorandum on Effluents Discharge Standard

6.23 Discharges of effluents are subject to control under the WPCO. The Technical Memorandum on Standards for Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal Waters (TM-DSS) gives guidance on permissible effluent discharges based on the type of receiving waters (foul sewers, storm water drains, inland and coastal waters). The limits control the physical, chemical and microbial quality of effluent. Any effluent discharge from the proposed construction activities must comply with the standards for effluent discharged into the foul sewers, inshore waters and marine waters of the Victoria Harbour WCZ and Western Buffer WCZ, as given in the TM-DSS.

Assessment Criteria for Corals

6.24 Potential impacts on benthic organisms, including corals, may arise through excessive sediment deposition. The magnitude of impacts on marine ecological sensitive receivers was assessed based on the predicted sedimentation rate. According to Pastorok and Bilyard ^([2]) and Hawker and Connell ^([3]), a sedimentation rate higher than 0.1 kg/m²/day would introduce moderate to severe impact upon corals. This criterion has been adopted for protecting the corals in Hong Kong under other approved EIAs such as Tai Po Sewage Treatment Works Stage 5 EIA ^{(^[4])}, Further Development of Tseung Kwan O Feasibility Study EIA, Wan Chai Reclamation Phase II EIA, Eastern Waters MBA Study ^([5]), West Po Toi MBA Study ^([6]) and Tai Po Gas Pipeline Study ^([7]). This sedimentation rate was used as the assessment criterion to guard against unacceptable impacts on marine ecological sensitive receivers.

6.25 The potential impacts on corals were also assessed with reference to the WQO of 30% from the ambient levels. However, in Eastern Buffer, Junk Bay and Southern WCZ with low baseline SS levels, it is proposed to adopt a SS criteria value of 10 mg/L for assessing the impacts upon corals. The SS value of 10 mg/L has been adopted as the SS criterion under many other EIA studies in Hong Kong ^([8]) for protection of sensitive coral sites in the eastern and southern waters of Hong Kong where the baseline SS levels were low. Based on previous EM&A monitoring results where this SS criterion of 10 mg/L has been adopted, no adverse impacts on corals have occurred.

EEFS Water Quality Criteria (WQC)

6.26 In addition to the statutory WQO stipulated under the WPCO, a set of water quality criteria (WQC) established under the “Environmental and Engineering Feasibility Assessment Studies in relation to the Way Forward of the HATS (EEFS)” was used as reference guidelines to assess the water quality impacts as per the requirement in Section 3.4.3.5 (iv) of the EIA Study Brief.

Background

6.27 The HATS is an overall sewage collection and treatment scheme for areas on both sides of Victoria Harbour. Stage 1 of HATS, fully commissioned in December 2001, collects sewage from the urban areas of Kowloon, Tsuen Wan, Kwai Tsing, Tseung Kwan O and the north-eastern part of Hong Kong Island and conveys it to SCISTW for CEPT treatment which has brought early water quality improvements especially in eastern Victoria Harbour, Eastern Buffer WCZ and Junk Bay WCZ. As the population grows and with further development on both sides of the harbour, deterioration of water quality in Victoria Harbour may occur if the full HATS is not completed.

6.28 In 2004, the Government of the Hong Kong SAR completed trials and studies on environmental impacts and engineering feasibility to assist in deciding the best way forward for the remaining stages of HATS. Detailed marine water quality, ecological and fisheries assessments were performed as part of the EEFS with the objective of assessing the potential impacts of different treatment and disposal schemes proposed for the remaining stages of HATS and the associated construction activities. The recommended option for HATS Stage 2 is to convey all sewage from the harbour area to SCISTW for centralized treatment. The EEFS recommended that biological treatment plus disinfection should be provided for the HATS on a long-term basis. The marine water quality, ecological and fisheries assessment conducted as part of the EEFS followed the guidelines set forth in the EIAO-TM. The potential water quality impacts were quantitatively assessed using various WQC outlined for the EEFS with a further qualitative assessment based on the collective professional opinion of a team of local and international experts in marine sciences and water quality management.

6.29 Setting of WQC has been recognized from the start of the EEFS as a key to assessing the acceptability and performance of different HATS options. In support of criteria setting, an extensive public consultation exercise on the proposed WQC for HATS was conducted in 2002 as part of the EEFS. These WQC were based on the statutory WQO stipulated under the WPCO, originally developed in the late 1980’s, and further refined by the consultant of the EEFS using the results of the Environmental Impact Assessment of the Strategic Sewage Disposal Scheme (SSDS EIA) and other recently completed studies. The set of HATS specific WQC, developed as a part of the EEFS, has integrated the concerns of various interested stakeholders and the general public through a presentation and briefing with the Advisory Council on the Environment and the Monitoring Group for HATS, public view-sharing workshops and receipt of public comments.

6.30 The findings of this consultation exercise and a full set of the proposed final WQC are documented in the “Report on Community Consultation for the Proposed Water Quality Criteria (October 2002)” prepared under the EEFS.

Water Quality Criteria (WQC)

6.31 The WQC as shown in Table 6.8 and Table 6.9 were used as reference guidelines for far field and near field impact assessment respectively. The far field water quality impact assessment and far field modelling performed under this EIA covered the North Western, Western Buffer, Victoria Harbour, Eastern Buffer, Junk Bay and Southern WCZ as well as the adjacent outer water to take into account all the major pollution sources (including the Pearl River) that may have a bearing on the environmental acceptability of the Project as required by the EIA Study Brief. The near field impact assessment was conducted by mathematical modelling to simulate the characteristics of the sewage plume in the vicinity of the submarine outfall to determine the zone of initial dilution (ZID), plume dimensions, rise height, merging and trapping in various flow and ambient conditions.

6.32 Relevant WQC were derived under the EEFS for Western Buffer, Eastern Buffer, Junk Bay, Southern and Victoria Harbour WCZ. Different WQC were provided for different zones or locations to protect marine resources and uses of the water bodies. EEFS did not derive WQC for the North Western WCZ. Various marine resources and water sensitive uses identified within the Study Area are shown in Figure 6.1.

Table 6.8 Reference Marine Water Quality Criteria for Far Field Water Quality Assessment

Parameter	Value	Type / Period	Applicable Zones / Uses ⁽¹⁾
E.coli	≤ 180/100ml	Geometric mean for bathing season (March to October)	Bathing waters
	≤ 610/100ml	Annual geometric mean	Secondary contact recreation zones and mariculture zones
	≤ 20,000/100ml	≥ 90% of occasions	Sea water intakes for flushing and industrial use
Dissolved oxygen (DO)	≥ 4 mg/l (water column average)	≥ 90% of occasions	Western Buffer, Eastern Buffer, Junk Bay and Victoria Harbour WCZ (except mariculture zones and fish spawning ground) (Figure 6.2)
	≥ 2 mg/l	at all times	All WCZ (except mariculture zones)
	≥ 5 mg/l (water column average)	Monthly average	Southern WCZ and fish spawning ground (Figure 6.2)
	≥ 5 mg/l (water column average)	≥ 90% of occasions	Mariculture zones only
	≥ 2 mg/l (bottom DO within 2 m from the seabed)	≥ 90% of occasions	Mariculture zones only
Depth-averaged total inorganic nitrogen (TIN)	≤ 0.2 mg(N)/l	Annual mean	Southern WCZ and fish spawning ground (Figure 6.2)
	≤ 0.4 mg(N)/l	Annual mean	Western Buffer, Eastern Buffer and Victoria Harbour WCZ (except fish spawning ground) (Figure 6.2)
	≤ 0.3 mg(N)/l	Annual mean	Junk Bay WCZ (Figure 6.2)
	≤ 0.1 mg(N)/l	Annual mean	Semi-enclosed bays (Figure 6.3)
Depth-averaged Unionized Ammonia (UIA)	≤ 0.021 mg(N)/l	Annual mean	All WCZ
Depth-averaged total inorganic phosphorus (PO₄)	≤ 0.02 mg(P)/l	Annual mean	Southern WCZ and fish spawning ground (Figure 6.2)
	≤ 0.04 mg(P)/l	Annual mean	Western Buffer, Eastern Buffer and Victoria Harbour WCZ (except fish spawning ground) (Figure 6.2)
	≤ 0.03 mg(P)/l	Annual mean	Junk Bay WCZ (Figure 6.2)
	≤ 0.01 mg(P)/l	Annual mean	Semi-enclosed bays (Figure 6.3)
Total residual chlorine (TRC)	≤ 0.008 mg/l	Daily maximum	All WCZ
Chronic toxicity	≤ one chronic toxicity unit (TUc), (derived from NOEC values based on whole effluent toxicity tests) ⁽²⁾	4-day average chronic toxicity exposure	All WCZ

(Source: EEFS Report on Community Consultation for the Proposed Water Quality Criteria)

⁽¹⁾ EEFS did not derive WQC for the North Western WCZ.

⁽²⁾ USEPA Technical Support Document for Water Quality-Based Toxics Control (March 1991), from which one chronic Toxicity Unit (TUc) is defined TUc = 100/NOEC, where NOEC = % of effluent which gives no observed effect on the most sensitive of the range of species tested.

Table 6.9 Reference Marine Water Quality Criteria for Near Field Water Quality Assessment

Parameter	Value	Type / Period	Applicable Zones / Uses
Unionized ammonia (UIA)	≤ 0.021 mg/l (as N) ⁽²⁾	Annual average	At edge of ZID ⁽¹⁾
	≤ 0.035 mg/l (as NH₃)⁽³⁾	4-day average	At edge of ZID ⁽¹⁾
	≤ 0.233 mg/l (as NH₃)⁽³⁾	1-hour average	At edge of ZID ⁽¹⁾
pH	6.5 – 8.5, and change ≤ 0.2	≥ 90% of occasions	At edge of ZID ⁽¹⁾
Temperature	change ≤ 2 ^oC	≥ 90% of occasions	At edge of ZID ⁽¹⁾
Sulphide	≤ 0.02 mg/l	≥ 90% of occasions	At edge of ZID ⁽¹⁾
Cyanide	≤ 0.005 mg/l	≥ 90% of occasions	At edge of ZID ⁽¹⁾
Total residual chlorine (TRC)	≤ 0.013 mg/l	Daily Maximum	At edge of ZID ⁽¹⁾
Surfactants	≤ 0.03 mg/l	≥ 90% of occasions	At edge of ZID ⁽¹⁾
Copper	≤ 0.005 mg/l	≥ 90% of occasions	At edge of ZID ⁽¹⁾
Nickel	≤ 0.005 mg/l	≥ 90% of occasions	At edge of ZID ⁽¹⁾
Total chromium	≤ 0.05 mg/l	≥ 90% of occasions	At edge of ZID ⁽¹⁾
Zinc	≤ 0.02 mg/l	≥ 90% of occasions	At edge of ZID ⁽¹⁾
Mercury	≤ 0.00021 mg/l	≥ 90% of occasions	At edge of ZID ⁽¹⁾
Arsenic	≤ 0.02 mg/l	≥ 90% of occasions	At edge of ZID ⁽¹⁾
Phenol	≤ 0.005 mg/l	≥ 90% of occasions	At edge of ZID ⁽¹⁾
Acute Toxicity	0.3 acute toxicity units (TUa) (derived from LC50 values based on whole effluent toxicity tests)⁽⁴⁾	One hour average condition not to exceed this value	At edge of ZID ⁽¹⁾

(Source: EEFS Report on Community Consultation for the Proposed Water Quality Criteria)

⁽¹⁾ For a surface plume, initial dilution is defined as the dilution obtained at the centre line of the plume when the sewage reaches the surface. For a trapped plume, initial dilution is defined as the dilution obtained at the center line of the plume where the plume reaches the maximum rise height when the vertical momentum / buoyancy of the plume becomes zero.

⁽²⁾ The WQC for annual averaged UIA was derived with reference to the WQO under WPCO which is expressed as N.

⁽³⁾ The WQC for 1-hour and 4-day averaged UIA was derived with reference to the USEPA which is expressed as NH₃.

⁽⁴⁾ USEPA Technical Support Document for Water Quality-Based Toxics Control (March 1991), from which one acute Toxicity Unit (TUa) is defined as TUa = 100/LC50, where LC50 = % of effluent which gives 50% survival of the most sensitive of the range of species tested.

6.33 The chronic and acute toxicity criteria as shown in Table 6.8 and Table 6.9 respectively were used to assess the toxicity of the sewage effluent with reference to the results of the whole effluent toxicity test (WETT). Details of the WETT results are given in Table 6.33 to Table 6.36.

Summary of Water Quality Criteria

6.34 Table 6.10 compares the WQC derived under the EEFS (in Table 6.8 and Table 6.9) with the statutory requirements stipulated under the WPCO (in Table 6.1 to Table 6.6) as well as the water quality criteria specified by WSD (in Table 6.7). It should be noted that some parameters considered under the EEFS for far field water quality assessment as listed in Table 6.8, including PO₄, TRC and chronic toxicity, are not controlled under the WPCO. In addition, most of the parameters considered under the EEFS for near field water quality assessment (in Table 6.9) are not controlled under the WPCO except for pH and temperature.

Applicable zones/uses ⁽¹⁾	Parameters	EEFS WQC	WPCO WQO	WSD Criteria ⁽²⁾
Far Field Water Quality Criteria
Bathing waters	E.coli	≤ 180/ 100ml	Geometric mean for the period from March to October	≤ 180/ 100ml	Geometric mean for the period from March to October	Not applicable	Not applicable
Secondary contact zones	E.coli	≤ 610/ 100ml	Annual Geometric mean	≤ 610/ 100ml	Annual geometric mean	Not applicable	Not applicable
WSD flushing water intakes	E.coli	≤ 20,000/ 100ml	≥ 90% of occasions	Not specified	Not specified	≤ 20,000/ 100ml	at all times
Seawater intakes for industrial use	E.coli	≤ 20,000/ 100ml	≥ 90% of occasions	Not specified	Not specified	Not applicable	Not applicable
Mariculture zones	E.coli	≤ 610/ 100ml	Geometric mean	≤ 610/ 100ml	Annual geometric mean	Not applicable	Not applicable
DO	Depth- averaged	≥ 5 mg/l	≥90% of occasions	≥ 5 mg/l	≥ 90% of occasions	Not applicable	Not applicable
within 2m from the seabed	≥ 2 mg/l	≥ 90% of occasions	≥ 2 mg/l	≥ 90% of occasions	Not applicable	Not applicable
Semi-enclosed bays	TIN	≤0.1 mg/l (as N)	Annual mean	Not specified	Not specified	Not applicable	Not applicable
PO₄	≤ 0.01 mg/l (as P)	Annual mean	Not specified	Not specified	Not applicable	Not applicable
Western Buffer, Eastern Buffer, and Victoria Harbour WCZ (except fish spawning ground)	DO	Depth- averaged	≥ 4 mg/l	≥ 90% of occasions	≥ 4 mg/l	≥ 90% of occasions	Not applicable	Not applicable
within 2m from the seabed	Not specified	Not specified	≥ 2 mg/l	≥ 90% of occasions	Not applicable	Not applicable
TIN	≤ 0.4 mg/l (as N)	Annual mean	≤ 0.4 mg/l	Annual mean	Not applicable	Not applicable
PO₄	≤ 0.04 mg/l (as P)	Annual mean	Not specified	Not specified	Not applicable	Not applicable
Southern WCZ and fish spawning ground	DO	Depth- averaged	≥ 5 mg/l	Monthly average	≥ 4 mg/l	≥ 90% of occasions	Not applicable	Not applicable
within 2m from the seabed	Not specified	Not specified	≥ 2 mg/l	≥ 90% of occasions	Not applicable	Not applicable
TIN	≤ 0.2 mg/l (as N)	Annual mean	≤ 0.1 mg/l	Annual mean	Not applicable	Not applicable
PO₄	≤ 0.02 mg/l (as P)	Annual mean	Not specified	Not specified	Not applicable	Not applicable
Junk Bay WCZ	DO	Depth-averaged	≥ 4 mg/l	≥ 90% of occasions	≥ 4 mg/l	≥ 90% of occasions	Not applicable	Not applicable
within 2m from the seabed	Not specified	Not specified	≥ 2 mg/l	≥ 90% of occasions	Not applicable	Not applicable
TIN	≤ 0.3 mg/l (as N)	Annual mean	≤ 0.3 mg/l	Annual mean	Not applicable	Not applicable
PO₄	≤ 0.03 mg/l (as P)	Annual mean	Not specified	Not specified	Not applicable	Not applicable
All WCZ	DO at any depth	≥ 2 mg/l	at all times	Not specified	Not specified	Not applicable	Not applicable
All WCZ	UIA	≤ 0.021 mg/l (as N)	Annual mean	≤ 0.021 mg/l	Annual mean	Not applicable	Not applicable
All WCZ	TRC	≤ 0.008 mg/l	Daily maximum	Not specified	Not specified	Not applicable	Not applicable
Near Field Water Quality Criteria
At edge of Initial dilution zone	pH ⁽³⁾	6.5 – 8.5, and change ≤ 0.2	≥ 90% of occasions	6.5 – 8.5, and change ≤ 0.2	at all times	Not applicable	Not applicable
Temperature ⁽³⁾	change ≤ 2^oC	≥ 90% of occasions	change ≤ 2^oC	at all times	Not applicable	Not applicable

CBP	Water Quality Criteria (µg/l)
Bromodichloromethane	22 ^a (marine water, annual avg.)
Bromoform	360 ^a (marine water, annual avg.)
Chloroform	12^b (marine water, annual avg.)
Dibromochloromethane	34 ^a (marine water, annual avg.)
Bromoacetic acid	See Note 1
Chloroacetic acid	See Note 1
Dibromoacetic acid	See Note 1
Dichloroacetic acid	See Note 1
Trichloroacetic acid	See Note 1
Methylene chloride	1,580^a (marine water, annual avg.)
Carbon tetrachloride	12 ^b (marine, annual avg.)
Chlorobenzene	25 ^c (marine water)
1,1-dichloroethane	See Note 1
1,2-dichloroethane	10 ^b (marine, annual avg.)
1,1-dichloroethylene	3.2 ^a (marine, annual avg.)
1,2-dichloropropane	See Note 1
Tetrachloroethylene	8.85 ^a (marine, annual avg.)
1,1,1-trichloroethane	100 ^b (marine annual avg.)
1,1,2-trichloroethane	100 ^b (marine annual avg.)
Trichloroethylene	10 ^b (marine annual avg.)
2-chlorophenol	50 ^b (marine annual avg.)
2,4-dichlorophenol	20 ^b (marine annual avg.)
p-chloro-m-cresol	40 ^b (marine annual avg.)
Pentachlorophenol	2 ^b (marine annual avg.)
2,4,6-trichlorophenol	See Note 1
Bis(2-chloroethoxy)methane	See Note 1
1,4-dichlorobenzene	See Note 1
Hexachlorobenzene	0.03 ^b (marine, annual avg.)
Hexachlorocyclopentadiene	See Note 1
Hexachloroethane	See Note 1
1,2,4-trichlorobenzene	5.4 ^c (marine water)
Alpha-BHC	See Note 1
Beta-BHC	0.046^a (marine, annual avg.)
Gamma-BHC	0.063^a (marine, annual avg.)

Contaminants	LCEL	UCEL
*Heavy Metal (mg/kg dry weight)*
Cadmium (Cd)	1.5	4
Chromium (Cr)	80	160
Copper (Cu)	65	110
Mercury (Hg)	0.5	1
Nickel (Ni)	40	40
Lead (Pb)	75	110
Silver (Ag)	1	2
Zinc (Zn)	200	270
*Metalloid (mg/kg dry weight)*
Arsenic	12	42
*Organic-PAHs (µg/kg dry weight)*
PAHs (Low Molecular Weight)	550	3160
PAHs (High Molecular Weight)	1700	9600
*Organic-non-PAHs (µg/kg dry weight)*
Total PCBs	23	180

Table 6.13 Baseline Water Quality Condition for North Western WCZ in 2006

Parameter		Lantau Island (North)	Pearl Island	Pillar Point	Urmston Road	Chek Lap Kok		WPCO WQO (in marine waters)	EEFS WQC (in marine waters)
Parameter		NM1	NM2	NM3	NM5	NM6	NM8	WPCO WQO (in marine waters)	EEFS WQC (in marine waters)
Temperature (^oC)		23.7 (17.6 – 27.4)	23.8 (17.5 – 27.6)	23.7 (17.7-27.6)	24.0 (17.9 – 27.8)	24.0 (17.7 – 29.2)	23.8 (17.5 – 28.3)	Not more than 2 ^oC in daily temperature range	Not available
Salinity		29.6 (22.2 – 33.1)	28.6 (19.0 – 33.1)	29.4 (23.7 – 33.1)	27.2 (16.4 – 32.8)	26.0 (10.5 – 33.3)	27.6 (11.9 – 33.4)	Not to cause more than 10% change	Not available
Dissolved Oxygen (DO) (mg/l)	Depth average	6.3 ((4.4 – 8.0)	6.5 (4.9 – 8.4)	6.3 (4.4 – 8.3)	6.3 (4.3 – 8.2)	6.7 (4.8 – 8.7)	6.8 (4.8 – 8.2)	Not less than 4 mg/l for 90% of the samples	Not available
Dissolved Oxygen (DO) (mg/l)	Bottom	5.9 (2.4 – 7.9)	6.3 (3.9 – 8.4)	6.1 ((3.3 – 8.0)	5.9 (3.4 – 7.9)	6.6 (4.2 – 8.6)	6.7 (4.6 – 8.3)	Not less than 2 mg/l for 90% of the samples	Not available
Dissolved Oxygen (DO) (% Saturation)	Depth average	87 (62 – 113)	90 (69 – 119)	88 (63 – 117)	87 (59 – 116)	92 (64 – 123)	94 (64 – 113)	Not available	Not available
Dissolved Oxygen (DO) (% Saturation)	Bottom	82 (35 – 111)	88 (56 – 118)	85 (47 – 113)	83 (49 – 108)	92 (60 – 122)	94 (63 – 118)	Not available	Not available
PH		7.9 (7.6 – 8.0)	7.9 (7.6 – 8.1)	7.9 (7.6 – 8.1)	7.9 (7.5 – 8.1)	7.9 (7.5 – 8.2)	7.9 (7.5 – 8.2)	6.5 - 8.5 (± 0.2 from natural range)	Not available
Secchi disc Depth (m)		1.6 (0.9 – 2.5)	1.7 (1.1 – 2.8)	1.5 (0.5 – 2.4)	1.4 (0.7 – 2.2)	1.2 (0.5 – 1.5)	1.3 (0.8 – 2.0)	Not available	Not available
Turbidity (NTU)		20.3 (6.8 – 78.7)	18.0 (5.8 – 75.3)	18.7 (8.1 – 55.8)	25.8 (11.0 – 75.3)	22.9 (8.0 – 47.8)	23.8 (9.3 – 45.5)	Not available	Not available
Suspended Solids (SS) (mg/l)		7.4 (2.5 – 17.4)	6.4 (2.9 – 21.3)	8.1 (3.0 – 14.0)	15.7 (3.8 – 53.8)	12.6 (4.1 – 35.9)	15.8 (2.7 – 56.7)	Not more than 30% increase	Not available
5-day Biochemical Oxygen Demand (BOD₅) (mg/l)		0.6 (0.4 – 1.1)	0.6 (0.2 – 1.0)	0.7 (0.4 – 1.2)	0.7 (0.5 – 0.9)	0.7 (0.3 – 1.3)	0.7 (0.3 – 1.9)	Not available	Not available
Ammonia Nitrogen (NH₃-N) (mgN/l)		0.15 (0.04 – 0.27)	0.15 (0.05 – 0.27)	0.15 (0.04-0.27)	0.22 (0.13 – 0.37)	0.17 (0.01 – 0.50)	0.10 (0.01 – 0.35)	Not available	Not available
Unionised Ammonia (UIA) (mgN/l)		0.005 (<0.001 – 0.010)	0.005 (0.001 – 0.011)	0.005 (0.001 – 0.011)	0.008 (0.003 – 0.017)	0.006 (0.002 – 0.022)	0.004 (0.001 – 0.019)	Not more than 0.021 mg/l for annual mean	Not available
Nitrite Nitrogen (NO₂-N) (mgN/l)		0.056 (0.017 – 0.150)	0.064 (0.020 – 0.170)	0.064 (0.018 – 0.187)	0.091 (0.024 – 0.250)	0.093 (0.026 – 0.200)	0.066 (0.009 – 0.150)	Not available	Not available
Nitrate Nitrogen (NO₃-N) (mgN/l)		0.23 (0.04 – 0.45)	0.28 (0.04 – 0.57)	0.28 (0.06 – 0.51)	0.37 (0.08 – 0.69)	0.39 (0.03 – 1.00)	0.28 (0.01 – 0.72)	Not available	Not available
Total Inorganic Nitrogen (TIN) (mgN/l)		0.43 (0.17 – 0.75)	0.49 (0.18 – 0.85)	0.50 (0.22 – 0.80)	0.67 (0.29 – 1.07)	0.66 (0.09 – 1.40)	0.44 (0.06 – 1.20)	Not more than 0.5 mg/l for annual mean	Not available
Total Nitrogen (TN) (mgN/l)		0.60 (0.29 – 0.92)	0.65 (0.28 – 1.05)	0.66 (0.33 – 0.94)	0.86 (0.41 – 1.31)	0.84 (0.22 – 1.66)	0.62 (0.15 – 1.39)	Not available	Not available
Orthophosphate Phosphorus (PO₄) (mgP/l)		0.03 (0.01 – 0.04)	0.03 (0.01 – 0.04)	0.03 (0.01 – 0.04)	0.03 (0.01 – 0.05)	0.02 (<0.01 – 0.05)	0.02 (<0.01 – 0.04)	Not available	Not available
Total Phosphorus (TP) (mgP/l)		0.05 (0.03 – 0.06)	0.04 (0.03 – 0.06)	0.05 (0.04 – 0.06)	0.07 (0.05 – 0.11)	0.05 (0.03 – 0.08)	0.04 (0.02 – 0.06)	Not available	Not available
Chlorophyll-a (µg/L)		3.6 (0.8 – 19.2)	2.8 (0.8 – 10.6)	3.3 (1.0 – 7.7)	4.2 (1.3 – 17.4)	3.9 (1.1 – 12.0)	3.5 (1.3 – 14.7)	Not available	Not available
E.coli (cfu/100 ml)		1100 (340 – 2600)	470 (280 – 1900)	500 (140 – 2100)	900 (220 – 2600)	64 (2 – 1900)	5 (1 – 420)	Not available	Not available
Faecal Coliforms (cfu/100 ml)		2800 (1100 – 7200)	1100 (570 – 3600)	1300 (260 – 6300)	2300 (460 – 6300)	140 (8 – 3600)	13 (1 – 950)	Not available	Not available

Note: 1. Except as specified, data presented are depth-averaged values calculated by taking the means of three depths: Surface, mid-depth, bottom.

2. Data presented are annual arithmetic means of depth-averaged results except for E.coli and faecal coliforms that are annual geometric means.

3. Data in brackets indicate the ranges.

4. EEFS did not derive WQC for North Western WCZ.

Table 6.14 Baseline Water Quality Condition for Victoria Harbour WCZ in 2006

Parameter		Victoria Harbour East		Victoria Harbour Central		Victoria Harbour West		Stonecutters Island		Rambler Channel		WPCO WQO (in marine waters)	EEFS WQC (in marine waters)
Parameter		VM1	VM2	VM4	VM5	VM6	VM7	VM8	VM15	VM12	VM14	WPCO WQO (in marine waters)	EEFS WQC (in marine waters)
Temperature (^oC)		23.2 (17.4 – 26.7)	23.5 (17.4 – 27.5)	23.6 (17.4 – 27.6)	23.7 (17.4 – 27.5)	23.6 (17.5 – 27.4)	23.8 (16.9 – 27.9)	23.8 (16.9 – 27.9)	23.7 (17.6 – 27.7)	23.8 (17.2 – 27.1)	24.2 (17.4 – 27.8)	Not more than 2 ^oC in daily temperature range	Not available
Salinity		32.2 (29.5 – 33.4)	31.7 (27.8 – 33.0)	31.6 (27.4 – 33.0)	31.4 (27.3 – 32.8)	31.5 (27.3 – 32.8)	30.8 (24.1 – 33.0)	20.7 (23.3 – 33.0)	31.1 (26.2 – 32.8)	30.8 (25.4 – 32.9)	28.7 (16.8 – 33.0)	Not to cause more than 10% change	Not available
Dissolved Oxygen (DO) (% Saturation)	Depth average	82 (46 – 101)	81 (49 – 97)	80 (61 – 94)	77 (65 – 91)	78 (60 – 89)	80 (60 – 107)	85 (61 – 105)	80 (67 – 93)	77 (46 – 98)	84 (61 – 108)	Not available	Not available
Dissolved Oxygen (DO) (% Saturation)	Bottom	80 (24 – 111)	81 (36 – 103)	79 (47 – 98)	76 (56 – 94)	75 (40 – 93)	77 (48 – 98)	82 (49 – 109)	78 (55 – 91)	75 (38 – 97)	83 (57 – 104)	Not available	Not available
Dissolved Oxygen (DO) (mg/l)	Depth average	5.9 (3.2 – 7.5)	5.8 (3.4 – 7.1)	5.7 (4.1 – 7.3)	5.5 (4.3 – 7.1)	5.5 (4.0 – 6.9)	5.7 (4.0 – 7.3)	6.0 (4.1 – 7.7)	5.7 (4.5 – 6.9)	5.5 (3.1 – 7.4)	6.0 (4.3 – 7.6)	Not less than 4 mg/l for 90% of the samples	Not less than 4 mg/l for 90% of the samples
Dissolved Oxygen (DO) (mg/l)	Bottom	5.7 (1.7 – 8.0)	5.8 (2.5 – 7.4)	5.6 (3.2 – 7.2)	5.4 (3.8 – 7.3)	5.4 (2.7 – 7.2)	5.5 (3.3 – 7.0)	5.8 (3.3 – 7.8)	5.5 (3.8 – 7.0)	5.3 (2.6 – 7.4)	5.9 (3.9 – 7.2)	Not less than 2 mg/l for 90% of the samples	Not less than 2 mg/l at all times
pH		7.9 (7.7 – 8.2)	7.9 (7.7 – 8.1)	7.9 (7.7 – 8.1)	7.9 (7.7 – 8.1)	7.9 (7.7 – 8.1)	8.0 (7..7 – 8.3)	8.0 (7.8 – 8.3)	7.9 (7.7 – 8.0)	8.0 (7.8 – 8.3)	8.0 (7.8 – 8.4)	6.5 - 8.5 (± 0.2 from natural range)	Not available
Secchi disc Depth (m)		2.1 (1.2 – 3.0)	2.0 (1.4 – 3.1)	2.0 (1.4 – 3.0)	1.8 (1.4 – 2.7)	2.1 (1.5 – 2.7)	2.0 (1.2 – 2.5)	1.9 (1.0 – 2.5)	2.0 (1.3 – 2.5)	1.5 (0.5 – 2.1)	1.5 (1.0 – 2.0)	Not available	Not available
Turbidity (NTU)		12.6 (6.9 – 17.6)	11.2 (5.4 – 23.4)	12.1 (5.6 – 23.4)	11.6 (6.5 – 18.5)	11.1 (5.2 – 14.2)	11.5 (5.8 – 20.5)	11.9 (5.3 – 19.0)	12.4 (6.4 – 22.9)	15.6 (10.0 – 28.0)	12..6 (7.0 – 22.2)	Not available	Not available
Suspended Solids (SS) (mg/l)		5.5 (2.1 – 18.0)	4.2 (1.2 – 12.8)	4.9 (1.1 – 12.3)	4.6 (1.7 – 8.9)	4.5 (2.0 – 8.9)	5.5 (1.8 – 11.6)	5.9 (2.1 – 11.0)	6.1 (1.8 – 13.9)	11.0 (3.1 – 19.0)	5.9 (2.9 – 12.7)	Not more than 30% increase	Not available
5-day Biochemical Oxygen Demand (BOD₅) (mg/l)		0.6 (0.1 – 1.5)	0.6 (0.1 – 1.2)	0.7 (0.1 – 1.4)	1.0 (0.2 – 1.9)	0.8 (0.1 – 1.8)	0.8 (0.1 – 1.2)	0.7 (0.2 – 1.1)	0.7 (0.1 – 1.7)	0.7 (0.2 – 1.6)	0.7 (0.2 – 1.2)	Not available	Not available
Nitrite Nitrogen (NO₂-N) (mgN/l)		0.019 (0.004 – 0.067)	0.024 (0.004 – 0.084)	0.024 (0.006 – 0.079)	0.027 (0.009 – 0.093)	0.027 (0.009 – 0.095)	0.033 (0.014 – 0.053)	0.035 (0.009 – 0.060)	0.033 (0.010 – 0.120)	0.036 (0.011 – 0.053)	0.053 (0.010 – 0.100)	Not available	Not available
Nitrate Nitrogen (NO₃-N) (mgN/l)		0.08 (0.03 – 0.20)	0.10 (0.03 – 0.25)	0.11 (0.03 – 0.25)	0.12 (0.04 – 0.27)	0.13 (0.04 – 0.28)	0.16 (0.06 – 0.38)	0.16 (0.04 – 0.40)	0.15 (0.06 – 0.36)	0.17 (0.08 – 0.37)	0.27 (0.06 – 0.64)	Not available	Not available
Ammonia Nitrogen (NH₃-N) (mgN/l)		0.07 (0.04 – 0.13)	0.11 (0.04 – 0.20)	0.13 (0.05 – 0.23)	0.16 (0.09 – 0.30)	0.16 (0.11 – 0.25)	0.21 (0.07 – 0.52)	0.17 (0.13 – 0.26)	0.18 (0.12 – 0.29)	0.18 (0.05 – 0.28)	0.16 (0.04 – 0.23)	Not available	Not available
Unionised Ammonia (UIA) (mgN/l)		0.002 (0.001 – 0.004)	0.004 (0.001 – 0.007)	0.004 (0.002 – 0.007)	0.005 (0.003 – 0.008)	0.005 (0.003 – 0.007)	0.008 (0.003 – 0.018)	0.007 (0.004 – 0.016)	0.006 (0.003 – 0.009)	0.006 (0.003 – 0.011)	0.007 (0.003 – 0.014)	Not more than 0.021 mg/l for annual mean	Not available
Total Inorganic Nitrogen (TIN) (mgN/l)		0.18 (0.07 – 0.29)	0.23 (0.07 – 0.40)	0.26 (0.08 – 0.44)	0.31 (0.14 – 0.50)	0.32 (0.16 – 0.51)	0.40 (0.24 – 0.67)	0.37 (0.20 – 0.64)	0.36 (0.18 – 0.67)	0.39 (0.26 – 0.49)	0.48 (0.21 – 0.88)	Not more than 0.4 mg/l for annual mean	Not more than 0.4 mg/l for annual mean
Total Nitrogen (TN) (mgN/l)		0.34 (0.22 – 0.49)	0.42 (0.20 – 0.64)	0.47 (0.22 – 0.69)	0.55 (0.30 – 0.83)	0.53 (0.34 – 0.73)	0.58 (0.45 – 0.75)	0.53 (0.38 – 0.86)	0.58 (0.35 – 0.86)	0.57 (0.39 – 0.75)	0.64 (0.31 – 1.06)	Not available	Not available
Orthophosphate Phosphorus (PO₄) (mgP/l)		0.02 (0.01 – 0.03)	0.03 (0.01 – 0.04)	0.03 (0.01 – 0.04)	0.03 (0.02 – 0.04)	0.03 (0.02 – 0.05)	0.03 (0.01 – 0.05)	0.03 (0.02 – 0.04)	0.03 (0.01 – 0.04)	0.03 (0.01 – 0.04)	0.03 (0.01 – 0.04)	Not available	Not more than 0.04 mg/l for annual mean
Total Phosphorus (TP) (mgP/l)		0.04 (0.03 – 0.07)	0.05 (0.03 – 0.06)	0.05 (0.03 – 0.07)	0.06 (0.03 – 0.08)	0.06 (0.03 – 0.08)	0.05 (0.03 – 0.08)	0.04 (0.03 – 0.06)	0.06 (0.04 – 0.07)	0.05 (0.03 – 0.08)	0.04 (0.03 – 0.06)	Not available	Not available
Chlorophyll-a (µg/L)		2.6 (0.8 – 6.8)	3.0 (1.0 – 8.9)	2.9 (1.0 – 9.2)	2.8 (0.9 – 8.6)	2.9 (0.9 – 10.1)	2.6 (0.6 – 9.0)	2.7 (1.0 – 11.3)	3.5 (1.1 – 16.6)	2.1 (0.8 – 6.9)	3.4 (0.9 – 19.5)	Not available	Not available
E coli (cfu/100 ml)		440 (44 – 3400)	1100 (58 – 14000)	2600 (510 – 12000)	7700 (1900 – 22000)	5500 (650 – 33000)	9400 (2600 – 57000)	6100 (350 – 11000)	1800 (590 – 11000)	3400 (1500 – 6600)	1300 (320 – 5100)	Not available	Not available
Faecal Coliforms (cfu/100 ml)		940 (74 – 8600)	2600 (130 – 25000)	6500 (1800 – 40000)	19000 (3500 – 65000)	13000 (2500 – 100000)	23000 (5400 – 180000)	15000 (930 – 55000)	4800 (1300 – 23000)	8100 (4200 – 14000)	2800 (460 – 13000)	Not available	Not available

Note: 1. Except as specified, data presented are depth-averaged values calculated by taking the means of three depths: Surface, mid-depth, bottom.

2. Data presented are annual arithmetic means of depth-averaged results except for E.coli and faecal coliforms that are annual geometric means.

3. Data in brackets indicate the ranges.

Table 6.15 Baseline Water Quality Condition for Western Buffer WCZ in 2006

Parameter		Hong Kong Island (West)	Tsing Yi (South)	Tsing Yi (West)	WPCO WQO (in marine waters)	EEFS WQC (in marine waters)
Parameter		WM2	WM3	WM4	WPCO WQO (in marine waters)	EEFS WQC (in marine waters)
Temperature (^oC)		23.8 (16.9 – 27.4)	23.6 (17.0 – 27.2)	23.6 (17.0 – 27.2)	Not more than 2 ^oC in daily temperature range	Not available
Salinity		30.9 (23.9 – 33.2)	31.5 (27.2 – 33.0)	31.0 (26.6 – 33.0)	Not to cause more than 10% change	Not available
Dissolved Oxygen (DO) (mg/l)	Depth average	6.1 (4.0 – 8.1)	5.8 (2.8 – 7.8)	5.7 (3.3 – 7.9)	Not less than 4 mg/l for 90% of the samples	Not less than 4 mg/l for 90% of the samples
Dissolved Oxygen (DO) (mg/l)	Bottom	6.0 (3.5 – 8.2)	5.7 (2.5 – 8.0)	5.4 (1.4 – 7.8)	Not less than 2 mg/l for 90% of the samples	Not less than 2 mg/l at all times
Dissolved Oxygen (DO) (% Saturation)	Depth average	86 (59 – 113)	82 (41 – 102)	80 (49 – 110)	Not available	Not available
Dissolved Oxygen (DO) (% Saturation)	Bottom	84 (52 – 114)	80 (37 – 111)	76 (20 – 109)	Not available	Not available
pH		8.0 (7.7 – 8.3)	8.0 (7.8 – 8.2)	8.0 (7.8 – 8.3)	6.5 - 8.5 (± 0.2 from natural range)	Not available
Secchi disc Depth (m)		2.0 (1.3 – 2.5)	1.7 (1.0 – 2.3)	1.7 (0.8 – 2.5)	Not available	Not available
Turbidity (NTU)		12.0 (6.5 – 20.3)	13.8 (6.2 – 25.1)	14.3 (6.3 – 23.0)	Not available	Not available
Suspended Solids (SS) (mg/l)		4.8 (1.4 – 10.3)	7.7 (2.8 – 15.0)	7.6 (3.2 – 13.0)	Not more than 30% increase	Not available
5-day Biochemical Oxygen Demand (BOD₅) (mg/l)		0.6 (0.1 – 1.2)	0.7 (0.2 – 1.2)	0.6 (0.1 – 1.1)	Not available	Not available
Ammonia Nitrogen (NH₃-N) (mgN/l)		0.10 (0.05 – 0.21)	0.13 (0.04 – 0.23)	0.12 (0.02 – 0.22)	Not available	Not available
Unionised Ammonia (UIA) (mgN/l)		0.004 (0.001 – 0.009)	0.005 (0.003 – 0.011)	0.005 (0.002 – 0.010)	Not more than 0.021 mg/l for annual mean	Not available
Nitrite Nitrogen (NO₂-N) (mgN/l)		0.034 (0.006 – 0.057)	0.032 (0.007 – 0.052)	0.037 (0.007 – 0.056)	Not available	Not available
Nitrate Nitrogen (NO₃-N) (mgN/l)		0.16 (0.03 – 0.43)	0.14 (0.04 – 0.32)	0.17 (0.05 – 0.38)	Not available	Not available
Total Inorganic Nitrogen (TIN) (mgN/l)		0.29 (0.12 – 0.55)	0.30 (0.15 – 0.45)	0.32 (0.17 – 0.48)	Not more than 0.4 mg/l for annual mean	Not more than 0.4 mg/l for annual mean
Total Nitrogen (TN) (mgN/l)		0.43 (0.23 – 0.79)	0.45 (0.27 – 0.60)	0.47 (0.28 – 0.65)	Not available	Not available
Orthophosphate Phosphorus (PO₄) (mgP/l)		0.02 (0.01 – 0.03)	0.02 (0.01 – 0.03)	0.02 (0.01 – 0.03)	Not available	Not more than 0.04 mg/l for annual mean
Total Phosphorus (TP) (mgP/l)		0.04 (0.02 – 0.06)	0.04 (0.02 – 0.07)	0.04 (0.02 – 0.07)	Not available	Not available
Chlorophyll-a (µg/L)		2.8 (0.9 – 10.9)	2.0 (0.8 – 6.0)	2.4 (1.0 – 9.9)	Not available	Not available
E coli (cfu/100 ml)		910 (16 – 5400)	3600 (1100 – 15000)	1400 (200 – 8700)	Not available	Not available
Faecal Coliforms (cfu/100 ml)		2000 (42 – 11000)	8600 (2900 – 31000)	3000 (360 – 28000)	Not available	Not available

Note: 1. Except as specified, data presented are depth-averaged values calculated by taking the means of three depths: Surface, mid-depth, bottom.

2. Data presented are annual arithmetic means of depth-averaged results except for E.coli and faecal coliforms that are annual geometric means.

3. Data in brackets indicate the ranges.

Table 6.16 Baseline Water Quality Condition for Junk Bay and Eastern Buffer WCZ in 2006

Parameter		Junk Bay WCZ				Eastern Buffer WCZ
Parameter		JM3	JM4	WPCO WQO (in marine waters)	EEFS WQO (in marine waters)	EM1	EM2	WPCO WQO (in marine waters)	EEFS WQC (in marine waters)
Temperature (^oC)		23.7 (17.4 – 27.2)	23.5 (17.4 – 26.6)	Not more than 2 ^oC in daily temperature range	Not available	23.4 (17.2-26.6)	23.4 (17.4 – 26.6)	Not more than 2 ^oC in daily temperature range	Not available
Salinity		31.8 (28.9 – 33.1)	32.3 (29.2 – 33.4)	Not to cause more than 10% change	Not available	32.2 (30.1 – 33.3)	32.3 (29.1 – 33.6)	Not to cause more than 10% change	Not available
Dissolved Oxygen (DO) (% Saturation)	Depth average	93 (78 – 119)	91 (69 – 112)	Not available	Not available	88 (63 – 101)	92 (69 – 112)	Not available	Not available
Dissolved Oxygen (DO) (% Saturation)	Bottom	90 (66 – 120)	88 (58 – 112)	Not available	Not available	84 (49 – 110)	88 (49 – 114)	Not available	Not available
Dissolved Oxygen (DO) (mg/l)	Depth average	6.6 (5.6 – 8.1)	6.4 (4.7 – 7.7)	Not less than 4 mg/l for 90% of the samples	Not less than 4 mg/l for 90% of the samples	6.2 (4.3 – 7.7)	6.5 (4.7 – 8.0)	Not less than 4 mg/l for 90% of the samples	Not less than 4 mg/l for 90% of the samples
Dissolved Oxygen (DO) (mg/l)	Bottom	6.4 (4.5-8.2)	6.2 (4.1 – 8.0)	Not less than 2 mg/l for 90% of the samples	Not less than 2 mg/l at all times	6.0 (3.4 – 7.9)	6.3 (3.4 – 8.2)	Not less than 2 mg/l for 90% of the samples	Not less than 2 mg/l at all times
pH		7.9 (7.7 – 8.3)	8.0 (7.7 – 8.3)	6.5 - 8.5 (± 0.2 from natural range)	Not available	8.0 (7.8 – 8.1)	8.0 (7.8 – 8.2)	6.5 - 8.5 (± 0.2 from natural range)	Not available
Secchi disc Depth (m)		2.2 (1.2 – 3.2)	2.2 (1.5 – 3.5)	Not available	Not available	2.2 (1.3 – 3.9)	2.1 (1.5 – 3.2)	Not available	Not available
Turbidity (NTU)		10.0 (2.5 – 16.4)	10.9 (4.1 – 14.9)	Not available	Not available	12.0 (4.9 -23.3)	10.8 (4.6 – 15.9)	Not available	Not available
Suspended Solids (SS) (mg/l)		3.2 (0.9 – 8.4)	4.5 (1.6 – 19.0)	Not more than 30% increase	Not available	3.9 (1.8 – 12.3)	3.9 (1.3-15.7)	Not more than 30% increase	Not available
5-day Biochemical Oxygen Demand (BOD₅) (mg/l)		0.7 (0.2 – 1.7)	0.6 (0.3 – 1.8)	Not available	Not available	0.6 (0.1 – 1.6)	0.5 (0.1 – 1.5)	Not available	Not available
Nitrite Nitrogen (NO₂-N) (mgN/l)		0.02 (0.004 – 0.084)	0.018 (0.003 – 0.060)	Not available	Not available	0.017 (0.004 – 0.047)	0.016 (0.002 – 0.058)	Not available	Not available
Nitrate Nitrogen (NO₃-N) (mgN/l)		0.09 (0.02 – 0.28)	0.07 (0.01 – 0.19)	Not available	Not available	0.07 (0.01 – 0.18)	0.07 (0.01 – 0.19)	Not available	Not available
Ammonia Nitrogen (NH₃-N) (mgN/l)		0.07 (0.03 – 0.14)	0.06 (0.02 – 0.10)	Not available	Not available	0.06 (0.02 – 0.14)	0.05 (0.01 – 0.09)	Not available	Not available
Unionised Ammonia (UIA) (mgN/l)		0.003 (0.001 – 0.005)	0.002 (0.001 – 0.004)	Not more than 0.021 mg/l for annual mean	Not available	0.003 (<0.001 – 0.006)	0.002 (<0.001 – 0.004)	Not more than 0.021 mg/l for annual mean	Not available
Total Inorganic Nitrogen (TIN) (mgN/l)		0.18 (0.06 – 0.44)	0.15 (0.04 – 0.33)	Not more than 0.3 mg/l for annual mean	Not more than 0.3 mg/l for annual mean	0.15 (0.04 – 0.30)	0.13 (0.02 – 0.32)	Not more than 0.4 mg/l for annual mean	Not more than 0.4 mg/l for annual mean
Total Nitrogen (TN) (mgN/l)		0.34 (0.19 – 0.61)	0.31 (0.16 – 0.45)	Not available	Not available	0.29 (0.17 – 0.47)	0.28 (0.15 – 0.48)	Not available	Not available
Orthophosphate Phosphorus (PO₄) (mgP/l)		0.01 (<0.01 – 0.03)	0.01 (<0.01 – 0.02)	Not available	Not more than 0.03 mg/l for annual mean	0.02 (<0.01 – 0.03)	0.01 (<0.01 – 0.02)	Not available	Not more than 0.04 mg/l for annual mean
Total Phosphorus (TP) (mgP/l)		0.03 (0.02 – 0.05)	0.03 (0.02 – 0.04)	Not available	Not available	0.03 (0.02 – 0.05)	0.03 (0.02 – 0.04)	Not available	Not available
Chlorophyll-a (µg/L)		4.8 (1.0 – 12.3)	3.6 (1.0 – 12.3)	Not available	Not available	3.3 (0.9 – 9.7)	3.3 (1.0 – 9.1)	Not available	Not available
E coli (cfu/100 ml)		38 (3 – 420)	76 (4 – 300)	Not available	Not available	74 (1 – 1800)	21 (2 – 110)	Not available	Not available
Faecal Coliforms (cfu/100 ml)		140 (18 – 740)	220 (19 – 1000)	Not available	Not available	190 (1 – 4900)	80 (12 – 430)	Not available	Not available

Note: 1. Except as specified, data presented are depth-averaged values calculated by taking the means of three depths: Surface, mid-depth, bottom.

2. Data presented are annual arithmetic means of depth-averaged results except for E.coli and faecal coliforms that are annual geometric means.

3. Data in brackets indicate the ranges.

Table 6.17 Baseline Water Quality Condition for Southern WCZ in 2006

Parameter		West Lamma Channel		Lantau Island (East)		WPCO WQO (in marine waters)	EEFS WQC (in marine waters)
Parameter		SM7	SM9	SM10	SM11	WPCO WQO (in marine waters)	EEFS WQC (in marine waters)
Temperature (^oC)		24.3 (18.0 – 29.0)	24.0 (17.8 – 27.5)	24.3 (17.8 – 27.9)	24.3 (17.9 – 28.2)	Not more than 2 ^oC in daily temperature range	Not available
Salinity		31.1 (26.4 – 33.2)	30.8 (26.5 – 33.0)	30.4 (24.8 – 33.0)	30.7 (25.7 – 33.0)	Not to cause more than 10% change	Not available
Dissolved Oxygen (DO) (mg/l)	Depth average	6.8 (5.1 – 9.3)	6.3 (4.4 – 8.0)	6.9 (5.2 – 9.9)	7.5 (5.6 – 11.3)	Not less than 4 mg/l for 90% of the samples	Not less than 5 mg/l for monthly average
Dissolved Oxygen (DO) (mg/l)	Bottom	6.8 (4.9 – 9.3)	6.3 (4.1 – 8.2)	6.9 (5.1 – 10)	7.3 (4.5 – 12)	Not less than 2 mg/l for 90% of the samples	Not less than 2 mg/l at all times
Dissolved Oxygen (DO) (% Saturation)	Depth average	97 (74 – 119)	89 (63 – 110)	98 (73 – 146)	107 (82 – 168)	Not available	Not available
Dissolved Oxygen (DO) (% Saturation)	Bottom	96 (71 – 119)	89 (59 – 113)	98 (73 – 147)	104 (66 – 170)	Not available	Not available
pH		8.2 (7.8 – 8.7)	8.1 (7.8 – 8.2)	8.1 (7.8 – 8.5)	8.1 (7.8 – 8.5)	6.5 - 8.5 (± 0.2 from natural range)	Not available
Secchi disc Depth (m)		2.0 (1.1 – 3.1)	1.5 (0.8 – 2.1)	1.2 (0.6 – 1.9)	1.5 (0.8 – 2.3)	Not available	Not available
Turbidity (NTU)		13.3 (5.4 – 33.9)	14.7 (6.2 – 26.7)	16.2 (4.9 – 39.8)	12.3 (4.4 – 21.5)	Not available	Not available
Suspended Solids (SS) (mg/l)		4.8 (1.2 – 9.0)	8.0 (2.6 – 17.6)	9.1 (3.8 – 32.0)	6.0 (3.2 – 10.7)	Not more than 30% increase	Not available
5-day Biochemical Oxygen Demand (BOD₅) (mg/l)		0.8 (0.4 – 1.9)	0.5 (0.2 – 1.0)	0.8 (0.2 – 2.5)	0.9 (0.3 – 2.4)	Not to exceed 5 mg/l	Not available
Ammonia Nitrogen (NH₃-N) (mgN/l)		0.07 (0.02 – 0.14)	0.13 (0.05 – 0.19)	0.11 (0.04 – 0.19)	0.09 (0.04 – 0.17)	Not available	Not available
Unionised Ammonia (UIA) (mgN/l)		0.004 (0.001 – 0.009)	0.006 (0.002 – 0.012)	0.006 (0.002 – 0.013)	0.005 (0.002 – 0.009)	Not more than 0.021 mg/l for annual mean	Not available
Nitrite Nitrogen (NO₂-N) (mgN/l)		0.027 (0.002 – 0.071)	0.040 (0.007 – 0.110)	0.038 (0.006 – 0.093)	0.035 (0.005 – 0.091)	Not available	Not available
Nitrate Nitrogen (NO₃-N) (mgN/l)		0.13 (<0.01 – 0.34)	0.17 (0.05 – 0.34)	0.17 (0.04 – 0.39)	0.15 (0.03 – 0.38)	Not available	Not available
Total Inorganic Nitrogen (TIN) (mgN/l)		0.23 (0.03 – 0.47)	0.34 (0.14 – 0.53)	0.32 (<0.10 – 0.60)	0.28 (0.09 – 0.51)	Not more than 0.1 mg/l for annual mean	Not more than 0.2 mg/l for annual mean
Total Kjeldahl Nitrogen (TKN) (mgN/l)		0.23 (0.13 – 0.29)	0.28 (0.16 – 0.35)	0.28 (0.19 – 0.38)	0.28 (0.15 – 0.42)	Not available	Not available
Total Nitrogen (TN) (mgN/l)		0.39 (0.15 – 0.63)	0.49 (0.37 – 0.74)	0.49 (0.33 – 0.75)	0.46 (0.32 – 0.71)	Not available	Not available
Orthophosphate Phosphorus (PO₄) (mgP/l)		0.01 (<0.01 – 0.02)	0.02 (0.01 – 0.03)	0.02 (<0.01 – 0.03)	0.02 (<0.01 – 0.03)	Not available	Not more than 0.02 mg/l for annual mean
Total Phosphorus (TP) (mgO/l)		0.03 (0.02 - .0.07)	0.05 (0.03 – 0.10)	0.05 (0.03 – 0.07)	0.04 (0.03 – 0.06)	Not available	Not available
Chlorophyll-a (µg/L)		4.4 (1.1 – 17.7)	2.7 (1.1 – 10.2)	7.4 (1.6 – 31.0)	9.1 (1.7 – 37.7)	Not available	Not available
E coli (cfu/100 ml)		17 (1 – 180)	170 (11 – 3000)	19 (1 - 310)	6 (1 – 80)	Not more than 610 per 100 ml for annual geometric mean, applicable to secondary contact recreation subzone (SM10 and SM11) only	Not more than 610 per 100 ml for annual geometric mean, applicable to secondary contact recreation subzone (SM10 and SM11) only
Faecal Coliforms (cfu/100 ml)		37 (1 – 330)	380 (34 – 8800)	58 (6 – 610)	19 (1 – 190)	Not available	Not available

Note: 1. Except as specified, data presented are depth-averaged values calculated by taking the means of three depths: Surface, mid-depth, bottom.

2. Data presented are annual arithmetic means of depth-averaged results except for E.coli and faecal coliforms that are annual geometric means.

3. Data in brackets indicate the ranges

HATS Survey Data

Total Residual Chlorine and Chlorination By-products

6.56 Two marine water sampling events were conducted under the Minor Works Contract ⁽[10]⁾ in December 2004 and May 2005 respectively to collect marine water samples for chemical analysis of nine CBP compounds (namely bromoform, chloroform, bromodichloromethane, dibromochloromethane, bromoacetic acid, chloroacetic acid, dibromoacetic acid, dichloroacetic acid and trichloroacetic acid). During each sampling event, replicate depth-integrated samples (composite of samples from near-surface, mid-depth and near-bottom) were collected at two marine stations, one near the existing outfall of SCISTW, namely SCI-1, and one at EPD’s monitoring station, namely SM18, in Southern WCZ. Locations of these monitoring stations are shown in Figure 6.1.

6.57 In February 2006, triplicate depth-integrated samples were collected at SCI-1 and SM18 under the ADF study for laboratory analysis of TRC and the other 25 identified CBP compounds.

6.58 Total (soluble plus insoluble) concentrations of CBP were measured in the marine water samples. Table 6.18 shows the maximum values measured in the samples. All the maximum levels were below the detection limits except for methylene chloride where its maximum level measured at station SM18 was 55 mg/l. No reason can be offered to explain the detection of methylene chloride at Station SM18. It should be noted that the chemical analysis was conducted by a HOKLAS accredited laboratory. Also, methylene chloride was detected in all three water samples collected at SM18, with satisfactory QA/QC results. The methylene chloride was detected at a level of 55 mg/l which complied well with the water quality objective of 1580 mg/l (refer to Table 6.11)

Table 6.18 Maximum Ambient Concentrations for TRC and CBP

Parameters	Station SCI-1	Station SM18
Total residual chlorine (mg/l)	<20	<20
Bromoform (mg/l)	<5	<5
Bromodichloromethane (mg/l)	<5	<5
Chloroform (mg/l)	<5	<5
Dibromochloromethane (mg/l)	<5	<5
Bromoacetic acid (mg/l)	<2	<2
Chloroacetic acid (mg/l)	<2	<2
Dibromoacetic acid (mg/l)	<2	<2
Dichloroacetic acid (mg/l)	<2	<2
Trichloroacetic acid (mg/l)	<2	<2
Methylene chloride (mg/l)	<20	55
Carbon tetrachloride (mg/l)	<0.5	<0.5
Chlorobenzene (mg/l)	<0.5	<0.5
1,1-dichloroethane (mg/l)	<0.5	<0.5
1,2-dichloroethane (mg/l)	<0.5	<0.5
1,1-dichloroethylene (mg/l)	<0.5	<0.5
1,2-dichloropropane (mg/l)	<0.5	<0.5
Tetrachloroethylene (mg/l)	<0.5	<0.5
1,1,1-trichloroethane (mg/l)	<0.5	<0.5
1,1,2-trichloroethane (mg/l)	<0.5	<0.5
Trichloroethylene (mg/l)	<0.5	<0.5
2-chlorophenol (mg/l)	<0.5	<0.5
2,4-dichlorophenol (mg/l)	<0.5	<0.5
p-chloro-m-cresol (mg/l)	<0.5	<0.5
Pentachlorophenol (mg/l)	<2.5	<2.5
2,4,6-trichlorophenol (mg/l)	<0.5	<0.5
Bis(2-chloroethoxy)methane (mg/l)	<0.5	<0.5
1,4-dichlorobenzene (mg/l)	<0.5	<0.5
Hexachlorobenzene (mg/l)	<0.5	<0.5
Hexachlorocyclopentadiene (mg/l)	<2.5	<2.5
Hexachloroethane (mg/l)	<0.5	<0.5
1,2,4-trichlorobenzene (mg/l)	<0.5	<0.5
Alpha-BHC (mg/l)	<0.5	<0.5
Beta-BHC (mg/l)	<1	<1
Gamma-BHC (mg/l)	<1	<1

Other Contaminants

6.59 Under the EEFS, marine water samples were collected in 2002 for laboratory analysis of toxic contaminants that are not routinely monitored by EPD. Two water sampling events were conducted in October and December respectively. Replicate depth-integrated water samples were collected at stations SCI-1 and SM18, as shown in Figure 6.1. Table 6.19 shows the maximum values measured in the marine water samples.

Table 6.19 Maximum Total Contaminant Concentrations in Ambient Marine Water

Parameters	Station SCI-1	Station SM18
Arsenic (mg/l)	1.38	1.48
Chromium, total (mg/l)	0.55	0.41
Copper (mg/l)	2.25	0.64
Mercury (ng/l)	0.06	0.06
Nickel (mg/l)	1.02	0.68
Zinc (mg/l)	3.54	1.2
Sulphide (mg/l)	48	44
Surfactants (mg/l)	400	390
Phenol (mg/l)	<1	<1
Cyanide (mg/l)	<2	<2

6.60 As shown in Table 6.19, most of the contaminants were detected in the samples except for phenol and cyanide which were not detected in all the collected water samples which indicated that these 2 chemicals would be negligible in the ambient water. No far field assessment criterion is available for these contaminants. When compared to the near field assessment criteria (Table 6.9), only the measured sulphide level exceeded the criteria value of 20 μg/l. The measured levels for the remaining contaminants were well below the near field criteria values.

Assessment Methodology

Operational Phase

Provision of Disinfection for HATS Effluent

6.61 The EIA study for ADF has concluded that provision of disinfection facilities would be needed for the HATS effluent in order to satisfy the requirement for protection of the identified water sensitive receivers and help to improve water quality in the Western Buffer water control zone (WCZ) and western Victoria Harbour.

6.62 Chlorination has been proposed as the disinfection technology for the HATS. Dechlorination would be applied to eliminate the discharge of total residual chlorine (TRC) into the marine environment. Sodium hypochlorite and sodium bisulphite would be adopted as the chlorination agent and dechlorination agent respectively in the latest scheme of HATS.

6.63 Detailed assessment was performed under the EIA study for ADF to estimate the optimum disinfection levels for the disinfection facilities, on one hand, to safeguard the beneficial uses of nearby water sensitive receivers and, on the other hand, to minimize the chlorine dose and thus the potential generation of chlorination by-products (CBP). Based on the findings of the ADF study, discharge standards for E.coli and TRC are recommended for the disinfected HATS effluent as shown in Table 6.20.

Table 6.20 Effluent Standards for the HATS Recommended under the ADF Study

Stage	E.coli (no. per 100 ml) ⁽¹⁾		TRC (mg/l)
Stage	Geometric Mean	95 Percentile	95 Percentile	Maximum
HATS – ADF Stage	200,000	3,000,000	0.2	0.4
HATS – Stage 2A	20,000	300,000	0.2	0.4
HATS – Stage 2B	20,000	300,000	0.2	0.4

Note:

(1) The effluent standards for E.coli are equivalent to 99% or above E.coli removal.

6.64 The EIA study for ADF concluded that, with the adoption of effluent standards recommended in Table 6.20, the discharge of HATS effluent after chlorination and dechlorination would not cause adverse environmental impact. This recommendation is based on detailed modelling assessment conducted under the ADF study. Details of the modelling assessment results can be referred to the separate EIA report for ADF. This water quality impact assessment is based on the effluent standards recommended under the ADF study to confirm their environmental acceptability. A summary of the key findings of the ADF study including the required chlorine dosage and the characteristics of chlorinated and dechlorinated (C/D) effluent is given in the subsequent sections.

Chemical Dosages

6.65 Bench-scale tests were conducted to estimate the chlorine demand and chlorine residual for meeting the geometric mean E.coli standards of 20,000 and 200,000 counts per 100 ml. The tests were conducted by HOKLAS accredited laboratories appointed by DSD in 2002 and 2005. Based on the data of the bench-scale chlorination studies, the recommended sodium hypochlorite and sodium bisulphite dosages for chlorination / dechlorination under different phases of HATS are summarized in Table 6.21.

Table 6.21 Recommended Sodium Hypochlorite and Sodium Bisulphite Dosages for HATS

Description		Operational Range
Advance Disinfection Facilities (ADF) Stage to meet the E.coli standard of 200,000 counts per 100 ml
Minimum Chlorine Contact Time		6.6 minutes (under peak flow condition)
Sodium Hypochlorite Dosage (mgCl₂/L)		11 - 15
Sodium Bisulphite Dosage(mgNaHSO₃/l)	Automatic Mode	4 – 7
Sodium Bisulphite Dosage(mgNaHSO₃/l)	Manual Mode	8 – 11
Stage 2A Disinfection Facilities to meet the E.coli standard of 20,000 counts per 100 ml
Minimum Chlorine Contact Time		20 minutes (under peak flow condition)
Sodium Hypochlorite Dosage (mgCl₂/L)		10 – 14
Sodium Bisulphite Dosage (mgNaHSO₃/l)	Automatic Mode	2 – 4
Stage 2B Disinfection Facilities to meet the E.coli standard of 20,000 counts per 100 ml
Minimum Chlorine Contact Time		20 minutes (under peak flow condition)
Sodium Hypochlorite Dosage (mgCl₂/L)		2 - 3
Sodium Bisulphite Dosage (mgNaHSO₃/l)	Automatic Mode	1 - 2

6.66 Under the ADF stage, two operation modes were developed for the control of chemical dosages – Manual Operation Mode and Automatic Operation Mode. The difference between the two operation modes is the methods for measuring the TRC levels at the outlet of the chlorination system. Under the Automatic Operation Mode, dosages of dechlorination agent (i.e. sodium bisulphite) would be adjusted automatically following sewage flow rate and online TRC measurements at the outlet of the chlorination system. The TRC measurements would be conducted continuously e.g. at 15-min intervals. Thus, dechlorination chemical can be adjusted based on the TRC data every 15 minutes. Under the Manual Operation Mode, chemical dosages would be adjusted based on sewage flow rate and manual TRC measurement. Practically, hourly TRC measurements would be conducted during the Manual Operation Mode and dechlorination chemical would be adjusted based on the TRC data every hour. A higher dechlorination chemical would be provided to achieve zero TRC in the discharge during the Manual Operation Mode and to allow sufficient safety margin to offset the potential fluctuation of TRC between the hourly measurements. Automatic Operation Mode would be the key control mode for SCISTW chlorination system, while Manual Operation Mode would be provided to allow operation flexibility and reliability in case of online TRC analyzer failure. As a conservative approach, the assessment of water quality impacts at the ADF stage has been based on the chemical dosages under Manual Operation Mode. Automatic operation mode will be used in the permanent disinfection facilities to be built for the HATS Stage 2. The design chemical dosage for Stage 2A and Stage 2B adopted in the EIA was therefore based on the automatic operation mode only.

6.67 At the ADF stage, the existing effluent culvert system would be used for chlorine contact at the ADF stage and the chlorine contact times would be less than 10 minutes. At Stage 2A and Stage 2B, the effluent flow rates would be considerably larger than the ADF flow. Under Stage 2A, a new chlorine contact tank would be built at the SCISTW to increase the chlorine contact time for the HATS effluent so as to effectively achieve the required disinfection level. Although the recommended dosage for ADF stage would be slightly higher than that for Stage 2A, the resulted disinfection level for ADF would be lower than that for Stage 2A due to the short contact time available under the ADF stage.

Effluent Characteristics

6.68 Laboratory pilot tests were conducted for the CEPT effluent from SCISTW and the secondary treated effluent from Tai Po Sewage Treatment Works (TPSTW) and Sha Tin Sewage Treatment Works (STSTW). The pilot tests aimed to simulate the chlorination and dechlorination process for sewage effluent and to determine the chemical characteristics of the chlorinated/dechlorinated (C/D) effluents. Table 6.22 summarizes the chemical analysis results.

6.69 Of the 34 CBP’s tested, only 8 were detected in the C/D CEPT effluent, the concentrations of 6 were less than 10 parts per billion while those for the remaining 2 were in the 10-50 parts per billion range. However, 5 of these 8 CBPs were also detected in the CEPT effluent before chlorination and dechlorination. Of these 5, the concentrations of 3 were less than 10 parts per billion while the remaining two had concentrations in the 10-50 parts per billion range. These results indicated that of the few CBP’s detected, the majority was already present in the CEPT effluent before chlorination and dechlorination, and the chlorination and dechlorination process introduced 3 CBP’s all showing concentrations of less than 10 parts per billion. The laboratory results also showed that the THM and HAA formed were in the parts per billion concentration range, well below USEPA’s drinking water standard for THM and HAA.

6.70 The laboratory results for C/D CEPT effluent were used to assess the water quality impacts due to the effluent discharged from SCISTW during ADF stage (Stage 1) and Stage 2A. The results for C/D secondary treated effluent were used to assess the water quality impacts during Stage 2B where the SCISTW would be upgraded to secondary treatment. The pilot tests were performed using a chlorine dosage of 15 and 20 mg/l for the CEPT effluent and 5 and 9 mg/l for the secondary treated effluent. The chlorine dosages adopted in the pilot tests are larger than the recommended dosages as shown in Table 6.21, which would provide conservative assessment.

Table 6.22 Summary of Chemical Analysis Results

Parameters	Raw CEPT Effluent	C/D CEPT Effluent	Raw Secondary Treated Effluent	C/D Secondary Treated Effluent
Ammonia (mg/l)	18 - 30	(17.4 – 21.5)	1.27 – 1.41	(0.43 – 1.16)
Sulfite (mg/l)	<0.1 – 1.5	(<0.1 – 1.5)	<0.1	0.7
Total residual chlorine (mg/l)	<0.02	(0.02 - 0.1)	<0.02	<0.02
Bromoform (mg/L)	<5	<5	<5	(32 – 49)
Bromodichloromethane (mg/L)	<5	<5	<5	<5
Chloroform (mg/L)	<5 to 5	(<5 – 7)	<5	<5
Dibromochloromethane (mg/L)	<5	<5	<5	(<5 – 8)
Bromoacetic acid (mg/L)	<2	<2	<2	<2
Chloroacetic acid (mg/L)	<2	(<2 – 4)	<2	<2
Dibromoacetic acid (mg/L)	<2	(<2 – 4)	<2	(6 – 10)
Dichloroacetic acid (mg/L)	11 to 17.3	(20 - 45.9)	<2	<2 - 3
Trichlroacetic acid (mg/L)	4.7 to 10	(7.19 – 22)	<2 to 2	(4 – 7)
Methylene chloride (mg/L)	<20	<20	<20	<20
Carbon tetrachloride (mg/L)	<0.5	<0.5	<0.5	<0.5
Chlorobenzene (mg/L)	<0.5	<0.5	<0.5	<0.5
1,1-dichloroethane (mg/L)	<0.5	<0.5	<0.5	<0.5
1,2-dichloroethane (mg/L)	<0.5	<0.5	<0.5	<0.5
1,1-dichloroethylene (mg/L)	<0.5	<0.5	<0.5	<0.5
1,2-dichloropropane (mg/L)	<0.5	<0.5	<0.5	<0.5
Tetrachloroethylene (mg/L)	1.0 to 1.3	(0.5 - 1.3)	<0.5	<0.5
1,1,1-trichloroethane (mg/L)	<0.5	<0.5	<0.5	<0.5
1,1,2-trichloroethane (mg/L)	<0.5	<0.5	<0.5	<0.5
Trichloroethylene (mg/L)	1.5 to 1.9	(1.1 – 2)	<0.5	<0.5
2-chlorophenol (mg/L)	<0.5	<0.5	<0.5	<0.5
2,4-dichlorophenol (mg/L)	<0.5	<0.5	<0.5	<0.5
p-chloro-m-cresol (mg/L)	<0.5	<0.5	<0.5	<0.5
Pentachlorophenol (mg/L)	<2.5	<2.5	<2.5	<2.5
2,4,6-trichlorophenol (mg/L)	<0.5	(1 – 2)	<0.5	<0.5
Bis(2-chloroethoxy)methane (mg/L)	<0.5	<0.5	<0.5	<0.5
1,4-dichlorobenzene (mg/L)	<0.5	<0.5	<0.5	<0.5
Hexachlorobenzene (mg/L)	<0.5	<0.5	<0.5	<0.5
Hexachlorocyclopentadiene (mg/L)	<2.5	<2.5	<2.5	<2.5
Hexachloroethane (mg/L)	<0.5	<0.5	<0.5	<0.5
1,2,4-trichlorobenzene (mg/L)	<0.5	<0.5	<0.5	<0.5
Alpha-BHC (mg/L)	<0.5	<0.5	<0.5	<0.5
Beta-BHC (mg/L)	<1	<1	<1	<1
Gamma-BHC (mg/L)	<1	<1	<1	<1

Note: Data in brackets indicate the ranges.

Near Field Modelling

6.71 The water quality impact assessment was conducted on two different scales – one in very close proximity to the point of discharge, which is usually called the near field and another across a larger area that ranges from the upper reaches of the Pearl River Estuary to points well offshore of Hong Kong, usually called the far field. The extent of the near field is limited to area of less than a couple of hundred meters around the point of discharge. In this near field area, the major water quality of concern is related to toxic impacts of specific chemicals including metals, arsenic, UIA, sulphide, TRC, surfactants, cyanide and phenol.

6.72 Near field modeling was conducted using the VISJET developed by the University of Hong Kong to simulate the near-field plume behaviour of the outfall discharges within a relatively short distance from the effluent discharge location. Hence, the zone of initial dilution (ZID) and vertical structure of the plume could be located. The initial dilution obtained from the near field model was used to assess the impact of toxic contaminants in the near field. The near field results were also used to determine where the effluent loading from the SCISTW outfall would be placed within the far field model (both horizontally and vertically).

6.73 The near field methodology and approach including the details of modelling tools, model input and modelling scenarios, are presented in Appendix 6.1. To compare the near field modelled results with the WQC derived under the EEFS, the concentration of a particular contaminant at the edge of the ZID was calculated by:

C zid = C background (Dilution -1) + C effluent

Dilution Dilution

Where

C zid = Concentration of the contaminant at the edge of ZID (and is the concentration to be compared with the WQC)

C background = Background or ambient concentration without the HATS discharge

C effluent = HATS effluent concentration

Dilution = Minimum dilution rate predicted by the near field model at the edge of ZID

Background Concentrations

6.74 The maximum contaminant concentrations measured under the HATS baseline survey (refer to Table 6.19) were used as the background concentrations for near field assessment. The maximum values for metals, sulphide, surfactants, cyanide and phenol were recorded at Station SCI-1 whilst the maximum value for arsenic was recorded at Station SM18. The measured ambient levels of phenol and cyanide were all under the detection limits of 1 mg/l and 2 mg/l respectively. It was assumed that the ambient concentrations of phenol and cyanide would be equal to one half of their detection limits, following the approach adopted under the EEFS.

6.75 As these baseline water quality data were collected in 2002 after the implementation of HATS Stage 1, some chemicals detected in the marine water during the sampling events would be associated with the existing HATS discharge (i.e. the CEPT effluent of SCISTW). Using these ambient data for near field assessment was therefore a conservative approach.

6.76 Based on the HATS survey data, the measured TRC levels in the marine water were all under the detection limit (refer to Table 6.18). Given the fact that TRC would undergo decay in marine water, it is anticipated that the ambient TRC concentration, if any, would be minimal. Thus, for the purpose of near field assessment, the ambient TRC concentration was assumed to be negligible.

6.77 The WQC was derived for pH and temperature for 90% of occasions (Table 6.9). There are also WQO available for pH, temperature and salinity which should be complied at all times (Table 6.1 to Table 6.6). In addition, two WQC for UIA would need to be compared with the predicted near field concentrations for 1-hour average and 4-day average respectively. For the purpose of near field impact assessment, the ambient levels of pH, salinity, temperature and UIA were compiled using the field data collected at EPD station WM3 in 2000. Year 2000 represents the pre-HATS situation without the SCISTW effluent discharge. Table 6.23 shows the assumed ambient values.

Table 6.23 Ambient Values for pH, Temperature, Salinity and Unionized Ammonia

Parameters	Ambient Concentrations
Parameters	Mean	90 Percentile	Range
Temperature (^oC)	22.8	26.5	16.3 – 27.2
Salinity (ppt)	31.0	32.0	29.0 – 32.8
Unionized ammonia (mg/l as N)	0.0030	0.0040	0.0020 – 0.0093
pH	7.9	8.1	7.6 – 8.1

HATS Effluent Concentrations

6.78 Sampling and analysis of toxic chemicals in sewage effluent was conducted under the EEFS in 2002. Under the EEFS, biological aerated filter (BAF) was assumed as the secondary treatment process for the HATS. Effluent samples were collected from the BAF pilot plant constructed under the Compact Sewage Treatment Technology Pilot Plant Trial Project (CSTTT), as well as the existing SCISTW for laboratory analysis. The CEPT plus BAF was assumed under the EEFS as the ultimate treatment process train for the HATS. The CEPT effluents were used as the influent of the BAF pilot plant under the CSTTT. Replicate samples were collected at each site from 31 October to 1 November 2002. One twenty-four hour composite sample was collected at each sampling occasion. For the purpose of near field water quality impact assessment, the maximum values measured in the CEPT effluent of SCISTW were assumed as the effluent concentrations for HATS Stage 1 and HATS Stage 2A. The maximum values measured in the BAF treated effluent were used as the effluent concentrations for HATS Stage 2B. Table 6.24 shows the effluent values adopted for near field modelling.

Table 6.24 Maximum Total Contaminant Concentrations in Effluent Samples

Parameters	CEPT Effluent (Stage 1 and Stage 2A)	Secondary Treated Effluent (Stage 2B)
Arsenic (mg/l)	1.49	0.88
Chromium, total (mg/l)	18.2	8.45
Copper (mg/l)	55.7	9.98
Mercury (ng/l)	29.4	3.48
Nickel (mg/l)	28.5	22.3
Zinc (mg/l)	44.1	11.8
Sulphide (mg/l)	4900	53
Surfactants (mg/l)	4600	380
Phenol (mg/l)	<1	<1
Cyanide (mg/l)	<2	<2

6.79 Ammonia concentrations in the CEPT effluent of SCISTW are routinely measured by DSD. Effluent sampling results for ammonia are presented for a one-year period from September 2005 to August 2006 in Figure 6.4. The effluent sampling frequency varies from daily to weekly. To compare the WQC for 1-hour average UIA, the maximum effluent concentration measured over the year was used for near field assessment as given in Table 6.25. To assess the UIA impact for 4-day average, the maximum 4-day running averaged effluent value measured over the year was used. Conversion of ammonia to unionized ammonia was based on the 90^th percentile values of the ambient pH, temperature and salinity ^([11]) as given in Table 6.23 which was a conservative approach.

Table 6.25 Assumed Effluent Concentrations for Ammonia

Parameters	Type/ Period	CEPT Effluent (Stage 1 and Stage 2A)	Secondary Treated Effluent (Stage 2B)
Ammonia (mg/l as N)	1-hour average	29 ⁽¹⁾	4.2 ⁽³⁾
4-day average	25 ⁽²⁾

Parameters

Type/ Period

CEPT Effluent

(Stage 1 and Stage 2A)

Secondary Treated Effluent (Stage 2B)

Ammonia

(mg/l as N)

1-hour average

29 ⁽¹⁾

4.2 ⁽³⁾

4-day average

25 ⁽²⁾

⁽¹⁾ Maximum effluent value based on actual measurements of CEPT effluent at SCISTW from September 2005 to August 2006 (The data are graphically presented in Figure 6.4).

⁽²⁾ Maximum 4-day running average value based on actual measurements of CEPT effluent at SCISTW from September 2005 to August 2006 (The data are graphically presented in Figure 6.4).

⁽³⁾ Maximum concentration measured in the BAF effluent collected under the EEFS for whole effluent toxicity testing.

6.80 The EEFS included an assessment of the toxicity of BAF effluent as measured by whole effluent toxicity testing. The ammonia concentration assumed for the secondary treated effluent (Stage 2B) was based on the maximum value measured in the BAF effluent collected under the EEFS for whole effluent toxicity testing. Based on the review of measured ammonia concentrations in the effluent of existing secondary treatment plants (Sha Tin STW and Tai Po STW), this EEFS value was considered to be conservative.

6.81 The assumed effluent values for pH, salinity and temperature were based on the historical effluent data collected at SCISTW (see Figure 6.4). As the measured pH values for both CEPT effluent (Figure 6.4) and ambient water (Table 6.23) were all within 6.5 to 8.5, there would be no exceedance of the WQO and WQC for pH under all the assessment scenarios. The assumed effluent and ambient values for salinity and temperature adopted in the near field assessment are detailed in Table 6.39 and Table 6.40.

6.82 Following the approach adopted under the ADF study, the 95^th percentile value of the TRC standards (i.e. 0.2 mg/l) was used as the effluent TRC concentration. This model input was conservative, given that 95%ile of the TRC loads were usually about twice their mean values. Dechlorination would be applied to eliminate the TRC contents in the HATS effluent discharge. Also, the pilot test data presented in Table 6.22 showed a lower TRC range of 0.02 to 0.1 mg/l only. Thus, use of the 95^th percentile TRC standard for near field assessment would be conservative.

6.83 Comprehensive review and identification of the contaminants of concerns (COC) in the SCISTW effluent and quantification of the adverse effects on the communities potentially at risk from exposure to the identified COC in marine water are provided under the human health and marine ecological risk assessment in Section 7 and Section 8 respectively.

Far Field Modelling

Modelling Tools

6.84 Computer modelling was used to assess the potential impacts on marine water quality associated with the Project. The Delft3D suite of models, namely Delft3D-FLOW and Delft3D-WAQ, developed by Delft Hydraulics, were used as the platforms for hydrodynamic and water quality modelling respectively. Delft3D is a state-of-the-art computer program that simulates three-dimensional flow and water quality processes and is capable of handling the interactions between different processes.

6.85 Delft3D-FLOW is a 3-dimensional hydrodynamic simulation programme with applications for coastal, river and estuarine areas. The model calculates non-steady flow and transport phenomena that result from tidal and meteorological forcing on a curvilinear, boundary fitted grid.

6.86 Delft3D-WAQ is a water quality model framework for numerical simulation of various physical, biological and chemical processes in 3 dimensions. It solves the advection-diffusion-reaction equation for a predefined computational grid and for a wide range of model substances.

6.87 In the present Study, the detailed HATS model developed using Delft3D-FLOW and Delft3D-WAQ was employed for simulation of the water quality changes due to the Project. The detailed HATS model was originally developed and applied under the EEFS to assess the potential water quality impacts of different treatment and disposal scheme proposed for the HATS and the associated construction activities.

6.88 The detailed HATS model covers the North Western, Western Buffer, Victoria Harbour, Junk Bay, Eastern Buffer and Southern WCZ. The model performance was fully calibrated and verified by comparing computational results with field measurements under the EEFS and the ADF study, and accepted by the EPD.

6.89 Set up of the detailed HATS model such as the meteorological forcing, flow aggregation, wind condition, initial and boundary conditions was based on the EEFS. The detailed HATS Model was linked to the regional Update Model, which was constructed, calibrated and verified under the project “CE42/97 Update on Cumulative Water Quality and Hydrological Effect of Coastal Development and Upgrading of Assessment Tool” (Update study)”. Computations were first carried out using the Update Model to provide open boundary conditions to the detailed HATS Model. The Update model covers the whole Hong Kong and the adjacent Mainland waters including discharges from the Pearl River. The influence on hydrodynamics and water quality in these outer regions would be fully incorporated into the detailed HATS Model.

Simulation Periods

6.90 For each assessment scenario, the simulation period of the hydrodynamic model covered two 15-day full spring-neap cycles (excluding the spin-up period) for dry and wet seasons respectively. The hydrodynamic results were used repeatedly to drive the water quality simulations for one complete calendar year (excluding the spin-up period) as specified in the EIA Study Brief. A spin-up period of 8 days and 30 days was provided for hydrodynamic simulation and water quality simulation respectively. These spin-up periods were tested under the EEFS and the ADF study and were found to be sufficient.

Model Setup for Discharges

6.91 The Pearl River estuary flows as well as the effluent flow from the SCISTW were incorporated in the hydrodynamic model. Flows from other storm and sewage outfalls within the whole Hong Kong waters were relatively small and would unlikely change the hydrodynamic regime in the area and were therefore excluded in the hydrodynamic model.

6.92 The diurnal flow variation of the Project effluent was incorporated in both the hydrodynamic and water quality model. The daily flow patterns measured at the SCISTW during August 2004 and January 2005 were reviewed. The review indicated that there was no substantial change in the diurnal patterns between the dry and wet seasons. The typical diurnal flow pattern measured at the SCISTW as shown in Table 6.28 below was applied to the projected daily Project flow and load to derive the hourly diurnal flow and load for different year horizons as model inputs. The same 24-hour diurnal flow pattern was used in the model throughout the simulation period. The exact vertical and horizontal grid cell(s) of the 3D far field model into which the Project flow and pollution loading were allocated were determined by the near field modelling as detailed in Appendix 6.1.

Table 6.28 Typical Hourly Flow Pattern for the SCISTW Effluent

Hour	% of Daily Flow	Hour	% of Daily Flow	Hour	% of Daily Flow	Hour	% of Daily Flow
0:00	4.45%	6:00	2.53%	12:00	5.03%	18:00	4.83%
1:00	3.70%	7:00	2.74%	13:00	4.94%	19:00	4.90%
2:00	3.48%	8:00	3.32%	14:00	4.81%	20:00	4.96%
3:00	3.17%	9:00	3.89%	15:00	4.27%	21:00	5.94%
4:00	2.90%	10:00	4.80%	16:00	4.29%	22:00	5.06%
5:00	2.66%	11:00	4.86%	17:00	4.40%	23:00	4.07%

6.93 Loading from the rest of the sewage outfalls was allocated in the bottom water layer. Pollution loads from storm outfalls and other point source discharges were included in the water quality model and were specified in the middle layer of the model. The loading from the Pear River flow was also included in the model at the upper surface layers, following the approach from the Update Study and EEFS.

Observation Points

6.94 All sensitive receiver points and monitoring stations as shown in Figure 6.1 were defined as observation points in the HATS model to store the simulated water quality results as a function of time.

Effluent Characteristics

6.95 Annual model simulations are required under the EIA Study Brief. The seasonal change in the measured SCISTW effluent concentrations is illustrated in Figure 6.4. To provide a realistic prediction of the water quality impacts of HATS Stage 2A, these measured effluent concentrations were applied to the projected SCISTW flow rates as given in Table 6.30 to create a time varying annual HATS loading profile for model input. The seasonal change in the effluent characteristics can therefore be taken into account.

6.96 The measured CEPT effluent data for BOD₅, TSS, TKN, NH₃-N and TP were available for one complete year (from September 2005 to August 2006) as shown in Figure 6.4 for annual model simulations. The measured effluent data for the period from January 2006 to August 2006 were applied for the simulation period from January to August whilst the measured effluent data for the period from September 2005 to December 2005 were applied for the simulation period from September to December. As shown in Figure 6.4, the effluent sampling frequency varied from daily to weekly. For the purpose of model input, it was assumed that the effluent concentrations would be uniform over the period between two successive sampling occasions. The effluent concentration available for a particular sampling time was also used for the subsequent time period until the next sampling occasion.

6.97 The historical data for PO₄ were only available for eight months from January 2006 to August 2006. The historical data indicated that there was no obvious difference in the trend of effluent quality for PO₄ between the wet and dry seasons. For the purpose of modelling assessment, the measured 6-month effluent profile for PO₄ was used repeatedly for annual simulations.

6.98 Changes of salinity (in 2006) and temperature (in 2002 – 2003) in the SCISTW effluent were also analyzed in Figure 6.4. Some changes in the salinity and temperature values were observed between the dry and wet seasons. The mean seasonal values were used for salinity and temperature. Table 6.29 summarizes the effluent values adopted for far field modelling.

Table 6.29 Effluent Characteristics Assumed for Far Field Water Quality Modelling

Parameters	HATS Stage 1 and HATS Stage 2A		HATS Stage 2B
Parameters	Dry	Wet	Dry	Wet
Temperature (^oC)	20	24	20	24
Salinity (ppt)	12	9.5	12	9.5
5-day Biochemical Oxygen Demand (mg/l)	43 (mean) and 64 (95%ile)⁽¹⁾ (EEFS value: 68)⁽²⁾		24⁽⁴⁾
Suspended Solids (mg/l)	37 (mean) and 48 (95%ile)⁽¹⁾(EEFS value: 42)⁽²⁾		16⁽⁴⁾
Total Kjeldahl Nitrogen (mgN/l)	26 (mean) and 31 (95%ile)⁽¹⁾(EEFS value: 28)⁽²⁾		10.5⁽⁴⁾
Ammonia Nitrogen (mgN/l)	20 (mean) and 25 (95%ile)⁽¹⁾(EEFS value: 18)⁽²⁾		7⁽⁴⁾
Total Phosphorus (mgP/l)	2.5 (mean) and 3.1 (95%ile)⁽¹⁾(EEFS value: 3)⁽²⁾		2⁽⁴⁾
Orthophosphate (mgP/l)	1.6 (mean) and 2 (95%ile)⁽¹⁾(EEFS value: 2)⁽²⁾		1.3⁽⁴⁾
Total Oxidized Nitrogen (mgN/l)	0⁽³⁾		23⁽³⁾
Copper (mg/l)	0.023⁽³⁾		0.012⁽³⁾
Silicate (mgSiO₂/l)	8.6⁽³⁾		8.6⁽³⁾
E.coli (no./100ml)	200,000 (at Stage 1) and 20,000 (at Stage 2A)⁽⁵⁾		20,000 ⁽⁵⁾

Notes:

(1) Time varying effluent concentrations as seen in Figure 6.4 were used for model input. The mean and 95 percentile values of the effluent profiles are provided in the table for reference.

(2) Values used under the EEFS are included in the table for reference.

(3) Based on assumptions used under the EEFS.

(4) Based on the EEFS’s recommendations.

(5) Based on the findings of the EIA Study for ADF.

6.99 For the rest of the parameters (total oxidized nitrogen and silicate) where no past effluent record was available, average values as adopted in the EEFS were used as the effluent concentrations for model input.

6.100 The effluent values for Stage 2B adopted under this EIA were based on the effluent standards recommended under the EEFS for CEPT plus nitrification. The treatment process requirements for Stage 2B will be confirmed by detailed investigation to be carried out at a later stage under separate study. The effluent values for Stage 2B as shown in Table 6.29 were input to the model constantly over the entire simulation period.

Whole Effluent Toxicity Test

6.101 Whole Effluent Toxicity Test (WETT) is a technique to directly measure the aggregate toxic effect of an effluent. In WETT, groups of organisms of a particular species are held in test chambers and exposed to different concentrations of an aqueous test sample (i.e. effluent). Observations are made at predetermined exposure periods. At the end of the test, the responses of test organisms are used to estimate the effects of the test sample.

6.102 Two phases of WETT were conducted in December 2004 and May/June 2005 under the Minor Works Contract to determine the whole effluent toxicity of composite CEPT effluent from SCISTW and chlorinated/ dechlorinated (C/D) CEPT effluent.

6.103 Two other phases of WETT were also conducted in December 2004 and April 2005 under a separate study ^([12]) to determine the whole effluent toxicity of composite secondary treated combined effluent from Tai Po Sewage Treatment Works and Sha Tin Sewage Treatment Works and (C/D) secondary treated effluent.

6.104 Five local species as listed below were used in the WETT which were considered as the “representative local species” of ecological and fisheries significance:

· Amphipod (Melita longidactyla), with 48-hour survival test

· Barnacle larvae (Balanus amphitrite), with 48-hour survival test

· Fish (Lutjanus malabaricus), with 48-hour survival test

· Shrimp (Metapenaeus ensis), with 48-hour survival test

· Diatom (Skeletonema costatum), with 7-day growth inhibition test

6.105 The toxicity tests for amphipod, barnacle larvae, fish and shrimp were to determine the acute toxicity of the effluents to the 4 animal species while the toxicity tests for diatom were to determine the chronic toxicity of the effluents to the plant species.

6.106 The species chosen and the testing protocols used for the WETT were developed by a previous study commissioned by the Agriculture, Fisheries and Conservation Department (AFCD) and were accepted by EPD and AFCD. It should be noted that the WETT results reflected the cumulative effects of all CBP species and other toxic contaminants (such as ammonia and heavy metals) that may be present in the CEPT and secondary treated effluent.

6.107 After the toxicity data for each species were obtained, the Lethal Concentration LC₅₀(foramphipod, barnacle larvae, fish and shrimp) and Inhibition Concentration IC₅₀ (for diatom) value and the No-Observable-Effect-Concentration (NOEC) of both raw effluent and (C/D) effluent were determined. The WETT results are presented and assessed in later sections.

Modelling Scenarios for Normal Operation

6.108 The assessment covers the impacts from HATS Stage 1, Stage 2A and Stage 2B. According to the Brief, construction of the Project (i.e. Stage 2A) is tentatively scheduled to commence in 2009 for commissioning the Stage 2A scheme by 2014. Construction of the advance disinfection facilities (before commissioning of Stage 2A) is tentatively scheduled to commence in 2008 for completion by 2009. The schedule of Stage 2B implementation would be subject to the review on population / flow build-ups and water quality conditions to be conducted in 2010/2011. For the purpose of water quality modelling, it was assumed under the base case scenarios that Stage 2B would be implemented in 2021. Sensitivity tests were conducted to address the possible changes of the schedule of Stage 2B implementation. The time horizons and projected HATS flows for operational phase assessment are shown in Table 6.30.

Table 6.30 Modelling Scenarios for Operational Phase Impact Assessment

Scenario	Year	Description	Projected SCISTW Flow Rate (m³/day)	Remarks
1a	2014	“Without Stage 2A” condition	1550000 ^(A)	Condition in 2014 under HATS Stage 1
1b	2014	Early phase of Stage 2A	2064000
2a	2021	“Without Stage 2A” condition	1628000 ^(A)	Condition in 2021 under HATS Stage 1
2b	2021	Late phase of Stage 2A	2170000
2c (Sensitivity Test)	2021	Late phase of Stage 2A	2170000	Use less conservative storm pollution loading (refer to S6.114 - 6.116)
2d (Sensitivity Test)	2021	Late phase of Stage 2A	1953000	Use less conservative flow rate for Stage 2A (refer to S6.117 - 6.118)
2e (Sensitivity Test)	Ultimate	Stage 2A under ultimate flow	2447000	Use design flow rate for Stage 2A
2f (Sensitivity Test)	2021	Late phase of Stage 2A	2170000	Use of enhanced phosphate removal
3a	2021	Early phase of Stage 2B	2170000
3b	Ultimate	Late phase of Stage 2B	2447000

Note:

(A) It is assumed that flows generated from HATS Stage 2 catchments are discharged into the marine waters via local PTW of individual catchments.

6.109 The Project flow rates were estimated under this Study using the latest population and employment forecast provided by the Planning Department and the approach from the GESF ^{(^[13])}. As the latest population forecasts provided by Planning Department were only available for 2010, 2020 and 2030, the flow rates were also compiled for 2010, 2020 and 2030 only. The ultimate flow was based on a 5% extrapolation of the projected flow for 2030. Safety margin was included in the flow projections for all the scenarios in Table 6.30 to address the uncertainties on the accuracy of the flow projection method. Based on the projected flows for 2010, 2020 and 2030, linear interpolation was used to determine the flow rates for 2014 and 2021.

Base Case Scenarios (Scenarios 1a, 1b, 2a, 2b, 3a and 3b)

6.110 Scenario 1a, Scenario 1b, Scenario 2a, Scenario 2b, Scenario 3a and Scenario 3b as shown in Table 6.30 were the base case scenarios for water quality modelling. Scenario 1a was to address the water quality impacts in 2014 without the implementation of HATS Stage 2A. This scenario represented a condition with HATS Stage 1 in 2014. On the other hand, Scenario 1b represented the condition during early commissioning of Stage 2A in 2014. It is anticipated that the HATS Stage 2A would bring water quality improvement in Victoria Harbour and southern part of the Western Buffer WCZ but may however reduce the assimilation capacity of pollutants in the central Western Buffer WCZ due to the increased loading in the SCISTW effluent. The model results were compared between Scenario 1a and Scenario 1b to quantify the changes of water quality condition within the Study Area due to the implementation of Stage 2A (a comparison on with and without Stage 2A). Year 2014 was selected as the time horizon for both Scenario 1a and Scenario 1b to facilitate the identification of the net effect from Stage 2A.

6.111 Scenario 2b was to address the water quality impacts under Stage 2A in 2021 just before the commissioning of Stage 2B. Year 2021 was selected as the time horizon for modelling as it represented the worst case condition in terms of the pollution loading in the Stage 2A effluent for organic pollutants, toxic contaminants and nutrients etc. On the other hand, Scenario 2a represented the “without Project” condition to investigate the water quality impacts in the Study Area with no implementation of Stage 2 in 2021. Scenario 3a represented the condition during early commissioning of Stage 2B in 2021. The model results were compared between Scenario 3a and Scenario 2b to quantify the water quality improvement due to the implementation of Stage 2B. Scenario 3b represents the late phase of Stage 2B under the ultimate condition.

6.112 It was assumed that CEPT with disinfection would be provided for the HATS effluent under Stage 1 and Stage 2A. In addition, secondary treatment with disinfection would be provided under Stage 2B.

6.113 The impact and toxicity of the SCISTW effluent were assessed in detail under the human health and ecological risk assessment in Section 7 and Section 8 of this EIA report. Impacts on the marine ecological resources due to the potential changes of water quality at different stages of the HATS are assessed in detail under the marine ecological impact assessment in Section 11 of this EIA report.

Sensitivity Test – Change of Residual Waste Load in Storm Systems (Scenario 2c)

6.114 Domestic, commercial and industrial activities are the principal wastewater sources in the HATS catchments. At present, a portion of the waste load enters the coastal waters via storm water drains. The waste load in the storm systems is contributed from unsewered developments as well as expedient connections from trade and residential premises and integrity problems of aged drainage and sewerage system. There should be a continuous reduction on the proportion of wastewater flow entering the storm system in future years due to the progressive works on sewerage connection and rectification of expedient connection being implemented by the HKSAR Government.

6.115 To address the uncertainties about the programme for implementing sewerage improvement and water pollution control measures in the catchments, it was assumed under the base case modelling scenarios that 10% of the pollution loads generated within the HATS catchments (except Tseung Kwan O) would be lost to the storm drainage network in all the future years. The sewerage network in Tseung Kwan O (TKO) is relatively new compared with other HATS catchments and only 5% of the load generated in the TKO area was assumed as the residual load in the storm system. The assumed proportions of sewage flow from domestic, commercial and industrial activities that would enter the storm system are given in Table 6.31. These assumptions are consistent with that adopted under other similar studies including the EIA Study for ADF and EEFS. Based on the approach of using a constant percentage of sewage interception for all the future years, the estimated storm loading would continue to increase as a result of future population growth.

Table 6.31 Assumed % of Pollution Load in the Storm System in Future Years

HATS Catchments	2014 / 2021 / Ultimate Year
Stage 1
Tseung Kwan O	5%
Chai Wan	10%
Shau Kei Wan	10%
Kwun Tong	10%
To Kwa Wan	10%
Northwest Kowloon	10%
Kwai Chung	10%
Tsing Yi	10%
Stage 2
Pokfulam Sandy Bay	10%
Cyberport	10%
Wah Fu	10%
Aberdeen	10%
Ap Lei Chau	10%
North Point	10%
Wan Chai	10%
Central	10%

6.116 The HATS Stage 2A effluent may contribute cumulative water quality impacts with the storm loading discharged into the Rambler Channel (from Tsuen Wan and Kwai Tsing) and the coastal water of West Kowloon (from Northwest Kowloon catchment). A sensitivity test, namely Scenario 2c (see Table 6.30) was included to examine the water quality effect due to the use of a less conservative storm loading under the 2021 scenario. An inventory of storm loading was compiled for the whole HKSAR using the population and employment statistics for year 2005 and the % sewage interception in Table 6.31. Sham Tseng catchment is a non-HATS catchment that may have a bearing on the water quality at Western Buffer WCZ including the Rambler Channel. For the purpose of sensitivity analysis, the inventory assumed that the sewerage system along the Castle Peak Road would be in place to serve unsewered village and properties around Ting Kau, Sham Tseng and Tsing Lung Tau and only 5% of the waste load generated in Sham Tseng catchment would be lost to the storm drainage network. The adjusted storm loading inventory was adopted under Scenario 2c as a sensitivity test to investigate the water quality effect.

Sensitivity Test – Change of Sewage Flow Interception (Scenario 2d)

6.117 Under the base case scenarios, it was assumed that the sewage flow discharged from the catchments of HATS would be more than 100% of the total estimated flow that would be generated in the catchments for conservative assessment. For example, it was assumed that 10% of the total sewage flow generated in the Wan Chai area would be lost to the storm drainage network. On the other hand, the HATS flow rates were projected based on the assumption that 100% of the total flow generated in the Wan Chai area would be discharged to the SCISTW for treatment (i.e. 110% of the total flow generated in Wan Chai would be discharged into the marine water). This was a conservative approach to address the uncertainty on the amount of residual load in the storm systems under the future conditions.

6.118 A sensitivity test, namely Scenario 2d (in Table 6.30), was conducted using a less conservative HATS flow rate. For example, to take account of the waste load lost to the storm as shown in Table 6.31, it was assumed that only 90% of the total sewage flow generated in the Wan Chai area would be diverted to the SCISTW. The adjusted HATS flow rate was adopted for the 2021 scenario as a sensitivity test to investigate the water quality changes. As previously mentioned, safety margin was included in the flow projections for all scenarios (including Scenario 2d) in Table 6.30 to address the uncertainties on the accuracy of the flow projection method. Let alone these safety factors, the 2021 base case scenarios (i.e. Scenario 2b) in Table 6.30 assumed that 100% of the wastewater generated in the HATS catchment would be discharged into the sewerage system. Under this sensitivity analysis (Scenario 2d), it was assumed that only about 90% of the wastewater generated in the HATS catchment would be discharged into the sewerage system and the rest would be lost to the storm systems. The flow rate proposed for Scenario 2d (1,953,000 m³/d) was therefore about 90% of the flow rate for Scenario 2b (2,170,000 m³/d).

Sensitivity Test –Change of Stage 2B Implementation Schedule (Scenario 2e)

6.119 The EEFS concluded that secondary treatment (Stage 2B) would be needed for the HATS in order to protect the Victoria Harbour on a long term basis. It was assumed under the base case modelling scenarios that Stage 2B would be implemented by 2021. A sensitivity test, namely Scenario 2e, was conducted to investigate the change in water quality impacts as a result of the possible change of Stage 2B implementation schedule. This sensitivity test assumed that the effluent flow from HATS Stage 2A was reaching its design flow rate of 2.45M m³/day to examine the water quality impacts of the CEPT effluent without Stage 2B.

Sensitivity Test – Enhanced Phosphate Removal from the CEPT Process (Scenario 2f)

6.120 This sensitivity test, namely Scenario 2f, was to investigate the water quality change due to an enhanced phosphate removal of up to 80% from the Stage 2A effluent in 2021. The phosphate removal rate from the current CEPT process at SCISTW was around 40% to 50% (equivalent to an effluent PO₄ concentration of about 2 mg/l). With an 80% enhanced P removal rate, the PO₄ concentration in the HATS effluent could be reduced to 0.6 mg/l (based on Table 2.16 of the EEFS Final Report). The recent sewage characterization exercise conducted under this EIA study indicated that 80% removal of PO₄ would be achievable from the CEPT process by increasing the ferric chloride dosage from the existing level of 10 mg/L to 40 mg/L. According to the test results, increasing the ferric chloride dosage to 40 mg/L would result in the PO₄ removal percentages of about 83% in the wet season and about 94% in the dry season and the corresponding PO₄ concentrations in the CEPT effluent were down to about 0.4 mg/L in the wet season and less than 0.2 mg/L in the dry season, which are generally in line with the effluent value of 0.6 mg/L assumed under the EEFS for 80% enhanced P removal. For the purpose of this sensitivity test, the EEFS value of 0.6 mg/L was assumed for conservative assessment.

Overflow at PTWs Due to Storm Events

6.121 The potential emergency discharges or temporary bypass of sewage effluent from HATS, which may arise during the operation and construction phase of the Project have been identified and discussed in detail in Appendix 6.1a.

6.122 Emergency discharges of diluted preliminarily treated effluent from the PTWs to the Harbour may occur during heavy rainfall via submarine outfalls or seawall bypasses when the existing submarine outfalls are no longer serviceable. Based on the historical records, emergency overflow due to heavy rainfall are low and the discharge volume is insignificant when compared with the total sewage from HATS. As a very conservative assumption, the volume and rate of emergency discharge at the PTWs due to heavy rainfall is calculated using the projected peak design flow of the PTWs as well as the design capacity difference between the PTWs and the tunnel segments. The duration of emergency discharge associated with heavy rainfall from PTWs is assumed to be 8 hours, which is made reference to the historical records (details refer to Appendix 6.1a). Emergency discharge via the submarine outfall of the PTW would achieve a greater dilution and dispersion effect as compared to the scenario via the seawall bypass. For the purpose of this EIA, it is assumed that all the emergency discharges occur at the PTWs during the operational phase would be via the seawall bypass for conservative assessment. The total volume of overflow discharge assumed in the modelling exercise was 358,848 m³.

6.123 The North West Kowloon Pumping Station (NWKPS) is physically distinct from the Stage 1 tunnel system. Under the upgrading of SCISTW, improvement measures to control emergency overflow of preliminary treated effluent associated with heavy rainfall will be provided. It is expected that the frequency and volume of emergency overflow of preliminary treated effluent associated with heavy rainfall from NWKPS will be significantly reduced after the upgrading of SCISTW. For assessment purpose, the maximum discharge rate of 12,000 m³/hr based on the historical overflow records is adopted as a worst-case scenario (details refer to Appendix 6.1a). Discharge duration of 8 hours is assumed based on the historical overflow records.

6.124 One modelling scenario, namely Scenario A, is proposed to address the worst case impacts for emergency overflow associated with heavy rainfall at the late phase of Stage 2A in 2021. Scenario A assumes that sewage overflow from the NWKPS for discharge at seawall bypass at Stonecutters Island (refer to Section 6.123) would occur together with the sewage overflow at the seawall bypass of all the Stages 1 & 2 PTWs (refer to Section 6.122) for a period of 8 hours to address the worst-case impact due to the capacity constraints as a result of heavy rain. The water quality of the discharge is calculated based on the 3:1 mix ratio of preliminary treated effluent and rain water following the approach adopted under the EEFS.

6.125 Four separate model runs have been conducted to simulate the impacts for four temporary discharge periods centred at namely neap tide high water, neap tide low water, spring tide mid-flood and spring tide mid-ebb respectively under both wet and dry seasons. The model results have been analyzed to determine the sensitivity of the water quality impacts under different discharge times. The sensitivity results indicated that the water quality impact at the identified sensitive receivers would be similar under the four discharge periods. Therefore only two tide conditions, namely neap tide low water and spring tide mid-ebb respectively, are selected for presentation. Thus, impact results are presented for four tidal statuses, namely neap low water in dry season, spring mid-ebb in dry season, neap low water in wet season and spring mid-ebb in wet season respectively.

Assumptions for TRC and CBP Modelling

6.126 The modelling scenarios have taken into account the discharge of TRC and CBP from the disinfection facilities. A tracer was defined at the discharge location of the Project to determine the dilution in the vicinity of the discharge point. Tracers were also defined at other identified concurrent discharges for cumulative impact assessment. The dilution information was used to determine the decreases in the concentrations and to evaluate the potential impacts. The modelling of TRC was conducted using a decay rate of 24 per day ^{(^[14])} which was based on the assumption used under the approved EIA for “Tai Po Sewage Treatment Works Stage 5” and was also used under the ADF study. Upon our review of relevant past EIA studies, this decay rate was the most conservative value. An even more conservative assumption of zero decay rate was applied to the CBP discharges.

6.127 Four major sewage effluent discharges were considered in the TRC and CBP modelling. They were the effluent from SCISTW, Pillar Point Sewage Treatment Works (PPSTW), Tai Po Sewage Treatment Works (TPSTW) and Sha Tin Sewage Treatment Works (STSTW) respectively. It should be noted that the effluent from TPSTW and STSTW would be discharged into the Victoria Harbour via the Kai Tak Nullah under the Tolo Harbour Effluent Export Scheme (THEES). UV radiation would be used as the disinfection method for TPSTW and STSTW. However, for the purpose of this water quality impact assessment and as a worst-case scenario study, chlorination and dechlorination were assumed to be the disinfection processes of the sewage effluent discharges from TPSTW and STSTW. Effluent from Sham Tseng STW and Siu Ho Wan STW was not included in the TRC modelling as UV disinfection is currently adopted for these two treatment works. Cumulative TRC and CBP impact from cooling water discharges within the Study Area was also incorporated into the model.

6.128 The initial TRC levels assumed in the effluent from TPSTW and STSTW as well as in the cooling water discharges were based on the assumptions adopted in the approved EIA for “Tai Po Sewage Treatment Works Stage 5”. The initial concentrations of CBP assumed in the TPSTW and STSTW effluents and the cooling water discharges were based on the maximum CBP levels measured in the C/D secondary treated effluent samples as shown in Table 6.22. As the SCISTW would eventually be upgraded to secondary treatment, the initial CBP levels in the SCISTW effluent under Stage 2B were also based on the maximum values measured in the C/D secondary treated effluent. The initial CBP levels in the HATS effluent during ADF stage (Stage 1) and Stage 2A were based on the maximum values measured in the C/D CEPT effluent samples collected from the SCISTW. The initial TRC and CBP levels in the C/D CEPT effluent from PPSTW were assumed to be the same as those in the C/D CEPT effluent from the SCISTW.

6.129 Modelling of TRC and CBP was to evaluate whether the Project discharges would cause any cumulative water quality impact with other concurrent discharges and to determine the size of the mixing zone for these compounds. The assumptions adopted for modelling of TRC and CBP represented the worst case scenarios for water quality impact assessment. Assessments of the acute and chronic toxicity of TRC and CBP were conducted under the Human Health and Ecological Risk Assessment in Section 7 and Section 8 of this EIA report.

Potential Oxygen Depletion Due to the Addition of Dechlorination Chemical

6.130 Sodium hypochlorite and sodium bisulphite would be adopted as the chlorination agent and dechlorination agent respectively. The operational range of sodium bisulphite dosage was derived from the data obtained from the bench-scale tests as shown in Table 6.21. The upper limits of operational range for sodium bisulphite dosage would be 11 mg/l, 4 mg/l and 2 mg/l for ADF (Stage 1), Stage 2A and Stage 2B respectively and these upper values were assumed as the sodium bisulphite concentration in the Project effluent for water quality modelling. An inert tracer (with zero decay rate) was defined in the far field model at the effluent discharge point to predict the sodium bisulphite concentrations in the marine water under normal operation. This would be a very worst case in terms of the predicted sodium bisulphite level in the marine water as it was assumed that the chemical did not react and remained inert. In reality, sodium bisulphite would not be inert as it would react with chlorine and other chemicals in the water.

6.131 The sodium bisulphite concentrations predicted by the far field water quality model were used to estimate the oxygen depletion in the marine water. For conservative assessment, all sodium bisulphate was assumed to undergo the following reaction with the oxygen in marine water:

2NaHSO₃ (sodium bisulphite) + O₂ (oxygen)  2NaHSO₄ (sodium bisulphate)

6.132 On a weight to weight basis, approximately 6.5 parts of sodium bisulphite would consume 1 part of oxygen. The level of oxygen depletion was calculated by dividing the predicted sodium bisulphite concentration in the marine water with a factor of 6.5. The level of oxygen depletion was then compared with the ambient DO levels to determine the extent of the potential impact. The assessment focused on areas in the vicinity of the Project discharge point.

Modelling Scenarios for Emergency Situations

Description of Emergency Situations

6.133 Under the current design, in the emergency event when the tunnel systems or the submarine outfall at Stonecutters Island are out of operation and power failure of SCISTW, preliminary treated effluent will be discharged directly from the PTWs into Victoria Harbour. The occurrence of malfunction of tunnel systems or submarine outfall would be very remote given the very robust design of these components. However, in the event that it did happen it may take considerable time to repair the tunnel systems and submarine outfall. The water quality of Victoria Harbour would go back to the pre-HATS situation. No water quality assessment would be conducted for these very remote events.

6.134 Item (1) – Provision of standby unit(s) would reduce the risk of equipment breakdown at the PTWs. In the emergency discharge event when one or two equipment are out of operation the raw sewage will be discharged directly to the Harbour via seawall bypass. A 50% of the projected ultimate flow (e.g. one unit is out of service) of each PTW is assumed to be the emergency discharge volume with a discharge duration of 8 hours.

6.135 Item (2) – In the emergency discharge event when power failure occurs at PTWs the raw sewage will be discharged directly to the Harbour via seawall bypass. It should be noted that no power failure has occurred at the PTWs since its commissioning. Discharge duration of 6 hours is assumed for EIA purpose based on the experience of other sewage treatment works e.g. Tai Po STW. The occurrence of this scenario is remote as dual power supply will be provided at all the PTWs.

6.136 Item (3) – When power failure occurs at SCISTW, the preliminary treated effluent will be discharged directly to the Harbour via seawall bypass at the PTWs. Discharge duration of 6 hours is assumed based on the experience of other sewage treatment works. It should be noted that no power failure has been occurred at SCISTW since its commissioning. The occurrence of this scenario is remote as dual power supply is provided at SCISTW.

6.137 Item (4) - Sufficient standby equipment and standby treatment facilities will be provided at the SCISTW. Emergency discharge due to equipment malfunction at SCISTW would be remote. It should be noted that no emergency discharge due to equipment failure has been occurred at SCISTW since its commissioning. Discharge duration of 6 hours is assumed based on the experience of other sewage treatment works. Discharge of the preliminary treated effluent directly to the Harbour via seawall bypass at the PTWs would cover the worst-case impact due to equipment failure at SCISTW.

6.138 Item (5) – Failure of dechlorination could be caused by the malfunction of the pumping system or power failure at the dechlorination unit. It is considered that the plant operator would be notified immediately after any power failure or pump failure via the control system of the treatment plant. It is also considered that the chlorination dosing process could be practically stopped within 30 minutes upon notification of the dechlorination plant failure. Thus, when failure of dechlorination occurs, the reaction time to shut down the chlorination unit to avoid excessive discharge of TRC into the marine environment would be about 30 minutes. During the period when the dechlorination unit is failed, a higher level of TRC would be present in the effluent.

Modelling Scenarios for Power / Equipment Failure

6.139 Two modelling scenarios, namely Scenarios B and C respectively, were simulated to address the worst case impacts for emergency discharge due to equipment or power failure at SCISTW or at individual Stage 2 PTWs during operational phase.

6.140 Scenario B covers the worst-case impact due to power or equipment failure at SCISTW as described in Item (3) and Item (4) above. This scenario assumes that the discharge of screened sewage at the seawall bypass of all the PTWs would occur at the same time in 2021 for a period of 6 hours. The projected ultimate flow was used to calculate the pollution loading of the emergency discharge of 6 hours (equivalent to a total discharge volume of 612,000 m³) for conservative assessment.

6.141 Scenario C is to address the worst-case impact from seawall bypass of raw sewage at individual PTW within the Stage 2 catchment due to power or equipment failure assuming that 100% of the ultimate flow would be discharged at the seawall bypass for a period of 8 hours in 2021 to cover the impacts described in Item (1) and Item (2) above. Based on the past records, power / equipment failure has not happened simultaneously at more than one PTW before. Therefore, model simulation was performed for only one PTW at a time to evaluate the extent of impact due to the sewage bypass at one individual PTW alone. The model simulation was performed for each of the Stage 2 PTW. The projected average ultimate flow for each PTW was used for model simulation.

6.142 Under each modelling scenario, four separate model runs were conducted to simulate the impacts for four temporary discharge periods centred at (i) neap tide low water in dry season, (ii) neap tide low water in wet season, (iii) spring tide mid-ebb in dry season and (iv) spring tide mid-ebb in wet season respectively.

6.143 More details of the emergency discharge scenarios are given in the TN3 in Appendix 6.1a.

Modelling Scenarios for Temporary Failure of Disinfection Facilities

6.144 Water quality impacts in relation to the temporary failure of disinfection facilities, refer to Item (5) above, have been fully assessed under the separate EIA for “HATS Provision of Disinfection Facilities at SCISTW (ADF)” and are therefore not re-modelled under this EIA (also see Section 6.18).

Temporary Discharge of Undisinfected Effluent

6.145 Water quality modelling was carried out under the ADF EIA to address the impact from the discharge of undisinfected effluent under temporary failure of disinfection facilities due to the lack of power supply as well as other incidents such as pump or equipment failure. The modelling also addressed the impacts from the discharge of undisinfected effluent in the event of temporary failure of dechlorination where the chlorination plant would need to be shutdown to avoid excessive discharge of TRC. In the event of emergency situations when the disinfection facilities fail or are shutdown, treated (but undisinfected) effluent would be discharged into the sea via the submarine outfall of SCISTW. Modelling was carried out under the ADF EIA for six scenarios as shown in Table 6.31a to simulate the impact due to the shutdown of the disinfection facilities.

Table 6.31a Modelling Scenarios for Emergency Discharge of Undisinfected CEPT Effluent

Scenario modelled under the ADF EIA	Stage	Total Discharge Period (hours)	*Assumed E.coli* Concentration in Undisinfected CEPT Effluent (no. per 100 ml)**
5a	Early phase of ADF stage	6	1.0E+7 ⁽²⁾
5b	Early phase of ADF stage	24 ⁽¹⁾
6a	Late phase of ADF stage	6
6b	Late phase of ADF stage	24 ⁽¹⁾
7a	Late phase of Stage 2A	6
7b	Late phase of Stage 2A	24 ⁽¹⁾

Notes:

(1) Past emergency discharge records available from various sewage treatment works indicated that the longest period of emergency discharge would be less than 6 hours. Thus, emergency discharge for 24 hours is an extremely remote scenario for the purpose of worst-case assessment only.

(2) The assumed E.coli level in undisinfected CEPT are based on the bench-scale chlorination tests conducted by DSD in 2002 and 2005.

6.146 Late phase of Stage 2A (Scenarios 7a and 7b) represents the worst case in terms of the E.coli loading from the undisinfected CEPT effluent. Scenarios 5a, 5b, 6a and 6b are selected to address the effect during the ADF stage. Although the HATS flow under the ultimate scenario would be larger than that under Stage 2A, the E.coli concentration in the undisinfected secondary treated effluent would be much smaller than that in the undisinfected CEPT effluent. The impact due to emergency discharge under the ultimate scenario was therefore not assessed. More detailed description of the modelling scenarios can be referred to the approved EIA report for ADF.

Temporary Discharge of Chlorinated Effluent

6.147 Table 6.31b shows the scenarios modelled under the EIA Study for ADF for temporary failure of dechlorination plant. Scenario 8a and Scenario 9a covered an emergency discharge period of 40 minutes which takes into account the reaction time for DSD to shut down the chlorination plant (30 minutes) plus the assumed chlorine retention time of 10 minutes. Scenario 10a covered an emergency discharge period of 60 minutes which takes into account the reaction time for DSD to shut down the chlorination plant (30 minutes) plus the assumed chlorine retention time of 30 minutes. Stage 2B was not assessed as Stage 2A already represents the worst case in terms of the Project discharge.

Table 6.31b Modelling Scenarios for Emergency Discharge of TRC

Scenario modelled under the ADF EIA	Stage	Assumed Chlorine Retention Time (minute)	Reaction Time to Shut Down the Chlorination Plant (minutes)	Total Discharge Period	Assumed Chlorine Residuals without Dechlorination (mg/l)
8a	Early phase of ADF stage	10	30	40 minutes	6.7
9a	Late phase of ADF stage	10	30	40 minutes	6.7
10a	Late phase of Stage 2A	30	30	60 minutes	5.5

6.148 Based on data from the bench-scale chlorination studies, the maximum chlorine residuals at the ADF stage and Stage 2A would be in the range of 4.8 - 6.3 mg/l and 4.3 – 5.3 mg/l respectively. The ranges identified from the bench-scale studies are based on the chlorine dosage of 15 and 20 mg/l. The chorine residuals assumed for the emergency scenarios as shown in Table 6.31b are 6.7 and 5.5 mg/l for ADF and Stage 2A respectively which are slightly higher than the ranges identified from the bench-scale studies. Based on the updated design information, the operational chlorine dosage would range from 11 to 15 mg/l to achieve geometric mean E.coli standard of 200,000 no. per 100ml for ADF and 10 to 14 mg/l to achieve the geometric mean standard of 20,000 no. per 100 ml for Stage 2A as shown in Table 6.21. The TRC at the dechlorination point of SCISTW will be controlled within range of 1 to 4 mg/l during the operation. The maximum chlorine dosage of 20 mg/l adopted in the bench-scale studies and the chlorine residuals adopted under the modelling scenarios are therefore conservative.

Construction Phase impact

General Construction Activities

6.149 No dredging and filling activity would be needed for Stage 2A construction. The general construction works would be primarily land-based. Key water quality issues associated with the land-based construction would include the impacts from site run-off, sewage from workforce, accidental spillage and discharges of wastewater from various construction activities.

6.150 The proposed upgrading works at Aberdeen PTW would require seawall re-construction. The seawall re-construction works would involve excavation of seawall and no dredging would be required. Excavation of the existing seawall would involve removal of seawall blocks and gravel only, which would not create adverse SS impact. Fines content in the filling materials in the seawall would be negligible and loss of fill material during seawall demolition was not expected.

6.151 The implementation of measures to control runoff and drainage would be important for the construction works adjacent to the marine water in order to prevent runoff and drainage water with high levels of SS from entering the water environment. Practical water pollution control measures / mitigation proposals were recommended to prevent construction materials from discharging into the sea, so that any effluent discharged from the construction site would comply with the criteria of WPCO.

Temporary Sewage Bypass

6.152 During the design stage of the Project, there is a need for the design consultant to prepare a construction sequence plan for the upgrading of SCISTW and the PTWs on Hong Kong Island in order to minimise interruption to the operation of existing plants and prevent discharge of untreated sewage to Victoria Harbour via seawall bypass or submarine outfall. This a common requirement for conducting upgrading works in existing treatment facilities. Referring to the approved EIA reports of similar project e.g. Shatin STW Stage 3 Upgrading & Tai Po STW Stage 5 Phase 1 Upgrading, discharge of untreated sewage to Tolo Harbour via seawall bypass or submarine outfall during the construction phase are not required.

6.153 Based on the requirements specified in the Consultancy Brief of the design consultancy “HATS Stage 2A Upgrading of the SCISTW and the PTWs - Investigation, Design and Construction”, potential sewage discharges from the PTWs in Kowloon may occur during the construction phase of the Project when the following modification works to the existing facilities are required under the Project:

· Item (a) - Modification works required for the existing main pumping station and Northwest Kowloon Pumping Station (NWKPS) at SCISTW to enhance maintainability, reliability, operational flexibility and pumping capacity for proper integration with the new works to be constructed under HATS Stage 2A.

· Item (b) - Provision of an interconnection between the existing main influent pumping station and the new influent pumping station on Stonecutters Island to enhance system reliability, flexibility and maintainability of the E&M equipment inside the pumping stations.

· It should be noted that Item (a) and Item (b) can be implemented at the same time. When the above modification works are required, sewage bypass from the Stage 1 tunnel system to Victoria Harbour via the submarine outfalls at the Stage 1 PTWs (except for NWK PTW) is likely to be required for a safe working environment. During the sewage bypass period, sewage flow from NWK PTW will still be under CEPT process at SCISTW and discharged via the submarine outfall of SCISTW.

· Item (c) - The existing NWKPS will be inadequate to handle the future projected flow. Upgrading of the existing NWKPS or diversion of excess flow to the new influent pumping station would be required. Discharge of preliminarily treated sewage from Northwest Kowloon PTW to the effluent box culvert via the emergency bypass chamber may occur when constructing the modification works to the NWKPS. The preliminarily treated sewage from NWKPTW will be discharged to the Harbour via the seawall bypass while the preliminarily treated sewage from other Stage 1 PTWs will be under CEPT process at SCISTW before discharging to Victoria Harbour via the submarine outfall at Stonecutters Island.

6.154 It should be noted that the upgrading scheme for the upgrading of SCISTW has not been formulated at this EIA stage. The temporary sewage bypass, if needed, would be scheduled to occur during the construction phase of the Project or early commissioning of the Project. For allowing more flexibility to the design consultant in formulating the upgrading scheme for SCISTW, the worst-case water quality impacts arising from the temporary sewage bypass in 2014 without the HATS Stage 2A have been assessed in this EIA assuming that the discharge of preliminary treated effluent at Stage 2 PTWs would occur together with the temporary sewage bypass at Stage 1 PTWs or NWK PTW. The water quality impact is considered smaller if the temporary bypass is scheduled after commissioning of Stage 2A in 2014 as the background water quality would be improved after the sewage effluents from the Stage 2 PTWs are intercepted to the SCISTW for treatment. In order to assess the magnitude of the worst-case water quality impacts, the predicted results from the sewage bypass scenarios are compared with the scenario under the normal condition in 2014 (without the HATS Stage 2A). The discharge duration and timing of the temporary sewage bypass are summarized in the table below.

Item

Expected Duration / Timing and Discharge Routes

Modelling Scenario

(a) Modification to the existing main pumping station (refer to Figure 6.4c) ^{See
Note 1}

Option 1:

Expected timing & duration: Two weeks in dry season at late phase of HATS Stage 1 in or before 2014

Discharge routes: Preliminary treated effluent ® submarine outfalls at the Stage 1 PTWs (except for NWK PTW) and Stage 2 PTWs

Sewage flow from NWK PTW will be under CEPT process at SCISTW and discharged via the submarine outfall of SCISTW

Water quality impact was modelled under Scenario D (refer to Section 6.157).

Option 1 (i.e. discharge of preliminary treated effluent from Stage 1 and Stage 2 PTWs) with projected flow and load in 2014 was assumed under Scenario D for worst-case assessment.

Option 2:

Expected timing & duration: Two weeks in dry season during early commissioning of HATS Stage 2A in 2014

Discharge routes: Preliminary treated effluent ® submarine outfalls at the Stage 1 PTWs (except for NWK PTW)

Sewage flow from NWK PTW and all the Stage 2 PTWs will be under CEPT process at SCISTW and discharged via the submarine outfall of SCISTW

(b) Interconnection between the existing main pumping station and the new pumping station (refer to Figure 6.4c) ^{See
Note 1}

Option 1:

Expected timing & duration: Two weeks in dry season at late phase of HATS Stage 1 in or before 2014

Discharge routes: Preliminary treated effluent ® submarine outfalls at the Stage 1 PTWs and Stage 2 PTWs (except for NWK PTW)

Sewage flow from NWK PTW will be under CEPT process at SCISTW and discharged via the submarine outfall of SCISTW

Water quality impact was modelled under Scenario D (refer to Section 6.157).

Option 1 (i.e. discharge of preliminary treated effluent from Stage 1 and Stage 2 PTWs) with projected flow and load in 2014 was assumed under Scenario D for worst-case assessment.

Option 2:

Expected timing & duration: Two weeks in dry season during early commissioning of HATS Stage 2A in 2014

Discharge routes: Preliminary treated effluent ® submarine outfalls at the Stage 1 PTWs (except for NWK PTW)

Sewage flow from NWK PTW and all the Stage 2 PTWs will be under CEPT process at SCISTW and discharged via the submarine outfall of SCISTW

Option 1:

Expected timing & duration: Two weeks at late phase of HATS Stage 1 in or before 2014

Discharge routes: Preliminary treated effluent from NWK PTW ® seawall bypass at Stonecutters Island

Sewage flow from Stage 1 PTWs (except NWK PTW) will be under CEPT process at SCISTW and discharged via the submarine outfall of SCISTW

Preliminary Treated effluent from Stage 2 catchments ® submarine outfalls of Stage 2 PTWs

Water quality impact was modelled under Scenario E (refer to Section 6.158).

Option 1 (i.e. discharge of preliminary treated effluent from Stage 2 PTWs) with projected flow and load in 2014 was assumed under Scenario E for worst-case assessment.

Option 2:

Expected timing & duration: Two weeks during early commissioning of HATS Stage 2A in 2014

Discharge routes: Preliminary treated effluent from NWK PTW ® seawall bypass at Stonecutters Island

Sewage flow from Stage 1 PTWs (except NWK PTW) and all the Stage 2 PTWs will be under CEPT process at SCISTW and discharged via the submarine outfall of SCISTW

Note 1: Item (a) and Item (b) could occur together at the same time or separately at different timing. As confirmed by the model prediction, water quality impact due to the temporary sewage bypass would return to normal condition quickly (within a few days) after the temporary sewage bypass. The water quality impact due to Item (a) alone, Item (b) alone and Item (a) and Item (b) together would be the same and covered under Scenario D.

6.155 As a worst-case scenario, the discharge duration of 2 weeks is assumed for the above temporary sewage bypass scenarios, based on the latest information provided by the design consultant of this Project. The design consultancy should take effort to schedule the modification and interconnection works, if needed, at the same time in order to minimise the discharge duration. The design consultancy should also take effort to reduce the discharge duration for the modification to the existing NWKPS e.g. by diverting the flow from NWK PTW to the new influent pumping station before constructing the modification works for the NWKPS. It is also recommended that the temporary sewage bypass at the Stage 1 PTWs should be scheduled in the dry season or low flow period (November to February) to minimize the potential water quality impact. For conservative sake, the total flow from NWK PTW is assumed to be discharged to the Harbour via the seawall bypass at Stonecutters Island. Figure 6.4a and Figure 6.4d show the sewage discharge routes during normal operation of HATS at Stage 1 and Stage 2A respectively. Figure 6.4b and Figure 6.4c show the sewage discharge routes during the modification / interconnection scenarios under the construction phase of the Project as described in Section 6.153.

6.156 Two worst-case modelling scenarios have been assessed for the temporary sewage bypass, namely Scenario D and Scenario E respectively.

Scenario D

6.157 Scenario D aims to assess the worst-case sewage bypass of screened sewage at the submarine outfalls ^([15]) of all the Stage 1 PTWs (except for NWK PTW) for a period of 2 weeks in 2014 during the construction phase. It should be noted that periodic flushing of these existing submarine outfalls is being carried out by DSD to maintain the serviceability of these outfalls. Sewage flow from NWK PTW will be under CEPT process at SCISTW and discharged via the submarine outfall of SCISTW during the sewage bypass period (refer to Figure 6.4c). Scenario D addresses the water quality impacts due to the modification works at the existing main pumping station and the interconnection between the existing main pumping station and the new pumping station, i.e. Items (a) and (b) respectively in Section 6.153. The projected average flow in 2014 for the Stage 1 PTWs was used to calculate the pollution loading for model input. It is assumed that the sewage bypass event will occur for a period of 2 weeks covering a typical spring-neap tidal cycle in the dry season.

Scenario E

6.158 Scenario E covers the worst-case impact due to the modification work at the existing NWK PTW during the late phase of Stage 1, i.e. Item (c) in Section 6.151. It is assumed that the sewage flow from the NWK PTW would be discharged to the Harbour via the seawall bypass at Stonecutters Island while the CEPT effluent from SCISTW (from other Stage 1 PTWs) would be discharged via the submarine outfall of SCISTW. The effluent flow of SCISTW will be assumed to be 1.14 Mm³ per day under this scenario. The measured effluent pollution concentrations for SCISTW (refer to Figure 6.4) were used for calculating the effluent pollution loading for SCISTW. The loading of NWK PTW was calculated based on the projected average flow in 2014. Similar to Scenario D, two alternative sewage bypass periods were asses to address the water quality changes under typical dry and wet season respectively. It is assumed that the sewage bypass event will occur for a period of 2 weeks covering a typical spring-neap tidal cycle in both dry and wet seasons.

6.159 More details of the sewage bypass scenarios are given in the TN3 in Appendix 6.1a.

Coastline Configurations

6.160 The coastline configurations adopted for the 2014 scenario are shown in Figure 6.5. The assumed coastline configurations for 2021 and ultimate stage are shown in Figure 6.6. Based on the latest information from the “Agreement No. CE54/2001 Wan Chai Development Phase II (WDII) Planning and Engineering Review”, construction of WDII is scheduled for commencement in 2009 for completion by 2016. Based on the construction programme, seawall construction for most of the WDII reclamation stages would be completed before 2014. Thus, the reclamation limit for WDII was included in the 2014, 2021 and ultimate coastline.

6.161 The reclamations for South East Kowloon Development (SEKD) and Yau Tong Bay Reclamation (YTBR) were excluded as their concept plans were not yet confirmed. It should be noted that the “no reclamation” scenario is being considered for the SEKD but the feasibility of such scenario is still subject to detailed investigation. Table 6.32 indicates the reclamation projects to be included in the far field model at different time horizon. The reclamation limit for each specific project can be referred to Figure 6.5 and Figure 6.6.

Table 6.32 Coastal Developments to be Incorporated in the 2014, 2021 and ultimate Coastline Configuration

Coastal Development	Information Source	Included in 2014 Scenario	Included in 2021 and Ultimate Scenario
Sunny Bay Reclamation	EIA Report for “Northshore Lantau Development Feasibility Study” (Register No.: AEIAR-031/2000).	Yes	Yes
Lantau Logistic Park Reclamation	EIA Report for “Northshore Lantau Development Feasibility Study” (Register No.: AEIAR-031/2000).	No	Yes
Penny’s Bay Reclamation	EIA Report for “Construction of an International Theme Park in Penny's Bay of North Lantau together with its Essential Associated Infrastructures” (EIAO Register No.: AEIAR-032/2000).	Yes	Yes
Lamma Power Station Extension	EIA Report for “1,800 MW Gas-fired Power Station at Lamma Extension” (EIAO Register No.: AEIAR-010/1999).	Yes	Yes
Further Development of Tseung Kwan O	EIA Report for “Further Development of Tseung Kwan O Feasibility Study” (EIAO Register No.: AEIAR-092/2005)	Yes	Yes
Wan Chai Reclamation Phase II	EIA Study Brief for “Wan Chai Development Phase II and Central-Wan Chai Bypass” (Study Brief No.: ESB-153/2006)	Yes	Yes
Tuen Mun Siu Lang Shui Reclamation	HATS EEFS	No	Yes
Hei Ling Chau Reclamation	HATS EEFS	No	Yes
Tai O Reclamation	HATS EEFS	No	Yes

Pollution Loading

6.162 The methodology for compiling the pollution loading inventory followed that adopted under the ADF study except that the latest planning data obtained under this EIA for 2010, 2020 and 2030 were used. Based on the planning data, the pollution loading generated in different catchments were compiled for 2010, 2020 and 2030 only. Loading for the ultimate scenario was based on a 5% extrapolation of the 2030 loading. Linear interpolation was adopted to determine the loading for 2014 and 2021. The inventory incorporated all possible pollution sources within the Hong Kong waters including those from landfill sites, non-point source surface run-off and sewage from cross connections etc. The inventory also took into account the removal of pollutants due to wastewater treatment facilities and the possible redistribution of pollution loads due to different sewage disposal plants and sewage export schemes at different year horizons. Appendix 6.2a gives the detailed methodology for compiling the pollution loading.

Prediction and Evaluation of Environmental Impacts

Operational Phase – Normal Plant Operation

Whole Effluent Toxicity Test

6.163 Whole effluent toxicity tests (WETT) were conducted to determine the whole effluent toxicity of C/D CEPT effluent from SCISTW and C/D secondary treated effluent from Tai Po Sewage Treatment Works (TPSTW) and Sha Tin Sewage Treatment Works (STSTW) for the following five local marine species:

l Amphipod (Melita longidactyla), with 48-hour survival test

l Barnacle larvae (Balanus amphitrite), with 48-hour survival test

l Fish (Lutjanus malabaricus), with 48-hour survival test

l Shrimp (Metapenaeus ensis), with 48-hour survival test

l Diatom (Skeletonema costatum), with 7-day growth inhibition test

6.164 The toxicity tests for amphipod, barnacle larvae, fish and shrimp were conducted to determine the acute toxicity of the effluents to the 4 animal species while the toxicity tests for diatom were to determine the chronic toxicity of the effluents to plants. Table 6.33 and Table 6.34 summarize the results obtained in the WETT for CEPT effluent whilst Table 6.35 and Table 6.36 give the results for secondary treated effluent.

Table 6.33 Summary of WETT Result for CEPT Effluent (Acute Toxicity)

Test Species	Composite CEPT Effluent		Chlorinated/Dechlorinated CEPT Effluent
Test Species	48-hr LC₅₀^a	NOEC ^b	48-hr LC₅₀^a	NOEC ^b
Amphipod	N.D	>85.0%	N.D	>85.0%
Barnacle Larvae	44.9%	26.6%	40.2%	26.6%
Fish	N.D	30.8%	N.D	>81.9%
Shrimp	N.D	>81.0%	N.D	>81.0%

Notes: ^a48-hr LC₅₀, the lethal concentration of effluent to 50% of test animals after 48 hours of exposure.

^bNo-Observable-Effect-Concentration, the highest concentration of effluent producing effects not significantly different from responses to controls

N.D = Not Determined, less than 50% mortality was recorded when animal species were exposed to the highest concentration of effluent

Table 6.34 Summary of WETT Result for CEPT Effluent (Chronic Toxicity)

Test Species	Composite CEPT Effluent		Chlorinated/Dechlorinated CEPT Effluent
Test Species	7-day IC₅₀^a	NOEC ^b	7-day IC₅₀^a	NOEC ^b
Diatom	34.9%	27.2%	39.7%	27.2%

Notes: ^a 7-day IC50, the inhibition concentration to 50% of organisms after 7 days of exposure.

^b No-Observable-Effect-Concentration, the highest concentration of effluent producing effects not significantly different from responses to controls

Table 6.35 Summary of WETT Result for Secondary Treated Effluent (Acute Toxicity)

Test Species	Composite Secondary Treated Effluent		Chlorinated/Dechlorinated Secondary Treated Effluent
Test Species	48-hr LC₅₀^a	NOEC	48-hr LC₅₀^a	NOEC
Amphipod	N.D¹	N.D²	N.D¹	N.D²
Barnacle Larvae	N.D¹	N.D²	N.D¹	N.D²
Fish	N.D¹	N.D²	N.D¹	N.D²
Shrimp	N.D1	N.D²	N.D¹	N.D²

N.D = Not Determined,

¹LC₅₀ could not be determined - less than 50% mortality was recorded when animal species were exposed to the highest concentration of effluent

²NOEC could not be determined - the highest concentration of effluent did not produce effects significantly different from controls

Table 6.36 Summary of WETT Result for Secondary Treated Effluent (Chronic Toxicity)

Test Species	Composite Secondary Treated Effluent		Chlorinated/Dechlorinated Secondary Treated Effluent
Test Species	7-day IC₅₀	NOEC	7-day IC₅₀	NOEC
Diatom	N.D¹	N.D²	N.D¹	N.D²

N.D = Not Determined,

¹IC₅₀ could not be determined - less than 50% growth inhibition was recorded when plant species were exposed to the highest concentration of effluent

²NOEC could not be determined - the highest concentration of effluent did not produce effects significantly different from controls

6.165 The WETT results indicated that both CEPT effluent with no C/D and CEPT effluent with C/D did not exert acute toxicity effect to amphipod, fish and shrimp as no 48-hr LC₅₀ was found (Table 6.33). However, LC₅₀ and IC₅₀ were observed for barnacle larvae and diatom respectively which implied that some degree of acute and chronic toxicity effect was exerted on barnacle larvae and diatom respectively from both the CEPT effluent with no C/D and CEPT effluent wth C/D.

6.166 Statistical analysis was conducted for the toxicity test data of barnacle larvae and diatom to determine whether the C/D process induced additional toxicity in the CEPT effluent. Two-way analysis of variance (ANOVA) test using software SigmaStat was conducted, which compared the difference between the toxicity data of composite CEPT effluent with no C/D and CEPT effluent with C/D. The analysis showed that the C/D process did not induce statistically significant difference to the toxic effect in CEPT effluent to barnacle larvae and diatom, i.e. the C/D process did not induce additional toxicity.

6.167 Based on the WETT tests conducted for the effluent from TPSTW and STSTW, no LC₅₀ values could be derived for fish, shrimp, amphipod and barnacle larvae (Table 6.35). Also, no IC₅₀ value could be calculated as the maximum growth inhibition for diatoms was low (Table 6.36). Based on the statistical tests, no significant difference was found between the seawater control and various strengths (up to 100%) of secondary treated effluent samples with and without C/D. Hence, it was concluded that the C/D secondary treated effluent samples would not pose any acute or chronic toxicity to the test organisms.

Near Field Water Quality Impacts

6.168 Only four scenarios, namely 1b, 2b, 2e and 3b (refer to Table 6.30), were presented for near field water quality assessment as they represented the worst-case in terms of the HATS pollution loading in 2014, 2021 and ultimate stage. Full description of the near field modelling approach and results are provided in Appendix 6.1.

Total Residual Chlorine

6.169 Concentrations of TRC at the edge of the zone of initial dilution (ZID) were calculated based on the near field modelling results. The TRC standard of 0.2 mg/l (at 95%ile) was assumed as the TRC concentration in the C/D HATS effluent for the purpose of near field modelling.

6.170 Based on the HATS survey data, the measured ambient TRC levels in the marine water were all under the detection limit. Given the fact that TRC would undergo decay in marine water, it was anticipated that the ambient TRC concentration, if any, would be minimal. For the purpose of calculation, the ambient TRC concentration was assumed to be negligible.

6.171 Based on the minimum initial dilutions predicted by the near field model, the TRC concentration at the edge of the ZID was calculated and the results are summarized in Table 6.37.

Table 6.37 Predicted TRC at Edge of ZID

Scenario (refer to Table 6.30)	Description	Minimum Initial Dilution	TRC concentration at Effluent (mg/l)¹	Predicted Concentration at Edge of ZID (mg/l)²	Criterion (mg/l) ³	% of Criterion
1b	2014 – Early Stage 2A	39	0.2	0.0051	<0.013	39%
2b	2021 – Late Phase of Stage 2A	38		0.0053		41%
2e (sensitivity Test)	Stage 2A at Design Flow	35		0.0057		44%
3b	Ultimate - Stage 2B	35		0.0057		44%

Notes: 1. TRC standard at 95%ile

2. TRC at edge of ZID = TRC at effluent / minimum initial dilution

3. TRC criterion at edge of ZID, daily maximum (see Table 6.9)

6.172 As shown in Table 6.37, with TRC concentration at effluent as 0.2 mg/l, the predicted TRC concentrations at the edge of ZID for the 2014, 2021 and ultimate scenarios would be 0.0051, 0.0053 and 0.0057 mg/l respectively which were well below the criterion of 0.013 mg/l.

Unionized Ammonia

6.173 Concentrations of UIA at the edge of the zone of initial dilution (ZID) were calculated for 1-hour average and 4-day average based on the minimum initial dilutions predicted by the near field model, and the results are summarized in Table 6.38. The annual mean UIA levels were assessed using the far field water quality modeling results provided in later sections.

Table 6.38 Predicted Unionized Ammonia at edge of ZID

UIA	Minimum Initial Dilution	Effluent Concentration	Ambient Concentration	Predicted Concentration at Edge of ZID	Criteria	Contribution from HATS
Scenario 1b – 2014 Early Phase of HATS Stage 2A
1-hour average (mg/l as NH₃)	39	2.08 ^{Notes 1, 6}	0.0093^{Note 4}	0.062	0.233	85%
4-day average (mg/l as NH₃)	39	1.79^{Notes 2, 6}	0.0040 ^{Note 5}	0.050	0.035	92%
Scenario 2b – 2021 Late Phase of HATS Stage 2A
1-hour average (mg/l as NH₃)	38	2.08 ^{Notes 1, 6}	0.0093^{Note 4}	0.064	0.233	85%
4-day average (mg/l as NH₃)	38	1.79^{Notes 2, 6}	0.0040 ^{Note 5}	0.051	0.035	92%
Scenario 2e – Sensitivity Test - HATS Stage 2A at Design Flow Rate
1-hour average (mg/l as NH₃)	35	2.08 ^{Notes 1, 6}	0.0093^{Note 4}	0.068	0.233	86%
4-day average (mg/l as NH₃)	35	1.79^{Notes 2, 6}	0.0040 ^{Note 5}	0.055	0.035	93%
Scenario 3b – Ultimate Stage of HATS Stage 2B
1-hour average (mg/l as NH₃)	35	0.30 ^{Notes 3, 6}	0.0093^{Note 4}	0.018	0.233	48%
4-day average (mg/l as NH₃)	35	0.30^{Notes 3, 6}	0.0040 ^{Note 5}	0.012	0.035	67%

Notes: 1. Maximum value from actual measurements of CEPT effluent at SCISTW from September 2005 to August 2006 (Table 6.25).

2. Maximum 4-day running average value from actual measurements of CEPT effluent at SCISTW from September 2005 to August 2006 (Table 6.25).

3. Maximum concentration measured in the BAF effluent collected under the EEFS for whole effluent toxicity testing.

4. Maximum value from actual measurements at EPD station WM3 in 2000 (during the pre-HATS period) (Table 6.23).

5. 90^th percentile value from actual measurements at EPD station WM3 in 2000 (during the pre-HATS period) (Table 6.23).

6. The total ammonia is converted to unionized ammonia by applying a contribution of unionised ammonia in total ammonia of 5.9% (based on 90^th percentile ambient values of 26.5 ^oC, 32.0 ppt and pH 8.1 in Table 6.23). The unionized ammonia level is converted from N to NH₃by dividing the measured level (as N) by a factor of 0.822.

6.174 The near field assessment results indicated that the Project effluent would not comply with the WQC for 4-day average UIA during the Stage 2A.

Salinity, pH and Temperature

6.175 Based on the WQO, changes in salinity due to human activity should not exceed 10% of ambient. Changes in pH and temperature should not exceed 0.2 and 2^oC respectively and the pH values should be within the range of 6.5 to 8.5. The EEFS also established the same criteria for pH and temperature. As the measured pH values for both CEPT effluent (Figure 6.4) and ambient water (Table 6.23) were all within 6.5 to 8.5, there would be no exceedance of the WQO and WQC for pH under all the assessment scenarios. The minimum initial dilution rates predicted by the near field model were used to calculate the maximum changes in salinity and temperature at the edge of the ZID and the results are given in Table 6.39 to Table 6.40.

Table 6.39 Predicted Salinity at Edge of ZID

Scenario	Description	Minimum Initial Dilution	Minimum Salinity at Effluent (ppt)¹	Maximum Ambient Salinity (ppt)²	Maximum change in Salinity from Ambient²	Criterion
1b	2014 – Early Stage 2A	39	8.27	32.8	1.9%	<10%
2b	2021 – Late Phase Stage 2A	38			2.0%
2e (sensitivity Test)	Stage 2A at Design Flow	35			2.1%
3b	Ultimate - Stage 2B	35			2.1%

Notes: 1 Minimum value from actual measurements of CEPT effluent at SCISTW from September 2005 to August 2006 (Figure 6.4).

2 Maximum value from actual measurements at EPD station WM3 in 2000 (Table 6.23).

Table 6.40 Predicted Temperature at Edge of ZID

Scenario	Description	Minimum Initial Dilution	Maximum Value at Effluent (^oC)¹	Minimum Ambient Value (^oC)²	Maximum change from Ambient (^oC)²	Criterion (^oC)
1b	2014 – Early Stage 2A	39	29	16.3	0.33	<2
2b	2021 – Late Phase Stage 2A	38			0.33
2e (sensitivity Test)	Stage 2A at Design Flow	35			0.36
3b	Ultimate - Stage 2B	35			0.36

Notes: 1 Maximum value from actual measurements of CEPT effluent at SCISTW from September 2005 to August 2006 (Figure 6.4).

2 Minimum value from actual measurements at EPD station WM3 in 2000 (Table 6.23).

6.176 The near field assessment results indicated that full compliance with the WQO and WQC for salinity and temperature would be achieved under all the assessment scenarios.

Other Contaminants of Concern

6.177 The near field water quality criteria provided for metals and other selected organic and inorganic contaminants were based on 90% of occurrences. The contaminant levels at the edge of the ZID were calculated by applying the 10^th percentile initial dilutions predicted by the near field model and the results are summarized in Table 6.41 and Table 6.42 for Stage 2A and Stage 2B respectively.

6.178 The concentrations predicted at the edge of the ZID for all selected parameters (except sulphide and surfactants) complied with their respective criteria. For sulphide (under Stage 2B) and surfactant (under Stage 2A and Stage 2B) in which the criteria were exceeded, the ambient concentrations exceeded the criteria themselves, and the amount contributed by the HATS effluent was small. The extent of sulphide exceedance contributed by the Stage 2A effluent was however larger than that by the Stage 2B effluent.

Parameter	Maximum Effluent Value (mg/l) ¹	10^th Percentile Initial Dilution	Maximum Background Value (mg/l) ²	Predicted Concentration at Edge of ZID (mg/l)	Criteria (mg/l)	Contribution from HATS Discharge
Arsenic	1.49	52	53	49	1.48	1.48	1.48	1.48	20	0.01%	0.01%	0.01%
Chromium, total	18.2	0.55	0.89	0.88	0.91	50	38%	38%	40%
Copper	55.7	2.25	3.28	3.26	3.34	5	31%	31%	33%
Mercury	0.0294	0.00006	0.0006	0.0006	0.0007	0.21	90%	90%	91%
Nickel	28.5	1.02	1.55	1.54	1.58	5	34%	34%	35%
Zinc	44.1	3.54	4.32	4.31	4.37	20	18%	18%	19%
Sulphide	4900	48	141	140	147	20	66%	66%	67%
Surfactants	4600	400	481	479	486	30	17%	16%	18%
Phenol	< 1	< 1	< 1	< 1	< 1	5	0%	0%	0%
Cyanide	< 2	< 2	< 2	< 2	< 2	5	0%	0%	0%

Parameter	Maximum Effluent Value (mg/l) ¹	10^th Percentile Initial Dilution (Stage 2B at Design Flow)	Maximum Background Value (mg/l) ²	Predicted Concentration at Edge of ZID (mg/l)	Criteria (mg/l)	Contribution from HATS Discharge
Arsenic	0.88	49	1.48	1.47	20	-0.83%
Chromium, total	8.45	0.55	0.71	50	23%
Copper	9.98	2.25	2.41	5	7%
Mercury	0.00348	0.00006	0.00013	0.21	54%
Nickel	22.3	1.02	1.45	5	30%
Zinc	11.8	3.54	3.71	20	5%
Sulphide	53	48	48	20	0%
Surfactants	380	400	400	30	0%
Phenol	< 1	< 1	< 1	5	0%
Cyanide	< 2	< 2	< 2	5	0%

Scenario	Minimum Initial Dilution	LC50 Detected for Barnacle Larvae in the WETT ¹	TUa at Effluent ²	Predicted TUa at Edge of ZID (mg/l) ³	Criterion ⁴	% of Criterion
1b – 2014 Early Stage 2A	39	40.2%	2.49	0.064	<0.3	21%
2b – 2021 Late Phase Stage 2A	38	0.065	22%
2e – Stage 2A at Design Flow	35	0.071	24%

6.219 From the 2006 records for Port Shelter, Mirs Bay and Tolo Harbour, it can be concluded that algal bloom could readily take off at a low TIN, PO₄ and SiO₂ level provided that other environmental conditions were suitable. Past research studies on long-term water quality data in Hong Kong suggested that, under favourable environmental conditions, there would be a sharp increase in red tide occurrence whenever the level of N and P rose above 0.1 mg/l and 0.02 mg/l respectively ⁽[21]⁾. Another recent report also indicated that the threshold nutrient level for algal bloom in Hong Kong would be 0.12 mg/l and 0.018 mg/l for N and P respectively ⁽[22]⁾. In the western waters (Victoria Harbour, Western Buffer, North Western and Deep Bay), the background nutrient levels are considered high enough to trigger algal bloom but the red tide occurrence was limited by light and / or other hydrodynamic factors (such as the degree of water circulation and vertical mixing).

6.220 Recent research studies on algal blooms indicated that red tides would be optimized in poorly flushed coastal water under calm wind condition⁽²⁰⁾. In the open water environment within the North Western and Western Buffer WCZ, the effect of tidal flushing was high.As these waters were frequently diluted by horizontal advection or vertical mixing, the accumulation of algal biomass and hence the chance of algal bloom were more effectively minimized. Although the nutrient level in Deep Bay was very high and the water was static, red tide occurrence in Deep Bay was limited due to the presence of high SS level in the water column which reduced the light penetration and limited the solar energy source for excessive algal growth.

6.221 Species of algae differ greatly in their nutrient requirements and efficiency in solar energy fixation (photosynthesis). A bloom of a species would depend on a combination of different environmental factors such as the flow condition, light penetration, salinity distribution, nutrient concentrations, nutrient ratios and species competition. The fact that 75% of past red tides in Hong Kong occurred in eastern waters with comparatively low nutrient levels implies that nutrient may not be a critical limiting factor for triggering red tide formation in Hong Kong waters.

6.222 In Hong Kong, the most common red tide species was the dinoflagellate Noctiluca scintillans, which accounted for ~30% of the reported red tides since the 1980s. In 2006, 13 red tides were recorded in Hong Kong waters, involving 6 species (i.e. Noctiluca scintillans, Skeletonema costatum, Ceratium furca, Dictyocha speculum, Phaeocystis globosa and Cochlodinium sp.) 3 of which were dinoflagellates. Of the remaining 3 species, 1 was diatom and the other 2 were minor algal species.

Impact from HATS Outfall

6.223 Figure 6.23 to Figure 6.25 illustrate the monthly variation of ammonia nitrogen, phosphate and silica contents in the receiving water before and after commissioning of HATS Stage 1. Only ammonia nitrogen is shown because the CEPT effluent contains minimal nitrate and nitrite. The current P removal rate from the CEPT process at the SCISTW was around 40% to 50%. As a result, there has been an overall reduction of the P loading in the receiving waters. The removal rate for ammonia nitrogen from the CEPT process would be lower (typically around 10%). As shown in Figure 6.23 to Figure 6.25), the CEPT effluent from Stage 1 outfall has not caused any obvious increase in the nutrient (ammonia nitrogen, phosphate and silica) contents in the Western Buffer and western Victoria Harbour where the SCISTW outfall is located.

6.224 In terms of the effect on DO level, monitoring results (Figure 6.15) indicated that the DO levels in the Study Area were not depleted during both the pre-HATS and post-HATS stages and no obvious decrease in the DO level was observed in the receiving water after the HATS Stage 1 implementation. So far, there has not been any sign of increasing red tide occurrence resulted from the CEPT discharge from HATS Stage 1. Though the TIN level currently exceeds the WQC and WQO in western Victoria Harbour and southern waters, the degree of exceedances is similar between the pre-HATS and post-HATS stages, which implies that the exceedences were not caused by the Stage 1 implementation.

6.225 As previously mentioned, Si would be potentially the most limiting nutrient (for diatoms only). However, the Si limiting condition would potentially occur in dry season only, when all concentrations of nutrients were also low. The bioassays conducted under the EEFS by adding CEPT effluent in ambient water samples collected from different water sources within the Study Area indicated that the addition of CEPT effluent caused Si in the water samples to become limiting since CEPT effluent contains higher N and P, but relatively low Si. Therefore, addition of CEPT effluent may further enhance the potential Si limitation and slow the growth of diatoms which could allow dinoflagellates to become more dominant. However, the bioassays did not take into account the dilution and hydrodynamics effects in the water environment. In actuality, the Stage 2A implementation would involve the change of distribution for sewage effluent only (from the local PTWs to the SCISTW outfall). Due to the better flushing achieved at the SCISTW outfall than that at the local PTW outfalls, the pollution levels including all the concentrations of nutrients would be reduced in the whole Study Area after the Stage 2A implementation as demonstrated by the modelling assessment conducted under this EIA. Although the CEPT effluent contains high N and P relatively to Si, the overall load reduction for P in the water environment would in fact be greater than that for N and Si as a result of the diversion of sewage effluent from the local PTWs to the SCISTW for CEPT treatment (the CEPT process would typically remove about 40% to 50% of the P loading in the sewage effluent whilst the load reduction for N and Si would be minimal from the CEPT process). In view of the above, it can be concluded that (i) the Stage 2A implementation would not increase the risk of diatom blooms because all concentrations of nutrients would be reduced after the Project completion whilst all the other environmental factors such as the hydrodynamic condition would remain unchanged (details refer to the modelling results presented in the subsequent sections); (ii) the Project would not further enhance the current Si limitation as the Stage 2A implementation would result in a higher load reduction for P as compared to Si; and (iii) the reduction of overall P loading achieved from the Stage 2A would further enhance the P limiting condition in the Study Area (as demonstrated by the modelling assessment) and minimize the growth of all phytoplankton species including both diatoms and dinoflagellates (as they all require P for growth). As such, no adverse effect on algal blooms would be expected from the Stage 2A implementation. The model results for different assessment scenarios are compared with the WQO and WQC in the subsequent sections. Chlorophyll-a was not modelled because although it is often used as an indicator in measuring phytoplankton biomass, it is not representative of potential algal bloom since a dominant algal bloom phytoplankton group, the dinoflagellates, has no chlorophyll.

Far Field Modelling Results for Base Case Modelling Scenarios (Scenarios 1a, 1b, 2a, 2b, 3a and 3b)

6.226 An assessment of water quality impact due to the operation of the Project was made using the Delft3D model. The far field water quality modelling scenarios are summarized in Table 6.30. The predicted water flow pattern was compared between different assessment scenarios in Figure 6.26 to Figure 6.28. The model results are also presented as contour plots for TIN (Figure 6.29 and Figure 6.30), PO₄ (Figure 6.31), UIA (Figure 6.32 to Figure 6.34), E.coli (Figure 6.35), DO (Figure 6.36 to Figure 6.37), BOD₅ (Figure 6.40), SS(Figure 6.41), sedimentation rate (Figure 6.42), TRC (Figure 6.43) and CBP (Figure 6.44 to Figure 6.46). The contour plots are presented as annual arithmetic averages for TIN (as N), PO₄ (as P), SS and BOD₅ and annual geometric means for E.coli. The UIA levels are presented as annual mean (as N), 4-day maximum (as NH₃) and hourly maximum values (as NH₃) for comparison with the relevant WQO and WQC. The contour plots for DO are presented as 10^th percentile depth-averaged values, 10^th percentile bottom values, annual minimum values and minimum monthly averaged values. The results for sedimentation rate, TRC and CBP are presented as maximum values over the simulation period. The flow pattern is presented as vector plots under typical flood and ebb tide conditions. Appendix 6.3 to Appendix 6.5 show the modelling results at identified water and marine ecological sensitive receivers.

Change of Flow Regime

6.227 The HATS effluent flow was included in the far field hydrodynamic model to assess the effect on the flow regime due to the increase in the effluent flow rate under Stage 2A and Stage 2B. The predicted flow patterns were compared between different assessment scenarios in Figure 6.26 to Figure 6.28. The model results showed that the increase in the effluent flow under Stage 2A and Stage 2B would not change the general flow pattern in the receiving water. There is no assessment criterion available for water flow and current in marine water.

Total Inorganic Nitrogen

6.228 The predicted TIN levels exceeded the WQO and WQC of 0.4 mg/l in Victoria Harbour and Western Buffer WCZ, 0.5 mg/l in North Western WCZ under all the assessment scenarios. The WQO of 0.1 mg/l and WQC of 0.2 mg/l in Southern WCZ were also exceeded under all the assessment scenarios. Figure 6.29 and Figure 6.30 show the areas with non-compliance for WQO and WQC respectively. The model results showed that implementation of HATS Stage 2A would reduce the TIN exceedances in the Victoria Harbour, Western Buffer and North Western WCZ. For example, the TIN level was reduced by about 5% at the nearest marine ecological sensitive receiver (i.e. Ma Wan FCZ) after the implementation of HATS Stage 2A. Comparing the model results between the “without Stage 2A” scenario and the “with Stage 2A” scenario indicated that there would be only very minor difference in the extent of TIN levels between the scenarios in the Southern, Junk Bay and Eastern Buffer WCZ.

6.229 Under HATS Stage 1 (without Stage 2A) in 2014, the predicted TIN levels marginally exceeded the WQO and WQC at the Rambler Channel to the east of Tsing Yi Island (Figure 6.29 and Figure 6.30). Recent EPD monitoring data indicated that the TIN levels measured at Rambler Channel (VM12 and VM14) also exceeded the WQO under the 2005/2006 condition. The model predicted that the TIN exceedances at Rambler Channel would greatly reduce after decommissioning of all the PTWs in Victoria Harbour under Stage 2A.

6.230 Although additional TIN loading would be transferred to the SCISTW outfall under Stage 2A, discharge from the SCISTW outfall would achieve better dilution and flushing in the open water of Western Buffer WCZ as compared to the local PTW outfalls in the harbour channel. Besides, the CEPT process at the SCISTW would typically remove about 40% of organic nitrogen, which is a key source of ammonia nitrogen in marine water via the mineralization process. Implementation of Stage 2A would therefore indirectly reduce the overall ammonia nitrogen level in the marine environment (via the reduction of the organic nitrogen loading) and would improve the overall TIN levels in the western Victoria Harbour and Western Buffer WCZ as compared to the “without Stage 2A” scenarios in 2014 and 2021. Implementation of Stage 2B (with nitrification process) would however increase the TIN levels in the receiving waters due to the substantial increase in the oxidized nitrogen loading in the BAF effluent (refer to Table 6.29). The mixing zone for TIN was quantified and compared between different assessment scenarios in Table 6.45.

Ortho-phosphate

6.231 The model results showed that implementation of HATS Stage 2A would cause an overall reduction of PO₄ levels in the Study Area due to the reduction of P loading from the CEPT process and the hydrodynamic effects as discussed in the above paragraph. As shown in Figure 6.31, the level of PO₄ exceedance in the semi-enclosed bay at the east of Lamma Island was predicted to decrease after commissioning of Stage 2A as compared to the baseline “without Stage 2A” scenarios.

6.232 Based on the model assumption in Table 6.29, the PO₄ concentration in the HATS effluent would be further reduced from the BAF process at Stage 2B. As shown in Figure 6.31, the exceendance at the east of Lamma Island predicted under Stage 2A would be eliminated after commissioning of Stage 2B. The level of exceedances in the marine embayments located at the Lantau Island was also predicted to decrease under Stage 2B as compared to the Stage 2A condition.

6.233 Some exceedances were predicted inside the marine embayments after commissioning of Stage 2A. However, similar or greater level of exceedance was also predicted under the baseline “without Stage 2A” scenarios. The predicted exceedances were not caused by the Stage 2A implementation. The Stage 2A implementation would in fact reduce the PO₄ level from the baseline (Stage 1) condition (Appendix 6.4 and Appendix 6.5).

Ammonia

6.234 The predicted annual mean unionized ammonia (UIA) levels exceeded the WQO and WQC of 0.021 mg/l at area in close vicinity of the SCISTW outfall under both the “with Stage 2A” and the “without Stage 2A” scenarios (Figure 6.32). The model results show that implementation of Stage 2A would reduce the annual mean UIA exceedances in the SCISTW outfall area. For annual mean UIA, the mixing zone of HATS effluent would be reduced by about 50% after the Stage 2A implementation (refer to Table 6.45).

6.235 The predicted maximum 4-day averaged UIA levels exceeded the WQC in the Victoria Harbour and Western Buffer WCZ under the “without Stage 2A” scenarios (Figure 6.33). Implementation of Stage 2A would remove the exceedances for 4-day averaged UIA level in the central and eastern Victoria Harbour and reduce the UIA plume size in the SCISTW outfall location in the Western Buffer and western Victoria Harbour. For maximum 4-day averaged UIA, the mixing zone of HATS effluent would be reduced by about 50% after the Stage 2A implementation (refer to Table 6.45). Full compliance with WQC for 1-hour averaged UIA levels would be achieved within the Study Area including the Western Buffer and western Victoria Harbour where the SCISTW outfall is located (Figure 6.34).

6.236 Some non-compliances of UIA levels were predicted in the Victoria Harbour at the marine embayments. As the degree of these exceedances was similar under all the modelled scenarios, these exceedances were not related to the HATS discharge and were due to the local pollution sources assumed in the model.

6.237 In general, the UIA levels would be improved in the North Western, Western Buffer and Victoria Harbour WCZ after commissioning of Stage 2A. Implementation of Stage 2B would further improve the UIA levels within the Study Area and remove the UIA exceedances at the HATS outfall area. The UIA exceedances were predicted in area close to the SCISTW outfall under the Stage 2A condition. Full compliances with the WQO and WQC for UIA would be achieved at all the identified marine ecological receivers which were farther away from the SCISTW outfall (Appendix 6.5). The level of ammonia nitrogen predicted at all the flushing water intakes also complied well with the WSD criterion of ≤ 1 mg/l under all the assessment scenarios (Appendix 6.3).

E.coli

6.238 With the provision of disinfection facilities at SCISTW, the E.coli removal efficiency of HATS would be equivalent to 99% or above. Comparison between the modelling results of baseline scenario (without Stage 2A) and operation phase scenario (with Stage 2A) indicated that Stage 2A implementation would improve the E.coli levels in Victoria Harbour and Western Buffer WCZ by transferring the untreated sewage from local PTWs to SCISTW for CEPT treatment and disinfection. Also, implementation of Stage 2B would cause only minor difference from the Stage 2A condition in terms of the predicted E.coli levels in the receiving waters (Figure 6.35). The influence of the E.coli impact from the disinfected HATS effluent would be localized and confined within the Victoria Harbour and Western Buffer WCZ.

6.239 The WQO and WQC for E.coli are geometric mean values which are only applicable to bathing beaches (≤ 180 E.coli per 100 ml), secondary contact recreation subzones (≤ 610 E.coli per 100 ml) and fish culture zones (≤ 610 E.coli per 100 ml). Based on the WSD criteria, the target level of E.coli at any flushing water intake should not be over 20,000 per 100 ml. Within the influence zone of the disinfected SCISTW effluent (i.e. Victoria Harbour and Western Buffer WCZ), full compliance with the above assessment criteria would be achieved under the Stage 2A and Stage 2B scenarios except that the predicted E.coli levels at Cheung Sha Wan flushing water intake and Approach Beach would marginally exceed the assessment criteria of 20,000 per 100 ml (for maximum value) and 180 per 100 ml (for geometric mean value) respectively as shown in Appendix 6.3 and Appendix 6.4. These exceedances were however not related to the effluent discharged from HATS and were caused by the background storm pollution loading assumed in the modelling exercise. The background storm loading was input into the model to assess the worst case cumulative effect with the HATS effluent.

Dissolved Oxygen

6.240 The model results indicated that implementation of HATS Stage 2A would improve the DO levels in the Victoria Harbour, Eastern Buffer and Junk Bay. Also, implementation of Stage 2B was predicted to further improve the DO levels in areas close to the HATS outfall.

6.241 The WQO for 10^th percentile depth-averaged DO and 10^th percentile bottom DO (≥4 mg/l and ≥2 mg/l respectively) are applicable to the whole Study Area except for mariculture zones where the 10^th percentile depth-averaged DO should be ≥5 mg/l. The WQC for 10^th percentile depth-averaged DO (≥4 mg/l) is applicable to Western Buffer, Eastern Buffer, Junk Bay and Victoria Harbour WCZ only. The WQC derived for Southern WCZ and fish spawning ground is a monthly averaged DO value (≥ 5 mg/l). The WQC also requires that the minimum DO level at any location of the marine water should not be less than 2 mg/l at all the times. There is no WQC available for 10^th percentile bottom DO. The model predicted that the assessment criteria would be fully complied under all the assessment scenarios for 10^th percentile depth-averaged DO, 10^th percentile bottom DO, monthly averaged DO and minimum DO (Figure 6.36 to Figure 6.39). The minimum DO levels predicted at all the flushing water intakes also complied well with the WSD criterion of ≥ 2 mg/l under all the assessment scenarios (Appendix 6.3).

6.242 Non-compliance with the WQO and WQC was only predicted at a few marine embayments in Victoria Harbour. These exceedances were caused by the discharges from local sources assumed in the modelling for the purpose of cumulative assessment and were not related to the HATS discharge.

Biochemical Oxygen Demand and Suspended Solids

6.243 The BOD and SS removal rate from the CEPT process is typically around 60% and 80% respectively. Implementation of Stage 2A can therefore reduce the overall BOD and SS loads discharged to the marine water. The model predicted that implementation of Stage 2A would improve the BOD levels in the western Victoria Harbour and central Western Buffer WCZ including the area where the SCISTW outfall is located. Comparing the model results between the “without Stage 2A” scenario and the “with Stage 2A” scenario indicated that there would be only very minor difference in the BOD level between the scenarios in the remaining Study Area (Figure 6.40). Also, the model predicted that implementation of Stage 2B would further improve the BOD levels in the area close to the HATS outfall. As predicted by the model, the SS level in the marine water was less sensitive to the Stage 2A implementation. The predicted SS levels are similar under all the assessment scenarios (Figure 6.41).

6.244 Full compliance with the WSD criteria for BOD (≤ 10 mg/l) was predicted at all the flushing water intakes (Appendix 6.3). However, non-compliance with the WSD criteria for SS (≤ 10 mg/l) was predicted for some flushing water intakes. These non-compliances were not caused by the Stage 2A implementation as similar degree of non-compliances was also predicted under the baseline (without Stage 2A) scenarios. These non-compliances were caused by the background pollution loading from local storm outfalls assumed in the modelling. Full compliance with the WQO for SS (i.e. elevation of less than 30% of ambient baseline level) would be achieved under both Stage 2A and Stage 2B scenarios in the whole Study Area including all the coral sites. There is no WQC and WQO available for BOD.

Sedimentation Rate

6.245 The model predicted that implementation of Stage 2A would slightly reduce the sedimentation rate in areas close to the SCISTW outfall including the western Victoria Harbour, Western Buffer and Northern Western WCZ. There was no observable difference in the extent of sedimentation rates between different assessment scenarios in the eastern Victoria Harbour, Junk Bay, Eastern Buffer and Southern WCZ. Also, the model predicted that implementation of Stage 2B would further cause minor reduction in the sedimentation rates within the Study Area as compared to the Stage 2A condition. As indicated in Appendix 6.5, the sedimentation rates predicted at all the marine ecological sensitive receivers including all the identified coral sites complied well with the assessment criterion of 100 g/m²/day under all the assessment scenarios.

TRC and CBP

6.246 Far field water quality modelling was conducted for TRC and 34 CBP compounds as listed in Table 6.11. The model results had taken into account other concurrent discharges such as Pillar Point STW, Sha Tin STW and Tai Po STW where chlorination and dechlorination were assumed to be their disinfection method for worst-case assessment. Possible discharges of TRC and CBP from spent cooling water were also included in the model for cumulative assessment. The initial CBP levels assumed in the effluent discharges and other concurrent discharges were based on the laboratory CBP testing in the C/D effluent samples as previously discussed. The effluent TRC level was assumed to be 0.2 mg/l at 95^th percentile standard.

6.247 The contour plots for TRC are shown in Figure 6.43 which represent the maximum values predicted over the whole model simulation period. The model results showed that the predicted maximum values at and near the SCISTW outfall complied well with the assessment criterion of 0.008 mg/l. The contour plots indicated some localized TRC exceedances along the coastlines. These localized exceedances were contributed from the background sources and were not related to the HATS discharges.

6.248 The predicted maximum values for all the identified CBP compounds complied well with their corresponding ambient water objectives as shown in Table 6.11. The predicted maximum levels of dichloroacetic acid and trichloroacetic acid were higher than the measured ambient levels near the outfall location under Stage 2A. The predicted maximum levels for bromoform were higher than the measured ambient levels near the outfall location during Stage 2B. The predicted maximum values for all the remaining CBP compounds were within the measured ambient levels. The model results are presented as contour plots for dichloroacidic acid, trichloroacetic acid and bromoform in Figure 6.44 to Figure 6.46 respectively.

6.249 Overall, there was no difference in the key findings from the assessment of TRC and CBP impacts between the EIA study for ADF and this EIA. All the TRC and CBP levels predicted at the SCISTW outfall complied well with the assessment criteria. However, it was observed that the plume size of conservative tracer (dichloroacetic acid and trichloroacetic acid) predicted under Stage 2A for this EIA was slightly larger than that presented in the EIA report for ADF but the plumes were still considered localized and confined in area close to the SCISTW outfall. This was in contrast to the assumption that the effluent flow rates used under the ADF study were more conservative than that used under this EIA.

6.250 The effluent plume would be moving around the SCISTW outfall as driven by the changing water current. As the model results for TRC and CBP were presented as the maximum values predicted over the entire simulation period, the TRC and CBP plumes shown in the contour plots did not represent the actual maximum plume size. They were considered as the areas which enveloped the moving plumes over the entire simulation period. The maximum instantaneous coverage of the effluent plume should be much smaller. The movement of the effluent plume would be affected by tides and water current. The modelling performed under this EIA covered a series of annual simulations which took into account more tidal condition and the corresponding envelop could be larger than that predicted under the EIA study for ADF which was based on a series of 30-day simulations only. TRC would be subject to decay once it is discharged to the sea and is therefore less susceptible to such hydrodynamic effect. The effect was also not noticeable for conservative tracer (bromoform) predicted for Stage 2B, because the flow rate adopted in the ADF study for Stage 2B (2.8M m³/d) was much larger than that used under this EIA (2.45M m³/d) which may neutralize the hydrodynamic effect. The flow difference under Stage 2A between the ADF study (2.34M m³/d) and this EIA (2.17M m³/d) was much smaller.

6.251 The TRC and CBP results in Figure 6.43 to Figure 6.46 indicated a sizable mixing zone for a number of background cooling water discharges. The TRC and CBP results for these background sources were based on some very conservative assumptions. The peak discharge flow rates of the cooling water systems were applied to the model continuously (that is, 24 hours daily). For cooling water where no information on the residual chlorine level was available, the maximum chlorine dose rates were directly applied to calculate the TRC loading for model input which, again, represented a very adverse scenario. In reality, the peak flow rates would occur during a short period of time within a day and the chlorine would be decayed within the cooling water systems. Therefore, the actual chlorine contents in the cooling water discharges should be smaller than that assumed in the model. The background cooling water discharges were included in the model only for the purpose of addressing the possible worst-case cumulative impact with the HATS effluent. The model results indicated that the C/D HATS effluent would not cause any cumulative TRC and CBP impact with all other concurrent discharges assumed in the model.

Discussion on the Need of Nutrient Removal

Size of Mixing Zone at the SCISTW Outfall

6.252 The model predicted that implementation of Stage 2A would improve the water quality in the receiving water (including the area close to the SCISTW outfall) for all the selected water quality parameters as compared to the baseline (without Stage 2A) condition. However, exceedances of the assessment criteria were still predicted at the SCISTW outfall area for two parameters (TIN and UIA respectively) at Stage 2A and one parameter (TIN) at Stage 2B. Table 6.45 shows the approximate dimensions of the mixing zones at the SCISTW outfall. The approximate location of the maximum mixing zone predicted for TIN and UIA at the late phase of Stage 2A with respect to the SCISTW outfall diffuser is shown in Figure 6.47.

Table 6.45 Approximate Dimensions of Mixing Zones at the SCISTW Outfall - Base Case Scenarios

Parameter	Approximate Dimension of Mixing Zone (km²)	Figure Reference
2014 – Stage 1 without Stage 2A – HATS Flow Rate of 1.55 M m³/d (Scenario 1a)
Total Inorganic Nitrogen (Annual Mean)	12.5	6.29 and 6.30
Unionized Ammonia (Annual Mean)	2.7	6.32
Unionized Ammonia (4-day Average)	3.9	6.33
2014 – Early Phase of Stage 2A – HATS Flow Rate of 2.06 M m³/d (Scenario 1b)
Total Inorganic Nitrogen (Annual Mean)	5.5	6.29 and 6.30
Unionized Ammonia (Annual Mean)	1.4	6.32
Unionized Ammonia (4-day Average)	1.9	6.33
2021 – Late Phase of Stage 2A – HATS Flow Rate of 2.17 M m³/d (Scenario 2b)
Total Inorganic Nitrogen (Annual Mean)	9.4	6.29 and 6.30
Unionized Ammonia (Annual Mean)	3.5	6.32
Unionized Ammonia (4-day Average)	4.9	6.33
Ultimate – Stage 2B (Scenario 3b)
Total Inorganic Nitrogen	30.3	6.29 and 6.30

6.253 A sensitivity test, namely Scenario 2c (refer to Table 6.30) was conducted to examine the water quality effect as a result of assuming a less conservative storm loading under the Stage 2A scenario in 2021. The model predicted that the change of storm loading input would not cause any obvious reduction in the degree of TIN and UIA exceedances from the base case scenario (Scenario 2b) as illustrated in Figure 6.29, Figure 6.30, Figure 6.32 and Figure 6.33.

6.254 Another sensitivity test, namely Scenario 2d, was conducted using a less conservative HATS flow rate of about 1.9M m³/d (refer to Table 6.30) under Stage 2A in 2021. It was found that the area of modelled exceedances predicted near the SCISTW outfall area for TIN and UIA would be slightly reduced as compared to the base case scenario (Scenario 2b - with a HATS flow rate of about 2.2M m³/d). The sensitivity analysis indicated that the background storm pollution loading would contribute only very minor cumulative impact with the HATS effluent discharge for TIN and UIA. The exceedances for UIA and TIN predicted near the SCISTW outfall area should mainly be contributed from the HATS effluent.

6.255 It was assumed under the base case modelling scenarios that Stage 2B would be implemented by 2021. A sensitivity test, namely Scenario 2e, was conducted to investigate the change in water quality impacts as a result of the possible change of Stage 2B implementation schedule (refer to Table 6.30). Under this sensitivity test the effluent flow from HATS Stage 2A was assumed to be reaching its design flow rate of 2.45M m³/day to examine the water quality impacts of the CEPT effluent without Stage 2B. As illustrated in Figure 6.29 to Figure 6.33, increasing the CEPT effluent flow to 2.45M m³/day would increase the area of modelled exceedances for both TIN and UIA. The increased CEPT effluent flow would also contribute a new mixing zone for PO₄ at the SCISTW outfall area but the extent of the mixing zone would be small and limited. Table 6.46 shows the approximate dimensions of the mixing zones predicted under Scenario 2e.

Table 6.46 Approximate Dimension of Mixing Zones at the SCISTW Outfall - Sensitivity Test

Parameter	Approximate Dimension of Mixing Zone (km²)	Figure Reference
Sensitivity Test – Use of Design HATS Flow Rate of 2.45 M m³/d for Stage 2A at Ultimate Year (Scenario 2e)
Ortho-phosphate (Annual Mean)	0.06	6.31
Total Inorganic Nitrogen	14.5	6.29 and 6.30
Unionized Ammonia (Annual Mean)	5.7	6.32
Unionized Ammonia (4-day Average)	6.2	6.33

Ammonia

6.256 The whole effluent toxicity tests (WETT), which reflected the cumulative effects of all toxic contaminants (including UIA) that were present in the CEPT effluent, were conducted to determine whether the CEPT effluent would induce any adverse effects on aquatic life and the results were used to supplement the water quality model results. Based on the minimum dilution at the edge of mixing zone for UIA as shown in Table 6.45 and Table 6.46, the chronic toxicity unit (TUc) ⁽[23]⁾ at the edge of the mixing zone for UIA was calculated for the CEPT effluent and the results are summarized in Table 6.47.

Table 6.47 Summary of Chronic Toxicity Unit (TUc) at Edge of Mixing Zone

Scenario	Minimum Dilution Factor ¹	NOEC Detected for Diatom in the WETT ²	TUc at Effluent ³	Predicted TUc at Edge of Mixing Zone ⁴	Criterion (mg/l) ⁵	% of Criterion
2014 – Early Phase of Stage 2A (Scenario 1b)	46	27.2%	3.68	0.08	1	8%
2021 – Late Phase of Stage 2A (Scenario 2b)	49			0.08		8%
Sensitivity Test - Use of Design Flow for Stage 2A (Scenario 2e)	56			0.07		7%

Notes:

1. Based on minimum dilution rate predicted at the edge of mixing zone.

2. NOEC of 27.2% was detected in C/D CEPT effluent for diatom (see Table 6.34)

3. TUc = 100/NOEC

4. TUc at edge of mixing zone = TUc at effluent / minimum dilution

5. Toxicity criterion for far field modelling (see Table 6.11)

6.257 As shown in Table 6.47, the predicted maximum TUc at the edge of mixing zone for Stage 2A would be 0.08 which fully complied with the established criterion of 1. There was a great safety margin of over 90% between the predicted chronic toxicity unit and the criteria value at the edge of the mixing zone. It should be noted that compliance with the chronic toxic criterion would be achieved with a dilution of 5 times only, which would be equivalent to a location at the immediate vicinity of the SCISTW outfall diffuser. Therefore, the extent of the chronic toxicity impact of the CEPT effluent would be limited. As predicted by the model, the WQC and WQO for UIA would be fully complied at all the identified water and marine ecological sensitive receivers. The non-compliance with the numeric WQO/WQC for UIA predicted near the SCISTW outfall area is considered acceptable due to the following:

Ø The UIA exceeedance would be localized and would not impair the integrity of the water body and the ecosystem as a whole (The exceedance was not caused by the Project; As predicted by the model, implementation of Stage 2A would decrease the UIA contents in the receiving waters to below the baseline Stage 1 level).

Ø The mixing zone would not endanger sensitive uses e.g. beaches, breeding grounds, or diminish beneficial uses.

Ø The mixing zone would not result in the accumulation of substances to such levels as to prduce significant toxic effects in human or aquatic organism (as demonstrated by the results of the Whole Effluent Toxicity Tests as well as the health and ecological risk assessments provided in Sections 7 and 8 of the EIA Report).

Ø The mixing zone would not induce objectionable deposits, floating debris, oil, scum, and other nuisances such as objectionable colour, odour, taste and turbidity. .

Total Inorganic Nitrogen and Ortho-Phosphate

6.258 As discussed before, the key water quality issue in relation to the high nutrient (N and P) level in marine water would be the potential enhancement of algal bloom. The non-compliance with the numeric WQO/WQC for N & P is considered acceptable due to the following:

Ø The N & P exceeedance would be localized and would not impair the integrity of the water body and the ecosystem as a whole: As predicted by the model, implementation of Stage 2A would decrease the P contents in the receiving waters to below the baseline Stage 1 level. Therefore, in theory, implementation of Stage 2A could reduce the chance of red tide formation by reducing the potential limiting nutrient (P) in the water. Increase in red tide occurrence would not be expected from the Project;

Ø The mixing zone would not endanger sensitive uses e.g. beaches, breeding grounds, or diminish beneficial uses.

Ø The mixing zone would not cause any toxic effects in human or aquatic organism;

Ø The mixing zone would not induce objectionable deposits, floating debris, oil, scum, and other nuisances such as objectionable colour, odour, taste and turbidity..

6.259 However, if Stage 2B is not implemented for HATS as scheduled, the nutrient contents (both P and N) in the marine water would ultimately increase to exceed the baseline Stage 1 level when the HATS flow is reaching its design capacity of 2.45M m³/day. The increased areas of modelled exceedance for PO₄ was only observed in areas very close to the SCISTW outfall diffuser as shown in Figure 6.31. There was no further increase in the predicted P contents in the Southern WCZ as compared to the baseline Stage 1 level (Appendix 6.4 and Appendix 6.5). The increased areas of modelled exceedance for TIN was however larger, which would cover the sensitive receivers in the Western Buffer WCZ. Therefore, if Stage 2B is not implemented for HATS in 2021 as scheduled, enhanced nutrient removal should be considered for HATS to minimize the potential exceedances for P and N. However, the N & P exceedances predicted under this EIA was based on the model assumption that the HATS effluent flow would reach 2.17M m³/day in 2021 and 2.45M m³/day in the ultimate year. It should be noted that the timing for implementation of Stage 2B will be reviewed in 2010-2011 taking into account sewage flow build-up and water quality conditions. It is recommended that the future review study for Stage 2B should review the validity of the model predictions provided in this EIA and confirm the need of enhanced nutrient removal for HATS after 2021.

6.260 It should be noted that the mixing zone for TIN predicted for Stage 2B was large with an area of about 30 km² as indicated in Table 6.45 and the area of exceedance would encroach on the nearby water sensitive receivers (e.g. Ma Wan Fish Culture Zone). This is due to the elevated oxidized nitrogen assumed for the proposed nitrification process at Stage 2B as well as the increased HATS effluent flow assumed for Stage 2B. It is recommended that these water quality issues should be further investigated / assessed under the future EIA for Stage 2B. Further mitigation measures / alternative treatment designs should also be considered under the future EIA for Stage 2B to mitigate / minimize the potential TIN exceedances.

The Need for Nutrient Removal for HATS

Introduction

6.261 Based on the nutrient ratios recorded at the EPD’s routine water quality monitoring stations as illustrated in Figure 6.21 and Figure 6.22, the Study Area (including the North Western, Southern, Western Buffer, Victoria Harbour, Junk Bay and Eastern Buffer WCZ) was under a P-limiting condition (i.e. the water was rich in N relative to P) in consistent with the EEFS findings. All phytoplankton species require both N & P for growth. Under a P-limiting condition and provided with favourable flow & atmospheric condition, algal growth may in theory respond to addition of P. By contract, further addition of N may not cause further effect on algal growth given the abundant N with limited P in the water.

6.262 It was however concluded that nutrient (both N & P) was not a critical limiting factor for algal bloom in the open water environment within the western waters including the Victoria Harbour, North Western and Western Buffer WCZ (where the SCISTW outfall is located), based on the fact that over 90% of past red tides in Hong Kong occurred in eastern & southern waters with comparatively low nutrient levels. Red tides rarely occurred in the western waters where the nutrient levels were much higher. Based on past research studies, algal blooms were mostly observed in weakly flushed coastal waters under calm wind condition and with adequate light penetration. In the open water environment within the Victoria Harbour, North Western and Western Buffer WCZ, the effect of tidal flushing was high.As these western waters were frequently diluted by horizontal advection or vertical mixing, the accumulation of algal biomass and hence the chance of algal bloom would be more effectively minimized.

6.263 As such, a more stringent WQC for P & N was established under EEFS for the poorly flushed semi-enclosed bays (Figure 6.3) to minimize occurrence of red tides. The WQC for N & P is 0.1 mg/l & 0.01 mg/l respectively for semi-enclosed bays. The WQC are less stringent for other open waters such as the Western Buffer WCZ (where the SCISTW outfall is located) with a limit of 0.4 mg/l & 0.04 mg/l for N & P respectively.

6.264 Based on the model results and past monitoring records, the WQO/WQC for N was exceeded in the Study Area under the existing Stage 1 condition (including the SCISTW outfall location, the Southern water, the North Western water and all the semi-enclosed bays). On the other hand, the WQC for P was exceeded at the semi-enclosed bays only. Full compliance with the WQC for P was achieved in all other open waters including the Western Buffer WCZ under the existing Stage 1 condition.

6.265 The HATS 2A implementation would reduce the nutrients (both N & P) in the whole Study Area (including the Western Buffer and all the semi-enclosed bays) as compared to the Stage 1 condition. Hence, HATS 2A would not increase the risk of red tide. However exceedance for N & P was still predicted after the HATS 2A implementation but the level of exceedance was smaller than that under the existing Stage 1 condition.

6.266 Stage 2B would cause a further reduction of P level in the receiving water as compared to the Stage 2A condition. Further increase in N caused by the nitrification process of Stage 2B would not increase the risk of algal bloom because their growth would be limited by the reduced P in the water.

6.267 Nutrient removal has been considered for HATS Stage 2A to minimize the nutrient levels in the receiving water. As the background source (Pearl River discharge) contains high N relative to P, the N loading from HATS would only have a relatively minor contribution to the N level within the Study Area. On the other hand, given that the background source (Pearl River discharge) contains low P content and based on the past monitoring records that the marine water in the Study Area generally had abundant N relative to P all year round, further enhancement in P removal from the Stage 2A effluent was considered a more effective option (as compared to N removal) in further minimizing the chance of algal bloom from the existing baseline level.

Effectiveness of Enhanced P Removal

6.268 To verify the occurrence of P exceedances at the semi-enclosed bays as predicted by the model, the relevant model predictions for HATS Stage 1 condition are compared with the EPD routine monitoring data collected in 2006 at or near the four semi-enclosed bays in the Southern waters as shown in Table 6.48. These four semi-enclosed bays were predicted to be under the greatest influence from the Stage 2A effluent. The comparison showed that the model predictions are reasonable and in line with the actual measurements. The locations of EPD monitoring stations are shown in Figure 6.1 and the semi-enclosed bays are indicated in Figure 6.3.

Table 6.48 Comparison between the Model Prediction and the EPD Monitoring Data

ID (Figure 6.3)	Semi-enclosed Bays (EPD Monitoring Station)	Annual Mean P Level under Stage 1 Condition, in mg/l
	Semi-enclosed Bays (EPD Monitoring Station)	Model Prediction	EPD Monitoring Data
	WQC :	0.01
1	Eastern Lantau (SM10)	0.014	0.02
2	Southeast Lantau (SM11)	0.014	0.02
3	Southern Lantau (SM13)	0.012	0.01
4	Northern Lamma (SM4)	0.010	0.01

6.269 A sensitivity test, namely Scenario 2f, was conducted to investigate the effect of enhancing the P removal rate at the SCISTW to around 80% under Stage 2A in 2021. The P removal rate from the current CEPT process at SCISTW was around 40% to 50% (equivalent to an effluent PO₄ concentration of about 2 mg/l). With an 80% enhanced P removal rate, the PO₄ concentration in the HATS effluent could be reduced to 0.6 mg/l (based on Table 2.16 of the EEFS Final Report). Based on the sensitivity test results as shown in Figure 6.48, the enhanced P removal would further reduce the areas of modelled exceedances at the semi-enclosed bays located in the Southern waters as compared to the baseline Stage 1 condition and the Stage 2A condition.

6.270 However, the enhanced P removal from the Stage 2A effluent of up to 80% would not completely eliminate the PO₄ exceedances (only the area of the modelled exceedances would be reduced). Table 6.49 to Table 6.51 tabulate the mean P level predicted at the four semi-enclosed bays. The values shown in the tables represent the P level predicted at the centre of each semi-enclosed bay.

Table 6.49 Predicted Depth-averaged Ortho-Phosphate Levels at Semi-enclosed Bays (Annual Mean)

Semi-enclosed Bay (Ref: Figure 6.3)	(1)	(2)	(3)	(4)	(5)
	HATS + Background source (mg/l)			% difference
	2021 without HATS 2A (under Stage 1 Condition)	2021 Late Phase of HATS 2A	2021 Late Phase of HATS 2A + Enhanced P-removal of 80%	Reduction Due to HATS 2A Implementation (2) - (1) (1)	Further Reduction Due to Enhanced P Removal for HATS 2A (3) - (2) (2)
Eastern Lantau (1)	0.0152	0.0142	0.0110	-6.9%	-22.3%
Southeast Lantau (2)	0.0146	0.0136	0.0109	-6.5%	-20.1%
Southern Lantau (3)	0.0125	0.0121	0.0109	-3.3%	-9.8%
Northern Lamma (4)	0.0104	0.0100	0.0092	-3.5%	-8.8%

Note: Bolded and shaded value indicate exceedance of WQC

Table 6.50 Predicted Depth-averaged Ortho-Phosphate Levels at Semi-enclosed Bays (Dry Season)

Semi-enclosed Bay (Ref: Figure 6.3)	(1)	(2)	(3)	(4)	(5)
	HATS + Background source (mg/l)			% difference
	2021 without HATS 2A (under Stage 1 Condition)	2021 Late Phase of HATS 2A	2021 Late Phase of HATS 2A + Enhanced P-removal of 80%	Reduction Due to HATS 2A Implementation (2) - (1) (1)	Further Reduction Due to Enhanced P Removal for HATS 2A (3) - (2) (2)
Eastern Lantau (1)	0.0170	0.0158	0.0119	-6.9%	-24.6%
Southeast Lantau (2)	0.0157	0.0147	0.0111	-6.7%	-24.2%
Southern Lantau (3)	0.0114	0.0110	0.0092	-4.3%	-15.6%
Northern Lamma (4)	0.0096	0.0093	0.0083	-3.9%	-10.4%

Note: Bolded and shaded value indicate exceedance of WQC

Table 6.51 Predicted Depth-averaged Ortho-Phosphate Levels at Semi-enclosed Bays (Wet Season)

Semi-enclosed Bay (Ref: Figure 6.3)	(1)	(2)	(3)	(4)	(5)
	HATS + Background source (mg/l)			% difference
	2021 without HATS 2A (under Stage 1 Condition)	2021 Late Phase of HATS 2A	2021 Late Phase of HATS 2A + Enhanced P-removal of 80%	Reduction Due to HATS 2A Implementation (2) - (1) (1)	Further Reduction Due to Enhanced P Removal for HATS 2A (3) - (2) (2)
Eastern Lantau (1)	0.0159	0.0158	0.0137	-1.2%	-13.2%
Southeast Lantau (2)	0.0173	0.0172	0.0157	-1.0%	-8.6%
Southern Lantau (3)	0.0189	0.0188	0.0185	-0.2%	-1.8%
Northern Lamma (4)	0.0145	0.0143	0.0134	-1.3%	-6.6%

Note: Bolded and shaded value indicate exceedance of WQC

6.271 Based on the mean values shown in tables above, the enhanced P removal for HATS would eliminate the P exceedance at only 1 semi-enclosed bay in dry season. In wet season, the WQC for P would still be exceeded at all the bays even with the adoption of enhanced P removal of up to 80%. Further modelling works were conducted to distinguish the impacts due to the HATS effluent discharge and those due to other background pollution sources; and to analyze the modelled data to detect the spatial and seasonal trends. A total of four sensitivity scenarios were tested for the late phase of Stage 2A in 2021 to isolate the impact of HATS as listed below:

Scenario 1 - With HATS effluent only (without other background pollution source), no enhanced P removal is assumed for HATS

Scenario 2 - With HATS effluent only (without other background pollution source), enhanced P removal (80%) is assumed for HATS

Scenario 3 - With HATS effluent plus other background pollution sources, no enhanced P removal is assumed for HATS

Scenario 4 - With HATS effluent plus other background pollution sources, enhanced P removal (80%) is assumed for HATS

6.272 The time series plots to show the seasonal trends of P levels predicted at the semi-enclosed bays under the four sensitivity scenarios are given below. The percentage (%) of time in compliance with the WQC for different sensitivity scenarios under different seasons are shown in Table 6.52 to Table 6.54.

Table 6.52 Percentage of Time in Compliance with the WQC for P (within a 1-year period)

Sensitivity Scenario	Description (Late Phase of Stage 2A with HATS flow of 2.17M m³/day)	Eastern Lantau (1)	Southeast Lantau (2)	Southern Lantau (3)	Northern Lamma (4)
1	HATS Stage 2A only (without Enhanced P Removal)	100%	100%	100%	100%
2	HATS Stage 2A only (with Enhanced P Removal)	100%	100%	100%	100%
3	HATS Stage 2A + Other Background Pollution Sources (without Enhanced P Removal)	55%	68%	52%	56%
4	HATS Stage 2A + Other Background Pollution Sources (with Enhanced P Removal)	85%	84%	72%	75%

Table 6.53 Percentage of Time in Compliance with the WQC for P (in Dry Season)

Sensitivity Scenario	Description (Late Phase of Stage 2A with HATS flow of 2.17M m³/day)	Eastern Lantau (1)	Southeast Lantau (2)	Southern Lantau (3)	Northern Lamma (4)
1	HATS Stage 2A only (without Enhanced P Removal)	100%	100%	100%	100%
2	HATS Stage 2A only (with Enhanced P Removal)	100%	100%	100%	100%
3	HATS Stage 2A + Other Background Pollution Sources (without Enhanced P Removal)	14%	36%	34%	64%
4	HATS Stage 2A + Other Background Pollution Sources (with Enhanced P Removal)	93%	100%	100%	96%

Table 6.54 Percentage of Time in Compliance with the WQC for P (in Wet Season)

Sensitivity Scenario	Description (Late Phase of Stage 2A with HATS flow of 2.17M m³/day)	Eastern Lantau (1)	Southeast Lantau (2)	Southern Lantau (3)	Northern Lamma (4)
1	HATS Stage 2A only (without Enhanced P Removal)	100%	100%	100%	100%
2	HATS Stage 2A only (with Enhanced P Removal)	100%	100%	100%	100%
3	HATS Stage 2A + Other Background Pollution Sources (without Enhanced P Removal)	11%	3%	0%	0%
4	HATS Stage 2A + Other Background Pollution Sources (with Enhanced P Removal)	29%	8%	0%	3%

6.273 With the HATS effluent discharge alone, all the P levels predicted at the semi-enclosed bays were well below the WQC with no significant variation between dry and wet seasons. With the addition of other background sources, the P level exceeded the WQC at all the semi-enclosed bays with an obvious trend of increasing P in the wet season. It was also predicted that the WQC exceedance for P occurred more frequently in the wet season. Appendix 6.6 summarizes the relative P contributions from individual pollution sources including the HATS discharge, the Pearl River discharge and other local pollution discharges at the four semi-enclosed bays. The model results showed that the HATS’s contribution to the P exceedance was very small in the wet season (ranged from 3% to 26% only). The P level predicted at the semi-enclosed bays in the wet season was mainly contributed by the local pollution sources (ranged from 27% to 92%).

6.274 Based on the model results, reduction of local pollution load would be a more effective means to reduce the P exceedance at the semi-enclosed bays. It should be noted that the local pollution load assumed in the model was derived from theoretical calculation. Following the common approach normally adopted in other approved EIAs, it was assumed in this EIA that 10% of the total load generated in the catchment would be discharged to the marine water under all the future assessment scenarios which is a conservative assumption. In fact, to tackle the local pollution problems, the government has been implementing the following local sewerage works to serve the unsewered villages in the outlying islands:

· Outlying Island Sewerage Stage 2 Mui Wo Village Sewerage Phase 1 (already commenced construction for completion in 2010)

· Outlying Island Sewerage Stage 2 Lamma Village Sewerage Phase II (scheduled to commence construction in 2010 for completion in 2013)

· Outlying Island Sewerage Stage 1 Phase I Part II and Phase I Yung Shue Wan and Sok Kwu Wan Sewerage, Sewage Treatment and Disposal (scheduled to commence construction in 2008 for completion in 2010)

· Upgrading of Mui Wo Village Sewerage Phase 2 and Mui Wo Sewage Treatment Works Investigation, Design and Construction (scheduled to commence construction in 2009 for completion in 2012)

6.275 With the continuous effort to implement sewerage improvement and pollution control measures to reduce the local pollution sources by the Government, there will be a further load reduction for the local pollution sources at the time when the Stage 2A is implemented by 2014.

6.276 The modelling assessment indicated that the risk of red tide occurrence would already be reduced as a result of the Stage 2A implementation alone. The model predicted that the Stage 2A implementation alone would improve the water quality (in terms of both N & P) in the receiving waters including the semi-enclosed bays in the Southern WCZ as compared to the current baseline Stage 1 condition. With the additional reduction of local pollution discharges being implemented under the regional sewerage schemes, the degree of water quality improvement (in terms of both N&P) is expected to be greater than that predicted by the model for the Stage 2A implementation alone. The P contents at the semi-enclosed bays are expected to be further improved to a level that is better than that predicted in this EIA. Although the enhanced P removal for HATS generally has an advantage of further reducing the P level in the water environment, it was predicted that there would be no substantial improvement in the extent of P exceedances at the semi-enclosed bays resulting from the adoption of the enhanced P removal. Furthermore, adopting the enhanced P removal for HATS will warrant environmental disbenefits in collecting, handling and disposal of increased quantities of sludge produced due to higher P removal efficiencies. It is therefore concluded that the enhanced P removal would not be an effective treatment option for HATS Stage 2A in minimizing the environmental impacts.

Potential Oxygen Depletion Due to the Addition of Dechlorination Chemical

6.277 The upper limits of the operational range for sodium bisulphite dosage of 11 mg/l, 4 mg/l and 2 mg/l for ADF, Stage 2A and Stage 2B respectively were assumed as the sodium bisulphite concentration in the C/D HATS effluent for water quality modelling which is a conservative approach.

6.278 The average and maximum levels of oxygen depletion predicted at the Project discharge point are given in Table 6.55. Based on the EPD routine monitoring data, the mean ambient oxygen levels measured near the Project discharge point were 5.7 and 5.8 mg/l at stations WM3 and VM8 respectively in 2005. It should be noted that the oxygen depletion as shown in Table 6.55 was based on the values extracted from the grid cell in which the SCISTW outfall was located. The size of the grid cells at or near the SCISTW outfall was approximately 140 m x 120 m. The oxygen depletion at the grid cell immediately adjacent to the outfall grid cells was at least 2 times smaller. The predicted oxygen depletions were considered minimal.

Table 6.55 Predicted Oxygen Depletion at the SCISTW Outfall Location

TScenario	Average		Maximum
TScenario	Value (mg/l)	% Decrease	Value (mg/l)	% Decrease
2014 – Late Phase of ADF Stage	0.024	0.43%	0.074	1.29%
2021 – Late Phase of Stage 2A	0.012	0.21%	0.034	0.60%
Ultimate Year – Stage 2B	0.007	0.12%	0.019	0.33%

Note: The % decrease was calculated using the mean ambient oxygen levels measured near the SCISTW outfall at EPD station WM3 in 2005.

6.279 The oxygen depletions shown in Table 6.55 represented a very conservative condition as the maximum chemical dosage was assumed to be the sodium bisulphite concentration in the effluent. In reality, the chemical would react with chlorine and be consumed within the dechlorination facilities and the diffuser systems. The actual concentration of the dechlorination chemical in the effluent should be much smaller. Thus, oxygen depletion caused by the Project would be negligible.

Occasional Overflow at PTW during Storm Events Under Normal Operation of HATS Stage 2A

6.280 The modelling results for the worst-case sewage overflow at all the PTWs within the HATS catchment due to heavy rain for a period of 8 hours under normal operation of the Project (namely Scenario A) are tabulated for all the water and marine ecological sensitive receivers identified within the Study Area in Appendix 6.3 to Appendix 6.5. Appendix 6.3 presents the peak concentrations for E.coli, NH₃-N, SS and BOD₅as well as the minimum concentrations for DO predicted at the WSD flushing water intakes for comparison with the WSD water quality standards. Appendix 6.4 gives the annual geometric means for E.coli and annual averages for UIA, PO4, TIN and UIA predicted at all the beaches for comparison with the WQO/WQC. Appendix 6.5 also gives the annual geometric means for E.coli and annual averages for UIA, PO4, TIN and UIA as well as the maximum sedimentation rates predicted at all the marine ecological sensitive receivers (including fish culture zones and coral sites) for comparison with the assessment criteria. It should be noted that, under Scenario A, several model runs were conducted to cover different discharge start times. However, the results provided in Appendix 6.3 to Appendix 6.5 are not presented for all the model runs. For the sake of simplicity, only the model run with greater water quality impact is selected and presented in these appendices.

6.281 Some of the model results presented in Appendix 6.4 and Appendix 6.5 for beaches and marine ecological sensitive receivers are mean values over the entire simulation period which cannot reflect the short term impacts of the temporary overflow over 8 hours. The elevation / trend of pollution levels during and after the sewage bypass period (as compared to the normal condition during late phase of HATS Stage 2A) is illustrated by means of time series plots. The time series plots for selected indicator points (beaches, fish culture zones and flushing water intakes) covering the periods before, during and after the temporary discharges are shown in Figure 6.49 to Figure 6.56. The results provided in the time series plots for the beaches and fish culture zones are depth-average values whereas those for the WSD flushing water intakes represent the middle water layer. As this scenario aims to address the water quality effects due to heavy rain, only wet season results are presented and assessed.

Secondary Contact Recreation Subzones, Beaches and Fish Culture Zones

6.282 The water quality impact on secondary contact recreation subzones, beaches and fish culture zones (FCZ) is assessed in terms of E.coli, which is used to measure the suitability of water recreation, bathing and marine cultural activities. Water quality objectives have been established for beaches at 180 no. per 100 ml, which is a geometric mean value for bathing season (March to October). Water quality objectives have also been established for secondary contact recreation subzones and FCZ at 610 no. per 100 ml, which is an annual geometric mean value. The model predicted that the worst-case overflow event would cause the following water quality effects:

· Short-term and very minor elevation of E.coli level at two beaches (B24, B26), refer to Figure 6.1 for their locations. The short-term sewage overflows did not cause the E.coli level predicted at these 2 beaches to exceed the criteria value of 180 no. per 100 ml. Only very minor or negligible increase of E.coli level was predicted at all the remaining beaches within the Study Area.

· Short-term and very minor elevation of E.coli level at the eastern part of the secondary contact recreation subzone in Southern WCZ. All the peak E.coli values predicted at the secondary contact recreation subzone during the overflow period were below the criteria value of 610 no. per 100 ml under all the discharge scenarios.

· Short-term elevation of E.coli level at two FCZ (F1, F4), refer to Figure 6.1 for their locations. All the peak E.coli values predicted at F4 during the overflow period were below the criteria value of 610 no. per 100 ml under all the discharge scenarios. However, the E.coli levels predicted at F1 would reach a peak value of about 700 no. per 100 mL under the spring-ebb tide condition which marginally exceeded the criteria value.

6.283 The time series plots for selected beaches (B9, B14, B24, B26, B27, B28) and FCZ (F1, F4, F5) are presented for E.coli in Figure 6.49 and Figure 6.50 for reference. Figure 6.1 shows these indicator points. It was found that all elevations of E.coli levels caused by the temporary overflow were minor and transient. The normal water quality conditions (i.e. the conditions under normal operation of Stage 2A) were predicted to recover quickly (within 1 day) after the end of the temporary overflow period.

6.284 The pollution levels for other parameters such as nutrients and DO were not predicted to be adversely affected by the sewage overflows. Figure 6.51 and Figure 6.52 presents the time series plots for DO (which is an important parameter for maintaining a healthy ecosystem) which indicated that the short-term overflows would only cause minor or negligible effect on the DO level predicted at all the beaches and FCZ.

SSSIs, Marine parks, Marine Reserves, Seagrass Beds, Artificial Reefs, Chinese White Dolphins, Green Turtle Nesting Grounds and Corals

6.285 As shown in Appendix 6.5, the maximum values for sedimentation rates predicted at all the coral sites complied well with the assessment criterion of 100g/m²/day under the overflow scenario. The SS levels predicted at all the ecological sensitive receivers would also fully comply with the WQO of no more than 30% increase from the ambient level.

WSD Flushing Water Intakes

6.286 The time series plots for WSD flushing water intakes are presented for E.coli in Figure 6.53 and Figure 6.54. The target E.coli limit specified by WSD at the flushing water intake points is 20,000 no. per 100 ml, expressed as a maximum value. The model predicted that the worst-case sewage overflows would cause short-term elevations of E.coli at 12 flushing water intakes (WSD9, WSD10, WSD11, WSD 12, WSD13, WSD15, WSD17, WSD18, WSD19, WSD20, WSD22, WSD21). Figure 6.1 shows these indicator points. However all the peak values predicted during the overflow periods complied well with the WSD criterion of 20,000 no. per 100mL except at only one flushing water intake (WSD10) where the peak E.coli level exceeded the criteria value. The peak value predicted at WSD10 was about 145,960 no. per 100 mL under the spring-ebb condition. For the discharge under the neap tide condition, the peak E.coli level predicted at WSD10 was much lower (about 30,000 no. per 100 mL) which only marginally exceeded the criteria value. It was found that all elevations of E.coli levels caused by the temporary overflows were transient. The normal water quality conditions (i.e. the conditions under normal operation of the Stage 2A) were predicted to recover quickly (within 1 day) after the end of the temporary overflow period.

6.287 The temporary overflows would not cause any exceedance at the flushing water intakes for DO, SS, NH₃-N and BOD₅ (refer to Appendix 6.3). Figure 6.55 and Figure 6.56 illustrate the trend of SS levels predicted at the WSD flushing water intakes, which indicated that the SS elevations caused by the temporary discharge were minor and acceptable.

Evaluation of Water Quality Impacts Due to Sewage Overflow as a Result of Heavy Rain under Normal Operation of Stage 2A

6.288 The maximum increase in E.coli at secondary contact recreation subzones, beaches and FCZ attributable to the overflows would be small. The impact of the overflow dissipated rapidly, due to the combination of natural die off of the bacteria in seawater and through dilution. By one day after the end of the discharge the effect would be completely gone. The temporary overflow would not cause any non-compliance with the assessment criteria at all the secondary contact recreation subzones, beaches and FCZ. Although the peak E.coli level predicted at Tung Lung Chau FCZ (F1) marginally breached the criteria value during the overflow period, the impact would be short term (for a few hours). Non-compliance with the WQO/WQC (which is an annual geometric mean) was not predicted.

6.289 The E.coli impact of the overflows on flushing water intakes was predicted to be more dramatic, mostly because the overflows were assumed to be discharged via seawall bypass locations which are often close to the flushing water intakes (also located along the seawall). However, it was predicted that E.coli exceedance would occur at only one intake at Cha Kwo Ling (WSD10) under the wet season scenario.

6.290 It should be highlighted that the adverse assumption of using the peak overflow value for continuous discharge of 8 hours was adopted for modelling. In reality, the peak flow rates would occur during a short period of time within the overflow period. It is also assumed that the peak overflow discharge would occur at all the PTWs at the same time. This is also extremely conservative due to the fact that not all the catchments will peak at the same time. From historical records for the Stage 1 PTWs, emergency sewage overflow due to storm events mostly occurred at NWKPS which will be modified under this Project to minimize the overflow discharge. For the rest of the catchment, sewage overflow as a result of heavy rain only occurred twice since 2005 at Chai Wan PTW and Kwun Tong PTW respectively. The recorded annual sewage overflow volume (6,028 m³ associated with heavy rainfall from the Chai Wan PTW and Kwun Tong PTW) is insignificant (as compared to the total overflow volume of 358,848 m³ assumed in this modelling exercise). Therefore, the model results presented in this EIA represent an extremely worst scenario for conservative assessment. The real situation that would happen would be much better than that simulated under this modelling exercise.

6.291 The water quality impact from the overflow discharge is considered minor and acceptable in view of the following:

· The temporary overflow would not cause any adverse effect on public health and health of biota or risk to life (the temporary discharge would not cause any WQO/WQC non-compliance at all the identified secondary contact recreation subzones, beaches and FCZ based on model predictions);

· The magnitude of the adverse water quality impact would be small and the potential water quality impact would be short-term (less than 1 day) and reversible (the normal water quality would resume quickly after the discharges);

· The frequency of the potential water quality impact would be low (less than 1 per year based on historical records);

· The water quality impact would not occur in areas or regions that are ecologically fragile or sensitive (the impact zone would be mainly confined in Victoria Harbour and the adjacent waters and would not encroach on any ecological sensitive receiver of great importance);

· The temporary bypass would not cause any disruption to the seabed and therefore no disruption to any site of cultural heritage;

· The potential impact area is not of international and regional importance (the area is currently subject to the pollution discharge from the urbanized areas in Kowloon and HK Island; Implementation of this Project would reduce the pollution discharge to the concerned area).

Operational Phase – Emergency Situation

Power / Equipment Failure at SCISTW and Stage 2 PTWs

6.292 The modelling results for emergency discharge from HATS due to equipment / power failure (namely Scenario B and Scenario C) are tabulated for all the water and marine ecological sensitive receivers identified within the Study Area in Appendix 6.3 to Appendix 6.5. Appendix 6.3 presents the peak concentrations for E.coli, NH₃-N, SS and BOD₅as well as the minimum concentrations for DO predicted at the WSD flushing water intakes for comparison with the WSD water quality standards. Appendix 6.4 gives the annual geometric means for E.coli and annual averages for UIA, PO4, TIN and UIA predicted at all the beaches for comparison with the WQO/WQC. Appendix 6.5 also gives the annual geometric means for E.coli and annual averages for UIA, PO4, TIN and UIA as well as the maximum sedimentation rates predicted at all the marine ecological sensitive receivers (including fish culture zones and coral sites) for comparison with the assessment criteria. It should be noted that, under Scenario B (i.e. total power / equipment breakdown at SCISTW), several model runs were conducted to cover different discharge start times. Under Scenario C (i.e. power / equipment failure at the Stage 2 PTWs), separate model runs were performed to simulate the impact for each of the 8 individual PTWs and for different discharge start times. However, the results provided in Appendix 6.3 to Appendix 6.5 are not presented for all the model runs. Only the model run with greater water quality impact is selected and presented in these appendices.

6.293 Some of the model results presented in Appendix 6.4 and Appendix 6.5 for beaches and marine ecological sensitive receivers are mean values over the entire simulation period which cannot reflect the short term impacts of the temporary overflow over 6 hours. The elevation / trend of pollution levels during and after the sewage bypass period (as compared to the normal condition during late phase of Stage 2A) is illustrated by means of time series plots. The time series plots for selected indicator points (beaches, FCZ and flushing water intakes) covering the periods before, during and after the temporary discharges are shown in Figure 6.57 to Figure 6.72. The results provided in the time series plots for the beaches and fish culture zones are depth-average values whereas those for the WSD flushing water intakes represent the middle water layer.

Scenario B – Temporary Seawall Bypass of Sewage from All the PTWs Due to Total Power / Equipment Failure at SCISTW

Secondary Contact Recreation Subzones, Beaches and Fish Culture Zones

6.294 The water quality impact on secondary contact recreation subzones, beaches and fish culture zones (FCZ) is assessed in terms of E.coli, which is used to measure the suitability of secondary contact recreation subzones, bathing and marine cultural activities. Water quality objectives have been established for beaches at 180 no. per 100 ml, which is a geometric mean value for bathing season (March to October). Water quality objectives have also been established for secondary contact recreation subzones and FCZ at 610 no. per 100 ml, which is an annual geometric mean value. The model predicted that the worst-case emergency discharge would cause the following water quality effects:

· Short-term elevation of E.coli level at four beaches (B7, B8, B9, B14, 24) in dry season and seven beaches (B7, B8, B9, B14, B24, B26, B31) in wet season (refer to Figure 6.1 for their locations). The short-term emergency discharge would cause the E.coli level predicted at these beaches to occasionally exceed the criteria value of 180 no. per 100 ml. The E.coli elevations were relatively minor or negligible for all the remaining beaches identified in the Study Area.

· Short-term and very minor elevation of E.coli level at the secondary contact recreation subzones in Tsuen Wan and Southern Districts. All the peak E.coli values predicted at the secondary contact recreation subzones during the overflow period were below the criteria value of 610 no. per 100 ml under all the discharge scenarios.

· Short-term elevation of E.coli level at three FCZ (F1, F4, F5) (refer to Figure 6.1 for their locations). However, the peak E.coli values predicted at all the three FCZ during the emergency period were below the criteria value of 610 no. per 100 ml under all the discharge scenarios, except at Tung Lung Chau FCZ (F1) where the E.coli levels would reach a peak value of 738 no. per 100 mL under the spring-ebb tide condition which marginally exceeded the criteria value.

6.295 The time series plots for selected beaches and FCZ are presented for E.coli in Figure 6.57 to Figure 6.60. Figure 6.1 shows these indicator points. The time series plot of E.coli for an indicator point located at the boundary of the secondary contact recreation subzone in Tsuen Wan District is given in Figure 6.60a. Elevations of E.coli levels were predicted immediately after the start of temporary discharge. The normal water quality conditions (i.e. the conditions under normal operation of the HATS Stage 2A) were predicted to recover within 1 days after the end of the emergency discharge.

6.296 The pollution levels for other parameters such as nutrients and DO were not predicted to be significantly affected by the emergency discharge. Figure 6.61 to Figure 6.64 present the time series plots for DO (which is an important parameter for maintaining a healthy ecosystem) which indicated that the short-term emergency discharge would only cause minor or negligible effect on the DO level predicted at all the beaches and FCZ.

SSSIs, Marine parks, Marine Reserves, Seagrass Beds, Artificial Reefs, Chinese White Dolphins, Green Turtle Nesting Grounds and Corals

6.297 As shown in Appendix 6.5, the maximum values for sedimentation rates predicted at all the coral sites complied well with the assessment criterion of 100g/m²/day under the emergency scenario. The SS levels predicted at all the ecological sensitive receivers would also fully comply with the WQO of no more than 30% increase from the ambient level.

WSD Flushing Water Intakes

6.298 The time series plots for WSD flushing water intakes are presented for E.coli in Figure 6.65 to Figure 6.68. The target E.coli limit specified by WSD at the flushing water intake points is 20,000 no. per 100 ml, expressed as a maximum value. The model predicted that the worst-case emergency discharge would cause E.coli exceedances at 10 flushing water intakes (WSD5, WSD9, WSD10, WSD11, WSD12, WSD13, WSD17, WSD18, WSD19, WSD22) in dry season and 8 flushing water intakes (WSD5, WSD9, WSD10, WSD11, WSD12, WSD13, WSD18, WSD19) in wet season. Figure 6.1 shows the intake locations. The exceedance values predicted at these intakes ranged from 24,880 to 126,100 no. per 100 mL as compared to the WSD criterion of 20,000 no. per 100mL. Elevations of E.coli levels were predicted immediately after the start of temporary discharge. The normal water quality conditions (i.e. the conditions under normal operation of the HATS Stage 2A) were predicted to recover within 1 day after the end of the emergency discharge.

6.299 The emergency discharge would not cause any exceedance at the flushing water intakes for DO, SS, NH₃-N and BOD₅ (refer to Appendix 6.3). Figure 6.69 to Figure 6.72 illustrate the trend of SS levels predicted at the WSD flushing water intakes, which indicated that the SS elevations caused by the temporary discharge were minor and acceptable.

Evaluation of Water Quality Impacts Due to Equipment / Power Failure at SCISTW during Operational Phase

6.300 Precautionary measures e.g. standby equipment and dual (back-up) power supply would be provided at the SCISTW to control emergency discharge. From historical records, no emergency discharge has happened at SCISTW before since its commissioning. Under the extremely remote case that it did happen, the bacterial level in the Victoria Harbour and the adjacent waters would be significantly elevated for a short period of time. The normal water conditions would be restored within 2 days from the start of the emergency discharge period. Elevations of pollution level for other parameters of concern were relatively minor and less sensitive to the emergency discharge. The overall water quality impact is considered transient and acceptable.

6.301 Under the emergency discharge event, water quality at the beaches in Tsuen Wan and Southern Districts would be potentially affected. The Leisure and Cultural Services Department (LCSD) is currently the “beach management authority”, responsible for determining the opening and closing of gazetted beaches. The decision is made with reference to the advice provided by EPD on the suitability of beach water quality for bathing purposes and the consideration of all other factors. Generally, a beach will be closed if it is ranked "Very Poor" repeatedly. Beaches having geometric mean E.coli densities greater than 610 per 100mL are ranked "Very Poor". As indicated in the in Figure 6.57 to Figure 6.60, all the peak E.coli values predicted at the beaches during and after the emergency discharge period are considered acceptable and well below 610 per 100 mL. As the potential elevation of E.coli level occurred only for a very short period (less than 1 day), non-compliance with the WQO/WQC (which is a geometric mean for bathing season, March to October) was not predicted. No unacceptable public health effect would be expected due to the temporary discharge. In case of the extremely remote event of total equipment or power failure at SCISTW, it is recommended that relevant government departments including EPD and LCSD should be informed by DSD as soon as possible of any emergency discharge. Water quality monitoring should be carried out at such a time to quantify the water quality impacts and to determine when the baseline water quality conditions are restored.

6.302 The E.coli impact was predicted at the flushing water intakes in Victoria Harbour due to the emergency discharge. The exceedance values predicted at these intakes ranged from 24,880 to 126,100 no. per 100 mL as compared to the WSD criterion of 20,000 no. per 100mL. Historical EPD monitoring data indicated that the E.coli levels measured in the Victoria Harbour during the pre-HATS period would be similar or even higher than these predicted exceedance values. It is understood that the seawater abstracted from all the WSD flushing water intakes would normally be treated by disinfection before it is distributed to the users for flushing purpose. Given that the E.coli exceedances predicted at the intakes occurred only for a very short period (maximum 8 hours), refer to Figure 6.65 to Figure 6.68. No unacceptable water quality and public health effect would be expected.

6.303 It was predicted that the E.coli level would exceed the criteria value of 610 no. per 100 mL at Tung Lung Chau FCZ due to the emergency discharge. However, the predicted peak level was 738 no. per 100 mL which only marginally exceeded the criteria value. As the exceedance occurred only for a very short period (about three hours), non-compliance with the WQO/WQC (which is an annual geometric mean value) was not predicted. No insurmountable water quality impact would be expected on the FCZ due to the emergency discharge.

6.304 The water quality impact from the emergency discharge is considered minor and acceptable in view of the following:

· The emergency discharge would not cause any adverse effect on public health and health of biota or risk to life (the emergency discharge would not cause any non-compliance with the WQO/WQC at all the identified secondary contact recreation subzones, beaches and FCZ based on model predictions);

· The frequency of the potential water quality impact would be extremely remote as dual power supply and standby facilities would be provided at the SCISTW to minimize the occurrence of emergency discharge (no emergency discharge has happened at SCISTW since its commissioning);

· The emergency discharge would not cause any disruption to the seabed and therefore no disruption to any site of cultural heritage;

Scenario C – Seawall Bypass of Sewage at individual Stage 2 PTWs for 8 Hours Due to Power / Equipment Failure

6.305 Precautionary measures e.g. standby equipment and dual (back-up) power supply would be provided at the PTWs to control emergency discharge.This scenario represents the worst-case impact from seawall bypass of raw sewage at individual PTW within the Stage 2 catchment due to power or equipment failure. Model simulation was performed for only one PTW at a time to evaluate the extent of impact due to the sewage bypass at one individual PTW alone. Model runs were performed for all the eight Stage 2 PTWs.

6.306 The model predicted that the emergency discharge from individual Stage 2 PTW would cause the following short-tem E.coli exceedances:

Scenario ^{See Note 1}	Location of exceedances (refer to Figure 6.1)	Season	Peak E.coli level predicted over the emergency discharge period (no./100 mL)	Maximum duration of exceedance (hours)
Total power / equipment failure at Wan Chai East PTW ^{See Note 2}	WSD17	Dry Season	33,813	2
	WSD22	Dry Season	99,389	7
Total power / equipment failure at North Point PTW ^{See Note 3}	WSD17	Dry Season	33,595	2
Total power / equipment failure at Central PTW ^{See Note 4}	WSD18	Dry Season	76,561	7
	WSD18	Wet Season	41,580	2
	WSD19	Dry Season	41,010	2
	WSD19	Wet Season	22,970	2
Total power / equipment failure at Sandy Bay PTW ^{See Note 5}	B31	Wet Season	425	6
Total power / equipment failure at Cyberport PTW ^{See Note 6}	B31	Wet Season	421	6
Total power / equipment failure at Ap Lei Chau PTW ^{See Note 7}	B31	Wet Season	358	6

Notes:

1. Power failure at individual PTW may involve failure of sewage transfer pumping stations causing sewage bypass at more than one PTWs (see Notes 2 to 7 below). No exceedance was predicted for power failure at Aberdeen and Wah Fu PTWs.

2. Power failure at Wan Chai East PTW may cause sewage bypass from North Point and Wan Chai East PTWs

3. Power failure at North Point PTW may cause sewage bypass from North Point PTW only

4. Power failure at Central PTW may cause sewage bypass from Central PTW only

5. Power failure at Sandy Bay PTW may cause sewage bypass from Sandy Bay, Cyberport, Wah Fu, Aberdeen and Ap Lei Chai PTWs

6. Power failure at Cyberport PTW may cause sewage bypass from Cyberport, Wah Fu, Aberdeen and Ap Lei Chai PTWs.

7. Power failure at Ap Lei Chau PTW may cause sewage bypass from Ap Lei Chau PTWs only.

6.307 The modelling results for Scenario C are tabulated for all the water and marine ecological sensitive receivers identified within the Study Area in Appendix 6.3 to Appendix 6.5. The time series plots for Scenario C are not presented as they are similar to those presented for Scenario A and Scenario B showing transient elevations of pollution levels at sensitive receivers.

Evaluation of Water Quality Impacts Due to Equipment / Power Failure at Stage 2 PTWs during Operational Phase

6.308 A total of 32 separate model runs were performed to cover the emergency discharges from all the eight Stage 2 PTWs under various discharge start times. Only transient E.coli exceedances would occur at isolated sensitive receivers under 10 out of 32 discharge scenarios and the water quality would return to normal condition within 1 day after the end of emergency discharge period. No exceedance was predicted under all the remaining 22 discharge scenarios. Precautionary measures e.g. standby equipment and dual (back-up) power supply would be provided at the PTWs to control emergency discharge.

6.309 Temporary elevation of E.coli level was predicted at Chung Hom Kok beach (B31). The Leisure and Cultural Services Department (LCSD) is currently the “beach management authority”, responsible for determining the opening and closing of gazetted beaches. The decision is made with reference to the advice provided by EPD on the suitability of beach water quality for bathing purposes and the consideration of all other factors. Generally, a beach will be closed if it is ranked "Very Poor" repeatedly. Beaches having geometric mean E.coli densities greater than 610 per 100mL are ranked "Very Poor". As indicated in the in table above, all the peak E.coli values predicted at B31 during and after the emergency discharge period are considered acceptable and well below 610 per 100 mL. As the potential exceedance of the E.coli criterion of 180 no. per 100 mL occurred only for a very short period (maximum 6 hours), non-compliance with the WQO/WQC (which is a geometric mean value for bathing season, March to October) was not predicted. Potential elevation of E.coli levels were also predicted at the secondary contact recreation subzones in Southern Districts. Similarly, as the potential elevation would be short-term non-compliance with the WQO/WQC (which is an annual geometric mean value) was not predicted at the secondary contact recreation subzones. No unacceptable public health effect would be expected due to the temporary discharge.

6.310 The E.coli impact was predicted at the flushing water intakes in Victoria Harbour due to the emergency discharge. The exceedance values predicted at these intakes ranged from 22,970 to 99,389 no. per 100 mL as compared to the WSD criterion of 20,000 no. per 100mL. Historical EPD monitoring data indicated that the E.coli levels measured in the Victoria Harbour during the pre-HATS period would be similar or even higher than these predicted exceedance values. It is understood that the seawater abstracted from all the WSD flushing water intakes would normally be treated by disinfection before it is distributed to the users for flushing purpose. Given that the E.coli exceedances predicted at the intakes occurred only for a very short period (maximum 7 hours). No unacceptable water quality and public health effect would be expected.

6.311 The water quality impact from the emergency discharge is considered minor and acceptable in view of the following:

· The emergency discharge would not cause any adverse effect on public health and health of biota or risk to life (the emergency discharge would not cause any WQO/WQC non-compliance at all the identified secondary contact recreation subzones, beaches and FCZ based on model predictions);

· Standby unit(s) will be provided at all the Stage 2 PTWs to minimize the risk of emergency discharge. The frequency of the potential water quality impact would be remote. Emergency discharge occurred only once at Kwun Tong PTW (due to equipment failure) for 2 hours since 2005 based on the review of historical emergency discharge records for all the Stage 1 PTWs;

· The water quality impact would not occur in areas or regions that are ecologically fragile or sensitive (the impact zone would be localized and would not encroach on any ecological sensitive receiver of great importance);

· The emergency discharge would not cause any disruption to the seabed and therefore no disruption to any site of cultural heritage;

· The potential impact area is not of international and regional importance (the area is currently subject to the pollution discharge from the urbanized areas in HK Island; Implementation of this Project would reduce the pollution discharge to the concerned area).

Temporary Failure of Disinfection Facilities

Temporary Discharge of Undisinfected Effluent from SCISTW

6.312 Water quality impacts in relation to the temporary failure of chlorination plant have been fully assessed under the separate EIA for “HATS Provision of Disinfection Facilities at SCISTW (ADF)” and are therefore not re-modelled under this EIA (also see Section 6.18).

6.313 The model results provided in the EIA report for ADF indicated that the bacteria levels at the gazetted beaches and the secondary contact recreation subzones in Tsuen Wan coastal waters would be significantly elevated due to the emergency release of undisinfected effluent at the SCISTW outfall during ADF stage and Stage 2A. The peak E.coli values predicted at the Tsuen Wan beaches would exceed the WQO of 180 no. per 100 mL but impacts are expected to be short-term. The water quality would return to normal conditions within 2 days after the end of the emergency discharge periods. In the event of emergency discharge of undisinfected effluent, all the beaches in the Tsuen Wan District should be closed. The Plant operators of SCISTW should inform EPD and LCSD as soon as possible so that appropriate actions can be taken. Water quality monitoring should be carried out to quantify the water quality impacts and to determine when the normal water quality conditions are recovered. In view of the temporary nature of the emergency discharge, no insurmountable water quality impact is expected. Detailed model results for the temporary discharge of undisinfected CEPT effluent from SCISTW can be referred to the approved EIA report for ADF.

Temporary Discharge of Chlorinated Effluent from SCISTW (Without Dechlorination)

6.314 The discharge of chlorinated effluent (without dechlorination) under the event of dechlorination plant failure has been modelled under the EIA study for ADF for various discharge durations and scenarios, refer to Table 6.31b. It is considered that the chlorination process could be practically stopped within 30 minutes upon the occurrence of any dechlorination plant failure. The model results provided under the ADF EIA indicated that, during the emergency discharge periods, the TRC levels near the SCISTW outfall would be elevated and the TRC levels would exceed the assessment criterion in the near field during the emergency discharge period. Since the impact zones for the emergency TRC discharge were predicted to be localized (close to the SCISTW outfall) and temporary (within a few hours), the associated water quality and ecological impacts should be limited. The model results indicated that the maximum TRC levels predicted at all the identified water and ecological sensitive receivers would comply well with the assessment criterion of 0.008 mg/l under the emergency situations. Detailed modelling results for the temporary discharge of undisinfected CEPT effluent from SCISTW can be referred to the approved EIA report for ADF.

Construction Phase

Land-Based Construction

6.315 The Project would involve the following major construction activities:

· Site formation & site establishment

· Piling (pre-bored H pile)

· Excavation and backfilling

· Tunnel construction works

· Erection of formwork and reinforcement

· Concreting

· Fabrication of steelwork & installation of E&M equipment

General Construction Activities

6.316 The land-based construction works would have the potential to cause water pollution. Various types of construction activities may generate wastewater. These include general cleaning and polishing, wheel washing, dust suppression and utility installation. These types of wastewater would contain high concentrations of suspended solids. Impacts could also result from the accumulation of solid and liquid waste such as packaging and construction materials, and sewage effluent from the construction work force involved with the construction.

6.317 However, the effects on water quality from general construction activities are likely to be minimal, provided that site drainage would be well maintained and good construction practices would be observed such that litter, fuels, and solvents are managed, stored and handled properly.

6.318 Based on the Sewerage Manual, Part I, 1995 of the Drainage Services Department (DSD), the sewage production rate for construction workers was estimated to be 0.35 m³ per worker per day. For every 50 construction workers working simultaneously at the construction site, about 17.5 m³ of sewage would be generated per day. The sewage should not be allowed to discharge directly into the surrounding water body without treatment. Sufficient chemical toilets should be provided for workers.

Construction Site Runoff

6.319 During a rainstorm, site runoff would wash away the soil particles. The runoff is generally characterised by high concentrations of suspended solids. Release of uncontrolled site runoff would increase the SS levels and turbidity in the nearby water environment.

6.320 Wind blown dust would be generated from exposed soil surfaces in the works areas. It is possible that wind blown dust would fall directly onto the nearby water bodies when a strong wind occurs. Dispersion of dust within the works areas may increase the SS levels in surface runoff causing a potential impact to the nearby sensitive receivers.

6.321 Construction run-off and drainage may cause local water quality impacts. Increase in SS arising from the construction site could block the drainage channels and may result in local flooding when heavy rainfall occurs. High concentrations of suspended degradable organic material in marine water could lead to a reduction in DO levels in the water column.

6.322 It is important that proper site practice and good site management be followed to prevent run-off with high level of SS from entering the surrounding waters. With the implementation of appropriate measures to control run-off and drainage from the construction site, disturbance of water bodies would be avoided and deterioration in water quality would be minimal.

Accidental Spillage

6.323 A large variety of chemicals may be used during construction activities. These may include surplus adhesives, spent paints, petroleum products, spent lubrication oil, grease and mineral oil, spent acid and alkaline solutions/solvent and other chemicals. Accidental spillage of chemicals in the works areas may contaminate the surface soils. The contaminated soil particles may be washed away by construction site runoff or storm runoff causing water pollution. Provided that mitigation measures are properly implemented to minimize and control accidental spillage, no adverse impact on the identified water sensitive receivers is anticipated.

Construction Works in Close Proximity of Storm Drain or Seafront

6.324 The proposed seawall re-construction at Aberdeen PTW would be conducted in close vicinity to the sea and may cause marine water quality impact due to potential release of construction wastes. Construction wastes are generally characterized by high concentration of SS and elevated pH. Adoption of good house keeping and mitigation measures would reduce the generation of construction wastes and potential water pollution. The implementation of measures to control runoff and drainage will be important for the construction works adjacent to the storm drains or marine water in order to prevent runoff and drainage water with high levels of SS from entering the water environment. With the implementation of adequate construction site drainage and the provision of mitigation measures, it is anticipated that unacceptable water quality impacts would not arise.

Temporary Sewage Bypass

6.325 The modelling results for temporary sewage bypass during construction phase (namely Scenario D and Scenario E) are tabulated for all the water and marine ecological sensitive receivers identified within the Study Area in Appendix 6.3 to Appendix 6.5. Appendix 6.3 presents the peak concentrations for E.coli, NH₃-N, SS and BOD₅as well as the minimum concentrations for DO predicted at the WSD flushing water intakes for comparison with the WSD water quality standards. Appendix 6.4 gives the annual geometric means for E.coli and annual averages for UIA, PO4, TIN and UIA predicted at all the beaches for comparison with the WQO/WQC. Appendix 6.5 also gives the annual geometric means for E.coli and annual averages for UIA, PO4, TIN and UIA as well as the maximum sedimentation rates predicted at all the marine ecological sensitive receivers (including fish culture zones and coral sites) for comparison with the assessment criteria.

6.326 Some of the model results presented in Appendix 6.4 and Appendix 6.5 for beaches and marine ecological sensitive receivers are mean values over the entire simulation period which cannot reflect the short term impacts of the temporary discharge for two weeks. The elevation / trend of pollution levels during and after the sewage bypass period (as compared to the normal condition during late phase of HATS Stage 1) is illustrated by means of time series plots. The time series plots for selected parameters covering the periods before, during and after the temporary discharges are shown in Figure 6.73 to Figure 6.76 for Scenario D and Figure 6.77 to Figure 6.84 for Scenario E. The predicted results for the normal operation scenarios are also included in these time series plots for comparison. The indicator points selected for presentation include ten beaches (namely B7, B8, B9, B10, B11, B12, B13, B14, B24 and B26), two fish culture zones (namely F1 and F5) and fourteen WSD flushing water intakes (namely WSD4, WSD5, WSD7, WSD9, WSD10, WSD11, WSD12, WSD13, WSD15, WSD17, WSD18, WSD19, WSD20 and WSD22) that may be potentially affected by the temporary bypass. The results provided in the time series plots for the beaches and fish culture zones are depth-average values whereas those for the WSD flushing water intakes represent the middle water layer. Figure 6.1 shows these indicator points.

Scenario D

6.327 Scenario D represents the worst-case temporary bypass of screened sewage at all the Stage 1 PTW submarine outfalls for a period of two weeks in the dry season at the late phase of HATS Stage 1 in 2014 due to the modification / interconnection works for the main pumping station at SCISTW.

Secondary Contact Recreation Subzones, Beaches and Fish Culture Zones

6.328 The potential water quality impacts for secondary contact recreation subzones, beaches and fish culture zones (FCZ) are assessed in terms of E.coli, which is used to measure the suitability of secondary contact recreation subzones, bathing and marine cultural activities. The time series plots for selected beaches and FCZ are presented in Figure 6.73 for Scenario D. The time series plot of E.coli for an indicator point located at the boundary of the secondary contact recreation subzone in Tsuen Wan District is given in Figure 6.60a. Water quality objectives have been established for beaches at 180 no. per 100 ml, which is a geometric mean value for bathing season (March to October). Water quality objectives have also been established for secondary contact recreation subzones and FCZ at 610 no. per 100 ml, which is an annual geometric mean value. The model predicted that the 2-week temporary sewage bypass during dry season at the Stage 1 PTWs would cause the following water quality effects:

· Short-term elevation of E.coli level at nine beaches (B7, B8, B9, B10, B11, B12, B13, B14, B24). The model predicted that the E.coli values at these beaches would occasionally breach the criteria value of 180 no. per 100 ml during the temporary bypass period.

· Short-term elevation of E.coli level at the secondary contact recreation subzone in Tsuen Wan District where the E.coli values would occasionally breach the criteria value of 610 no. per 100 ml during the temporary bypass period. The peak E.coli values predicted at all other secondary contact recreation subzones (such as those in the Southern District) were all below the criteria value of 610 no. per 100 ml.

· Short-term elevation of E.coli level at two FCZ (F1, F5). However, all the peak E.coli values predicted at the FCZ were below the criteria value of 610 no. per 100 ml.

6.329 Elevations of E.coli levels were predicted immediately after the start of temporary discharge. The trends of E.coli levels shown in Figure 6.73 indicated that the E.coli levels would sharply increase after the start of temporary sewage bypass. The normal water quality conditions (i.e. the conditions under normal operation of the HATS Stage 1) were predicted to recover within 2 days after the end of the temporary bypass period.

6.330 The pollution levels for other parameters such as nutrients and DO were not predicted to be significantly affected by the sewage bypass. Figure 6.74 presents the time series plots for DO (which is an important parameter for maintaining a healthy ecosystem) which indicated that the temporary sewage bypass would only cause minor or negligible effect on the DO level predicted at all the beaches and FCZ.

SSSIs, Marine parks, Marine Reserves, Seagrass Beds, Artificial Reefs, Chinese White Dolphins, Green Turtle Nesting Grounds and Corals

6.331 As shown in Appendix 6.5, the maximum values for sedimentation rates predicted at all the coral sites complied well with the assessment criterion of 100g/m²/day under the temporary bypass scenario. The SS levels predicted at all the ecological sensitive receivers would also fully comply with the WQO of no more than 30% increase from the ambient level except for two isolated coral sites (C25 and CR28) in Junk Bay where the SS increase was predicted to be 30.9% and 31.9% of the baseline ambient values respectively which only marginally breached the WQO of 30% increase. The SS exeedances at these two coral sites would be very short-term (only last for 1 hour at CR25 and 4 hours at CR28), with a peak SS level of 5.2 mg/L only. These two coral sites are located in the eastern waters where the baseline SS levels were low. In terms of the absolute values, the peak SS levels predicted at these two coral sites were low and well below the value of 10 m/L. The SS value of 10 mg/L has been adopted as the SS criterion under many other EIA studies in Hong Kong ⁽[24]⁾ for protection of sensitive coral sites in the eastern and southern waters of Hong Kong where the baseline SS levels were also low. Based on previous EM&A monitoring results where this SS criterion of 10 mg/L has been adopted, no adverse impacts on corals have occurred. In summary, the SS exceedances at the two coral sites in Junk Bay are temporary (occurred for a few hours only) and the water quality impact would be minor (with a peak SS level of 5.2 mg/L only) and reversible (the normal water quality would resume quickly after the discharges), no adverse water quality impacts on these two coral sites would be expected.

WSD Flushing Water Intakes

6.332 The time series plots for WSD flushing water intakes are presented for E.coli in Figure 6.75. The target E.coli limit specified by WSD at the flushing water intake points is 20,000 no. per 100 ml, expressed as a maximum value. The model predicted that the 2-week temporary sewage bypass would cause E.coli exceedances at only one flushing water intake (WSD5) with a peak value of 61,461 no. per 100 mL.

6.333 The temporary bypass would not cause any exceedance at the flushing water intakes for DO, SS, NH₃-N and BOD₅ (refer to Appendix 6.3). Figure 6.76 illustrates the trend of SS levels predicted at the WSD flushing water intakes, which indicated that all the SS elevations caused by the temporary discharge were minor and acceptable.

Scenario E

6.334 Scenario E represents the scenario of temporary bypass of screened sewage from NWK PTW at the seawall for a period of two weeks at the late phase of HATS Stage 1 in 2014 due to the modification to the existing NWKPS.

Secondary Contact Recreation Subzones, Beaches and Fish Culture Zones

6.335 The potential water quality impacts for secondary contact recreation subzones, beaches and fish culture zones (FCZ) are assessed in terms of E.coli, which is used to measure the suitability of secondary contact recreation subzones, bathing and marine cultural activities. The time series plots for selected beaches and fish culture zones (FCZ) are presented for E.coli in Figure 6.77 and Figure 6.78 for Scenario E. Water quality objectives have been established for beaches at 180 no. per 100 ml, which is a geometric mean value for bathing season (March to October). Water quality objectives have also been established for secondary contact recreation subzones and FCZ at 610 no. per 100 ml, which is an annual geometric mean value. The model indicated that the 2-week temporary sewage bypass from the NWK PTW would cause only minor and short term E.coli elevations at four beaches (B7, B8, B9, B14) in dry season and two beaches (B7, 14) in wet season. The 2-week temporary sewage bypass from the NWK PTW would also cause minor and short term E.coli elevations at the secondary contact recreation subzone in Tsuen Wan District. The E.coli elevations were negligible for all the remaining secondary contact recreation subzones and beaches as well as all the FCZ identified in the Study Area. Non-compliance with the WQO/WQC was not predicted.

6.336 The pollution levels for other parameters such as nutrients and DO were not predicted to be significantly affected by the sewage bypass. Figure 6.79 and Figure 6.80 present the time series plots for DO (which is an important parameter for maintaining a healthy ecosystem) which indicated that the temporary sewage bypass would only cause minor or negligible effect on the DO level predicted at all the beaches and FCZ.

SSSIs, Marine parks, Marine Reserves, Seagrass Beds, Artificial Reefs, Chinese White Dolphins, Green Turtle Nesting Grounds and Corals

6.337 As shown in Appendix 6.5, the maximum values for sedimentation rates predicted at all the coral sites complied well with the assessment criterion of 100g/m²/day under the temporary bypass scenario. The SS levels predicted at all the ecological sensitive receivers would also fully comply with the WQO of no more than 30% increase from the ambient level.

WSD Flushing Water Intakes

6.338 The temporary bypass from NWK PTW would not cause any exceedance at the flushing water intakes for all the parameters of concern including E.coli, DO, SS, NH₃-N and BOD₅ (refer to Appendix 6.3). Figure 6.81 to Figure 6.84 show the trend of pollution levels for selected parameters (E.coli and SS) predicted at the WSD flushing water intakes for reference.

Evaluation of Water Quality Impacts Due to Temporary Sewage Bypass During Construction Phase

Scenario D

6.339 The bacterial levels at 9 beaches (B7, B8, B9, B10, B11, B12, B13, B14, B24) and the secondary contact recreation subzone in the Tsuen Wan District were predicted to be potentially affected by the temporary sewage bypass under Scenario D. The geometric means of the E.coli levels predicted during the 2-week temporary sewage bypass period are shown in the table below and compared with the geometric mean values predicted under the normal HATS operation for the same period.

Scenario	B7- Anglers'	B8 - Gemini	B9 - Hoi Mei Wan	B10 - Casam	B11 - Lido	B12- Ting Kau	B13 - Approach	B14 - Ma Wan	B24 - Big Wave Bay
Scenario	Geometric Mean * (no. per 100 ml) for the 2-week Sewage Bypass Period
Normal operation of HATS during late phase of Stage 1	122	131	103	118	86	186	376	18	2
Temporary sewage bypass at Stage 1 PTWs in dry season during late phase of Stage 1	238	279	232	166	135	236	514	57	20

6.340 The Leisure and Cultural Services Department (LCSD) is currently the “beach management authority”, responsible for determining the opening and closing of gazetted beaches. The decision is made with reference to the advice provided by EPD on the suitability of beach water quality for bathing purposes and the consideration of all other factors. Generally, a beach will be closed if it is ranked "Very Poor" repeatedly. Beaches having geometric mean E.coli densities greater than 610 per 100mL are ranked "Very Poor". As indicated in the table above, the geometric mean E.coli values predicted at the nine beaches are below 610 per 100 mL. Also, the temporary bypass will be scheduled to occur in the dry (or non-bathing) season (November to February) to minimize the potential impact. As the potential elevation of E.coli level occurred only for a very short period (about two weeks in dry season), non-compliance with the WQO/WQC (which is expressed as a geometric mean for bathing season, i.e. March to October) was not predicted at the beaches. In addition, the geometric mean E.coli levels predicted at the secondary contact recreation subzone in Tsuen Wan District were all below 610 no. per 100 ml. Non-compliance with the WQO/WQC was not predicted at the secondary contact recreation zone. No unacceptable effect on public health would be expected due to the temporary discharge. The potential elevation of E.coli level would be short term (about 2 weeks) and the water quality would return to the normal conditions quickly (within 2 days) after the temporary bypass. Relevant government departments including EPD and LCSD should be informed of the planned sewage bypass prior to any discharge. During the sewage bypass event, water quality monitoring should be carried out at such a time to quantify the water quality impacts at the beaches and to determine when the baseline water quality conditions are restored. No unacceptable impact would be expected with proper implementation of the recommended mitigation measures.

6.341 The E.coli impact was still predicted at the flushing water intake at Tsing Yi (WSD5) under the temporary bypass scenario. The peak E.coli level predicted at this intake was about 60,000 no. per 100 ml under the dry season scenario. E.coli should not be a critical parameter of concern for flushing water intake as it is understood that the seawater abstracted from all the WSD flushing water intakes would be treated by disinfection before it is distributed to the users for flushing purpose. Historical EPD monitoring data measured in 1998 at VM13 ^([25]) (in close vicinity of WSD5) indicated that the E.coli level at this intake point could reach 130,000 no. per 100 mL during the pre-HATS period (much higher than the peak value predicted under the temporary bypass scenario). Given that the sewage bypass would be short-term (for 2 weeks) and the water quality would return to normal condition quickly after the end of sewage bypass period as demonstrated by the model predictions. No unacceptable water quality and public health effect would be expected.

6.342 The SS impact was predicted at two isolated coral sites (C25 and CR28) in Junk Bay where the SS increase was predicted to be 30.9% and 31.9% of the baseline ambient values respectively which only marginally breached the WQO of 30% increase. The SS exeedances at these two coral sites would be very short-term (only last for 1 hour at CR25 and 4 hours at CR28), with a peak SS level of 5.2 mg/L only. These two coral sites are located in the eastern waters where the baseline SS levels were low. In terms of the absolute values, the peak SS levels predicted at these two coral sites were low and well below the value of 10 m/L. The SS value of 10 mg/L has been adopted as the SS criterion under many other EIA studies in Hong Kong (refer to Footnote no. 23 above) for protection of sensitive coral sites in the eastern and southern waters of Hong Kong where the baseline SS levels were also low. Based on previous EM&A monitoring results where this SS criterion of 10 mg/L has been adopted, no adverse impacts on corals have occurred. The SS exceedances at the two coral sites in Junk Bay are temporary (occurred for a few hours only) and the water quality impact would be minor (with a peak SS level of 5.2 mg/L only) and reversible (the normal water quality would resume quickly after the discharges). No adverse impact on the coral sites would be expected. In conclusion, the water quality impact from the temporary sewage bypass is considered minor and acceptable in view of the following:

· The temporary sewage bypass would not cause any adverse effect on public health and health of biota or risk to life (the temporary discharge would not cause any WQO/WQC exceenance at all the identified secondary contact recreation subzones, beaches and FCZ based on model predictions);

· The magnitude of the adverse water quality impact would be small and the potential water quality impact would be short-term (about 2 weeks) and reversible (the normal water quality would resume quickly after the discharges);

· The temporary bypass would not cause any disruption to the seabed and therefore no disruption to any site of cultural heritage;

Scenario E

6.343 No exceendance of the assessment criteria values was predicted at all the identified water and ecological sensitive receivers during the temporary sewage bypass from NWK PTW as demonstrated from the modelling assessment. As minor elevations of bacteria levels were predicted at some of the Tsuen Wan beaches, it is recommended that relevant government departments including EPD and LCSD should be informed of the planned sewage bypass prior to any discharge. During the sewage bypass event, water quality monitoring should be carried out at such a time to quantify the water quality impacts and to determine when the baseline water quality conditions are restored. No unacceptable residual impact would be expected with proper implementation of the recommended mitigation measures.

Mitigation of Adverse Environmental Impacts

Operation Phase

6.344 No adverse water quality impact associated with normal operation of the Project was predicted. Emergency discharges of screened sewage at the Stage 1 and Stage 2 PTWs would be the consequence of power or equipment failure at SCiSTW. Dual power supply would be provided at the SCISTW to minimize the occurrence of power failure. In addition, standby facilities for the main treatment units and standby equipment parts / accessories would also be provided at the SCISTW in order to minimize the chance of emergency discharge. To provide a mechanism to minimise the impact of emergency discharges and facilitate subsequent management of any emergency, an emergency contingency plan has been formulated to clearly state the response procedure in case of total power or equipment failure at SCISTW (details refer to the standalone EM&A Manual). The plant operators of SCISTW should closely communicate with relevant departments including EPD and LCSD during the emergency discharge. An event and action plan and a detailed water quality monitoring programme for the emergency discharge is given in a standalone EM&A Manual.

6.345 No unacceptable water quality impact was predicted associated with power or equipment failure at individual Stage 2 PTWs. Precautionary measures e.g. standby equipment and dual (back-up) power supply would be provided at all the Stage 2 PTWs to control emergency discharge.

6.346 In case of total power outage of the dechlorination plant, the uninterruptible power supply (UPS) system to be provided would switch the power supply of the sodium bisulphite dosing pump to a backup battery almost instantaneously, allowing continuous dosage of sodium bisulphite for at least half an hour so that sufficient time can be provided for shutting down the chlorination plant to avoid the possibility of discharge of chlorinated effluent. With proper implementation of these mitigation measures, the occurrence of discharge of non-dechlorinated effluent would be very remote. In case that the dechlorination process fails, the chlorination process could be practically stopped within 30 minutes to avoid discharge of TRC into the marine water. The E.coli impact due to no chlorination for a short period of time would be temporary and acceptable as supported by the water quality impact assessment conducted under the ADF study. A framework of the emergency response plan and water quality monitoring requirements for temporary failure of disinfection facilities are provided in the separate EM&A Manual.

6.347 The model predicted that if Stage 2B is not implemented for HATS in 2021 as scheduled, the nutrient contents (both P and N) in the marine water would ultimately increase to exceed the baseline Stage 1 level when the HATS flow is reaching its design capacity of 2.45M m³/day. It should be noted that the schedule of Stage 2B implementation would be subject to the review of flow build-ups and water quality conditions to be conducted in 2010/2011. It is recommended that the future review study for Stage 2B should review the validity of the model predictions provided in this EIA and confirm the need of enhanced nutrient removal for HATS after 2021.

6.348 It should be noted that the mixing zone for TIN predicted for Stage 2B was large with an area of about 30 km² as indicated in Table 6.45 and the area of exceedance would encroach on the nearby water sensitive receivers (e.g. Ma Wan Fish Culture Zone). This is due to the elevated oxidized nitrogen assumed for the proposed nitrification process at Stage 2B as well as the increased HATS effluent flow assumed for Stage 2B. It is recommended that these water quality issues should be further investigated / assessed under the future EIA for Stage 2B. Further mitigation measures / alternative treatment designs should also be considered under the future EIA for Stage 2B to mitigate / minimize the potential TIN exceedances.

Construction Phase

Construction Site Runoff and General Construction Activities

6.349 To minimize the potential water quality impacts from construction site runoff and various construction activities, the practices outlined in ProPECC PN 1/94 Construction Site Drainage should be adopted.

Surface Run-off

6.350 Surface run-off from construction sites should be discharged into storm drains via adequately designed sand/silt removal facilities such as sand traps, silt traps and sedimentation basins. Channels or earth bunds or sand bag barriers should be provided on site to properly direct stormwater to such silt removal facilities. Perimeter channels at site boundaries should be provided where necessary to intercept storm run-off from outside the site so that it will not wash across the site. Catchpits and perimeter channels should be constructed in advance of site formation works and earthworks.

6.351 Silt removal facilities, channels and manholes should be maintained and the deposited silt and grit should be removed regularly, at the onset of and after each rainstorm. Any practical options for the diversion and re-alignment of drainage should comply with both engineering and environmental requirements in order to provide adequate hydraulic capacity of all drains. Minimum distances of 100 m should be maintained between the discharge points of construction site runoff and the existing saltwater intakes.

6.352 Construction works should be programmed to minimize soil excavation works in rainy seasons (April to September). If excavation in soil could not be avoided in these months or at any time of year when rainstorms are likely, for the purpose of preventing soil erosion, temporary exposed slope surfaces should be covered e.g. by tarpaulin, and temporary access roads should be protected by crushed stone or gravel, as excavation proceeds. Intercepting channels should be provided (e.g. along the crest / edge of excavation) to prevent storm runoff from washing across exposed soil surfaces. Arrangements should always be in place in such a way that adequate surface protection measures can be safely carried out well before the arrival of a rainstorm.

6.353 Earthworks final surfaces should be well compacted and the subsequent permanent work or surface protection should be carried out immediately after the final surfaces are formed to prevent erosion caused by rainstorms. Appropriate drainage like intercepting channels should be provided where necessary.

6.354 Measures should be taken to minimize the ingress of rainwater into trenches. If excavation of trenches in wet seasons is necessary, they should be dug and backfilled in short sections. Rainwater pumped out from trenches or foundation excavations should be discharged into storm drains via silt removal facilities.

6.355 Open stockpiles of construction materials (e.g. aggregates, sand and fill material) on sites should be covered with tarpaulin or similar fabric during rainstorms.

6.356 Manholes (including newly constructed ones) should always be adequately covered and temporarily sealed so as to prevent silt, construction materials or debris from getting into the drainage system, and to prevent storm run-off from getting into foul sewers. Discharge of surface run-off into foul sewers must always be prevented in order not to unduly overload the foul sewerage system.

6.357 Good site practices should be adopted to remove rubbish and litter from construction sites so as to prevent the rubbish and litter from spreading from the site area. It is recommended to clean the construction sites on a regular basis.

Groundwater

6.358 Groundwater pumped out of wells, etc. for the lowering of ground water level in basement or foundation construction, and groundwater seepage pumped out of tunnels or caverns under construction should be discharged into storm drains after the removal of silt in silt removal facilities.

Boring and Drilling Water

6.359 Water used in ground boring and drilling for site investigation or rock / soil anchoring should as far as practicable be recirculated after sedimentation. When there is a need for final disposal, the wastewater should be discharged into storm drains via silt removal facilities.

Wastewater from Concrete Batching and Precast Concrete Casting

6.360 Wastewater generated from the washing down of mixing trucks and drum mixers and similar equipment should whenever practicable be used for other site activities. The discharge of wastewater should be kept to a minimum and should be treated to meet the appropriate standard as specified in the TM-DSS before discharging.

6.361 To prevent pollution from wastewater overflow, the pump sump of any wastewater system should be provided with an on-line standby pump of adequate capacity and with automatic alternating devices.

6.362 Under normal circumstances, surplus wastewater may be discharged into foul sewers after treatment in silt removal and pH adjustment facilities (to within the pH range of 6 to 10). Disposal of wastewater into storm drains will require more elaborate treatment.

Wheel Washing Water

6.363 All vehicles and plant should be cleaned before they leave a construction site to minimize the deposition of earth, mud, debris on roads. A wheel washing bay should be provided at every site exit if practicable and wash-water should have sand and silt settled out or removed before discharging into storm drains. The section of construction road between the wheel washing bay and the public road should be paved with backfall to reduce vehicle tracking of soil and to prevent site run-off from entering public road drains.

Bentonite Slurries

6.364 Bentonite slurries used in diaphragm wall and bore-pile construction should be reconditioned and used again wherever practicable. If the disposal of a certain residual quantity cannot be avoided, the used slurry may be disposed of at the marine spoil grounds subject to obtaining a marine dumping licence from EPD on a case-by-case basis.

6.365 If the used bentonite slurry is intended to be disposed of through the public drainage system, it should be treated to the respective effluent standards applicable to foul sewer, storm drains or the receiving waters as set out in the TM-DSS (refer to Section 6.23).

Water for Testing & Sterilization of Water Retaining Structures and Water Pipes

6.366 Water used in water testing to check leakage of structures and pipes should be used for other purposes as far as practicable. Surplus unpolluted water could be discharged into storm drains.

6.367 Sterilization is commonly accomplished by chlorination. Specific advice from EPD should be sought during the design stage of the works with regard to the disposal of the sterilizing water. The sterilizing water should be used again wherever practicable.

Wastewater from Building Construction

6.368 Before commencing any demolition works, all sewer and drainage connections should be sealed to prevent building debris, soil, sand etc. from entering public sewers/drains.

6.369 Wastewater generated from building construction activities including concreting, plastering, internal decoration, cleaning of works and similar activities should not be discharged into the stormwater drainage system. If the wastewater is to be discharged into foul sewers, it should undergo the removal of settleable solids in a silt removal facility, and pH adjustment as necessary.

Acid Cleaning, Etching and Pickling Wastewater

6.370 Acidic wastewater generated from acid cleaning, etching, pickling and similar activities should be neutralized to within the pH range of 6 to 10 before discharging into foul sewers. If there is no public foul sewer in the vicinity, the neutralized wastewater should be tinkered off site for disposal into foul sewers or treated to a standard acceptable to storm drains and the receiving waters.

Wastewater from Site Facilities

6.371 Wastewater collected from canteen kitchens, including that from basins, sinks and floor drains, should be discharged into foul sewer via grease traps capable of providing at least 20 minutes retention during peak flow.

6.372 Drainage serving an open oil filling point should be connected to storm drains via a petrol interceptors with peak storm bypass.

6.373 Vehicle and plant servicing areas, vehicle wash bays and lubrication bays should as far as possible be located within roofed areas. The drainage in these covered areas should be connected to foul sewers via a petrol interceptor. Oil leakage or spillage should be contained and cleaned up immediately. Waste oil should be collected and stored for recycling or disposal in accordance with the Waste Disposal Ordinance.

Sewage from Workforce

6.374 The presence of construction workers generates sewage. It is recommended to provide sufficient chemical toilets in the works areas. The toilet facilities should be more than 30 m from any watercourse. A licensed waste collector should be deployed to clean the chemical toilets on a regular basis.

6.375 Notices should be posted at conspicuous locations to remind the workers not to discharge any sewage or wastewater into the nearby environment. Regular environmental audit on the construction site can provide an effective control of any malpractices and can encourage continual improvement of environmental performance on site. It is anticipated that sewage generation during the construction phase of the project would not cause water pollution problem after undertaking all required measures.

Effluent Discharge

6.376 There is a need to apply to EPD for a discharge licence for discharge of effluent from the construction site under the WPCO. The discharge quality must meet the requirements specified in the discharge licence. All the runoff and wastewater generated from the works areas should be treated so that it satisfies all the standards listed in the TM-DSS. Minimum distances of 100 m should be maintained between the discharge points of construction site effluent and the existing saltwater intakes. The beneficial uses of the treated effluent for other on-site activities such as dust suppression, wheel washing and general cleaning etc., can minimise water consumption and reduce the effluent discharge volume. If monitoring of the treated effluent quality from the works areas is required during the construction phase of the Project, the monitoring should be carried out in accordance with the WPCO license which is under the ambit of regional office (RO) of EPD.

Accidental Spillage of Chemicals

6.377 Contractor must register as a chemical waste producer if chemical wastes would be produced from the construction activities. The Waste Disposal Ordinance (Cap 354) and its subsidiary regulations in particular the Waste Disposal (Chemical Waste) (General) Regulation should be observed and complied with for control of chemical wastes.

6.378 Any service shop and maintenance facilities should be located on hard standings within a bunded area, and sumps and oil interceptors should be provided. Maintenance of vehicles and equipment involving activities with potential for leakage and spillage should only be undertaken within the areas appropriately equipped to control these discharges.

6.379 Disposal of chemical wastes should be carried out in compliance with the Waste Disposal Ordinance. The Code of Practice on the Packaging, Labelling and Storage of Chemical Wastes published under the Waste Disposal Ordinance details the requirements to deal with chemical wastes. General requirements are given as follows:

· Suitable containers should be used to hold the chemical wastes to avoid leakage or spillage during storage, handling and transport.

· Chemical waste containers should be suitably labelled, to notify and warn the personnel who are handling the wastes, to avoid accidents.

· Storage area should be selected at a safe location on site and adequate space should be allocated to the storage area.

Construction Works in Close Proximity of Storm Drains or Seafront

6.380 To minimize the potential water quality impacts from the construction works located at or near any watercourse, the practices outlined below should be adopted where applicable.

· The use of less or smaller construction plants may be specified to reduce the disturbance to the storm water courses or marine environment.

· Temporary storage of materials (e.g. equipment, filling materials, chemicals and fuel) and temporary stockpile of construction materials should be located well away from any water courses during carrying out of the construction works.

· Stockpiling of construction materials and dusty materials should be covered and located away from any water courses.

· Construction debris and spoil should be covered up and/or disposed of as soon as possible to avoid being washed into the nearby water receivers.

· Construction activities, which generate large amount of wastewater, should be carried out in a distance away from the waterfront, where practicable.

· Proper shoring may need to be erected in order to prevent soil/mud from slipping into the storm culvert or sea.

Temporary Sewage Bypass

6.381 It is recommended that the temporary sewage bypass required for (i) the modification to the existing pumping station at SCISTW and (ii) the interconnection between the existing main pumping station and the new pumping station on Stonecutters Island, if needed, should be scheduled at the same time in order to minimise the temporary discharge duration. It is also recommended that all the modification and interconnection to the existing facilities (including the modification to the existing NWKPS) should be programmed to avoid temporary sewage bypass in wet or bathing season (March to October) to minimize the potential impacts. Relevant government departments including EPD and LCSD should be informed of the planned sewage bypass prior to any discharge. During the sewage bypass period, water quality monitoring should be carried out at the water sensitive receivers to quantify the water quality impacts and to determine when the baseline water quality conditions are restored. Also, a framework of the response procedures has been formulated to minimize the impact of temporary discharges. Details are provided in the standalone EM&A Manual.

Evaluation of Residual Impacts

Construction Phase

6.382 The construction phase water quality impact would generally be temporary and localized during construction. No unacceptable residual water quality impacts would be expected during the construction phase of the Project, provided that all the recommended mitigation measures are properly implemented.

Operation Phase

6.383 The HATS Stage 2A implementation would improve the water quality in the receiving water as compared to the existing baseline condition. No adverse water quality impact associated with normal operation of the Project was predicted from the assessment. No unacceptable residual water quality impacts would be expected for the operation of the Project.

Environmental Monitoring and Audit

6.384 Monitoring of effluent quality is recommended for operational stage and under the perspective of the WPCO. A post project monitoring (PPM) programme will be implemented to confirm the water quality predictions made in the EIA report. The PPM would consist of one-year baseline monitoring before commissioning and at least one-year impact monitoring after commissioning of the Project. The extent of PPM programme is subject to the prevailing environmental conditions at the time before commissioning of the Project. A more detailed description of the PPM requirements is given in the standalone EM&A Manual.

6.385 Marine water quality monitoring is recommended during the emergency discharge (as a result of equipment / power failure at SCISTW) during operational phase and the temporary sewage bypass (required for the modification / interconnection works) during construction phase or early commissioning of the Project. A baseline monitoring programme is proposed to establish the baseline water quality conditions at selected monitoring points. In case of emergency discharge or temporary sewage bypass, daily marine water monitoring should be conducted throughout the whole discharge period until the normal water quality resumes. A more detailed description of the water quality requirements is given in the standalone EM&A Manual.

Conclusions

Construction Phase

6.386 The general construction works for the Project would be land-based except seawall re-construction at the Aberdeen PTW. No dredging would be required for the Project. The fine content in the fill material during seawall re-construction at the Aberdeen PTW should be negligible. Key water quality issues associated with land-based construction would include the impacts from site run-off, sewage from workforce, accidental spillage and discharges of wastewater from various construction activities. With well maintained site drainage and the implementation of good site practices, impacts would be controlled to comply with the WPCO standards by implementing the recommended mitigation measures. No unacceptable water quality impact would therefore be expected.

6.387 Temporary bypass of sewage effluent via individual PTW would be required during the construction stage. The water quality impacts during the temporary sewage bypass were assessed using the Delft3D model. The predicted water quality impact associated with the temporary discharge would be short-term and the water quality would return to the normal condition quickly after the sewage bypass period. Water quality monitoring is recommended to be carried out during the temporary sewage bypass to quantify the water quality impacts and to determine when the baseline water quality conditions are restored. Also, a framework of the response procedures has been formulated to minimize the impact of temporary discharges. No insurmountable water quality impact would be expected.

Operation Phase

6.388 The water quality impacts during operation of the Project were assessed using the Delft3D model. Impacts were assessed over a series of one-year simulation periods. The assessment area included the Victoria Harbour, Western Buffer, Eastern Buffer, Junk Bay, Northwestern, and Southern Water Control Zones. Sensitive receivers within the assessment area identified for water quality impact assessment included cooling water intakes, flushing water intakes, fish culture zones, beaches, sites of special scientific interest, marine parks, marine reserves, seagrass beds, artificial reefs, corals, the Chinese White Dolphin and Green Turtle nesting grounds.

6.389 The water quality modelling results showed that the Project would not cause any adverse impact on the marine water quality and on the identified sensitive receivers during normal operations of the SCISTW. The total residual chlorine from the chlorination/dechlorination disinfection process would meet the criterion set for the edge of the zone of initial dilution, with a large safety margin. Whole effluent toxicity tests showed that the chlorination/dechlorination disinfection process did not introduce additional toxic effects to the test organisms. A Post Project Monitoring (PPM) programme was proposed to confirm the model predictions made in this EIA.

6.390 Overflow at PTW may occasionally occur only during storm events and the extent of impact was predicted to be minor and acceptable. Mitigation measures, including dual power supply, standby pumps, treatment units and equipment, would be provided at SCISTW to minimize the occurrence of emergency discharge. Standby unit(s) and dual (backup) power supply would also be provided at all the Stage 2 PTWs to reduce the risk of equipment breakdown at the PTWs. The model suggested that the water quality impacts associated with power or equipment failure at SCISTW and the Stage 2 PTWs would be short term. The water quality would return to the normal condition quickly after the emergency sewage. In case of power outage of the dechlorination plant, the uninterruptible power supply (UPS) system to be provided would switch the power supply of the sodium bisulphite dosing pump to a backup battery almost instantaneously, allowing continuous dosage of sodium bisulphite for at least half an hour so that sufficient time can be provided for shutting down the chlorination plant to avoid the possibility of discharge of chlorinated effluent. An emergency contingency plan has been formulated to minimise the impact of emergency discharges and facilitate subsequent management of the emergency. An event and action plan and a detailed EM&A programme are recommended to collect water quality information and to mitigate the potential impact due to emergency discharge. The monitoring results shall be employed to identify areas for any further necessary mitigation measures to avoid, rectify and eliminate environmental damage associated with the Project.

^{(^[1])} Rehan Sadiq, Manuel J. Rodriguez (2003). Disinfection By-products (DBPs) in Drinking Water and Predictive Models for their Occurrence: A Review.

^([2]) Pastorok, R.A. and Bilyard, G.R. (1985). “Effects of sewage pollution on coral-reef communities.” Marine Ecology Progress Series 21: 175-189.

^([3]) Hawker, D. W. and Connell, D. W. (1992). “Standards and Criteria for Pollution Control in Coral Reef Areas” in Connell, D. W and Hawker, D. W. (eds.), Pollution in Tropical Aquatic Systems, CRC Press, Inc.

^{(^[4])} Maunsell Consultants Asia Limited (2003). Tai Po Sewage Treatment Works Stage 5, EIA Report, Drainage Services Department, 2003

^{(^[5])} Hyder (1997). Sand Dredging and Backfilling of Borrow Pits at the Potential Eastern Waters Marine Borrow Area, EIA Report, CED, 1997.

^{(^[6])} ERM-Hong Kong, Limited (2001). Focused Cumulative Water Quality Impact Assessment of Sand Dredging at the West Po Toi Marine Borrow Area Final Report.

^{(^[7])} ERM-Hong Kong, Limited (2003). The Proposed Submarine Gas Pipelines from Cheng Tou Jiao Liquefied Natural Gas Receiving Terminal, Shenzhen to Tai Po Gas Production Plant, Hong Kong, EIA Report, The Hong Kong and China Gas Company Limited, 2003

^{(^[8])} Studies that have adopted the SS elevation criterion of 10 mg/L include:

Sai Kung Sewage Treatment Works EIA Study;

Tai Po Sewage Treatment Works Stage 5 EIA Study

The Proposed Submarine Gas Pipelines from Cheng Tou Jiao Liquefied Natural Gas Receiving Terminal, Shenzhen to Tai Po Gas Production Plant, Hong Kong, EIA Report, The Hong Kong and China Gas Company Limited, 2003

Civil Engineering Department (1997) Sand dredging and backfilling of Borrow Pits at the potential Eastern Waters Marine Borrow Area EIA Report;

Civil Engineering Department (1998) Environmental Impact Assessment of backfilling Marine Borrow Areas at East Tung Lung Chau;

CLP Power (2001) Environmental consultancy services for the proposed 11 kV cable circuits from Tsai Mong Tsai to Kiu Tsui;

CLP Power (2002) Environmental consultancy services for the proposed 132 kV cable circuits from A Kung Wan to Sai Kung Pier;

The Hongkong Electric Co (2002) 132 kV submarine cable installation for Wong Chuk Hang – Chung Hom Kok 132 kV circuits.

^([9])Compliance with the WQO was achieved in 8 out of 10 stations in the Victoria Harbour WCZ (i.e. 80% compliance).

^([10]) Tender Ref 2004T140 Testing of Chlorination / Dechlorination CEPT Effluent from SCISTW.

^{(^[11])} Bower C. E. and Bidwell J. P. 1978, Ionization of ammonia in seawater: Effect of temperature, pH and Salinity. J. Fish. Res. Board Can. Vol 35, pp. 1012 – 1016.

^([12]) Tender Ref. CE43/2001-002 Testing of Chlorinated/Dechlorinated Sewage Effluent from Tai Po Sewage Treatment Works (TPSTW) and Sha Tin Sewage Treatment Works (STSTW)

⁽[13]⁾ Guidelines for Estimating Sewage Flows for Sewage Infrastructure Planning (Version 1.0), EPD, March 2005

^([14]) A.M. Shams El Din, Rasheed A. Arain, A.A. Hammoud (2000). “On the chlorination of seawater”, Desalination 129, page 53-62, www.elsevier.com.

^{(^[15])} As confirmed by DSD, these submarine outfalls are currently kept in a serviceable condition by flushing the outfalls annually together with dye test and diver inspection if required.

^([16])Acute Toxicity Unit (TUa) is defined as TUa = 100/LC50, where LC50 = % of effluent which gives 50% survival of the most sensitive of the range of species examined under the 48-hour survival tests.

^([17]) Agreement No. CE 73/95 Strategic Sewage Disposal Scheme (Harbour Area Treatment Scheme) Stage 1, Baseline Monitoring and Performance Verification Final Report, August 2004

^{(^[18])} Redfield A.C., On the proportions of organic derivations in seawater and their relation to the composition of plankton. In James Johnson Memorial Volume (ed. R.J. Daniel). University Press of Liverpool, pp. 177-192, 1934.

^{(^[19])} Arrigo, K. R., Marine microorganisms and global nutrient cycles, Nature Vol 437, pp. 349-355, 2005.

^{(^[20])} I. J. Hodgkiss & K. C. Ho (1997) Are changes in N:P ratios in coastal waters the key to increased red tide blooms? Hydrobiologia 352:141-147.

^{(^[21])} Hodgkiss, I. J., Ho, K. C., 1997. Are changes in N:P ratios in coastal waters the key to increased red tide blooms. Hydrobiologia 352, 141 – 147.

^{(^[22])} Ken T. M. Wong, Joseph H. W. Lee, I. J. Hodgkiss, 2007. A simple model for forecast of coastal algal blooms. Estuarine, Coastal and Shelf Science 74, 175 – 196.

^{(^[23])} Chronic Toxicity Unit (TUc) is defined as: TUc = 100/NOEC, where NOEC = % of effluent which gives no observed effect on the diatom examined under the 7-day growth inhibition test.

^{(^[24])} Studies that have adopted the SS elevation criterion of 10 mg/L include:

Sai Kung Sewage Treatment Works EIA Study;

Tai Po Sewage Treatment Works Stage 5 EIA Study

Civil Engineering Department (1997) Sand dredging and backfilling of Borrow Pits at the potential Eastern Waters Marine Borrow Area EIA Report;

Civil Engineering Department (1998) Environmental Impact Assessment of backfilling Marine Borrow Areas at East Tung Lung Chau;

CLP Power (2001) Environmental consultancy services for the proposed 11 kV cable circuits from Tsai Mong Tsai to Kiu Tsui;

CLP Power (2002) Environmental consultancy services for the proposed 132 kV cable circuits from A Kung Wan to Sai Kung Pier;

The Hongkong Electric Co (2002) 132 kV submarine cable installation for Wong Chuk Hang – Chung Hom Kok 132 kV circuits.

^{(^[25])} Monitoring at EPD station VM13 has been cancelled since 1999.

WQC	Occurrence of Red Tide (1980 – 2006) **		Occurrence of Red Tide in 2006 **		Occurrence of Red Tide Species (1980 – 2006) **			Pollution Levels in 2006 (Annual Mean)
WQC	No.	% Contribution	No.	% Contribution	Diatom	Dinoflagellates	Other Species	TIN (mg/l)	PO₄ (mg/l)	SiO₂ (mg/l)	SS (mg/l)	Chlorophyll-a (μg/l)
Western Buffer	20	3%	1	8%	12	7	0	0.19 – 0.32	0.01 – 0.02	0.9 – 1.2	4.7 – 7.7	2.0 – 2.8
Victoria Harbour	14	2%	0	0%	11	2	0	0.18 – 0.48	0.02 - 0.03	0.7 – 1.6	4.2 – 11.0	2.1 – 3.5
Junk Bay	6	1%	0	0%	5	0	2	0.15 – 0.18	0.01	0.6	3.2 – 4.5	3.6 – 4.8
Eastern Buffer	1	0%	0	0%	1	0	0	0.12 – 0.15	0.01 – 0.02	0.6 – 0.7	3.9 – 5.0	3.0 – 3.3
Southern	126	17%	3	23%	26	67	29	0.11 – 0.34	<0.01 – 0.02	0.6 – 1.1	3.4 – 13.4	2.5 – 9.1
North Western	25	3%	1	8%	4	12	9	0.43 – 0.67	0.02 – 0.03	1.4 – 2.4	6.4 – 15.8	2.8 – 4.2
Deep Bay	9	1%	0	0%	3	2	4	0.83 – 3.86	0.04 – 0.35	2.4 – 4.9	8.3 – 58.5	2.13 – 25.5
Port Shelter	108	14%	5	38%	3	89	17	0.03 – 0.05	<0.01	0.5 – 0.6	2.2 – 4.5	2.3 – 3.3
Mirs Bay	139	18%	1	8%	10	106	24	0.04 – 0.13	<0.01	0.5 – 0.6	1.5 – 5.0	2.0 – 7.7
Tolo Harbour & Channel	315	41%	2	15%	75	241	60	0.06 – 0.18	<0.01 – 0.01	0.6 – 1.2	1.7 – 5.3	2.3 – 9.5
Total	763	100%	13	100%	150	526	145	-	-	-	-	-

Operational Phase

Construction Phase

Environmental Impacts from the Provision of Disinfection at SCISTW

Environmental Legislation, Policies, Plans, Standards and Criteria

Environmental Impact Assessment Ordinance (EIAO)

Marine Water Quality Objectives under WPCO

Water Supplies Department (WSD) Water Quality Criteria

Practice Note

Technical Memorandum on Effluents Discharge Standard

Assessment Criteria for Corals

EEFS Water Quality Criteria (WQC)

Background

Water Quality Criteria (WQC)

Summary of Water Quality Criteria

Assessment Criteria for CBP

Sediment Quality Assessment Criteria

Existing Baseline Marine Water Quality

EPD Routine Monitoring Data

North Western Waters

Central Waters

Southern Waters

HATS Survey Data

Total Residual Chlorine and Chlorination By-products

Other Contaminants

Operational Phase

Provision of Disinfection for HATS Effluent

Chemical Dosages

Effluent Characteristics

Near Field Modelling

Background Concentrations

HATS Effluent Concentrations

Far Field Modelling

Modelling Tools

Simulation Periods

Model Setup for Discharges

Observation Points

Effluent Characteristics

Whole Effluent Toxicity Test

Modelling Scenarios for Normal Operation

Base Case Scenarios (Scenarios 1a, 1b, 2a, 2b, 3a and 3b)

Sensitivity Test – Change of Residual Waste Load in Storm Systems (Scenario 2c)

Sensitivity Test – Change of Sewage Flow Interception (Scenario 2d)

Sensitivity Test –Change of Stage 2B Implementation Schedule (Scenario 2e)

Sensitivity Test – Enhanced Phosphate Removal from the CEPT Process (Scenario 2f)

Overflow at PTWs Due to Storm Events

Assumptions for TRC and CBP Modelling

Potential Oxygen Depletion Due to the Addition of Dechlorination Chemical

2NaHSO3 (sodium bisulphite) + O2 (oxygen)  2NaHSO4 (sodium bisulphate)

Modelling Scenarios for Emergency Situations

General Construction Activities

Temporary Sewage Bypass

Pollution Loading

Operational Phase – Normal Plant Operation

Whole Effluent Toxicity Test

Near Field Water Quality Impacts

Total Residual Chlorine

Unionized Ammonia

Salinity, pH and Temperature

Other Contaminants of Concern

Acute Toxicity Unit

Dimension of ZID

Evaluation of Near Field Water Quality Impacts

Far Field Water Quality Impacts

Past Water Quality Trend - EPD Data

Total Inorganic Nitrogen

Background TON

Total Inorganic Nitrogen

Unionized Ammonia

Ortho-Phosphate

Dissolved Oxygen

E.coli

Summary

Sediment Quality

HATS Stage 1 Baseline Monitoring and Performance Verification

Nutrients

Possible Implication of WQO/WQC Exceedances for Nutrients

The Most Limiting Nutrient

Past Record on Red Tide Occurrence

Impact from HATS Outfall

Change of Flow Regime

2NaHSO₃ (sodium bisulphite) + O₂ (oxygen)  2NaHSO₄ (sodium bisulphate)