4. WATER Quality.. 4-1

4.1 Introduction. 4-1

4.2 Water Sensitive Receivers. 4-1

4.3 Environmental Legislations, Policies, Plans, Standards and Criteria. 4-1

4.4 Description of the Environment 4-10

4.5 Assessment Methodology. 4-17

4.6 Identification of Environmental Impacts. 4-25

4.7 Prediction and Evaluation of Environmental Impacts. 4-26

4.8 Mitigation of Adverse Environmental Impacts. 4-38

4.9 Evaluation of Residual Impact 4-40

4.10 Environmental Monitoring and Audit 4-41

4.11 Conclusions. 4-41

List of Tables

Table 4.1 Summary of Water Quality Objectives for North Western WCZ 4-2

Table 4.2 Summary of Water Quality Objectives for Western Buffer WCZ 4-4

Table 4.3 Summary of Water Quality Objectives for Deep Bay Water Control Zone 4-6

Table 4.4 WSD Standards at Flushing Water Intakes 4-9

Table 4.5 Not Used

Table 4.6 Baseline Water Quality Condition for North Western WCZ in 2005 4-12

Table 4.7 Baseline Water Quality Condition for Western Buffer WCZ in 2005 4-13

Table 4.8 Baseline Water Quality Condition for Deep Bay WCZ in 2005 4-15

Table 4.9 Typical Hourly Flow Pattern for the Project Effluent 4-18

Table 4.10 Design Influent and Effluent TSS and BOD5 of the Upgraded PPSTW 4-19

Table 4.11 Not Used

Table 4.12 Comparison of Effluent E.coli Discharge Standards for Existing CEPT Plants in Hong Kong 4-19

Table 4.13 Assumed Effluent Loadings from the Upgraded PPSTW 4-20

Table 4.14 Assumed Effluent Quality of Secondary Treatment with Nitrogen Removal and Disinfection 4-21

Table 4.15 Modelling Scenarios for Emergency Discharge of Untreated Effluent from PPSTW 4-22

Table 4.16 Modelling Scenarios for Emergency Bypass of Treated Effluent from PPSTW 4-23

Table 4.17 Coastal Developments to be Incorporated in the 2012 and UDS Coastline Configurations 4-24

Table 4.18 Occurrence and Distribution of Red Tides in Hong Kong 4-31

List of Figures

Figure 4.1 Locations of Water Quality Sensitive Receivers

Figure 4.2 Not Used

Figure 4.3 Not Used

Figure 4.4 Ultimate Coastline Configurations

Figure 4.5 10 Percentile Depth-averaged Dissolved Oxygen – Normal Operation - Overview

Figure 4.6 10 Percentile Bottom Dissolved Oxygen - Normal Operation - Overview

Figure 4.7 Arithmetic Mean Depth-averaged Total Inorganic Nitrogen - Normal Operation – Overview

Figure 4.8 Arithmetic Mean Depth-averaged Suspended Solids - Normal Operation – Overview

Figure 4.9 Arithmetic Mean Depth-averaged Unionized Ammonia - Normal Operation – Overview

Figure 4.10 Geometric Mean Depth-averaged E.coli Levels - Normal Operation – Overview

Figure 4.11 Arithmetic Mean Depth-averaged 5-day Biochemical Oxygen Demand - Normal Operation – Overview

Figure 4.12 Not Used

Figure 4.13 Not Used

Figure 4.14 10 Percentile Depth-averaged Dissolved Oxygen – Normal Operation – Close up at North Western WCZ

Figure 4.15 10 Percentile Bottom Dissolved Oxygen - Normal Operation - Close up at North Western WCZ

Figure 4.16 Arithmetic Mean Depth-averaged Total Inorganic Nitrogen - Normal Operation – Close up at North Western WCZ

Figure 4.17 Arithmetic Mean Depth-averaged Suspended Solids - Normal Operation – Close up at North Western WCZ

Figure 4.18 Arithmetic Mean Depth-averaged Unionized Ammonia - Normal Operation – Close up at North Western WCZ

Figure 4.19 Geometric Mean Depth-averaged E.coli Levels - Normal Operation – Close up at North Western WCZ

Figure 4.20 Arithmetic Mean Depth-averaged 5-day Biochemical Oxygen Demand - Normal Operation – Close up at North Western WCZ

Figure 4.21 Not Used

Figure 4.22 Not Used

Figure 4.23 10 Percentile Depth-averaged Dissolved Oxygen – Normal Operation – Change of Treatment Level – 2012

Figure 4.24 10 Percentile Depth-averaged Dissolved Oxygen – Normal Operation – Change of Treatment Level - UDS

Figure 4.25 10 Percentile Bottom Dissolved Oxygen - Normal Operation – Change of Treatment Level – 2012

Figure 4.26 10 Percentile Bottom Dissolved Oxygen - Normal Operation – Change of Treatment Level - UDS

Figure 4.27 Arithmetic Mean Depth-averaged Total Inorganic Nitrogen - Normal Operation – Change of Treatment Level – 2012

Figure 4.28 Arithmetic Mean Depth-averaged Total Inorganic Nitrogen - Normal Operation – Change of Treatment Level - UDS

Figure 4.29 Arithmetic Mean Depth-averaged Suspended Solids - Normal Operation – Change of Treatment Level – 2012

Figure 4.30 Arithmetic Mean Depth-averaged Suspended Solids - Normal Operation – Change of Treatment Level - UDS

Figure 4.31 Arithmetic Mean Depth-averaged Unionized Ammonia - Normal Operation – Change of Treatment Level – 2012

Figure 4.32 Arithmetic Mean Depth-averaged Unionized Ammonia - Normal Operation – Change of Treatment Level - UDS

Figure 4.33 Geometric Mean Depth-averaged E.coli Levels - Normal Operation – Change of Treatment Level – 2012

Figure 4.34 Geometric Mean Depth-averaged E.coli Levels - Normal Operation - Change of Treatment Level - UDS

Figure 4.35 Arithmetic Mean Depth-averaged 5-day Biochemical Oxygen Demand - Normal Operation – Change of Treatment Level – 2012

Figure 4.36 Arithmetic Mean Depth-averaged 5-day Biochemical Oxygen Demand - Normal Operation – Change of Treatment Level - UDS

Figure 4.37 Minimum Depth-averaged Dissolved Oxygen – Emergency Discharge at Twin Submarine Outfalls

Figure 4.38 Minimum Bottom Dissolved Oxygen – Emergency Discharge at Twin Submarine Outfalls

Figure 4.39 Maximum Depth-averaged Suspended Solids – Emergency Discharge at Twin Submarine Outfalls

Figure 4.40 Maximum Depth-averaged E.coli Levels – Emergency Discharge at Twin Submarine Outfalls

Figure 4.41 Maximum Depth-averaged 5-day Biochemical Oxygen Demand – Emergency Discharge at Twin Submarine Outfalls

Figure 4.42 Minimum Depth-averaged Dissolved Oxygen – Emergency Discharge at Bypass Location

Figure 4.43 Minimum Bottom Dissolved Oxygen – Emergency Discharge at Bypass Location

Figure 4.44 Maximum Depth-averaged Suspended Solids – Emergency Discharge at Bypass Location

Figure 4.45 Maximum Depth-averaged E.coli Levels – Emergency Discharge at Bypass Location

Figure 4.46 Maximum Depth-averaged 5-day Biochemical Oxygen Demand – Emergency Discharge at Bypass Location

Figure 4.47 Maximum Depth-averaged Total Inorganic Nitrogen – Emergency Discharge at Bypass Location

Figure 4.48 Maximum Depth-averaged Unionized Ammonia – Emergency Discharge at Bypass Location

Figure 4.49 Emergency Discharge of Untreated Effluent at Twin Submarine Outfalls – UDS Dry Season – Dissolved Oxygen

Figure 4.50 Emergency Discharge of Untreated Effluent at Twin Submarine Outfalls – UDS Wet Season – Dissolved Oxygen

Figure 4.51 Emergency Discharge of Untreated Effluent at Twin Submarine Outfalls – UDS Dry Season – Suspended Solids

Figure 4.52 Emergency Discharge of Untreated Effluent at Twin Submarine Outfalls – UDS Wet Season – Suspended Solids

Figure 4.53 Emergency Discharge of Untreated Effluent at Twin Submarine Outfalls – UDS Dry Season – E.coli

Figure 4.54 Emergency Discharge of Untreated Effluent at Twin Submarine Outfalls – UDS Wet Season – E.coli

Figure 4.55 Emergency Discharge of Untreated Effluent at Twin Submarine Outfalls – UDS Dry Season – 5-day Biochemical Oxygen Demand

Figure 4.56 Emergency Discharge of Untreated Effluent at Twin Submarine Outfalls – UDS Wet Season – 5-day Biochemical Oxygen Demand

Figure 4.57 Emergency Discharge of Untreated Effluent at Bypass Location – UDS Dry Season – Dissolved Oxygen

Figure 4.58 Emergency Discharge of Untreated Effluent at Bypass Location – UDS Wet Season – Dissolved Oxygen

Figure 4.59 Emergency Discharge of Untreated Effluent at Bypass Location – UDS Dry Season – Suspended Solids

Figure 4.60 Emergency Discharge of Untreated Effluent at Bypass Location – UDS Wet Season – Suspended Solids

Figure 4.61 Emergency Discharge of Untreated Effluent at Bypass Location – UDS Dry Season – E.coli

Figure 4.62 Emergency Discharge of Untreated Effluent at Bypass Location – UDS Wet Season – E.coli

Figure 4.63 Emergency Discharge of Untreated Effluent at Bypass Location – UDS Dry Season – 5-day Biochemical Oxygen Demand

Figure 4.64 Emergency Discharge of Untreated Effluent at Bypass Location – UDS Wet Season – 5-day Biochemical Oxygen Demand

Figure 4.65 Emergency Discharge of Untreated Effluent at Bypass Location – UDS Dry Season – Total Inorganic Nitrogen

Figure 4.66 Emergency Discharge of Untreated Effluent at Bypass Location – UDS Wet Season – Total Inorganic Nitrogen

Figure 4.67 Emergency Discharge of Untreated Effluent at Bypass Location – UDS Dry Season – Unionized Ammonia

Figure 4.68 Emergency Discharge of Untreated Effluent at Bypass Location – UDS Wet Season – Unionized Ammonia

Figure 4.69 Minimum Depth-averaged Dissolved Oxygen – Emergency Bypass of Treated Effluent

Figure 4.70 Minimum Bottom Dissolved Oxygen – Emergency Bypass of Treated Effluent

Figure 4.71 Maximum Depth-averaged Suspended Solids –Emergency Bypass of Treated Effluent

Figure 4.72 Maximum Depth-averaged E.coli Levels – Emergency Bypass of Treated Effluent

Figure 4.73 Maximum Depth-averaged 5-day Biochemical Oxygen Demand – Emergency Bypass of Treated Effluent

Figure 4.74 Maximum Depth-averaged Total Inorganic Nitrogen – Emergency Bypass of Treated Effluent

Figure 4.75 Maximum Depth-averaged Unionized Ammonia – Emergency Bypass of Treated Effluent

Figure 4.76 Not Used

Figure 4.77 Emergency Bypass of Treated Effluent – UDS Dry Season – Dissolved Oxygen

Figure 4.78 Emergency Bypass of Treated Effluent – UDS Wet Season – Dissolved Oxygen

Figure 4.79 Emergency Bypass of Treated Effluent – UDS Dry Season – Suspended Solids

Figure 4.80 Emergency Bypass of Treated Effluent – UDS Wet Season – Suspended Solids

Figure 4.81 Emergency Bypass of Treated Effluent – UDS Dry Season – E.coli

Figure 4.82 Emergency Bypass of Treated Effluent – UDS Wet Season – E.coli

Figure 4.83 Emergency Bypass of Treated Effluent – UDS Dry Season – 5-day Biochemical Oxygen Demand

Figure 4.84 Emergency Bypass of Treated Effluent – UDS Wet Season – 5-day Biochemical Oxygen Demand

Figure 4.85 Emergency Bypass of Treated Effluent – UDS Dry Season – Total Inorganic Nitrogen

Figure 4.86 Emergency Bypass of Treated Effluent – UDS Wet Season – Total Inorganic Nitrogen

Figure 4.87 Emergency Bypass of Treated Effluent – UDS Dry Season – Unionized Ammonia

Figure 4.88 Emergency Bypass of Treated Effluent – UDS Wet Season – Unionized Ammonia

Figure 4.89 Area with Water Quality Improvement and Approximate Location of Initial Dilution Zone

List of Appendices

Appendix 4-1 Near Field Modelling Results

Appendix 4-2 Assumed Effluent Flow and Concentrations for Pillar Point Effluent

Appendix 4-3 Pollution Loading Inventory for Deep Bay

Appendix 4-4 Model Results at Indicator Points for 2012 – Dry Season

Appendix 4-5 Model Results at Indicator Points for 2012 – Wet Season

Appendix 4-6 Model Results at Indicator Points for UDS – Dry Season

Appendix 4-7 Model Results at Indicator Points for UDS – Wet Season

Appendix 4-8 Emergency Response Procedures

4. WATER Quality

4.1 Introduction

4.1.1.1 This section evaluates the potential water quality impacts that are likely to be generated during the construction and operation phase of the proposed Project. Appropriate mitigation measures were identified, where necessary, to mitigate the potential water quality impacts to acceptable levels.

4.2 Water Sensitive Receivers

4.2.1.1 In order to evaluate the potential water quality impacts from the Project, the water sensitive receivers within the North Western Water Control Zone, Western Buffer Water Control Zone and Deep Bay Water Control Zone are considered. The identified water sensitive receivers include:

l Cooling Water Intakes

l Fish Culture Zone

l Beaches

l Secondary Contact Recreation Subzones

l Site of Special Scientific Interest (SSSIs)

l Ramsar Site

l Marine Park

l WSD Flushing Water Intakes

l Corals

l Artificial Reefs

l Horseshoe Crab

l Seagrass

l Mangrove Communities

l Chinese White Dolphins

l Mai Po Nature Reserve Area

4.2.1.2 Figure 4.1 shows the locations of the water quality sensitive receivers. Locations of ecological resources are shown in Figure 6.1 in Section 6.

4.3 Environmental Legislations, Policies, Plans, Standards and Criteria

4.3.1.1 The criteria for evaluating water quality impacts in this EIA Study include:

4.3.2 Environmental Impact Assessment Ordinance (EIAO)

4.3.2.1 The 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 this Study. 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:

l Annex 6 Criteria for Evaluating Water Pollution

l Annex 14 Guidelines for Assessment of Water Pollution.

4.3.3 Marine Water Quality Objectives

4.3.3.1 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 Ordinance 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. The effluent from Pillar Point STW would potentially impact the marine water quality within the North Western Water Control Zone, Western Buffer Water Control Zone and Deep Bay Water Control Zone. Their corresponding WQOs are listed in Table 4.1, Table 4.2 and Table 4.3 respectively.

Table 4.1 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
	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
	Not to cause the annual median to exceed 20 mg/L	Tuen Mun (A), Tuen Mun (B) and Tuen Mun (C) Subzones and Water Gathering Ground Subzones
	Not to cause the annual median to exceed 25 mg/L	Inland waters
Unionized Ammonia (UIA)	Annual mean not to exceed 0.021 mg/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/L	Castle Peak Bay Subzone
	Annual mean depth-averaged inorganic nitrogen not to exceed 0.5 mg/L	Marine waters excepting Castle Peak Bay Subzone
Bacteria	Not exceed 610 per 100ml, 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 running median 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 running median 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. Samples should be taken at least 3 times in one calendar month at intervals of between 3 and 14 days.	Bathing Beach Subzones
Colour	Not to exceed 30 Hazen units	Tuen Mun (A) and Tuen Mun (B) Subzones and Water Gathering Ground Subzones
	Not to exceed 50 Hazen units	Tuen Mun (C) Subzone and other inland waters
5-Day Biochemical Oxygen Demand (BOD5)	Not to exceed 3 mg/L	Tuen Mun (A), Tuen Mun (B) and Tuen Mun (C) Subzones and Water Gathering Ground Subzones
	Not to exceed 5 mg/L	Inland waters
Chemical Oxygen Demand (COD)	Not to exceed 15 mg/L	Tuen Mun (A), Tuen Mun (B) and Tuen Mun (C) Subzones and Water Gathering Ground Subzones
	Not to exceed 30 mg/L	Inland waters
Toxins	Should not cause a risk to any beneficial uses of the aquatic environment	Whole zone
	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 C6 H5OH	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 4.2 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 excepting fish culture subzones
	Not less than 2.0 mg/L for 90% of samples	Fish culture subzones
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)	Not to exceed 3 mg/L	Water Gathering Ground Subzones
	Not to exceed 5 mg/L	Inland waters
Chemical Oxygen Demand (COD)	Not to exceed 15 mg/L	Water Gathering Ground Subzones
	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
	Not to cause the annual median to exceed 20 mg/L	Water Gathering Ground Subzones
	Not to cause the annual median to exceed 25 mg/L	Inland waters
Unionized Ammonia (UIA)	Annual mean not to exceed 0.021 mg/L as unionised 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/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
	Waste discharges should not cause a risk to any beneficial use of the aquatic environment	Whole zone
Bacteria	Not exceed 610 per 100ml, 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 100ml, 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 100ml, calculated as the geometric mean of the most recent 5 consecutive samples taken at intervals of between 7 and 21 days	Inland waters
Colour	Not to exceed 30 Hazen units	Water Gathering Ground
	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 4.3 Summary of Water Quality Objectives for Deep Bay Water Control Zone

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	Outer Marine Subzone excepting Mariculture Subzone
Dissolved Oxygen (DO) within 1 m below surface	Not less than 4.0 mg/L for 90% of samples	Inner Marine Subzone excepting Mariculture Subzone
	Not less than 5.0 mg/L for 90% of samples	Mariculture Subzone
Depth-averaged DO	Not less than 4.0 mg/L for 90% of samples	Outer Marine Subzone excepting Mariculture Subzone
	Not less than 4.0 mg/L	Yuen Long & Kam Tin (Upper and Lower) Subzones, Beas Subzone, Indus Subzone, Ganges Subzone, Water Gathering Ground Subzones and other inland waters of the Zone
5-Day Biochemical Oxygen Demand (BOD5)	Not to exceed 3 mg/L	Yuen Long & Kam Tin (Upper) Subzone, Beas Subzone, Indus Subzone, Ganges Subzone and Water Gathering Ground Subzones
	Not to exceed 5 mg/L	Yuen Long & Kam Tin (Lower) Subzone and other inland waters
Chemical Oxygen Demand (COD)	Not to exceed 15 mg/L	Yuen Long & Kam Tin (Upper) Subzone, Beas Subzone, Indus Subzone, Ganges Subzone and Water Gathering Ground
	Not to exceed 30 mg/L	Yuen Long & Kam Tin (Lower) Subzone and other 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 excepting Yung Long Bathing Beach Subzone
	To be in the range of 6.5 – 8.5	Yuen Long & Kam Tin (Upper and Lower) Subzones, Beas Subzone, Indus Subzone, Ganges Subzone 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, change due to waste discharges not to exceed 0.5	Yung Long Bathing Beach Subzone
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
	Not to cause the annual median to exceed 20 mg/L	Yuen Long & Kam Tin (Upper and Lower) Subzones, Beas Subzone, Ganges Subzone, Indus Subzone, Water Gathering Ground Subzones and other inland waters
Unionized Ammonia (UIA)	Annual mean not to exceed 0.021 mg/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 0.7 mg/L	Inner Marine Subzone
	Annual mean depth-averaged inorganic nitrogen not to exceed 0.5 mg/L	Outer Marine Subzone
Bacteria	Not exceed 610 per 100ml, calculated as the geometric mean of all samples collected in one calendar year	Secondary Contact Recreation Subzones and Mariculture Subzones
	Should be zero per 100 ml, calculated as the running median of the most recent 5 consecutive samples taken between 7 and 21 days.	Yuen Long & Kam Tin (Upper) Subzone, Beas Subzone, Indus Subzone, Ganges Subzone and Water Gathering Ground Subzones
	Not exceed 180 per 100ml, calculated as the geometric mean of the collected from March to October inclusive in one calendar year. Samples should be taken at least 3 times in a calendar month at intervals of between 3 and 14 days.	Yung Long Bathing Beach Subzone
	Not exceed 1000 per 100ml, calculated as the running median of the most recent 5 consecutive samples taken at intervals of between 7 and 21 days	Yuen Long & Kam Tin (Lower) Subzone and other inland waters
Colour	Not to exceed 30 Hazen units	Yuen Long & KamTin (Upper) Subzone, Beas Subzone, Indus Subzone, Ganges Subzone and Water Gathering Ground Subzones
	Not to exceed 50 Hazen units	Yuen Long & KamTin (Lower) Subzone and other inland waters
Turbidity	Shall not reduce light transmission substantially from the normal level	Yuen Long Bathing Beach Subzone
Phenol	Quantities shall not sufficient to produce a specific odour or more than 0.05 mg/L as C6 H5OH	Yuen Long Bathing Beach Subzone
Toxins	Should not cause a risk to any beneficial uses of the aquatic environment	Whole Zone
	Should not attain such levels as to produce toxic carcinogenic, mutagenic or teratogenic effects in humans, fish or any other aquatic organisms.	Whole Zone

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

4.3.4 Technical Memorandum on Effluents Discharge Standard

4.3.4.1 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), issued under Section 21 of the WPCO, 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 sewage from the proposed construction activities must comply with the standards for effluent discharged into the foul sewers, inshore waters and marine waters of the North Western WCZ provided in the TM-DSS.

4.3.5 Water Supplies Department (WSD) Water Quality Criteria

4.3.5.1 Besides the WQO set under the WPCO, the WSD has also specified a set of seawater quality objectives for water quality at their flushing water intakes. The list is shown in Table 4.4.

Table 4.4 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. / 100 ml)	< 20,000

4.3.6 Practice Note

4.3.6.1 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 minimise the water quality impact due to construction site drainage.

4.3.7 Water Quality Assessment Criteria for Coral

4.3.7.1 Since potential impacts on corals may arise through excessive sediment deposition, the magnitude of impacts on corals was assessed based on this water quality parameter.

4.3.7.2 According to Pastorok and Bilyard ⁽[1]⁾ and Hawker and Connell ⁽[2]⁾, a sedimentation rate higher than 0.2 kg/m²/day would introduce moderate to severe impact upon corals. This criterion has been adopted in other recently approved EIA such as Tai Po STW Stage 5 Study ⁽[3]⁾, Eastern Waters MBA Study ⁽[4]⁾, West Po Toi MBA Study ⁽[5]⁾ and Tai Po Gas Pipeline Study ⁽[6]⁾. This sedimentation rate criterion is considered to offer sufficient protection to corals and is anticipated to guard against unacceptable impacts.

4.3.7.3 The assessment criteria used in this Project for protection of corals is also based on the WQO for SS established under the WPCO, i.e. the SS elevations should be less than 30% of ambient baseline conditions. The WQO for SS has also been adopted under the approved Tai Po Sewage Treatment Works Stage 5 EIA as one of the assessment criteria for evaluating the water quality impact from the sewage effluent on corals.

4.4 Description of the Environment

4.4.1 EPD Marine Water Quality Monitoring Data

4.4.1.1 The marine water quality monitoring data routinely collected by EPD were used to establish the baseline condition. The EPD monitoring stations in North Western WCZ (NM1-NM3, NM5-NM6 and NM8), Western Buffer WCZ (WM2-WM4) and Deep Bay WCZ (DM1-DM5) are shown in Figure 4.1. A summary of EPD monitoring data collected in 2005 is presented in Table 4.6, Table 4.7 and Table 4.8 for North Western, Western Buffer and Deep Bay WCZ respectively. As the HATS Stage I was commissioned in late 2001, the data shown in Table 4.6 to Table 4.8 represent the situation after the commissioning of HATS Stage I. Descriptions of the baseline conditions for individual WCZ provided in the subsequent sections are extracted from the EPD’s report “20 years of Marine Water Quality Monitoring in Hong Kong” which contains the latest information published by EPD on marine water quality at the moment of preparing this EIA report.

4.4.1.2 Due to the effect of the Pearl River, the North Western WCZ has historically experienced higher levels of TIN, particularly to the west closest to the river's outflow. In addition to this, the WCZ is affected by local discharges, in particular those from the Stonecutters, Pillar Point and San Wai Sewage Treatment Works, as well as discharges from village houses in unsewered areas. The levels of E.coli and ammonia nitrogen at stations NM1, NM2, NM3 and NM5 located near the outfalls were generally higher compared with other stations. Full compliance with the WQO for DO and UIA was achieved at all stations in 2005. All stations (except NM5 and NM6) complied with the WQO for TIN

4.4.1.3 Since the commissioning of the HATS Stage 1, the water near the SCISTW has experienced a marked increase of faecal bacteria. The water quality in Western Buffer WCZ was largely stable in 2005 as compared to that in 2004 except that there were some increases of E.coli at WM3. The increase in the E.coli levels in the central area (WM3) may be related to the increased discharges from the SCISTW. However, full compliance with the WQO for depth-averaged and bottom DO, TIN and UIA was achieved at WM2, WM3 and WM4 in 2005.

4.4.1.4 Pollution flows into the Deep Bay from the catchments and rivers on both the Hong Kong and Shenzhen sides. This has resulted in poor water quality especially in Inner Deep Bay. In 2005, the water quality of Deep Bay remained poor, in particular in the Inner Deep Bay, characterised by high organic and inorganic pollutants and low DO. The BOD₅, SS and nitrogenous nutrients showed a distinct increase gradient from the outer Deep Bay to the inner part. The levels of nitrogen compounds in Deep Bay continued to be the highest in the territory. In 2005, the inner bay (DM1 and DM2) failed to comply with the DO objective while the whole Deep Bay WCZ failed to meet the TIN objective. Of all the monitoring stations, only DM4 and DM5 in the outer bay met the WQO for UIA, which is toxic to marine organisms. The overall WQO compliance in the Deep Bay WCZ improved slightly from 20% in 2003 to 33% in 2003.

Table 4.6 Baseline Water Quality Condition for North Western WCZ in 2005

Parameter		Lantau Island (North)	Pearl Island	Pillar Point	Urmston Road	Chek Lap Kok		WPCO WQO (in marine waters)
		NM1	NM2	NM3	NM5	NM6	NM8
Temperature (^oC)		22.8 (16.0-28.1)	23.2 (16.1-28.1)	23 (16.2-28.2)	23.1 (16.2-28.2)	23.3 (15.4-28.7)	23.1 (15.5-28.3)	Not more than 2 ^oC in daily temperature range
Salinity		31 (27.4-33.1)	28.9 (21.0-33.1)	29.6 (22.8-33.1)	28.4 (21.8-32.8)	27.6 (17.2-33.1)	28.7 (18.0-33.3)	Not to cause more than 10% change
Dissolved Oxygen (DO) (mg/l)	Depth average	5.9 (3.7-7.2)	6.4 (4.8-7.6)	6.2 (4.1-7.5)	6.1 (4.1-7.4)	6.7 (4.7-9.1)	6.9 (5.1-8.8)	Not less than 4 mg/l for 90% of the samples
	Bottom	5.5 (2.3-7.2)	6.1 (4.0-7.3)	5.9 (3.2-7.2)	5.8 (3.3-7.3)	6.5 (3.3-8.6)	6.6 (3.6-8.5)	Not less than 2 mg/l for 90% of the samples
Dissolved Oxygen (DO) (% Saturation)	Depth average	81 (53-102)	88 (68-108)	86 (58-107)	84 (58-103)	92 (66-123)	94 (71-119)	Not available
	Bottom	77 (33-99)	84 (57-109)	82 (46-107)	79 (47-101)	89 (47-117)	91 (51-116)	Not available
PH		8.1 (7.7-8.4)	8.1 (7.4-8.4)	8.0 (7.6-8.4)	8.0 (7.6-8.4)	8.1 (7.6-8.4)	8.1 (7.7-8.5)	6.5 - 8.5 (± 0.2 from natural range)
Secchi disc Depth (m)		1.8 (1.2-3.0)	1.7 (0.8-2.8)	1.7 (0.8-2.7)	1.5 (0.5-2.3)	1.5 (1.0-2.8)	1.4 (0.8-2.3)	Not available
Turbidity (NTU)		13.3 (5.6-27.3)	12.7 (5.0-28.2)	13.9 (4.9-22.6)	19.2 (6.1-30.3)	16.0 (5.5-27.5)	16.5 (7.8-38.5)	Not available
Suspended Solids (SS) (mg/l)		8.4 (3.9-22.7)	8.2 (3.1-26.0)	9.9 (2.5-17.0)	16.1 (4.7-32.7)	11.3 (4.5-26.0)	11.7 (4.9-30.3)	Not more than 30% increase
5-day Biochemical Oxygen Demand (BOD5) (mg/l)		0.7 (0.1-1.4)	0.9 (0.4-1.9)	0.7 (0.3-1.3)	0.8 (0.5-1.1)	0.8 (0.4-2.4)	0.7 (0.3-2.0)	Not available
Ammonia Nitrogen (NH3-N) (mgN/l)		0.11 (0.06-0.15)	0.12 (0.07-0.18)	0.13 (0.07-0.22)	0.20 (0.12-0.30)	0.13 (0.05-0.28)	0.06 (0.01-0.16)	Not available
Unionised Ammonia (UIA) (mgN/l)		0.005 (0.002-0.012)	0.007 (<0.001-0.015)	0.007 (0.001-0.014)	0.009 (0.003-0.016)	0.007 (0.001-0.017)	0.004 (<0.001-0.012)	Not more than 0.021 mg/l for annual mean
Nitrite Nitrogen (NO2-N) (mgN/l)		0.05 (0.01-0.10)	0.06 (0.02-0.13)	0.06 (0.02-0.12)	0.08 (0.03-0.15)	0.08 (0.02-0.16)	0.06 (0.01-0.14)	Not available
Nitrate Nitrogen (NO3-N) (mgN/l)		0.2 (0.05-0.56)	0.31 (0.06-0.87)	0.27 (0.07-0.65)	0.34 (0.11-0.86)	0.38 (0.07-1.15)	0.3 (0.05-1.02)	Not available
Total Inorganic Nitrogen (TIN) (mgN/l)		0.36 (0.17-0.75)	0.49 (0.17-1.13)	0.47 (0.18-0.87)	0.62 (0.31-1.18)	0.58 (0.20-1.46)	0.42 (0.09-1.27)	Not more than 0.5 mg/l for annual mean
Total Nitrogen (TN) (mgN/l)		0.49 (0.29-0.88)	0.64 (0.25-1.27)	0.61 (0.30-1.03)	0.77 (0.43-1.35)	0.74 (0.30-1.65)	0.55 (0.17-1.43)	Not available
Orthophosphate Phosphorus (PO4) (mgP/l)		0.02 (0.02-0.03)	0.02 (0.02-0.03)	0.03 (0.02-0.03)	0.03 (0.02-0.06)	0.03 (0.01-0.06)	0.02 (0.01-0.04)	Not available
Total Phosphorus (TP) (mgP/l)		0.04 (0.03-0.05)	0.04 (0.03-0.05)	0.04 (0.03-0.05)	0.06 (0.04-0.08)	0.04 (0.03-0.07)	0.03 (0.02-0.05)	Not available
Chlorophyll-a (µg/L)		2.7 (0.4-8.2)	3.9 (0.6-15.0)	2.4 (0.6-10.5)	2.5 (0.4-8.8)	4.3 (0.9-21.0)	4.2 (0.8-19.7)	Not available
E.coli (cfu/100 ml)		700 (80-7200)	370 (98-1800)	460 (140-1400)	740 (380-1500)	55 (10-510)	2 (1-25)	Not available
Faecal Coliforms (cfu/100 ml)		1500(120-16000)	910 (220-6000)	1000 (320-5600)	1600 (700-2800)	130 (16-1200)	6 (1-140)	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 4.7 Baseline Water Quality Condition for Western Buffer WCZ in 2005

Parameter		Hong Kong Island (West)	Tsing Yi (South)	Tsing Yi (West)	WPCO WQO (in marine waters)
		WM2	WM3	WM4
Temperature (^oC)		23 (15.5-27.7)	22.9 (15.7-27.7)	22.9 (15.7-27.6)	Not more than 2 ^oC in daily temperature range
Salinity		31.3 (27.9-33.2)	31.6 (29.3-33.1)	31.3 (27.7-33.1)	Not to cause more than 10% change
Dissolved Oxygen (DO) (mg/l)	Depth average	6 (4.8-7.3)	5.7 (3.9-6.9)	5.6 (3.1-6.8)	Not less than 4 mg/l for 90% of the samples
	Bottom	5.9 (3.3-7.1)	5.7 (2.8-7.1)	5.5 (2.3-6.7)	Not less than 2 mg/l for 90% of the samples
Dissolved Oxygen (DO) (% Saturation)	Depth average	84 (68-108)	80 (55-104)	78 (43-100)	Not available
	Bottom	82 (47-103)	79 (40-107)	76 (33-93)	Not available
pH		8.1 (7.8-8.3)	8.1 (7.8-8.3)	8.1 (7.7-8.3)	6.5 - 8.5 (± 0.2 from natural range)
Secchi disc Depth (m)		1.9 (1.4-2.8)	1.8 (1.2-2.5)	2 (1.3-2.8)	Not available
Turbidity (NTU)		10.8 (5.3-15.6)	11.5 (6.8-16.4)	12.6 (7.8-19.5)	Not available
Suspended Solids (SS) (mg/l)		3.8 (2.5-6.5)	4.7 (2.6-8.0)	5 (1.9-9.7)	Not more than 30% increase
5-day Biochemical Oxygen Demand (BOD5) (mg/l)		0.8 (0.4-1.7)	0.7 (0.4-1.3)	0.6 (0.4-1.1)	Not available
Ammonia Nitrogen (NH3-N) (mgN/l)		0.13 (0.02-0.26)	0.16 (0.09-0.29)	0.13 (0.05-0.24)	Not available
Unionised Ammonia (UIA) (mgN/l)		0.006 (0.001-0.011)	0.007 (0.004-0.011)	0.006 (0.002-0.010)	Not more than 0.021 mg/l for annual mean
Nitrite Nitrogen (NO2-N) (mgN/l)		0.04 (0.01-0.08)	0.03 (0.01-0.08)	0.04 (0.01-0.09)	Not available
Nitrate Nitrogen (NO3-N) (mgN/l)		0.16 (0.04-0.43)	0.14 (0.08-0.32)	0.17 (0.08-0.36)	Not available
Total Inorganic Nitrogen (TIN) (mgN/l)		0.32 (0.14-0.67)	0.34 (0.26-0.56)	0.34 (0.22-0.56)	Not more than 0.4 mg/l for annual mean
Total Nitrogen (TN) (mgN/l)		0.45 (0.21-0.77)	0.46 (0.38-0.66)	0.47 (0.32-0.66)	Not available
Orthophosphate Phosphorus (PO4) (mgP/l)		0.02 (0.01-0.03)	0.03 (0.01-0.04)	0.02 (0.02-0.03)	Not available
Total Phosphorus (TP) (mgP/l)		0.03 (0.02-0.05)	0.04 (0.03-0.05)	0.03 (0.02-0.04)	Not available
Chlorophyll-a (µg/L)		2.5 (0.9-8.3)	2 (0.7-7.3)	1.9 (0.8-4.7)	Not available
E coli (cfu/100 ml)		1500 (170-9700)	4500 (450-23000)	1500 (360-6800)	Not available
Faecal Coliforms (cfu/100 ml)		2900 (210-23000)	11000 (1200-46000)	3400 (820-13000)	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 4.8 Baseline Water Quality Condition for Deep Bay WCZ in 2005

Parameter		Inner Deep Bay			Outer Deep Bay		WPCO WQOs (in marine waters)
		DM1	DM2	DM3	DM4	DM5
Temperature (^oC)		24.2 (15.7 - 31.8)	24.2 (15.6 - 32.1)	24.2 (16.1 - 31.5)	24.2 (16.2 - 30.1)	23.9 (16.3 - 29.2)	Not more than 2 ^oC in daily temperature range
Salinity		16.6 (1.8 - 24.1)	18.5 (3.1 - 27.0)	21.5 (4.1 - 30.0)	23.0 (4.3 - 31.6)	26.1 (7.6 - 32.6)	Not to cause more than 10% change
Dissolved Oxygen (DO) (mg/L)	Depth average	2.9 (1.3 - 6.3)	4.3 (2.2 - 9.0)	5.3 (4.1 - 7.2)	6.0 (4.8 - 9.7)	6.1 (4.8 - 9.0)	Not less than 4 mg/L for 90% of the samples
	Bottom	Not measured	Not measured	Not measured	6.1 (4.8 - 10.2)	6.1 (4.7 - 8.1)	Not less than 2 mg/L for 90% of the samples
Dissolved Oxygen (DO) (% Saturation)	Depth average	38 (16 - 87)	57 (26 - 106)	72 (56 - 89)	82 (63 - 139)	84 (65 - 129)	-
	Bottom	Not measured	Not measured	Not measured	83 (63 - 145)	84 (64 - 116)	-
pH		7.5 (7.0 – 8.0)	7.6 (7.1 - 8.2)	7.8 (7.3 - 8.3)	7.9 (7.4 - 8.9)	8.0 (7.5 - 8.6)	6.5 - 8.5 (± 0.2 from natural range)
Secchi disc Depth (m)		0.3 (0.2 - 0.5)	0.4 (0.2 – 0.6)	0.6 (0.2 – 1.0)	1 (0.5 – 1.5)	1.6 (0.5 – 2.0)	-
Turbidity (NTU)		42.7 (21.2 – 80.7)	40.4 (16.1 - 96.4)	38.0 (12.9 - 87.6)	35.6 (10.4 - 150.5)	18.6 (9.3 - 47.4)	-
Suspended Solids (mg/L)		19.5 (12.0 - 130)	42.4 (15.0 - 100)	40.7 (7.8 - 93.0)	26.8 (5.3 - 88.5)	13.7 (5.8 - 36.4)	Not more than 30% increase
5-day Biochemical Oxygen Demand (BOD5) (mg/L)		5.0 (2.1 – 8.9)	3.8 (1.7 - 7.7)	1.7 (0.4 - 3.9)	1.3 (0.5 - 3.0)	0.9 (0.5 - 2.1)	-
Ammonia Nitrogen (NH3-N) (mg/L)		4.84 (2.1 – 6.8)	3.00 (1.20 - 4.90)	0.75 (0.27 - 1.40)	0.43 (0.17 - 0.80)	0.27 (0.05 - 0.48)	-
Unionised Ammonia (UIA) (mg/L)		0.096 (0.023 – 0.46)	0.060 (0.015 - 0.164)	0.025 (0.005 – 0.075)	0.020 (0.004 – 0.068)	0.012 (0.003 - 0.033)	Not more than 0.021 mg/L
Nitrite Nitrogen (NO2-N) (mg/L)		0.29 (0.13 – 0.48)	0.38 (0.20 - 0.75)	0.25 (0.16 – 0.37)	0.17 (0.06 – 0.30)	0.12 (0.03 – 0.31)	-
Nitrate Nitrogen (NO3-N) (mg/L)		0.35 (0.08 – 1.2)	0.51 (0.23 - 1.70)	0.73 (0.27 - 2.00)	0.67 (0.21 - 1.95)	0.50 (0.12 - 1.47)	-
Total Inorganic Nitrogen (TIN) (mg/L)		5.48 (3.68 – 7.01)	3.89 (3.20 - 5.58)	1.73 (0.99 - 2.87)	1.27 (0.68 - 2.75)	0.89 (0.34 - 2.11)	Not more than 0.5mg/L
Total Kjeldahl Nitrogen (TKN) (mg/L)		5.79 (2.60 – 8.40)	3.68 (1.50 - 6.10)	1.19 (0.47 - 2.00)	0.71 (0.40 - 1.40)	0.49 (0.24 - 0.89)	-
Total Nitrogen (TN) (mg/L)		6.43 (4.18-8.61)	4.56 (3.50 - 6.78)	2.17 (1.47 - 3.18)	1.55 (0.82 - 2.99)	1.11 (0.46 - 2.44)	-
Orthophosphate Phosphorus (Ortho P) (mg/L)		0.48 (0.23 - 0.65)	0.35 (0.18 - 0.53)	0.13 (0.07 - 0.19)	0.07 (0.03 - 0.14)	0.04 (0.01 - 0.09)	-
Total Phosphorus (TP) (mg/L)		0.68 (0.45 – 0.93)	0.48 (0.23 - 0.75)	0.21 (0.10 - 0.31)	0.11 (0.07 - 0.29)	0.07 (0.04 - 0.13)	-
Silica (as SiO2) (mg/L)		6.1 (1- 11.0)	5.0 (1.2 - 9.9)	3.5 (0.8 - 7.3)	2.9 (0.3 - 7.6)	2.3 (0.3 - 7.5)
Chlorophyll-a (µg/L)		35.9 (0.6 – 260)	15.7 (0.4 - 140)	6.3 (0.2 - 39.0)	5.3 (0.3 - 43.0)	4.5 (0.5 - 34.0)	-
E. coli (cfu/100 mL)		9800 (2100 – 360000)	1300 (160 - 26000)	150 (2 - 3800)	610 (51 - 2300)	490 (61 - 1700)	-
Faecal Coliforms (cfu/100 mL)		17000 (3000 – 740000)	3100 (270 - 91000)	430 (8 - 5600)	1500 (600 - 5000)	1100 (260 - 4400)	-

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.

4.5 Assessment Methodology

4.5.1 Construction Phase

4.5.1.1 Construction of the Project would not involve marine works such as dredging or filling. The construction works would be land-based and would be designed not to affect normal operation of the PPSTW and the sewage effluent quality. Construction phase water quality issues would include the impacts from site run-off, sewage from workforce, accidental spillage and discharges of wastewater from various construction activities. The potential impact from these activities was reviewed. Practical water pollution control measures / mitigation proposals were recommended to ensure that any effluent discharged from the construction site would comply with the criteria of WPCO.

4.5.2 Operational Phase

Far Field Modelling Tools

4.5.2.1 The Delft3D suite of models were used to provide a platform for hydrodynamic and water quality modelling. A Delft3D far field model, namely the Pillar Point model, developed under the “Review of North District and Tolo Harbour Sewerage Master Plans (SMP Study)”, was used for simulation of the hydrodynamics and water quality changes in this Project. The Pillar Point Model was fully calibrated, verified and adopted under the SMP Study and has also been applied for the approved EIA for “Upgrading and Expansion of San Wai Sewage Treatement Works and Expansion of Ha Tsuen Pumping Station (Upgrading of San Wai STW)”.

4.5.2.2 The Pillar Point Model covers the North Western, Western Buffer, Deep Bay and Victoria Harbour WCZ. The Pillar Point Model is linked to the 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 (Cumulative Study)”. Computations were first carried out using the Update Model to provide open boundary conditions to the Pillar Point Model. The Update model covers the whole HKSAR and the adjacent Mainland waters including the discharges from Pearl River. The influence on hydrodynamics and water quality in these outer regions would be fully incorporated into the Pillar Point Model.

Simulation Periods

4.5.2.3 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 at least 15 days (excluding the spin-up period) for each of dry and wet seasons 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. In order to determine whether sufficient spin-up period is provided for the simulation, a test was conducted by repeating the model run for one more simulation period. It was found that the results of the two successive model runs were consistent with each other which indicated that the spin-up period was sufficient.

Model Setup for Discharges

4.5.2.4 The Pearl River estuary flows were incorporated in the hydrodynamic model. Flows from other storm and sewage outfalls within the HKSAR waters are relatively small and would unlikely change the hydrodynamic regime in the area and were therefore not included in the hydrodynamic model.

4.5.2.5 The diurnal variation of the Project effluent was incorporated in the water quality model. The daily flow patterns measured at the PPSTW during August 2004 and January 2005 were reviewed. The review indicated that there was no significant change in the diurnal patterns between the dry and wet seasons. The typical diurnal flow pattern measured at the PPSTW as shown in Table 4.9 below was applied to the projected daily Project flow and load to derive the hourly diurnal load for different year horizons as model inputs.  The same 24-hour diurnal flow pattern was used in the model throughout the simulation periods. The exact vertical and horizontal grid cell(s) of the far field model into which the Project flow and pollution loading were allocated were determined by the near field modelling as detailed in Appendix 4-1.

Table 4.9 Typical Hourly Flow Pattern for the Project Effluent

Hour	% of Daily Flow	Hour	% of Daily Flow	Hour	% of Daily Flow	Hour	% of Daily Flow
0:00	5.38%	6:00	2.20%	12:00	4.62%	18:00	4.29%
1:00	5.17%	7:00	2.32%	13:00	4.59%	19:00	4.43%
2:00	4.29%	8:00	3.23%	14:00	4.50%	20:00	4.84%
3:00	3.23%	9:00	4.29%	15:00	4.46%	21:00	5.18%
4:00	2.62%	10:00	4.57%	16:00	4.13%	22:00	5.25%
5:00	2.34%	11:00	4.56%	17:00	4.13%	23:00	5.38%

4.5.2.6 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 specified in the middle layer of the water quality model.

Modelling Scenarios

Normal Operation

4.5.2.7 In accordance with the Review of the Tuen Mun and Tsing Yi Sewerage Master Plan (RTMTYSMP) and the Preliminary Project Feasibility Study (PPFS) subsequently completed in June 2001, CEPT with disinfection is recommended as the sewage treatment process for the upgraded PPSTW and the minimum removal efficiency of TSS and BOD₅ are 70% and 55% respectively. With reference to the preliminary design of the upgraded PPSTW, the average design loading and the respective average effluent TSS and BOD₅ are summarized in Table 4.10.

Table 4.10 Design Influent and Effluent TSS and BOD₅ of the Upgraded PPSTW

	Parameters
	TSS	BOD5
Design Average Influent Loadings (UDS)	260 (mg/L)	277 (mg/L)
Minimum Removal Efficiency	70%	55%
Average Effluent Loadings (UDS)	80 (mg/L)	120 (mg/L)

4.5.2.8 With reference to the preliminary design, the design average influent E.coli is 3 x 10⁷ no. per 100ml and the respective minimum removal efficiency is 99.9%. As such, the average effluent E.coli would be 3 x 10⁴ no. per 100ml. A comparison of the discharge standards regarding the E.coli counts for 4 existing CEPT sewage treatment plants are summarized in Table 4.12. As shown in Table 4.12, the effluent standards of E.coli are 300,000 no. per 100ml (95%ile) and 20,000 no. per100ml (geometric mean) for most of the plants.

Table 4.12 Comparison of Effluent E.coli Discharge Standards for Existing CEPT Plants in Hong Kong

CEPT STW	E.coli (counts/ 100ml)	Remarks	E.coli (counts/100ml)	Remarks
Siu Ho Wan STW	20,000	geometric mean	300,000	95%ile
Stonecutters Island STW	20,000	geometric mean	300,000	95%ile
Cyber Port STW	20,000	geometric mean	300,000	95%ile
Sham Tseng STW	4,000	geometric mean	60,000	95%ile

4.5.2.9 The 95%ile effluent standards proposed for the upgraded PPSTW (i.e. 120 mg/l, 180 mg/l and 300,000 no./100ml for TSS, BOD and E.coli respectively) were adopted for water quality modelling in this EIA.

4.5.2.10 For the purpose of water quality impact assessment and based on the common approach normally adopted in past approved EIA studies, the 95%ile effluent standards proposed for the upgraded PPSTW were applied to the water quality model continuously (that is, 24 hours daily) throughout the entire simulation period. This is a very adverse scenario because, in reality, the 95%ile values would occur only for a short period of time within the assessment period. The actual performance of the upgraded PPSTW should be better than that assumed in the water quality model. It should be highlighted that the assessment provided in this EIA aimed to address the possible worst-case impact as a conservative approach. It is likely that the actual water quality impact caused by the Project under the real situation would be smaller than that simulated by the water quality model. The model results indicated that, even with the adoption of such an adverse model assumption, the Project discharges would not cause any adverse water quality impact.

4.5.2.11 According to the programme, the upgraded PPSTW is scheduled to start commissioning in 2012 tentatively. The time horizons for water quality modelling would be the 2012 scenario for early stage of commissioning of the Project and the ultimate development scenario (UDS). The water quality model runs covered the following scenarios:

l Scenario 1a - Year 2012 without the proposed upgrading works

l Scenario 1b - Year 2012 with the proposed upgrading works

l Scenario 2a - UDS without the proposed upgrading works

l Scenario 2b - UDS with the proposed upgrading works

4.5.2.12 Two scenario runs (Scenario 1a and Scenario 2a) for the case without the upgrading works were included to give the baseline conditions for the two selected time horizons. The baseline conditions assumed that the existing treatment level and design capacity at PPSTW would remain unchanged.

4.5.2.13 Under normal circumstances, the treated effluent would be discharged into the sea via the existing twin submarine outfalls. The submarine outfalls of PPSTW are shown in Attachment I of Appendix 4-1. It is assumed that CEPT with disinfection would be provided for the Project effluent under Scenario 1b (for 2012) and Scenario 2b (for ultimate condition) and the new effluent loadings from the upgraded PPSTW as shown in Table 4.13 would be adopted. There is no reduction in nitrogen compounds from the CEPT process. In accordance with the preliminary design of the upgraded PPSTW, UV irradiation would be used as the disinfection method. Details of the effluent concentrations adopted under different modelling scenarios are given in Appendix 4-2.

Table 4.13 Assumed Effluent Loadings from the Upgraded PPSTW

	Parameters
	TSS (mg/L)	BOD5 (mg/L)	E. coli (counts/100mL)
Effluent Loadings at 95 percentile	120	180	300,000

4.5.2.14 Details of the modelling scenarios are given below:

Scenario 1a

4.5.2.15 Scenario 1a represents the baseline condition in 2012 without the upgrading works. The baseline conditions assumed that the average effluent flow would reach 199,000 m³ per day and the existing treatment level (i.e. preliminary treatment) would remain unchanged by 2012. This average effluent flow value was applied to quantity the pollutants for the purpose of water quality modelling. The net effect from the change of treatment level from preliminary treatment to CEPT plus disinfection was assessed by comparing the model results between Scenario 1a (without upgrading works) and Scenario 1b (with upgrading works, also see subsequent section).

Scenario 1b

4.5.2.16 Scenario 1b represents normal operation of PPSTW after the Project commissioned in 2012. The treatment level in the PPSTW would be upgraded to CEPT with disinfection and the new effluent loadings from the upgraded PPSTW were adopted. Same as Scenario 1a, it is assumed that the effluent flow would reach 199,000 m³ per day by 2012. The 95%ile effluent quality as shown in Table 4.13 was applied to the effluent flow to calculate the loading for discharge continuously throughout the simulation period, which is a conservative assumption. The 95%ile value means that the effluent quality can meet the defined value over 95% of the time. The average loading of the Project effluent would be much smaller.

Scenario 2a

4.5.2.17 Scenario 2a represents the UDS without the upgrading works. Major differences of Scenario 2a from Scenario 1a include (i) the increase of effluent flow from PPSTW to reach its existing design capacity of 230,000 m³ per day and (ii) the change in background pollution loading and coastline configuration between the two time horizons due to planned developments. This effluent flow value was applied to quantity the pollutants for the purpose of water quality modelling. The net effect from the change of treatment level from preliminary treatment to CEPT plus disinfection was assessed by comparing the model results between Scenario 2a (without upgrading works) and Scenario 2b (with upgrading works, also see subsequent section).

Scenario 2b

4.5.2.18 Scenario 2b represents the normal operation of the PPSTW under UDS. The difference of Scenario 2b from Scenario 2a would be the use of the new effluent loadings as shown in Table 4.13.

Sensitivity Test

4.5.2.19 Two sensitivity runs, namely Scenario 1c (for 2012) and Scenario 2c (for UDS) respectively, were conducted to investigate the change in the water quality effects due to the adoption of a higher treatment level (i.e. secondary treatment with nitrogen removal and disinfection). The effluent concentrations assumed for the Project discharges in these sensitivity runs are given in Table 4.14.

Table 4.14 Assumed Effluent Quality of Secondary Treatment with Nitrogen Removal and Disinfection

	Parameters
	BOD 5 (mg/L)	TSS (mg/L)	E. coli (no/100mL)	TN (mg/L)
Effluent quality	20	30	15,000	27.99

Remarks: Effluent standards for 95%ile values as adopted in the Tai Po Sewage Treatment Works Stage V EIA Study

Emergency Situations

Emergency Discharge of Untreated Effluent

4.5.2.20 Water quality modelling was carried out to address the impact from the discharge of untreated effluent under temporary failure of power supply as well as other incidents such as pump or equipment failure.

4.5.2.21 In the event of emergency situations during operation phase of the Project, untreated effluent would be directly discharged into the sea via the twin submarine outfalls. Under a very remote condition when malfunctioning of the twin outfalls occurs during the emergency situation, untreated effluent would be diverted to the sea via the emergency bypass as shown in the as-built drawing provided in Attachment I of Appendix 4-1.

4.5.2.22 Modelling was carried out for four scenarios as shown in Table 4.15 to simulate the impact due to the emergency discharge of untreated effluent from PPSTW.

Table 4.15 Modelling Scenarios for Emergency Discharge of Untreated Effluent from PPSTW

Scenario	Year	Discharge Point	Discharge Period (hours)	Assumed Concentration in Untreated Effluent ^(1)
				BOD 5 (mg/L)	TSS (mg/L)	E.coli (no/100mL)
3a	2012	Twin submarine outfalls	6	268	264	1.75E+7
3b	2012	Emergency bypass	6
4a	UDS	Twin submarine outfalls	6
4b	UDS	Emergency bypass	6

Notes:

(1) Based on the influent concentrations of existing PPSTW.

4.5.2.23 According to the information provided by DSD, emergency discharge has not happened at the existing PPSTW. The historical records of emergency discharge at the PTW and STW in both HATS Stage 1 and HATS Stage 2 catchments were reviewed for the period from 2002 to 2007. Emergency discharge due to equipment failure occurred only once at Kwun Tong PTW in 2005 but the PTW had resumed quickly to normal operation and the duration of emergency discharge was only about 2 hours. Emergency discharge due to power supply failure has not happened at all the PTWs and STWs within the HATS Stage 1 and Stage 2 catchments since 2002..

4.5.2.24 Based on the approved EIA reports for Tai Po Sewage Treatment Works (STW) Stage 5, emergency discharge of raw sewage at Tai Po STW had occurred only once since 1995 due to power supply failure.  The duration of the emergency discharge was less than 3 hours. Based on the historical records, emergency discharge due to power failure had not happened before at the SCISTW and San Wai PTW. The proposed scenarios therefore cover a discharge period of 6 hours (which is a reasonable assumption based on past emergency discharge records). The assumed emergency discharge period of 6 hours is consistent with the approach adopted in the recent approved EIA for Tai Po STW Stage 5.

4.5.2.25 In accordance with the requirements in Appendix C of the EIA Study Brief, the worst case scenario of the discharge at the slack water of neap tide was simulated for both dry and wet seasons. Meanwhile, the hourly diurnal pattern shown in Table 4.9 was adopted for the Project effluent under the emergency discharge scenario. During the discharge period of 6 hours, the six highest diurnal flow rates (i.e. 4.84%, 5.18%, 5.25%, 5.38%, 5.38%, 5.17% of the daily flow as indicated in Table 4.9) and discharge at the slack water of neap tide are assumed for model input respectively.

Emergency Bypass of Treated Effluent

4.5.2.26 Modelling was also conducted for two other scenarios as shown in Table 4.16 to cover the impact due to emergency bypass of treated effluent from PPSTW in case when malfunctioning of the twin submarine outfalls occurs. Based on the information provided by DSD, temporary discharge of treated effluent via the emergency bypass has not happened before. With reference to the information provided in the approved EIA report for “Upgrading of San Wai STW”, the longest substantial emergency repairing and maintenance works, though very remote, would be for NWNT tunnel which could have up to 12 days. The worst case of the emergency bypass duration of 12 days would therefore be considered to be taken for assessing the very worst scenario for the upgraded PPSTW.

4.5.2.27 It was advised by DSD that the outfall maintenance work currently conducted for PPSTW is regular flushing, which is about once a month.  The flushing activity does not require suspension of the outfall service. Moreover, leakage test (Dye Test) and diffuser inspection (Underwater Inspection) for monitoring the outfall condition is also conducted by DSD currently. Apart from the outfall flushing, the general maintenance work for the outfall also includes replacement of check valves at the diffusers, which requires suspension of outfall service.  Normally, such replacement work is required at every ten years and one day is sufficient for completing the work. Therefore, the assumption of sewage bypass of 12 days adopted in this EIA as stated in Section 4.5.2.27 above represents a very adverse scenario for conservative assessment. The actual water quality impact caused by the sewage bypass should be smaller than that simulated by the model under this EIA. It is assumed that the discharge would occur at the beginning of flood tide and during neap tide in both dry and wet seasons.

Table 4.16 Modelling Scenarios for Emergency Bypass of Treated Effluent from PPSTW

Scenario	Year	Discharge Point	Discharge Period (days)	Assumed Concentration in Treated Effluent ^(2)
				BOD 5 (mg/L)	TSS (mg/L)	E.coli (no/100mL)
5	2012	Emergency bypass	12	180	120	300,000
6	UDS	Emergency bypass	12

Notes:

(2) Based on effluent discharge standards at 95 percentile.

Pollution Loading

4.5.2.28 The pollution loading inventory for different assessment years was compiled using the latest planning data for domestic, commercial and industrial activities. The inventory had also incorporated all possible pollution sources including those from landfill sites, non-point source surface run-off and sewage from cross connections. The inventory had also taken into account the removal of pollutants due to wastewater treatment facilities including Stonecutters Island STW, San Wai STW, Siu Ho Wan STW, Sham Tseng STW, Yuen Long STW and Shek Wu Hui STW and the possible redistribution of pollution loads due to different sewage disposal plans and sewage export schemes. The inventory covered all storm and sewage outfalls within the modelling areas for cumulative assessment for 2012 and UDS. Details of the pollution loading inventory compiled for the HKSAR are given in the “Water Quality Impact Assessment Methodology Paper” prepared for this EIA Study.

4.5.2.29 The pollution loading discharged into the Deep Bay from the Mainland side was input into the water quality model for cumulative assessment. The latest information contained in the “Guangdong Province Shenzhen Environmental Quality Report 2004” issued by the Shenzhen Environmental Protection Bureau was reviewed. This Mainland document provides the pollutant concentrations measured at various storm outfalls in Deep Bay including Xixiang River, Nanshan Outfall, Sekou River, Dasha River and Shenzhen River. However, no flow measurement data is available from this Mainland document for these storm outfalls. Based on the review of the “Guangdong Province Shenzhen Environmental Quality Report 2004” and the recently approved EIA reports for the “Upgrading of San Wai STW” and the “Shenzhen Western Corridor”, it is proposed to use the Mainland loading data provided in the EIA for the “Upgrading of San Wai STW” for model input as these loading data represent the best information available for use in this modelling exercise. Appendix 4-3 tabulates the assumed pollution loading discharged into the Deep Bay.

Coastline Configurations

4.5.2.30 The coastline configurations adopted for the UDS are shown in Figure 4.4. The reclamations for South East Kowloon Development (SEKD), Wan Chai Development II (WDII) and Yau Tong Bay Reclamation (YTBR) are excluded as they are still subject to planning review. It should be noted that the reclamation for Central Reclamation Phase III (CRIII) has been incorporated into the existing coastline as shown in Figure 4.4. Table 4.17 indicates the reclamation projects to be included in the far field model for 2012 and UDS. The reclamation limit for each specific project can be referred to Figure 4.4.

Table 4.17 Coastal Developments to be Incorporated in the 2012 and UDS Coastline Configurations

Coastal Development	Information Source	Included in 2012 Coastline Configuration	Included in UDS Coastline Configuration
Sunny Bay Northshore 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	Further Development of Tseung Kwan O Feasibility Study	No	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

4.6 Identification of Environmental Impacts

4.6.1 Construction Phase

General Construction Activities

4.6.1.1 The general construction works would be primarily land-based but 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. If uncontrolled, these could lead to deterioration in water quality. Increased nutrient level from contaminated discharges and sewage effluent could also lead to a number of secondary water quality impacts including localised increase in ammonia and nitrogen concentrations.

Construction Site Runoff

4.6.1.2 During a rainstorm, site runoff generated 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.

4.6.1.3 Wind blown dust would be generated from exposed soil surface 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.

Accidental Spillage

4.6.1.4 There would be a large variety of chemicals to be used for carrying out 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.

4.6.2 Operation Phase

4.6.2.1 During the operational phase, the potential water quality impacts will be mainly related to the effluent discharge from PPSTW. Key concerns are the water quality effects on the receiving water and the change in the risk level that imposes to human health and ecological resources due to the effluent discharged from the Project under normal plant operation.

4.6.2.2 In case of emergency discharge as a result of equipment or power supply failure, there would be transient increase in the pollution level in the receiving water as compared to the normal operation condition after upgrading of the PPSTW. Dual power supply, standby facilities and equipment would be provided at the upgraded PPSTW to avoid the occurrence of emergency discharge.

4.6.2.3 It is considered that the Project (upgrade the sewage treatment level at PPSTW from preliminary treatment to CEPT process with disinfection with slightly increase the treatment capacity from 5.79 m³/s to 6.08 m³/s and not to change the effluent discharge location) would provide a net decrease of pollution load discharge from PPSTW outfall when compared with the present situation. Hence, the Project would have beneficial effects on the water quality, human health and ecological risk. This Project would not change the hydrology, flow regime, sediment quality and salinity profile in the nearby marine environment.

4.7 Prediction and Evaluation of Environmental Impacts

4.7.1 Construction Phase

General Construction Activities

4.7.1.1 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 to ensure that litter, fuels, and solvents are managed, stored and handled properly.

4.7.1.2 Based on the Sewerage Manual, Part I, 1995 of the Drainage Services Department (DSD), the sewage production rate for construction workers is estimated at 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. Existing toilets within the PPSTW could also be made available for use as necessary.

Construction Runoff and Drainage

4.7.1.3 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 reduction in DO levels in the water column.

4.7.1.4 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. Thus, unacceptable impacts on the water quality are not expected, provided that the recommended measures described in Section 4.8 are properly implemented.

4.7.2 Operational Phase

Water Quality Impact under Normal Operation

4.7.2.1 The water quality modelling results are presented as contour plots for dissolved oxygen (DO), total inorganic nitrogen (TIN), suspended solids (SS), unionized ammonia (UIA), E.coli and 5-day biochemical oxygen demand (BOD₅). The model outputs for the whole Study Area are compared between different assessment scenarios, namely Scenario 1a, Scenario 1b, Scenario 2a and Scenario 2b, in Figure 4.5 to Figure 4.11. Figure 4.14 to Figure 4.20 show the close up of the model output at North Western WCZ. The contour plots contained in these figures are presented as arithmetic average over a 15-day simulation period except for E.coli levels, which are geometric means and the 10 percentile DO levels, which are 10 percentile values. The water quality assessment focused on 3 water control zones (WCZ) including Deep Bay, North Western and Western Buffer.

Year 2012

4.7.2.2 Scenario 1a and Scenario 1b were to assess the water quality effects during the early stage of commissioning of the Project for the year 2012. The water quality contour plots in Figure 4.5 to Figure 4.20 showed that the predicted 10 percentile depth averaged DO would comply with the marine WQO of 4 mg/L in the Study Area except the inner Deep Bay area during dry season. In wet season, the predicted 10 percentile depth averaged DO would marginally exceed the WQO of 4 mg/L in North Western WCZ and outer Deep Bay which is likely due to the influence from the pollution discharges from Pearl River and Deep Bay assumed in the modelling exercise. The 10%ile bottom DO complied with the marine WQO of 2 mg/L for all 3 WCZ except in the inner Deep Bay area for both dry and wet seasons.

4.7.2.3 The predicted BOD₅ levels were low which were in general less than 1 mg/L within the North Western and Western Buffer WCZs for both dry and wet season. The predicted UIA levels complied well with the WQO of 0.021 mg/L for North Western and Western Buffer WCZ except for some localized area in the vicinity of other background discharges. For the Deep Bay area, high levels of BOD₅ and UIA were observed due to the influence from the loading discharges from the Shenzhen River and other background sources within the Deep Bay and from the Pearl Estuary.

4.7.2.4 The predicted TIN levels failed to comply with the WQO for all 3 WCZ. The predicted TIN levels were subject to the influence from the background sources from the Pearl Estuary as well as other concurrent discharge such as the NWNT outfall assumed in the modelling.

4.7.2.5 Comparing the “without upgrading” case (Scenario 1a) and “with upgrading” case (Scenario 1b) in 2012, there was no significant change in the water quality between the two scenarios for all the selected water quality parameters except for E.coli. The predicted E.coli levels were significantly reduced under Scenario 1b due to the proposed disinfection process as compared to Scenario 1a without disinfection.

Ultimate Development Scenario

4.7.2.6 Scenario 2a and Scenario 2b were to assess the water quality effects under UDS. Figure 4.5 to Figure 4.20 showed that there was no significant change in the water quality between the “without upgrading” case (Scenario 2a) and “with upgrading” case (Scenario 2b) under the UDS for all of the selected water quality parameters except for E.coli. The predicted E.coli levels were significantly reduced under Scenario 2b due to the proposed disinfection process as compared to Scenario 2a without disinfection.

4.7.2.7 Figure 4.5 to Figure 4.20 also showed that there was no significant change in the WQO compliance between the 2012 scenarios and the UDS. However, the predicted TIN levels in North Western and Western Buffer waters were observed to be higher under the UDS when compared to the 2012 scenarios. This was due to the increase in the nutrient loading caused by the increase in the projected population within the modelling area. The assessment area was subject to the direct influence from the nutrient discharges from the Pearl River. As the background source (such as Pearl River discharge) contains high TIN level, the nitrogen loading from PPSTW under UDS is estimated to have a very minor contribution (less than 0.4%) to the total TIN loading discharged into the marine waters from the catchments of Deep Bay, North Western and Pearl River. Therefore, the TIN exceedances predicted in 2012 and UDS were not caused by the PPSTW effluent. The model results also showed that the E.coli levels in North Western and Western Buffer WCZ would be generally reduced under the UDS as compared to the 2012 scenarios due to the provision of disinfection facilities assumed for the effluent discharged from PPSTW, NWNT tunnel and SCISTW in the UDS.

Sensitivity Test

4.7.2.8 The contour plots for the sensitivity runs using a higher treatment level (secondary treatment plus nitrogen removal and disinfection) for the PPSTW effluent under 2012 scenarios and UDS are shown in Figure 4.23 to Figure 4.36. The contour plots for the scenario with the provision of CEPT plus disinfection for the PPSTW effluent are also included in these figures for comparison.

4.7.2.9 The model results showed that, with the provision of a higher treatment level (i.e. secondary treatment plus nitrogen removal and disinfection), the BOD₅, E.coli and the nutrient levels in the receiving water would be slightly reduced. However, the improvement was very minor.

Water Quality Impacts at Indicator Points

4.7.2.10 Appendix 4-4 to Appendix 4-7 tabulate the model results at the water and marine ecological sensitive receivers identified within the Study Area. Some exceedances of WQO were predicted at the sensitive receivers. The comparison between the results of the “without Project” scenario and the “with Project” scenario indicated that there was no obvious difference in the extent of WQO exceedances between the scenarios. Therefore, it can be concluded that these WQO exceedances are not related to the effluent discharged from the PPSTW and were mainly caused by the background pollution sources assumed in the modelling exercise.

4.7.2.11 Appendix 4-4 to Appendix 4-7 showed that some WQO exceedances were found at the beaches in the Tuen Mun and Tsuen Wan Districts most of which were not contributed by the PPSTW effluent and were caused by the background sources adopted in the model. The model results showed that the provision of the disinfection for the CEPT effluent discharged from PPSTW would cause some reduction of the bacterial levels at the Tuen Mun beaches as compared to the baseline condition. The residual exceedances were essentially due to the pollutant discharges from the nearby stormwater outfalls assumed in the modelling. Appendix 4-4 to Appendix 4-7 also showed that the E.coli improvement predicted at all the identified beaches due to the use of a higher treatment level (secondary treatment plus nitrogen removal and disinfection) was negligible.

Nutrients

Possible Implication of WQO Exceedances for TIN

4.7.2.12 As a result of adoption of the CEPT and UV disinfection process for the PPSTW, the predicted BOD, SS, DO and E.coli levels in the marine water would be improved from the existing baseline level. Based on the model predictions, full compliance with the WQO for UIA would also be achieved in the receiving waters after the Project completion under both the 2012 and UDS scenarios.

4.7.2.13 Based on the recent monitoring data, the WQO for TIN was already exceeded in the North Western WCZ under the existing 2005 baseline condition. Morevore, the existing baseline TIN levels in the North Western and Western Buffer waters are predicted to be persistently exceeded under the ultimate condition (refer to Section 4.7.2.7). as the background source (such as Pearl River discharge) contains high TIN level. The nitrogen loading from PPSTW under UDS is estimated to have a very minor contribution (less than 0.4%) to the total TIN loading discharged into the marine waters from the catchments of Deep Bay, North Western and Pearl River. Therefore, the TIN exceedances predicted in the North Western and Western Buffer were not caused by the PPSTW effluent.

4.7.2.14 Nutrients in general are not harmful to marine organism and fish. The key issue in relation to these nutrient exceedances would be the possible enhancement of excessive algal formation which could lead to various indirect water quality impacts such as oxygen depletion.

4.7.2.15 Both nitrogen and phosphorus are essential components of phytoplankton biomass. Inorganic nutrients such as TIN and PO₄ can therefore be taken up by phytoplankton. Excessive nutrients in water could enhance excessive phytoplankton growth (often called algal bloom), which may adversely affect marine life because the water can become completely deprived of oxygen when a bloom declines rapidly, since the biological degradation of dead algal material consumes large amounts of oxygen. Marine water in hypoxic condition (DO < 2 mg/l) is often considered as one of the signals for excess algal formation. Although the primary concern would be oxygen depletion, algal bloom could also cause other side effects such as discoloration of marine water. Some species of phytoplankton may also produce toxins and induce toxic effect on marine life and cultured fish. However, only a minority of blooms consist of species that synthesize toxins.  In actuality, most algal blooms would be non-toxic.

4.7.2.16 It should be noted that inorganic nitrogen could exist in water in two different forms, namely ammonia and nitrate both of which can support algae growth. As ammonia is the preferred nitrogen nutrients over nitrate for phytoplankton growth, the presence of a certain level of algae in water may in fact be beneficial to the environment by consuming the ammonia which could be toxic to marine life. It should therefore be highlighted that the presence of algae in water is generally not harmful. Only their uncontrolled growth as algal bloom would adversely affect the environment.

Past Record on Red Tide Occurrence

4.7.2.17 Table 4.18 shows the occurrence and distribution of red tides in Hong Kong extracted from the EPD’s report “20 years of Marine Water Quality Monitoring in Hong Kong”. From the past records (1980 – 2005), majority (over 90%) of the red tides happened in the eastern waters (Port Shelter, Mirs Bay and Tolo Harbour) and southern waters where the nutrient level was lower than that of the western waters (Victoria Harbour, Western Buffer, North Western and Deep Bay).

Table 4.18 Occurrence and Distribution of Red Tides in Hong Kong

WQC	Occurrence of Red Tide (1980 – 2005)		Occurrence of Red Tide in 2005		Pollution Levels in 2005 (Annual Mean), mg/l
	No.	% Contribution	No.	% Contribution	TIN	PO4	SS
Western Buffer	18	2%	0	0%	0.18 – 0.34	0.01 – 0.03	3.8 - 5
Victoria Harbour	13	2%	0	0%	0.22 – 0.49	0.02 – 0.04	3.4 – 7.2
Junk Bay	7	1%	0	0%	0.15 – 0.16	0.01 – 0.02	2.7 – 3.2
Eastern Buffer	1	0%	0	0%	0.09 – 0.16	0.01 – 0.02	3 – 3.2
Southern	119	15%	6	14%	0.10 – 0.35	0.01 – 0.02	3.7 – 10.4
North Western	24	3%	3	7%	0.36 – 0.62	0.02 – 0.03	8.2 – 16.1
Deep Bay	8	1%	1	2%	0.89 - 5.48	0.04 – 0.48	13.7 – 49.5
Port Shelter	103	13%	10	24%	0.06 – 0.14	0.01	1.5 – 5.1
Mirs Bay	139	17%	9	21%	0.06 – 0.07	0.01	1.9 – 5.2
Tolo Harbour	374	46%	13	31%	0.07 – 0.11	<0.01 - 0.01	1.5 – 2.6
Total	806	100%	42	100%	-	-	-

4.7.2.18 In 2005, over 75% of the red tides occurred in the eastern waters where the nutrient level was lowest in Hong Kong. From the 2005 records for Port Shelter, Mirs Bay and Tolo Harbour, it can be concluded that algal bloom could readily take off at a low TIN and PO₄ level provided that the 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 ^([7]). 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 ^([8]). In the western waters (Western Buffer, North Western), the background nutrient levels are considered high enough to trigger algal bloom. However, algal bloom was seldom observed in these waters. It is believed that light and other hydrodynamic factors (such as the salinity distribution and the degree of water circulation and vertical mixing) should be the more important limiting factors controlling the onset of algal blooms and red tides in these waters.

4.7.2.19 Probably, the water flushing effect and vertical mixing in the North Western and Western Buffer WCZ are too strong to allow accumulation of algal biomass and hence the chance of algal bloom. Red tide occurrence was also limited in Deep Bay WCZ where the nutrient level was very high and the water was static. This may be partly due to the presence of high SS level in the water column which may reduce the light penetration and limit the solar energy source for excessive algal growth.

4.7.2.20 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. From the past red tide records in Hong Kong, no direct link could be found between excessive nutrients and red tide occurrence. It is considered that nutrient may not be a critical limiting factor for controlling red tide formation in our Study Area.

The Need for Nutrient Removal for PPTSTW

4.7.2.21 As the background source (such as Pearl River discharge) contains high TIN level, the nitrogen loading from PPSTW would only have a minor contribution to the TIN exceedances recorded in the Study Area. The model predicted that adoption of a higher treatment level (secondary treatment plus nitrogen removal and disinfection) for PPSTW would not remove the TIN exceedances. As shown in Figure 4.47 and Figure 4.48, the reduction of TIN level caused by the adoption of a higher treatment level for PPSTW was very minor and is mainly observed in areas close to the PPSTW outfall. Due to the high TIN loading in the background sources, it would not be possible to reduce the TIN level in the receiving water to below the threshold level for algal bloom of 0.12 mg/l (refer to Section 4.7.2.18) by adoption of any enhanced nitrogen removal from the PPSTW effluent.

4.7.2.22 An analysis of past red tide data concluded that the occurrence of red tides in Study Area was not directly driven by nutrient enrichment. Currently, algal bloom rarely occurred in North Western, Deep Bay and Western Buffer WCZ where the measured nutrient level is already considered high enough to support algal bloom. Probably, light and other hydrodynamic factors should be the more important limiting factors for algal bloom formation in these waters. As nutrient is not the critical limiting factor for algal bloom in the Study Area, adoption of a higher treatment level (secondary treatment plus nitrogen removal and disinfection) for PPSTW would unlikely have any significant effect in reducing the red tide formation in the receiving waters. This is also in consistent with the model prediction: The water quality model used in this EIA incorporates various physical / biochemical processes. Biochemical processes such as nitrification, algal growth and decay and the decay of organic matter, were taken into account in the modelling exercise. Chlorophyll-a, which is often used as an indicator in measuring algal biomass, was modelled. Review of the model prediction showed that no significant change in the chlorophyll-a level was observed within the Study Area as a result of adoption of a higher treatment level (secondary treatment plus nitrogen removal and disinfection) for PPSTW.

4.7.2.23 However, if the water quality of the Pearl River Estuary and other Mainland discharges could be significantly improved in the future, adoption of a higher treatment level (including enhanced nutrient removal) could be considered for PPSTW to minimize the chance of red tide occurrence in the receiving waters.

Water Quality Impacts under Emergency Situation

Emergency Discharge of Untreated Effluent

4.7.2.24 This section addresses the potential water quality impacts in case of emergency discharge of untreated effluent for 6 hours under temporary failure of power supply as well as other incidents such as pump or equipment failure. According to the information provided by DSD, emergency discharge has not happened at the existing PPSTW. The historical records of emergency discharge at all the PTWs and STWs in both HATS Stage 1 and HATS Stage 2 catchments were reviewed for the period from 2002 to 2007. Emergency discharge due to equipment failure occurred only once at Kwun Tong PTW in 2005 but the PTW had resumed quickly to normal operation and the duration of emergency discharge was only about 2 hours. Emergency discharge due to power failure has not happened at all the PTWs and STWs within the HATS Stage 1 and Stage 2 catchments since 2002. Based on the approved EIA reports for Tai Po Sewage Treatment Works (STW) Stage 5, emergency discharge of raw sewage at Tai Po STW had occurred only once since 1995 due to power supply failure.  The duration of the emergency discharge was less than 3 hours. Based on the historical records, emergency discharge due to power failure had not happened before at the SCISTW and San Wai PTW. The proposed scenarios therefore cover a discharge period of 6 hours (which is a reasonable assumption based on past emergency discharge records). The assumed emergency discharge period of 6 hours is consistent with the approach adopted in the recent approved EIA for Tai Po STW Stage 5.

4.7.2.25 Various scenarios of emergency discharge at the twin submarine outfalls (Scenario 3a for 2012 and Scenario 4a for UDS) and the emergency bypass location (Scenario 3b for 2012 and Scenario 4b for UDS) were modelled. The model outputs for the emergency discharge at the twin submarine outfalls are presented as contour plots in Figure 4.37 to Figure 4.41 for DO, SS, E.coli and BOD₅ respectively. The contour plots for the emergency bypass scenarios are given in Figure 4.42 to Figure 4.48 for DO, SS, E.coli, BOD₅, TIN and UIA respectively. The contour plots for TIN and UIA are not presented for the discharge at twin outfalls as it is assumed that there is no reduction in nitrogen compounds from the CEPT process. The contour plots for DO are presented as the minimum instantaneous values over the model simulation period whilst the contour plots for SS, E.coli, TIN UIA and BOD₅ are presented as the maximum values over the simulation period. The model outputs for the normal operation scenarios are also included in these figures for comparison.

4.7.2.26 Comparison of the contour plots for emergency discharge scenarios and normal operation scenarios indicated that the impacts from the emergency discharge at the twin submarine outfalls would be negligible for DO, SS and BOD₅. The emergency discharge of untreated effluent at the twin submarine outfall would however significantly elevate the E.coli levels in the northern part of the North Western WCZ (Figure 4.40).

4.7.2.27 The predicted water quality impacts from the emergency bypass of untreated effluent are negligible for DO and SS (Figure 4.42 to Figure 4.44). The model results showed that the emergency bypass would increase the BOD₅, TIN and UIA level in the receiving water but the impact zone was found to be very small and localized within close proximity of the bypass location (Figure 4.46 to Figure 4.48). The influence zone of the emergency bypass for E.coli is however much larger (Figure 4.45). It was found that the emergency bypass would cause a relatively larger E.coli impact on the Tuen Mun coast as compared to the emergency discharge at the twin submarine outfalls.

4.7.2.28 To address the potential water quality at the sensitive receivers, the model results for UDS are presented as time series plots for various water quality parameters covering the periods before, during and after the emergency discharges. Figure 4.49 to Figure 4.56 show the time series plots for DO, SS, E.coli and BOD₅ under the emergency discharge at the twin submarine outfalls. Figure 4.57 to Figure 4.68 provide the time series plots for DO, SS, E.coli, BOD₅, TIN and UIA for the emergency bypass situations. The predicted results for the normal operation scenarios are also included in these time series plots for comparison. The time series plots for the 2012 scenarios are similar to that for the UDS and are therefore not presented.

4.7.2.29 Based on the review of the influence zones of the emergency discharge as shown in the contour plots (Figure 4.37 to Figure 4.48), fifteen indicator points were selected for presentation. These indicator points include seven beaches namely Butterfly (B1), Castle Peak (B2), Kadoorie (B3), Cafeteria Old (B4), Cafeteria New (B5), Golden (B6) and Anglers’ (B7) and four habitat areas for Chinese White Dophin at the north of Lung Kwu Chau, the north of airport, the Brothers, and the Marine Park respectively as well as three WSD flushing water intakes namely the Butterfly Beach (WSD1), the LRT Terminus (WSD2) and the Hong Kong Garden (WSD3) and one cooling water intake for Shui Wing Steel Mill (C4). The results provided for the WSD flushing water intakes and the cooling water intake represents the middle water layer whereas those for the remaining indicator points are depth-average values. The Ma Wan fish culture zones (FCZ) are located far away from the PPSTW discharges and it is not anticipated that the PPSTW discharge would adversely affect the FCZ under the emergency situations. Figure 4.1 shows these indicator points.

4.7.2.30 As expected, the time series plots showed that there would be substantial increases in the E.coli levels at the selected indicator points during the emergency discharge period in both dry and wet seasons (Figure 4.53, Figure 4.54, Figure 4.61 and Figure 4.62). Slight increases in the BOD₅, TIN and UIA are also observed at some indicator plots during the emergency periods (Figure 4.55, Figure 4.56, Figure 4.63 to Figure 4.68). The levels of these parameters increased immediately after the start of the emergency discharge and, however, reduced rapidly and returned to the baseline levels within 1 day after the termination of emergency discharge. No observable elevation was predicted at the water sensitive receivers for DO and SS under all the emergency discharge scenarios.

4.7.2.31 Appendix 4-4 to Appendix 4-7 tabulate the model results at all the water and marine ecological sensitive receivers identified within the Study Area. The model results showed that the emergency discharge of untreated effluent would not contribute any WQO at any of the identified sensitive receivers under the emergency situations.

Emergency Bypass of Treated Effluent

4.7.2.32 Emergency discharge of treated effluent via the emergency bypass has not happened before. The routine maintenance of the twin submarine outfalls in mainly flushing once a month, and the discharge of effluent via the outfall is not affected by the flushing work. Apart from the routine maintenance work, the major maintenance work for the outfalls comprises replacement of check valves at the diffusers of the outfalls. The operation of the outfalls is required to be suspended for carrying out the replacement work. Such replacement work is anticipated to be carried out in every ten years and take 12 hours to complete. Under this EIA, a more conservative value (12 days) was used under this EIA. The duration adopted in this EIA is based on the assumption used under the approved EIA for upgrading and expansion of San Wai STW. Upon our review of relevant past EIA studies, this duration (12 days) is the most conservative value in terms of the duration for maintenance of sewage outfalls and was therefore applied to the model for conservative assessment. It is expected that the actual water quality impact caused by the sewage bypass should be smaller than that simulated by the model under this EIA.

4.7.2.33 The model outputs for the emergency bypass of treated effluent for 12 days are presented as contour plots in Figure 4.69 to Figure 4.75 for DO, SS, E.coli, BOD₅, TIN and UIA respectively. Figure 4.69 and Figure 4.70 showed that the emergency bypass would cause a slight reduction (<0.05 mg/L) of the minimum depth-averaged and bottom DO around the coast of Pillar Point, Lung Kwu Tan and Black Point during the dry season. In wet season, the DO impact from the emergency bypass would be negligible.

4.7.2.34 Figure 4.72 and Figure 4.73 showed that the emergency bypass would cause an elevation of the E.coli and BOD₅ levels in the receiving water. The influence zones of the emergency bypass for E.coli and BOD₅ were however predicted to be very localized within close proximity of the bypass location. Figure 4.71, Figure 4.74 to Figure 4.75 indicated that the influence of the emergency bypass on the SS, TIN and UIA levels in the receiving water would be insignificant.

4.7.2.35 Figure 4.77 to Figure 4.88 provide the time series plots for various water quality parameters covering the periods before, during and after the emergency bypass at selected water sensitive receivers for the UDS. The time series plots for the 2012 emergency bypass scenarios are similar to that for the UDS and are therefore not presented. Figure 4.77 to Figure 4.80 indicated that the emergency bypass of treated effluent would cause negligible impacts on the water sensitive receivers for DO and SS.

4.7.2.36 No observable elevation of E.coli was predicted at all the selected water sensitive receivers during the emergency bypass period except for the WSD flushing water intake at Butterfly Beach (WSD1) and the cooling water intake for Shui Wing Steel Mill (C4). The predicted elevations at these two seawater intake points are however considered acceptable. The peak E.coli values predicted at WSD1 and C4 are below 400 no. per 100 ml during the emergency bypass period as compared to the WSD water quality criteria of 20,000 no. per 100 ml. Elevations of BOD₅, TIN and UIA are also observed at some sensitive receivers but the predicted elevations are considered minor as compared to the baseline levels. Within 1 day after the termination of emergency bypass, all the pollutant levels would drop to the levels almost the same as the baseline conditions.

4.7.2.37 Appendix 4-4 to Appendix 4-7 showed that there would not be any significant change in the predicted values for all selected water quality parameters between the normal operation scenarios and the emergency bypass scenario. It is also noted that the Ma Wan fish culture zones (FCZ) are located far away from the PPSTW discharges and it is not anticipated that the PPSTW discharge would adversely affect the FCZ under the emergency situations. It is considered that the emergency discharge of treated effluent at the bypass location would not contribute any WQO at any of the sensitive receivers identified within the Study Area.

Summary of Operational Phase Impacts

Water Quality Impacts under Normal Operation

4.7.2.38 As a result of the upgrading of the PPSTW, the overall loading of BOD₅, TSS and E.coli in the effluent would be reduced due to the CEPT and disinfection as compared to the current preliminary treatment process. The modelling results showed that there would be water quality improvement for BOD₅, TSS and E.coli due to the upgrading works. The approximate size and location of area with water quality improvement for BOD₅, TSS and E.coli are indicated in Figure 4.89. The water quality improvement in terms of E.coli levels in the receiving water was predicted to be relatively more significant as compared to the BOD₅ and SS levels. The model however predicted that the TIN levels in the receiving waters under the ultimate condition were higher than the existing baseline level which was due to the increase in the nutrient loading assumed in the modelling area as a result of the increase of the projected population growth. However, the assessment concluded such increase in the TIN level would not increase the chance of algal bloom in the receiving waters.

4.7.2.39 A sensitivity test was carried out to investigate the water quality effect due to the change from CEPT with disinfection to a higher treatment level (i.e. secondary treatment with nitrogen removal and disinfection).  The sensitivity test indicated that there would be no substantial differences in the water quality impacts due to this change. The modelling results showed negligible reduction in E.coli levels at the water sensitive receivers due to the higher treatment level.  The reduction of BOD₅ and nutrient levels in the receiving water was also insignificant.  Based on water quality impact assessment results, CEPT plus disinfection would be the most effective treatment option for the PPSTW in minimizing the water quality impacts.

Water Quality Impacts under Emergency Discharge of Untreated Effluent

4.7.2.40 Emergency discharge from the PPSTW would be the consequence of interruption of the electrical power supply or failure of treatment units or equipment failure.  In the event when shutdown of the PPSTW occurs due to power or equipment failure, untreated effluent would be directly discharged into the sea via the twin submarine outfalls. Under a very remote condition when malfunctioning of the twin outfalls occurs during the shutdown of the PPSTW, the untreated effluent would be diverted to the sea via the emergency bypass location. The modelling results indicated that the emergency discharge of untreated effluent under various discharge scenarios would cause short-term elevations of bacterial levels at the water sensitive receivers. The bacterial levels would however reduce rapidly and returned to the baseline levels within only 1 day after the termination of the emergency discharge under all the discharge scenarios.

4.7.2.41 Standby pumps and back-up power, standby treatment units and equipment will be installed for the upgraded PPSTW, the chance of emergency discharge of untreated effluent is therefore very remote. However, if shutdown of the PPSTW due to the failure of power supply or treatment units ever happened, it is expected that the normal operation should be able to recover in hours as normally experienced in Hong Kong. The model results indicated that the predicted elevations of bacterial levels at the sensitive receivers would be more significant under the emergency bypass of untreated effluent as compared to the emergency discharge of untreated effluent at the twin outfalls. It is however important to note that there has not been any emergency bypass due to malfunctioning of the twin outfalls occurred for PPSTW to date. The outfall maintenance work of regular flushing as currently conducted would be maintained by the future PPSTW operator after the PPSTW upgrading works. Moreover, the leakage test (Dye Test) and diffuser inspection (Underwater Inspection) for monitoring the outfall would be maintained by DSD as well. As such, shutting down of both the PPSTW and the twin outfalls together is even extremely remote and the probability for the occurrence of the emergency bypass of untreated effluent is anticipated to be extremely low. It is not anticipated that the emergency discharge of untreated effluent from the PPSTW would cause any long term residual impact to the receiving water.

Water Quality Impacts under Emergency Bypass of Treated Effluent

4.7.2.42 A worst scenario was considered to assume that the emergency bypass of treated effluent from the PPSTW would continue for a period of 12 days in the event of emergency repair or regular maintenance of the twin submarine outfalls. The model results for the emergency bypass did not show any significant deviations from the normal operation conditions. The model results indicated that the pollutant levels would be similar to the normal operation conditions under the emergency bypass of treated effluent.

Water Quality Impacts on Cooling Water Intakes at Shui Wing Steel Mill and Recovery Park

4.7.2.43 As shown in Appendix 4-4 to Appendix 4-7, upon commissioning of the PPSTW upgrading works, the water quality of Western waters will be improved. . It is therefore anticipated that the upgrading of the PPSTW would not cause any adverse water quality impacts at the cooling water intakes including the intakes at the Shui Wing Steel Mill and the proposed Recovery Park in Tuen Mun Area 38 under normal operation or various emergency situations.

Impact to Corals

4.7.2.44 As shown in Appendix 4-4 to Appendix 4-7, the change in the PPSTW effluent quality would cause negligible change in the sedimentation flux at the nearby water sensitive receivers. It is therefore not anticipated that the upgrading of the PPSTW would cause any significant change in the sediment erosion and deposition pattern under normal operation or various emergency situations. Full compliance with the assessment criteria for sedimentation rate and WQO for SS elevation was predicted at all the identified coral sites.

4.8 Mitigation of Adverse Environmental Impacts

4.8.1 Construction Phase

Construction Site Runoff and General Construction Activities

4.8.1.1 To minimise 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. It is recommended to install perimeter channels in the works areas to intercept runoff at site boundary prior to the commencement of any earthwork. To prevent storm runoff from washing across exposed soil surfaces, intercepting channels should be provided. Drainage channels are also required to convey site runoff to sand/silt traps and oil interceptors. Provision of regular cleaning and maintenance can ensure the normal operation of these facilities throughout the construction period. Any practical options for the diversion and re-alignment of drainage should comply with both engineering and environmental requirements in order to ensure adequate hydraulic capacity of all drains.

4.8.1.2 There is a need to apply to EPD for a discharge licence under the WPCO for discharging effluent from the construction site. The discharge quality is required to 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. Reuse and recycling of the treated effluent can minimise water consumption and reduce the effluent discharge volume. The beneficial uses of the treated effluent may include dust suppression, wheel washing and general cleaning. It is anticipated that the wastewater generated from the works areas would be of small quantity. 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.

4.8.1.3 The construction programme should be properly planned to minimise soil excavation, if any, in rainy seasons. This prevents soil erosion from exposed soil surfaces. Any exposed soil surfaces should also be properly protected to minimise dust emission. In areas where a large amount of exposed soils exist, earth bunds or sand bags should be provided. Exposed stockpiles should be covered with tarpaulin or impervious sheets at all times. The stockpiles of materials should be placed at locations away from any stream courses so as to avoid releasing materials into the water bodies. Final surfaces of earthworks should be compacted and protected by permanent work. It is suggested that haul roads should be paved with concrete and the temporary access roads protected using crushed stone or gravel, wherever practicable. Wheel washing facilities should be provided at all site exits to ensure that earth, mud and debris would not be carried out of the works areas by vehicles.

4.8.1.4 Good site practices should be adopted to clean the rubbish and litter on the 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.

Sewage from Workforce

4.8.1.5 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. The construction workers can also make use of the existing toilet facilities within the PPSTW as necessary.

4.8.1.6 Notices should be posted at conspicuous locations to remind the workers not to discharge any sewage or wastewater into the nearby environment during the construction phase of the project. Regular environmental audit on the construction site can provide an effective control of any malpractices and can achieve 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.

Accidental Spillage of Chemicals

4.8.1.7 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.

4.8.1.8 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.

4.8.1.9 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 labeled, 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.

4.8.2 Operation Phase

4.8.2.1 Emergency discharges of untreated effluent from PPSTW would be the consequence of interruption of the electrical power supply or failure of treatment units.  In case of emergency discharge of untreated effluent, elevations of the bacterial levels would be expected at the Tuen Mun and Tsuen Wan waters. It is recommended that relevant government departments including EPD, LCSD and DSD should be informed by the upgraded PPSTW operator as soon as possible of any emergency discharge of untreated effluent so that appropriate actions can be taken to prevent any bathing or water sports activities to be carried out within the Tuen Mun and Tsuen Wan Districts. The PPSTW operators should maintain good communications with various concerned parties including AFCD and WSD. A list of address, email address, phone and fax number of key persons in relevant departments responsible for action should be made available to the PPSTW operators. Water quality monitoring should be carried out at such a time to quantify the water quality impacts and to determine when baseline water conditions are recovered.

4.8.2.2 Dual power supply should be provided to prevent the occurrence of power failure.  In addition, standby facilities for the main treatment units and standby pump, equipment parts / accessories ^([9]) should also be provided in order to minimize the chance of emergency discharge.  The occurrence of such emergency events would therefore be very remote.

4.8.2.3 To provide a mechanism to minimise the impact of emergency discharges, a framework of the emergency response procedures has been formulated and are given in Sections 3.2.1.24 to 3.2.1.26 of the standalone EM&A Manual. The relevant information is extracted from the EM&A Manual and provided in Appendix 4-8 for easy reference.

4.9 Evaluation of Residual Impact

4.9.1 Construction Phase

4.9.1.1 The construction phase water quality impact would generally be temporary and localised 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.

4.9.2 Operation Phase

4.9.2.1 The water quality impact assessment concluded that the Project would not cause any adverse water quality impacts under normal plant operation. The water quality impact due to the emergency discharges is expected to be short-term. Implementation of mitigation measures such as dual power supply and standby equipment and treatment units would minimise the occurrence of any emergency discharge. In the remote case that it occurs, a framework of the emergency response procedures has been formulated to minimise the impact of emergency discharges. No insurmountable water quality impact is expected from these temporary discharges provided that all the recommended mitigation measures are properly implemented.

4.10 Environmental Monitoring and Audit

4.10.1.1 Marine water quality monitoring is recommended during and after any emergency discharge of untreated effluent from PPSTW. A six-month baseline monitoring programme covering both dry and wet seasons is proposed at a frequency of once per month to establish the baseline water quality conditions at selected monitoring points during normal operation of the Project. In case of emergency discharge, daily marine water monitoring should be conducted throughout the whole discharge period until the normal water quality resumes after the normal plant operation is restored. Monitoring of effluent quality is also recommended for operational stage and under the perspective of the WPCO. 

4.10.1.2 A Post-Project Water Quality Monitoring (PPWQM) programme will be implemented to confirm the water quality predictions made in the EIA report. A general outline of the PPWQM requirements is given in the standalone EM&A Manual. The extent of PPWQM programme will be subject to the prevailing environmental conditions at the time before commissioning of the Project. Details of the monitoring programme are given in the standalone EM&A Manual.

4.11 Conclusions

4.11.1 Construction Phase

4.11.1.1 Minor water quality impact would be associated with land-based construction. Impacts may result from the surface runoff and sewage from on-site construction workers. Impacts could be controlled to comply with the WPCO standards by implementing the recommended mitigation measures. Unacceptable residual impacts on water quality would not be expected.

4.11.2 Operation Phase

4.11.2.1 An assessment of water quality impact due to the operation of the Project was made using the Delft3D model. The water quality modelling results showed that the discharge of effluent from the upgraded PPSTW after CEPT and UV disinfection would not cause adverse water quality impacts.   A sensitivity test was carried out to investigate the water quality effect due to the change from CEPT with disinfection to a higher treatment level (i.e. secondary treatment with nitrogen removal and disinfection).  The sensitivity test indicated that there would be no substantial differences in the water quality impacts due to this change. Based on the impact assessment results, the selection of CEPT with disinfection is considered the most effective option for PPSTW in minimizing the water quality impacts.

4.11.2.2 The model predicted that the bacteria levels in the Tuen Mun and Tsuen Wan coastal waters would be elevated due to the emergency discharge of untreated effluent at the PPSTW. Mitigation measures, including dual power supply, standby pumps, back-up treatment units and equipment, would be provided to avoid the occurrence of any emergency discharge. A frame work of the emergency response procedures has been formulated to minimise the impact due to any emergency discharge of untreated effluent from the PPSTW. A detailed EM&A programme are also recommended to collect water quality information and to mitigate the potential impact. 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 emergency release of untreated effluent from the PPSTW. No insurmountable water quality impact would be expected from these emergency discharges under emergency situation provided all the recommended mitigation measures are properly implemented.

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^([1]) Pastorok, R.A. and Bilyard, G.R. (1985). “Effects of sewage pollution on coral-reef communities.” Marine Ecology Progress Series 21: 175-189.

^([2]) 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.

^([3]) Maunsell Consultants Asia Limited (2003). Tai Po Sewage Treatment Works Stage 5 Environmental Impact Assessment (EIA) Report, DSD.

^([4]) Hyder Consulting Limited (1997). Sand Dredging and Backfilling of Borrow Pits at the Potential Eastern Waters Marine Borrow Area, EIA Report, CED.

^([5]) ERM-Hong Kong, Limited (2001). Focused Cumulative Water Quality Impact Assessment of Sand Dredging at the West Po Toi Marine Borrow Area Final Report.

^([6]) 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.

^([7]) 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.

^([8]) 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.

^([9]) The main treatment units include the sedimentation tanks, sludge treatment facilities including the sludge holding tanks and sludge dewatering building and the UV disinfection facility. There are in total 9 nos. of sedimentation tanks in which 8 nos. are duty, which are capable for the treatment during peak design flow and 1 no. of sedimentation tank is reserved for standby. There are in total 3 nos. of sludge holding tanks in which one tank is in use normally and the second tank is for in use over weekend due to weekend stopage and the third tank retained for emergency storage. There are in total 4 nos. of centrifuges installed in the sludge dewatering building, with 3 in duty and 1 in standby mode for sludge dewatering. There are spare UV lamps and accessories, like ballasts etc., for the UV disinfection facility. They will be stored in the spares storage areas as shown in Figure 2.3 for replacement of worn parts/accessories in the UV tanks during non-peak period. At least one standby pump will be provided for each pumping station for maintenance purpose.