5

5.1.1 The village sewers, treatment works and a large section of the effluent export pipeline will be constructed within sensitive areas. The construction works would cause potential water quality impacts to the water gathering ground. The water quality of local streams will need to be protected from impacts from site runoff, sewage from workforce, accidental spillage and discharges of groundwater from excavations.

5.1.2 During the operational phase, the discharges of the export effluent may affect the water quality in the receiving water body and the nearby water quality sensitive receivers. Impacts could also occur as a result of emergency situations such as interruption of power supply or damage to the effluent pipeline.

5.1.3 The Project would in fact yield high benefits for water quality in the Ngong Ping area and protect the quality of water diverted from the catchment to Shek Pik reservoir.

5.2 Water Quality Sensitive Receivers and Beneficial Uses

5.2.1 The export effluent will be of high quality and the potential impacts on the water quality would be localised. The assessment was based on the discharge via an effluent pipeline at the western mouth of Tai O creek. Given the tidal nature of this location, the area of interest extends throughout the creek as well as to the bay immediately to the west.

5.2.2 The main water quality sensitive receivers are:

	the existing mangrove stands along Tai O creek;
	the future sheltered boat anchorage and its associated navigational access routes and the new salt pan mangrove habitat.

5.2.3 The shorelines along both sides of outer Tai O Bay have recently been gazetted as secondary recreational subzones and are thus considered to be sensitive receivers. Drawing No. 23400/EN/001 shows the existing and future water sensitive receivers.

5.2.4 Sha Chau and Lung Kwu Chau Marine Park and the potential South Lantau Marine Park are several km away from the site. The impacts of this Project would be minimal on these distant areas.

5.3 Relevant Legislation and Guidelines

Environmental Impact Assessment Ordinance (EIAO), Cap.499, S16

5.3.1 The Project is a Designated Project under Schedule 2 of the EIAO. Under Section 16 of the EIAO, EPD issued the "Technical Memorandum on Environmental Impact Assessment Process (TM on EIA Process)" which specifies the assessment methods and criteria for EIA. This Study will follow the TM on EIA process to assess the potential water quality impacts that may arise during the construction and operational phases of the Project. Sections in the TM on EIA Process relevant to the water quality impact assessment are:

	Annex 6 - Criteria for Evaluating Water Pollution; and
	Annex 14 - Guidelines for Assessment of Water Pollution.

Water Quality Objectives (WQOs)

5.3.2 The Water Pollution Control Ordinance (WPCO) (Cap.358) provides the major statutory framework for the protection and control of water quality in Hong Kong. According to the Ordinance and its subsidiary legislation, the whole Hong Kong waters are divided into ten Water Control Zones (WCZs). Water Quality Objectives (WQOs) were established to protect the beneficial uses of water quality in WCZs. Specific WQOs are applied to each WCZ. Tai O Bay is located within the North Western WCZ and the corresponding WQOs for the parameters of concern in the area of interest are listed in Table 5.1.

Table 5.1 Water Quality Objectives for the North Western Supplementary Water Control Zone

Parameters	General Marine Waters	Secondary Contact Recreation
E. coli	Not specified	Annual geometric mean not to exceed 610 per 100mL
Depth Average Dissolved Oxygen (DO)	Not less than 4 mg/L for 90% samples
pH value	To be in the range 6.5-8.5. Change due to human activity not to exceed 0.2
Salinity	Change due to human activity not to exceed 10% of natural ambient level
Suspended Solids	Human activity not to raise the natural ambient level by 30% nor cause the accumulation of suspended solids which may adversely affect aquatic communities
Unionised Ammonia	Annual mean not to exceed 0.021 mg/L
Nutrients	Not to be present in quantities that cause excessive algal growth. Annual mean depth average inorganic nitrogen not to exceed 0.5 mg/L

Technical Memorandum on Effluent Discharge Standards

5.3.3 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) sets limits for effluent discharges. Specific limits apply for different areas and are different between surface waters and sewers. The limits vary with the rate of effluent flow. Relevant TM Standards are presented in Tables 5.2 to 5.6.

Table 5.2 Standards for Effluents Discharged into the Inshore Waters of North Western Water Control Zone

Flow rate (m³/day) Determinant	<10	>10 and <200	>200 and <400	>400 and <600	>600 and <800	>800 and <1000	>1000 and <1500	>1500 and <2000	>2000 and <3000	>3000 and <4000	>4000 and <5000	>5000 and <6000
pH (pH units)	6-9	6-9	6-9	6-9	6-9	6-9	6-9	6-9	6-9	6-9	6-9	6-9
Temperature (^oC)	40	40	40	40	40	40	40	40	40	40	40	40
Colour (lovibond units) (25mm cell length)	1	1	1	1	1	1	1	1	1	1	1	1
Suspended solids	50	30	30	30	30	30	30	30	30	30	30	30
BOD	50	20	20	20	20	20	20	20	20	20	20	20
COD	100	80	80	80	80	80	80	80	80	80	80	80
Oil & Grease	30	20	20	20	20	20	20	20	20	20	20	20
Iron	15	10	10	7	5	4	3	2	1	1	0.8	0.6
Boron	5	4	3	2	2	1.5	1.1	0.8	0.5	0.4	0.3	0.2
Barium	5	4	3	2	2	1.5	1.1	0.8	0.5	0.4	0.3	0.2
Mercury	0.1	0.001	0.001	0.001	0.001	0.001	0.001	0.001	0.001	0.001	0.001	0.001
Cadmium	0.1	0.001	0.001	0.001	0.001	0.001	0.001	0.001	0.001	0.001	0.001	0.001
Other toxic metals individually	1	1	0.8	0.7	0.5	0.4	0.3	0.2	0.15	0.1	0.1	0.1
Total toxic metals	2	2	1.6	1.4	1	0.8	0.6	0.4	0.3	0.2	0.1	0.1
Cyanide	0.2	0.1	0.1	0.1	0.1	0.1	0.05	0.05	0.03	0.02	0.02	0.01
Phenols	0.5	0.5	0.5	0.3	0.25	0.2	0.1	0.1	0.1	0.1	0.1	0.1
Sulphide	5	5	5	5	5	5	2.5	2.5	1.5	1	1	0.5
Total residual chlorine	1	1	1	1	1	1	1	1	1	1	1	1
Total nitrogen	100	100	80	80	80	80	50	50	50	50	50	30
Total phosphorus	10	10	8	8	8	8	5	5	5	5	5	5
Surfactants (total)	20	15	15	15	15	15	10	10	10	10	10	10
E. coli (count/100 mL)	1000	1000	1000	1000	1000	1000	1000	1000	1000	1000	1000	1000

Notes:

All units in mg/L unless otherwise stated; and
All figures are upper limits unless otherwise indicated.

Table 5.3 Standards for Effluents Discharged into Group A Inland Waters

Flow rate (m3/day)

Determinant

<10

>10 and <10

>100 and <500

>500 and <1000

>1000 and <2000

pH (pH units)

Temperature (oC)

Colour (lovibond units)

(25mm cell length)

Conductivity (µ s/cm at 20 oC)

Suspended solids

Dissolved oxygen

BOD

COD

Oil & Grease

Boron

Barium

Iron

Arsenic

Total chromium

Mercury

Cadmium

Selenium

Copper

Lead

Manganese

Zinc

Other toxic metals individually

Total toxic metals

Cyanide

Phenols

Hydrogen sulphide

Sulphide

Fluoride

Sulphate

Chloride

Total reactive phosphorus

Ammonia nitrogen

Nitrate + nitrite nitrogen

E. coli (count/100 ml)

6.5-8.5

35

1

1000

10

>4

10

50

1

2

2

2

0.05

0.05

0.001

0.001

0.01

0.2

0.1

0.5

1

0.1

0.3

0.05

0.1

0.05

0.2

1

800

800

1

1

15

<1

6.5-8.5

35

1

1000

10

>4

10

50

1

2

2

2

0.05

0.05

0.001

0.001

0.01

0.2

0.1

0.5

1

0.1

0.3

0.05

0.1

0.05

0.2

1

600

500

0.7

1

15

<1

6.5-8.5

30

1

1000

5

>4

5

20

1

1

1

1

0.05

0.05

0.001

0.001

0.01

0.2

0.1

0.5

1

0.1

0.2

0.05

0.1

0.05

0.1

1

500

500

0.7

1

15

<1

6.5-8.5

30

1

1000

5

>4

5

20

1

0.5

0.5

0.5

0.05

0.05

0.001

0.001

0.01

0.2

0.1

0.5

1

0.1

0.2

0.05

0.1

0.05

0.1

1

400

200

0.5

1

10

<1

6.5-8.5

30

1

1000

5

>4

5

20

1

0.5

0.5

0.5

0.05

0.05

0.001

0.001

0.01

0.1

0.1

0.5

1

0.1

0.15

0.02

0.1

0.05

0.1

0.5

200

200

0.5

0.5

10

<1

Notes:

Group A inland waters include all waters in water gathering grounds and within the boundaries of country parks:
All units in mg/L unless otherwise stated; and
All figures are upper limits unless otherwise indicated.

Table 5.4 Standards for Effluents Discharged into Group B Inland Waters

Flow rate (m3/day) Determinand	<200	>200 and <400	>400 and <600	>600 and <800	>800 and <1000	>1000 and <1500	>1500 and <2000	>2000 and <3000
pH (pH units) Temperature (oC) Colour (lovibond units) (25mm cell length) Suspended solids BOD COD Oil & Grease Iron Boron Barium Mercury Cadmium Selenium Other toxic metals individually Total toxic metals Cyanide Phenols Sulphide Fluoride Sulphate Chloride Total phosphorus Ammonia nitrogen Nitrate + nitrite nitrogen Surfactants (total) E. coli (count/100 ml)	6.5-8.5 35 1 30 20 80 10 10 5 5 0.001 0.001 0.2 0.5 2 0.1 0.1 0.2 10 800 1000 10 5 30 5 100	6.5-8.5 30 1 30 20 80 10 8 4 4 0.001 0.001 0.2 0.5 1.5 0.1 0.1 0.2 10 800 1000 10 5 30 5 100	6.5-8.5 30 1 30 20 80 10 7 3 3 0.001 0.001 0.2 0.2 1 0.1 0.1 0.2 8 600 800 10 5 30 5 100	6.5-8.5 30 1 30 20 80 10 5 2.5 2.5 0.001 0.001 0.2 0.2 0.5 0.08 0.1 0.2 8 600 800 8 5 20 5 100	6.5-8.5 30 1 30 20 80 10 4 2 2 0.001 0.001 0.2 0.2 0.5 0.08 0.1 0.2 8 600 800 8 5 20 5 100	6.5-8.5 30 1 30 20 80 10 3 1.5 1.5 0.001 0.001 0.1 0.1 0.2 0.05 0.1 0.2 8 400 600 8 5 20 5 100	6.5-8.5 30 1 30 20 80 10 2 1 1 0.001 0.001 0.1 0.1 0.2 0.05 0.1 0.2 5 400 600 5 5 10 5 100	6.5-8.5 30 1 30 20 80 10 1 0.5 0.5 0.001 0.001 0.1 0.1 0.2 0.03 0.1 0.2 3 400 400 5 5 10 5 100

Notes:

Group B inland waters are those draining agricultural areas;
All units in mg/L unless otherwise stated; and
All figures are upper limits unless otherwise indicated.

Table 5.5 Standards for Effluents Discharged into Group C Inland Waters

Flow rate (m3/day) Determinand	<100	>100 and <500	>500 and <1000	>1000 and <2000
pH (pH units) Temperature (oC) Colour (lovibond units) (25mm cell length) Suspended solids BOD COD Oil & Grease Boron Barium Iron Mercury Cadmium Silver Copper Selenium Lead Nickel Other toxic metals individually Total toxic metals Cyanide Phenols Sulphide Fluoride Sulphate Chloride Total phosphorus Ammonia nitrogen Nitrate + nitrite nitrogen Surfactants (total) E. coli (count/100 ml)	6-9 30 1 20 20 80 1 10 1 0.5 0.001 0.001 0.1 0.1 0.1 0.2 0.2 0.5 0.5 0.05 0.1 0.2 10 800 1000 10 2 30 2 1000	6-9 30 1 10 15 60 1 5 1 0.4 0.001 0.001 0.1 0.1 0.1 0.2 0.2 0.4 0.4 0.05 0.1 0.2 7 600 1000 10 2 30 2 1000	6-9 30 1 10 15 40 1 4 1 0.3 0.001 0.001 0.1 0.05 0.05 0.2 0.2 0.3 0.3 0.05 0.1 0.2 5 400 1000 8 2 20 2 1000	6-9 30 1 5 5 20 1 2 0.5 0.2 0.001 0.001 0.1 0.05 0.05 0.1 0.1 0.2 0.2 0.01 0.1 0.1 4 200 1000 8 1 20 1 1000

Notes:

Group C inland waters are those running through areas where there are large numbers of fish ponds;
All units in mg/L unless otherwise stated; and
All figures are upper limits unless otherwise indicated.

Table 5.6 Standards for Effluents Discharged into Group D Inland Waters

Flow rate (m³/day) Determinand	<200	>200 and <400	>400 and <600	>600 and <800	>800 and <1000	>1000 and <1500	>1500 and <2000	>2000 and <3000
pH (pH units) Temperature (^oC) Colour (lovibond units) (25mm cell length) Suspended solids BOD COD Oil & Grease Iron Boron Barium Mercury Cadmium Other toxic metals individually Total toxic metals Cyanide Phenols Sulphide Sulphate Chloride Fluoride Total phosphorus Ammonia nitrogen Nitrate + nitrite nitrogen Surfactants (total) E. coli (count/100 ml)	6-10 30 1 30 20 80 10 10 5 5 0.1 0.1 1 2 0.4 0.4 1 800 1000 10 10 20 50 15 1000	6-10 30 1 30 20 80 10 8 4 4 0.05 0.05 1 2 0.4 0.3 1 600 800 8 10 20 50 15 1000	6-10 30 1 30 20 80 10 7 3.5 3.5 0.001 0.001 0.8 1.6 0.3 0.2 1 600 800 8 10 20 50 15 1000	6-10 30 1 30 20 80 10 5 2.5 2.5 0.001 0.001 0.8 1.6 0.3 0.1 1 600 800 8 8 20 30 15 1000	6-10 30 1 30 20 80 10 4 2 2 0.001 0.001 0.5 1 0.2 0.1 1 600 600 5 8 20 30 15 1000	6-10 30 1 30 20 80 10 2.7 1.5 1.5 0.001 0.001 0.5 1 0.1 0.1 1 400 600 5 8 20 30 15 1000	6-10 30 1 30 20 80 10 2 1 1 0.001 0.001 0.2 0.5 0.1 0.1 1 400 400 3 5 20 30 15 1000	6-10 30 1 30 20 80 10 1.3 0.7 0.7 0.001 0.001 0.2 0.4 0.05 0.1 1 400 400 3 5 10 20 15 1000

Notes:

Group D inland waters are those large enough to permit secondary contact recreation and those draining urban and semi-urban areas;
All units in mg/L unless otherwise stated; and
All figures are upper limits unless otherwise indicated.

Practice Note for Professional Persons on Construction Site Drainage

5.3.4 The Practice Note for Professional Persons (ProPECC Note PN1/94) on Construction Site Drainage provides guidelines for the handling and disposal of construction discharges. This note is applicable to this study for control of site runoff and wastewater generated during the construction phase of the Project. The types of discharges from construction sites outlined in the note include:

	Surface run-off;
	Groundwater;
	Boring and drilling water;
	Wastewater from concrete batching and precast concrete casting;
	Wheel washing water;
	Bentonite slurries;
	Water for testing and sterilisation of water retaining structures and water pipes;
	Wastewater from building construction and site facilities; and
	Acid cleaning, etching and pickling wastewater.

5.4 Construction Phase Impacts

5.4.1 The discharge location will be at the mouth of Tai O Creek. No dredging and reclamation will be required. Potential sources of water quality impacts are likely to be construction site runoff, sewage from workforce, accidental spillage of chemicals and wastewater from various construction activities, including groundwater collected from excavations.

Construction Site Runoff and Wastewater from Construction Activities

Potential Impact

5.4.2 Construction of Ngong Ping STW, village sewerage, Ngong Ping trunk sewer and Tai O effluent pipeline will require soil excavation. During a rainstorm, site runoff would be generated washing 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.

5.4.3 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 such as the Ngong Ping SSSI and the nearby stream courses within the water gathering ground.

5.4.4 Various types of construction activities may generate wastewater. These include excavation and filling, general cleaning and polishing, wheel washing, dust suppression and utility installation. These types of wastewater would contain high concentrations of suspended solids. Excavation and filling activities generate stockpiles of excavated soils. Good site practices should be implemented to handle and treat the excavated soils and fill materials on site.

Mitigation Measures

5.4.5 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. The recommendation to install perimeter drains to collect site runoff and to properly treat the runoff by settlement tank/treatment system shall apply to all sites including those for mainlaying works.

5.4.6 A discharge licence needs to be applied from EPD for discharging effluent from the construction site. The discharge quality is required to meet the requirements specified in the discharge licence. As project location is an environmentally sensitive area, all the runoff and wastewater generated from the works areas within the water gathering ground should be treated so that it satisfies with all the standards listed in the Technical Memorandum for Group A inland waters. In addition, substances listed in Clause 8.4 of the Technical Memorandum shall not discharge into the water gathering ground. A wastewater treatment system should be provided for removal of suspended solids and to adjust pH prior to final discharge. Suitable coagulants and neutralising chemicals should be used to enhance the efficiency of the treatment system. 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 in small quantity. Monitoring of the discharge quality of treated effluent should be part of the Environmental Monitoring and Audit (EM&A) programme. Detailed effluent sampling programme for water quality control during construction phase should be submitted to EPD and WSD for approval prior to commencement of the construction works. Details of the monitoring requirements are specified in a separate EM&A Manual.

5.4.7 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. Hydroseeding could be applied to protect exposed slope surfaces, if any. No earth, building materials, soil and other materials should be allowed to be stockpiled on site within the water gathering ground. All surplus spoil should be removed from the water gathering ground as soon as practicable. All mud and debris should be removed from any waterworks access roads and associated drainage systems within the water gathering ground. In areas outside the water gathering ground 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 time. The stockpiles of materials should be placed in the 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 are 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.

5.4.8 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 dropping into the nearby environment. It is recommended to clean the construction sites on a regular basis.

Sewage from Workforce

5.4.9 The presence of workforce for the construction generates sewage. Sewage discharge is subject to control and illegal discharge of untreated sewage would not be acceptable. To avoid introducing additional pollution loads into the nearby waters, it is recommended to provide chemical toilets in the works areas. Provision of temporary toilet facilities within the water gathering ground is subject to the approval of the Director of Water Supplies. All waste should be cleared away daily and disposed outside the water gathering ground. The toilet facilities should not be less than 30 m from any watercourse.

5.4.10 All canteens/kitchens should be located outside the water gathering ground. Wastewater generated from kitchens, if any, should be collected in a temporary storage tank. A licensed waste collector should be deployed to clean the chemical toilets and temporary storage tank on a regular basis. The collected sewage and wastewater could then be transported to the sewage treatment plants for disposal.

5.4.11 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. Implementation of 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

5.4.12 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.

5.4.13 Accidental spillage of chemicals in the works areas contaminates the surface soils. The contaminated soil particles may be washed away by construction site runoff or storm runoff causing water pollution.

5.4.14 It is required to 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.

5.4.15 Any service shop and minor maintenance facilities should be located outside the water gathering ground and should be 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 with the areas appropriately equipped to control these discharges.

5.4.16 Storage of oils/chemicals/waste within the water gathering ground should be limited to absolute minimum volume and are to be removed from sites at the earliest opportunity. No storage and discharge of flammable or toxic solvents, petroleum oil or tar and other toxic substances should be allowed within the water gathering ground. Any construction plant which causes pollution to catchwater or water gathering ground due to leakage of oil or fuel should be removed off site immediately. Any soil contaminated with chemicals/oils should be removed off site and the voids arising from removal of contaminated soil should be replaced by suitable material to the approval of the Director of Water Supplies. Any chemicals to be used including disinfectants and deodorants within the water gathering ground should be subject to the approval of the Director of Water Supplies.

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

	Suitable containers should be used to hold the chemical wastes to avoid leakage or spillage during storage, handling and transport.
	Chemical waste containers should be suitably labelled to notify and warn the personnel who are handling the wastes to avoid accidents.
	Storage area should be selected at a safe location on site and adequate space should be allocated to the storage area.

5.4.18 Emergency plans and clean up procedures should be approved before the commencement of the construction work to deal with accidental spillage of chemicals. Leakage and spillage of chemicals should be contained and cleaned up immediately so as to minimise the impact to the water quality. The emergency plans should include the procedures for:

	Spill prevention and precaution;
	Response actions; and
	Spill clean up and disposal.

5.4.19 Spill prevention and precaution cover the following areas:

	Good housekeeping practices;
	Chemical storage precaution; and
	Chemical transfer and transport precaution.

5.4.20 Detailed response actions should be clearly stated in the emergency plans and training on implementation of the response actions should be provided to the staff. A detailed emergency call-out procedure should be formulated and a number of departments such as EPD, FSD, WSD and Police may need to be informed in case of accidental spillage of chemicals within the water gathering ground. The emergency plan and clean up procedures are given in Table 5.6a. The emergency plans should also include a list of contact persons/parties and their phone numbers in the event of an accident. Detailed emergency plans and clean up procedures should be approved by EPD/WSD before the commencement of the construction work.

Table 5.6a Emergency Plan for Accidental Spillage of Chemicals

1. Spill Prevention and Precaution

General Precaution

	Any service shop and minor maintenance facilities should be located outside the water gathering ground and should be 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 with the areas appropriately equipped to control these discharges.
	Any soil contaminated with chemicals/oils shall be removed from site and the void created shall be filled with suitable materials. Washing the chemicals away is not acceptable as they will contaminate the water gathering ground.
	Storage of oils/chemicals/waste within the water gathering ground should be limited to absolute minimum volume and are to be removed from sites at the earliest opportunity. No storage and discharge of flammable or toxic solvents, petroleum oil or tar and other toxic substances should be allowed within the water gathering ground.
	Any construction plant which causes pollution to catchwater or water gathering ground due to leakage of oil or fuel should be removed off site immediately.
	Any chemicals to be used including disinfectants and deodorants within the water gathering ground should be subject to the approval of the Director of Water Supplies.
	Suitable containers should be used to hold the chemical wastes to avoid leakage or spillage during storage, handling and transport.
	Chemical waste containers should be suitably labelled to notify and warn the personnel who are handling the wastes to avoid accidents.
	Storage area should be selected at a safe location on site and adequate space should be allocated to the storage area.
	Avoid disorder and storage of unnecessary materials in working areas.
	Prevent obstructions and tripping hazards.
	Lock all dangerous goods and chemical stores.

Storage Precautions

	Use solid and impermeable enclosure walls or storage shelves.
	Reduce the danger of falling of stacked containers.
	Provide tightly closed lips to avoid leakage of chemical wastes to further reduce the danger of container falling.
	Store compatible chemical wastes in the same storage area.
	Inspect the storage area to detect if any leaking or defective containers on a regular basis.
	Check the conditions of the storage containers regularly.
	Identify and provide suitable notices in storage area.
	Store large and heavy containers on the floor as far as possible or avoid storage of these containers higher than 2 feet from the floor.
	Keep chemical waste containers below eye level.
	Provide adequate space for handling of the containers.
	Maintain a log of chemical wastes.
	Separate incompatible chemicals from each other.

Transfer and Transport Precautions

	Consider the size of the container to avoid overfilling.
	Use pumps to transfer chemical wastes instead of simple pouring.
	Provide containment structure to hold the chemical wastes when leakage or spillage of chemical waste occurs.
	Use suitable carriers to transfer the chemical waste containers from one location to another.
	Employ licensed waste collectors to be responsible for chemical waste transport.

2. Responses Action

Workers should be aware of emergency telephone numbers and locations of spill kits. The response actions to an accident would include the following steps:

	Keep untrained personnel away from the spillage area or evacuate all personnel and call the emergency service if the spills are highly toxic and volatile.
	Allow only trained persons who have equipped with protective clothing and equipment to enter the spillage area for clean up.
	Transfer the spills back into containers using suitable equipment wherever practicable.
	Use suitable absorbing materials to clean up the spills and dispose the absorbing materials as chemical wastes.
	Use suitable solvent to clean the spillage area after removing the spills.
	Inform Environmental Protection Department, Fire Services Department and Police in the case where the spillage of chemicals would cause serious contamination of an area or risk of pollution. Inform Water Services Department in the case where the spillage of chemicals would cause contamination of the water gathering ground.
	Prepare necessary protective devices, safety equipment, containers and clean up materials for emergency use.
	Train staff to handle the spillage of chemicals.
	Evaluate the potential hazard of the chemical wastes.

3. Spill Clean Up and Disposal

	Control the leakage of the chemical wastes and absorb the spills using suitable absorbing materials
	Use acidic or alkaline solution for neutralisation wherever appropriate
	Take special precautions for flammable wastes and wastes in powder form
	Keep and label the clean up wastes
	Clean the spillage area and equipment used in the response actions
	Dispose the clean up wastes as chemical wastes
	Safety equipment and personal protective equipment for handling of chemical wastes would be similar to that for handling of chemicals.

The equipment includes:

Fire extinguishers

	Brush, dustpan, mop and bucket
	Dry sand Tissue and toweling
	Containers including plaster bags, drums, etc
	Absorbing materials
	Pumps
	Sampling devices

Personal protective equipment:

Safety helmet and goggles

	Gloves which can resist chemical reaction
	Protective boot and clothing
	Respirators and gas masks
	First-aid kits
	Face visor

5.5 Operational Phase Impacts Associated with Emergency Situations

5.5.1 In view of the sensitivity of the area, several special precautionary measures will be adopted:

	Ductile iron pipe will be used for all the Ngong Ping village sewers and effluent pipeline for its robustness, because the area is within the water gathering ground. Sealed pipe joints with hatchboxes along the pipeline will also be adopted;
	The maximum distance between manholes would be limited to 60 m to facilitate over-pumping operations during sewer inspection or maintenance. This would also facilitate flow diversion in case of emergency situation during pipe leakage;
	Standby units, emergency power generation and emergency storage facilities will be provided at Ngong Ping STW to avoid the need for emergency discharges. It is proposed to construct an emergency storage tank to temporary store both the raw sewage from Ngong Ping sewerage catchment and the effluent of STW to cater for the STW breakdown and bursting of effluent pipe. Furthermore, it is also proposed that the size of the emergency storage tank will be large enough to store 72hr. Sewage/effluent flow (48 hours peak day and 24 hours average day i.e. 2 x 2956 + 1524 = 7436m³) in ultimate stage. Thus, the volume of the emergency storage tank is about 7600m³ and the size is about 50m(L) x 40m(W) x 3.8m (D).
	The effluent will be treated to standards listed in Table 1.2 such that the impact to local water quality in the unlikely event of pipe leakage would be small. As a result of the high quality effluent and steep gradient, there would minimal chance of effluent pipeline blockage.
	Installation of 150mm diameter borehole with 100mm diameter slotted iron sleeve for water pollution monitoring, around the STW site at suitable location.
	The drainage of any high contamination risk areas such as the DG store will be physically separated from the drainage system of the STW site.

5.5.2 Nevertheless, careful monitoring of the export pipeline would still be necessary to protect the highly sensitive environment at Ngong Ping. Routine flow monitoring will be carried out at both the upstream end (STW) and downstream of the water gathering ground to ensure early detection of any major leakage. A flow monitoring chamber will be provided east of Tai O for this purpose. An emergency plan listing the procedures to be followed in the event that pipe leakage is suspected or identified is given in Table 5.6b. Temporary diversion of effluent to the emergency storage tank at the STW could be arranged to provide no-flow condition for the repair of the effluent pipeline.

5.5.3 No pumping station will be located in the vicinity of any watercourse or along the effluent pipeline.

Table 5.6b Emergency Plan for Pipe Leakage

If possible leakage is detected from routine flow monitoring, carry out investigation to determine the reason of such detection and identify the location of any leakage.
If pipe leakage is confirmed, inform EPD and WSD;
Determine possible remedial measures such as temporary flow diversion and pipe repairing work;
Ensure remedial actions are properly implemented;
Assess effectiveness of the remedial actions and keep EPD and WSD informed of the results;
If leakage continues, consider what portion of the work is responsible and reassess the remedial actions;
Arrange meeting with EPD and WSD to discuss the required remedial actions if necessary and ensure all necessary remedial actions are properly implemented until the leakage is abated.

5.6 Operational Phase Impacts due to the Export Effluent at Tai O

Baseline Condition

Presentation of Field Data

5.6.1 EPD routinely monitors water quality at six stations within the North Western WCZ, namely NM1 to NM3, NM5, NM6 and NM8. Baseline conditions will be established from the nearest routine monitoring station to Tai O (NM8) which is located in open marine waters north of Sai Tso Wan. It is however considered that the water quality data obtained for this monitoring station is representative of the regional water quality conditions in vicinity of Tai O, but not of Tai O Bay and its immediate surrounds.

5.6.2 Water quality monitoring was carried out during dry season (13 to 14 February 1999) under the Project 'Tai O Sheltered Boat Anchorage' to provide information on water quality in the Tai O creek and Tai O Bay. The in-situ results at eight stations in the bay or creek are presented in Table 5.7. Samples were also taken for more detailed water quality analysis at seven stations in the bay, and the results are summarised in Table 5.8. Parameters selected for presentation include 5-day Biochemical Oxygen Demand (BOD5), Chemical Oxygen Demand (COD), E. coli, Total Phosphorus (TP), Ammonia Nitrogen (NH4), Total Inorganic Nitrogen (TIN), Total Nitrogen (TN), Suspended Solids (SS) and heavy metals. Locations of the water quality sampling stations are shown in Drawing No. 23400/EN/001.

Table 5.7 In-situ Water Quality Results from the Tai O Sheltered Boat Anchorage Study

Parameter	HHW	LLW		HHW	LLW	HHW		LLW	HHW		LLW
	Position 1			Position 2		Position 3			Position 4
Temperature °C	19.35		18.09	18.79	18.06	18.30	17.79		18.52	17.90
Salinity ppt	31.09		32.04	32.04	31.76	32.23	32.23		32.23	32.23
DO mg/L	8.73		5.60	8.85	7.01	8.52	7.36		8.80	7.30
pH	8.16		7.75	8.12	7.87	8.09	7.93		8.10	7.94
	Position 5			Position 6		Position 9			Position 10
Temperature °C	18.56		17.50	18.60	18.10	18.92	17.94		19.02	-
Salinity ppt	32.13		32.23	31.78	32.13	32.04	31.85		31.86	-
DO mg/L	8.16		7.29	8.67	6.87	8.22	6.75		7.64	-
pH	8.10		7.92	8.09	7.90	8.17	7.92		8.22	-

(Source: Tai O Sheltered Boat Anchorage Environmental and Drainage Impact Assessment Final Assessment Report May 2000)

Table 5.8 Laboratory Analysis Results from the Tai O Sheltered Boat Anchorage Study

Station	1		2		3		4		5		6		7
Station	HHW	LLW	HHW	LLW	HHW	LLW	HHW	LLW	HHW	LLW	HHW	LLW	HHW	LLW
SS (mg/L)	18	16	13	10	8	8	10	6	6	10	6	9	10	78
E. coli (count/100mL)	410	390	16	3400	9	14	22	19	13	7	2	1900	25	1600
Cd (µg/L)	<1	<1	<1	<1	<1	<1	<1	<1	<1	<1	<1	<1	<1	<1
Cr (µg/L)	<5	<5	<5	<5	<5	<5	<5	<5	<5	<5	<5	<5	<5	<5
Cu (µg/L)	10	10	10	10	10	55	10	10	10	10	10	10	10	10
Ni (µg/L)	<5	<5	<5	<5	<5	<5	<5	<5	<5	<5	<5	<5	<5	<5
Pb (µg/L)	<5	<5	<5	<5	<5	<10	<5	<5	<5	<5	<5	<5	<5	<5
Zn (µg/L)	5	10	5	10	5	30	5	5	10	5	<5	5	5	5
Hg (µg/L)	<0.5	<0.5	<0.5	<0.5	<0.5	<0.5	<0.5	<0.5	<0.5	<0.5	<0.5	<0.5	<0.5	<0.5
NH₄ (mg/L)	<0.01	0.03	<0.01	0.02	0.01	0.06	0.02	0.02	0.06	0.02	<0.01	0.03	0.01	0.06
TN (mg/L)	0.3	0.5	0.3	0.4	0.3	0.4	0.3	0.3	0.5	0.4	0.6	0.5	0.3	0.5
TIN (mg/L)	0.01	0.05	<0.01	0.04	0.02	0.06	0.03	0.02	0.06	0.02	0.02	0.05	0.02	0.08
TP (mg/L)	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
COD (mg/L)	<10	12	15	<10	<10	10	12	<10	<10	14	16	12	13	<10
BOD (mg/L)	<1	<1	<	<1	<1	<1	<1	<1	<1	<1	<1	<1	<1	<1

(Source: Tai O Sheltered Boat Anchorage Environmental and Drainage Impact Assessment Final Assessment Report May 2000)

5.6.3 Water quality survey was also carried out at Tai O Creek in March 2000 under the PPFS for this Project. Results of the water quality surveys are presented in Table 5.9. The locations of the relevant sampling points are shown on Drawing No. 23400/EN/001.

Table 5.9 Water Quality Sampling Results from the PPFS March 2000 Survey

Parameter	Sampling Station Name
Parameter	C	K	L	E	B	J
E. coli (count/100mL)	4000	5500	1000	2800	5500	5900
pH	8.0	8.0	8.2	8.2	8.0	8.1
NH₄ (mg/L)	0.1	0.1	0.1	0.1	0.2	0.2
COD (mg/L)	40	40	30	30	40	30
BOD (mg/L)	< 2	< 2	< 2	< 2	< 2	< 2

(Source: Outlying Islands Sewerage Master Plan Stage 2 Review Environmental Review of Ngong Ping Sewerage Project April 2001)

5.6.4 Wet season water quality survey data (July - August 2001) collected under the PPFS for this Project are presented in Table 5.10. Two surveys were conducted in the wet season during spring and neap tides respectively. Each survey covered both the mid-flood and mid-ebb periods. The 9 locations (Stations A to I) for water quality sampling are shown in Drawing No. 23400/EN/001.

Table 5.10 Water Quality Sampling Results from the PPFS Wet Season Survey

Station	pH	DO	*E. coli*	SS	NH₄	TIN	BOD	Salinity
Spring Ebb
A	7.95	5.49	2947	32	0.0782	0.3909	1.00	10.77
B	8.02	5.08	702	12	0.0900	0.3400	1.00	10.90
C	7.73	4.97	1533	13	0.0533	0.3333	1.00	7.23
D	7.75	5.05	1740	23	0.0775	0.3000	1.00	8.00
E	8.00	4.83	1650	15	0.1167	0.2667	1.00	11.07
F	7.80	5.00	1476	10	0.0600	0.2200	1.00	7.70
G	7.99	4.94	966	11	0.0800	0.2714	1.00	13.39
H	7.99	5.19	1133	15	0.0771	0.2714	1.00	12.57
I	8.08	4.93	495	19	0.0763	0.3000	1.00	12.00
Spring Flood
A	7.84	5.17	783	21	0.0871	0.2857	1.00	9.74
B	7.73	4.40	5750	13	0.1025	0.3000	1.00	7.73
C	7.68	5.38	1300	18	0.0850	0.2750	1.00	6.95
D	7.83	5.00	2173	20	0.1033	0.3000	1.00	9.77
E	7.75	4.28	4125	27	0.1050	0.3750	1.00	9.53
F	7.73	4.55	1300	13	0.0725	0.3250	1.00	7.98
G	7.93	4.83	288	44	0.1043	0.3000	1.00	10.33
H	7.84	4.69	332	23	0.0929	0.2143	1.00	9.31
I	7.79	4.73	135	14	0.1063	0.3625	1.00	10.00
Neap Ebb
A	8.26	7.05	1408	20	0.0475	0.3563	1.00	12.05
B	8.53	8.90	613	20	0.0433	0.4100	1.33	9.90
C	8.33	8.10	440	14	0.0233	0.3533	1.67	9.13
D	8.40	7.77	2067	17	0.0267	0.4200	1.33	11.00
E	8.37	7.73	3300	10	0.0533	0.3933	1.33	9.87
F	8.26	7.82	216	22	0.0400	0.3740	1.40	9.34
G	8.47	8.34	187	14	0.0267	0.3656	1.44	13.24
H	8.47	8.38	343	17	0.0333	0.3467	1.50	12.85
I	8.57	9.69	20	17	0.0257	0.4443	2.29	13.51
Neap Flood
A	8.12	5.98	1248	19	0.0633	0.4633	1.00	13.53
B	8.03	5.93	1928	16	0.0700	0.3900	1.00	9.40
C	7.86	5.88	1524	11	0.0680	0.3720	1.00	8.56
D	8.13	6.08	1560	24	0.0575	0.4000	1.00	12.23
E	7.85	5.98	9800	15	0.0925	0.4250	1.00	8.75
F	7.87	5.53	1510	10	0.0700	0.4000	1.33	9.13
G	8.37	7.40	503	15	0.0657	0.3114	1.29	13.69
H	8.30	6.94	1512	9	0.0486	0.2571	1.29	12.31
I	8.33	7.53	28	14	0.0643	0.2900	1.14	12.90

Discussion of Field Investigation Results for Selected Parameters

5.6.5 All the pH values measured during both the PPFS and Tai O Boat Anchorage Surveys complied with the WQO except for only one occasion where a pH value of 8.57 was measured at Station I during the PPFS wet season survey (Table 5.10) which marginally exceeded the WQO by 0.8%. The measured DO data ranged from 4.28 to 9.69 mg/L as compared to the WQO of ³ 4mg/L for 90% depth-averaged samples. The salinity levels ranged from 31.09 to 32.23 ppt for dry season and 7.23 to 13.69 ppt for wet season.

5.6.6 The SS concentrations varied from 6 to 78 mg/L for dry season and 10 to 44 mg/L for wet season. The relatively higher SS reading of 78 mg/L obtained at Station 7 at low tide may be due to boat disturbance of the seabed during the sampling as indicated in the Tai O Sheltered Boat Anchorage EIA Report. Relatively higher SS level of 44 mg/L were found during the wet season spring ebb tide survey at Station G under the PPFS. All other measured SS levels are considered relatively lower.

5.6.7 The ammonia concentrations measured during the PPFS and Tai O Sheltered Boat Anchorage EIA Study varied from <0.01 to 0.2 mg/L. As derived under the Tai O Sheltered Boat Anchorage EIA Study, approximately 3.23% of the ammonia within the water column of Tai O Bay may be present in an unionised state. Therefore, unionised ammonia (UIA) levels are considered to vary between <0.0003 and 0.0064 mg/L. These concentrations are below the WQO for UIA of 0.021 mg/L for annual average. The TIN levels varied from <0.01 to 0.46 mg/L which are below the WQO for TIN of 0.5 mg/L for annual water column average.

5.6.8 The E. coli data collected in 1999 during the Tai O Sheltered Boat Anchorage Study showed that Tai O Creek and Nam Chung Tsuen are key sources of E. coli which result in relatively high levels within the inner part of Tai O Bay during low tide (see Table 5.8). During the PPFS March 2000 Survey, it was observed that none of the stilted dwellings along the Tai O Creek has connection to DSD's existing sewer and that human waste was disposed of directly into the creek. As revealed by the field data collected at the Tai O creek during the PPFS March 2000 Survey (Table 5.9), relatively high E. coli levels were observed at all the sampling points which agreed with the findings of the water quality survey undertaken during the Tai O Sheltered Boat Anchorage Study. The 2001 wet season survey conducted under the PPFS also indicated that the water samples collected at Tai O creek and inner Tai O Bay contained relatively high E. coli (Table 5.10).

5.6.9 The conclusion from the water quality monitoring programme undertaken during the PPFS and Tai O Sheltered Boat Anchorage Study were that water in Tai O Bay had relatively low levels of ammonia, nitrogen species, BOD but had relatively high levels of E. coli within the inner part of Tai O Bay and Tai O Creek.

Modelling Tools and Model Grid Layout

5.6.10 The Delft3D suite of models was used to provide a platform for hydrodynamic and water quality modelling. A 3-dimensional Tai O detailed model was used to provide a detailed assessment on the water quality impact at Tai O. The model is extended to cover the tidal reach of Tai O creek. The grid size ranges from 40 x 40 m2 in the area of interest to 200 x 200 m2 at the open boundary. Drawing No. 23400/EN/016 shows the schematic layout of the modelling grid.

5.6.11 The Tai O detailed model was linked to the Fine Grid model. The hydrodynamic model of the Fine Grid model was previously calibrated and verified and has been used to provide boundary and initial hydrodynamic conditions to the detailed model. The hydrodynamic modelling of the Fine Grid model covers the outer regions of Pearl River Estuary, Macau, Lamma Channel and Deep Bay. All major influences on hydrodynamics in the outer regions were therefore incorporated. Drawing No. 23400/EN/003 shows the grid layout of the Fine Grid model.

5.6.12 The initial conditions for initiating the hydrodynamic computations of the Tai O detailed models were based on the computed results from the Fine Grid model. A mapping technique was applied to project the Fine Grid model results onto the model grid of the detailed model.

Performance of Fine Grid Model - Water Quality

5.6.13 The hydrodynamic model of the Fine Grid model was previously calibrated and verified under another study. However, the performance of the water quality model of the Fine Grid model is needed to be confirmed under the current study. The results for confirmation of the model performance are presented in this section.

Key Modelling Parameters

5.6.14 The key modelling parameters of the Fine Grid model are summarised in Table 5.11 below.

Table 5.11 Key Water Quality Modelling Parameters

Parameter	Description
Salinity	Salinity
ModTemp	Water Temperature
E Coli	E. coli Bacteria
Oxy	Oxygen
CBOD₅	Carbonaceous 5-day Biochemical Oxygen Demand
NO₃	Nitrate
NH₄	Ammonium
PO₄	Ortho-Phosphorus
AAP	Adsorbed Ortho-Phosphorus
Si	Silica
Diat	Diatoms
Green	Algae
DetC	Detritus Carbon
DetN	Detritus Nitrogen
DetP	Detritus Phosphorus
DetSi	Detritus Silica
BOD₅	5-day Biochemical Oxygen Demand
Chlfa	Chlorophyll a
SS	Suspended Solids
TotN	Total Nitrogen
TotP	Total Phosphorus
NH₃	Unionised Ammonia

Key Parameters Selected for Presentation

5.6.15 The key parameters of interest in the verification of water quality modelling are summarised in Table 5.12 below.

Table 5.12 Key Parameters for Model Verification

Parameters

Dissolved Oxygen

Salinity

Total Inorganic Nitrogen (TIN)

Unionised Ammonia (UIA)

Suspended Solids (SS)

E. coli

5.6.16 The verification of the water quality models was based on the comparisons with the model outputs from the Update Model (developed under the EPD Cumulative Effect Study). The Update Model covered the entire area of HKSAR as well as the discharges from the major Pearl River estuaries, which include Humen, Jiaomen, Hongqili, Hengmen, Muodaomen, Jitimen, Hutiaomen and Aimen. Monthly average flows from the estuaries were used in the model for dry and wet seasons. As the Update Model provided boundary conditions to the Fine Grid model, the influences on hydrodynamics due to the discharges from Pearl River estuaries were incorporated. In the Fine Grid model, part of the Pearl estuaries were also included in the model set-up including Humen, Jiaomen, Hongqili and Hengmen.

5.6.17 Water quality data collected in the calibration year at EPD's marine water sampling stations were used to verify the water quality predictions from the Fine Grid model. It should be noted that there are no data available for the EPD Station NM8 (north of Tai O) during the calibration year 1998. The verification have made use of contour plots to assess and compare the models during high water spring tide and low water neap tide.

Simulation Period

5.6.18 The extended calibrations of the Update Model under the Cumulative Impact Study demonstrated the successful application of a full representative spring-neap cycle for the wet and dry seasons. Use of the representative spring-neap cycle aimed to give a better representation of the flow dynamics in the modelling system. The hydrodynamic database of the representative spring-neap cycle was used in the water quality model.

5.6.19 The calibration simulation periods for wet and dry seasons are summarised in Table 5.13 below. The simulation periods covered 7-day of model spin-up prior to the actual 15-days spring-neap tidal cycle. A computational time step of one minute was adopted in the model simulation.

Table 5.13 Model Simulation Periods

Season	Spin Up	Model Start Time	Model End Time
Wet	19 July 1998 04:00 – 26 July 1998 04:00	26 July 1998 04:00	10 August 1998 04:00
Dry	2 Feb 1998 12:00 – 9 Feb 1998 12:00	9 Feb 1998 12:00	24 Feb 1998 12:00

5.6.20 Due to the large number of active grid cells in the Fine Grid model and the subsequent huge computation files generated in size, hydrodynamic simulations of the Fine Grid model were divided into four separate runs. The first run consisted of the 7-days spin-up while the 15-days spring-neap cycle simulation periods were divided evenly into three separate runs. The four sets of computation results were then combined and compared with the Update Model.
Wind

5.6.21 The wind conditions applied in the hydrodynamic simulation were 5 m/s NE for dry season and 5 m/s SW for the wet season. The same average wind speed and direction were adopted in the Update Model.

Initial and Boundary Conditions

5.6.22 The Fine Grid Model was linked to the Update Model. Hydrodynamic computations were first carried out using the Update Model. A restart file from the hydrodynamic computations with the Update Model was used to provide initial conditions. The initial conditions for the Fine Grid model were selected to be the same as that of the Update Model. This was done by using a utility program to map the information contained in the restart file of the Update Model to the restart file of the Fine Grid model.

5.6.23 The Update Model provided open boundary conditions to the Fine Grid model through the nesting process. In order to transfer information from the Update Model to the Fine Grid model thoroughly, the open boundaries were divided into small segments.

5.6.24 There were in total three open boundaries with 24 segments as adopted in the hydrodynamic calibrated Fine Grid model. These segments generally cover four to twelve cells in the open boundaries, depending on the sizes and locations. All the 24 segments on the open boundaries were defined as water levels.

Water Quality Modelling

5.6.25 The Fine Grid model was constructed as 3-dimensional detailed model to simulate the vertical structure of the water body and the distribution of pollutants in the water column.

5.6.26 The hydrodynamic computations of the Fine Grid model provide input data to drive the water quality computations. The model set up for water quality simulation incorporated suitable meteorological forcing, initial and boundary conditions, flow aggregation and modelling substances. The substance file used for water quality simulation incorporated the substances of salinity, temperature, dissolved oxygen, suspended solids, BOD, E. coli, phytophankton, organic and inorganic nitrogen, phosphorus and silicate. In addition, the water quality simulation took into account air-water exchange and benthic processes.

5.6.27 The ambient environmental conditions are closely linked to the processes of water quality changes. Meteorological forcing including wind speed, solar surface radiation and water temperature for the dry and wet seasons need to be defined in the hydrodynamic and water quality computations of the Fine Grid model. The data for meteorological forcing were based on the past records from Hong Kong Observatory.

5.6.28 The wind conditions applied in the water quality simulation were 5 m/s for both dry and wet seasons, which was identical to the hydrodynamic model. On the other hand, monthly averaged values of solar surface radiation and water temperature were used in the model. It is assumed that wind speed, solar radiation and water temperature are constant over the entire domain of the model. Solar radiation is recorded only at King's Park station by Hong Kong Observatory. The monthly averaged solar radiation was calculated based on the hourly data recorded at this station. The average values of solar radiation adopted were 132 W/m2 in the dry season and 237 W/m2 in the wet season.

5.6.29 The ambient water temperature was determined based on the EPD routine monitoring data collected within the WCZs. The average water temperature values used in the water quality model were 16 °C in the dry season and 29 °C in the wet season.

5.6.30 Aggregation of the hydrodynamics model was performed in order to optimise the long computation time without reducing significantly the accuracy and quality of the results. The aggregation involved the reduction of the vertical resolution from 10 layers to 5 layers. The vertical distribution of the layers for the water quality model is 10%, 20%, 20%, 30% and 20% of the hydrodynamic layers from surface to bottom. In the spatial level, a 2x2 flow aggregation was applied on the existing hydrodynamic grid layout.

5.6.31 The detailed water quality model simulated the transport of substances and associated water quality processes. A number of process coefficients was involved in the modelling work. Table 5.14 presents the state variables and process coefficients used in the water quality modelling. The listed process coefficients were the same as those adopted in the Update Model. Detailed description of each of the variables is presented in the Delft3D-WAQ manual.

Table 5.14 State Variables and Process Coefficients for Water Quality Modelling

Variable	Description	Value	Unit
VsedIM1	Settling velocity IM1	1	m/d
VsedIM2	Settling velocity IM2	15	m/d
VsedDetC	Settling velocity DetC	1	m/d
VsedDiat	Settling velocity Diat	1	m/d
VsedGreen	Settling velocity Greens	1	m/d
VsedBOD5	Settling velocity BOD5	1	m/d
TauCSIM1	Critical shear sedimentation IM1	0.075	Pa
TauCSIM2	Critical shear sedimentation IM2	1.0	Pa
TauCSdetC	Critical shear sedimentation DetC	0.075	Pa
TauCSDiat	Critical shear sedimentation Diat	0.075	Pa
TauCSGreen	Critical shear sedimentation Green	0.075	Pa
TauCSBOD	Critical shear sedimentation BOD	0.075	Pa
PpmaxGreen	Potential maximum production rate	2.3	1/d
TcGroGreen	Temperature coefficient grow	1.07	-
MrespGreen	Maintenance respiration	0.036	1/d
GrespGreen	Growth respiration factor	0.11	-
Mort0Green	Mortality rate Green at salm 1 green	0.32	1/d
MortSGreen	Mortality rate Green at salm 2 green	2	1/d
SalM1Green	Lower salinity limit for Mort0Green	5	ppt
SalM2Green	Upper salinity limit for MortSGreen	10	ppt
Ppmaxdiat	Potential maximum production rate	2.3	1/d
MrespDiat	Maintenance respiration	0.036	1/d
GrespDiat	Growth respiration factor	0.11	-
Mort0Diat	Mortality rate Diat at salm 1 diat	2	1/d
MortSDiat	Mortality rate Diat at salm 2 diat	0.32	1/d
SalM1diat	Lower salinity limit for Mort0diat	5	ppt
SalM2Diat	Upper salinity limit for MortSdiat	10	ppt
RcDetC	Mineralisation rate DetC	0.1	1/d
RcDetN	Mineralisation rate DetN	0.1	1/d
RcDetP	Mineralisation rate DetP	0.08	1/d
RcDetsi	Mineralisation rate DetSi	0.01	1/d
RcBOD	Decay rate CBOD	0.1	1/d
RcNit	Nitrification rate	0.1	1/d
TaucRS1DM	Critical shear resuspension DM	0.1	Pa
Klrear	Mass transport coefficient reaeration	1.5	1/d
ExtVLBak	Background extinction	0.5	1/m
ExtVL1M1	Specific extinction IM1	0.075	m²/g
ExtVLDetC	Specific extinction DetC	0.47	m²/g
ExtVLGreen	Specific extinction Green	0.15	m²/g
ExtVLDiat	Specific extinction Diat	0.15	m²/g
ExtUVBak	Background extinction	0.5	1/m
ExtUVIM1	Specific extinction IM1	0.075	m²/g
ExtUVIM2	Specific extinction IM2	0.075	m²/g
ExtUVDetC	Specific extinction DetC	0.47	m²/g
ExtUVGreen	Specific extinction Green	0.15	m²/g
ExtUVDiat	Specific extinction Diat	0.15	m²/g
ZresDM	Zero order resuspension flux DM	30	g/m²/d

5.6.32 The Fine Grid model was linked to the Update Model. In order to start the water quality model run from a more realistic condition, a longer spin-up period of two full spring/neap cycles was adopted prior to the actual water quality simulation. After performing the spin-up, the influence on initial conditions would be subsided and would not affect the concentrations of the simulated parameters.

5.6.33 The open boundary condition for the Fine Grid model was defined using information extracted from the Update Model database for water quality simulations in the vicinity of the Fine Grid model boundary segments. For the interested parameters, depth-averaged values were derived and the averages of the whole 15-days spring-neap cycle were calculated. These averages were then used as the boundary conditions for the Fine Grid model.

5.6.34 The pollution flows and loads derived for year 1997 under the Cumulative Impact Study were used in both the Update Model and the Fine Grid model for water quality computations. The data covered the outfall discharges from Hong Kong Island, Kowloon Peninsula, New Territories and outlying islands.

5.6.35 Storm outfall discharges are usually made in shallow water and are near the water surface. Discharges from sewage outfalls are mostly at the sea bottom due to the use of submerged outfall pipes. The near-field spreading of effluent plume and buoyancy effects redistribute the pollutants in the vertical direction of the water column. In this modelling exercise, the flow and load inputs for storm outfall discharges were specified only in the surface layer. For sewage outfall, the discharges were allocated in the middle layer. This arrangement was made consistent with the approach adopted in the Cumulative Impact Study.

Model Performance Results

Dry Season

Salinity

5.6.36 Figures 4A1 and 4A2 of Appendix 5A show the predicted surface layer salinity patterns from the Update and Fine Grid model during high water spring tide and low water neap tide. In general, the two models had reproduced very similar salinity patterns in all regions. The Fine Grid model had successfully replicated the high salinity gradient due to fresh water discharges from Pearl River Estuary. In addition, the Fine Grid model also produced the high salinity levels in the southern and eastern area. Overall, the predicted salinity patterns from the Update and Fine Grid models were in good agreement.

5.6.37 In comparison with EPD monitoring stations, the predicted salinity levels matched well with field data in the region where the salinity levels were high and with levels ranged from 32 to 34 mg/L. However, both the Update and Fine Grid models had underestimated the salinity levels in the regions near the Pearl Estuary and in Outer Deep Bay.

Dissolved Oxygen (DO)

5.6.38 Figures 4A3 and 4A4 of Appendix 5A show the predicted surface layer DO levels from the Update and Fine Grid models during high water spring tide and low water neap tide. The Fine Grid model had predicted slightly lower DO in the Outer Deep Bay, Sha Chau and Lantau west regions. However, the predicted DO levels from other area, including the Inner Deep Bay, southern Lantau Island and Lamma Island were consistent with the Update Model.

5.6.39 In comparison with data recorded at EPD monitoring stations, the two models agreed reasonably well with the field data in the Deep Bay area. However, for other areas, both models underestimated the DO levels. This may be caused by the use of uniform wind speed throughout the entire modelling domain. This transfer of oxygen or reaeration rate would influence the DO levels in sea water, particularly in the surface layer, and is a function of many factors, including wind velocity and water depths. In the two models, a constant wind speed of 5 m/s was utilised for all areas. However, realistically, the wind characteristics across open water were complicated and fluctuated. It is anticipated that wind speed at the open sea would be higher as there would be less friction above the sea surface. The fluctuations in wind velocity in the real world would therefore not be accounted for in the two models.

Suspended Solids (SS)

5.6.40 Figures 4A5 and 4A6 of Appendix 5A show the predicted surface layer SS patterns from the Update and Fine Grid models during high water spring tide and low water neap tide. In general, the SS patterns predicted from the two models were quite similar. The high SS gradient due to Pearl River Estuary in the Update Model and the SS levels in region east and south of Lantau Island were successfully reproduced in the Fine Grid model. The Fine Grid model had predicted higher SS levels in the Deep Bay area, possibly due to the refined grids. Overall, the predicted SS patterns from the Update and Fine Grid models were in good agreement.

5.6.41 In comparison with EPD monitoring stations, the predicted salinity levels from the models were in good agreement with field data in region east and south of Lantau Island, where the SS levels were low. However, higher predicted SS levels in comparison with field data were observed in the Outer Deep Bay regions.

E. coli

5.6.42 Figures 4A7 and 4A8 of Appendix 5A show the predicted surface layer E. coli patterns from the Update and Fine Grid models during high water spring tide and low water neap tide. Both models had produced high E. coli levels in regions where there were stormwater and sewage outfalls and near Pearl River Estuary. Relatively high levels of E. coli were observed in both models in Victoria Harbour and near Shenzhen River regions. Meanwhile, in other area, low levels were predicted in the two models with E. coli concentration of less than 10 count/100ml.

5.6.43 In comparison with the data recorded at EPD monitoring stations, the predicted E. coli levels from the models were in good agreement with most of the field data.

Total Inorganic Nitrogen (TIN)

5.6.44 Figures 4A9 and 4A10 of Appendix 5A show the predicted surface layer TIN concentration for high water spring tide and low water neap tide. The TIN patterns were similar between the Update and Fine Grid model. In outer Deep Bay area, the Fine Grid model had predicted a higher TIN levels in the area where the influences from Pearl River Estuary were great. Comparison with EPD's field data, the Fine Grid model seemed to perform better in this area. For southern and eastern regions, both Update and Fine Grid models predicted similar levels, and agreed well with EPD data. The TIN concentration at this region was low, with levels of below 0.2 mg/L.

Unionised Ammonia Nitrogen (UIA)

5.6.45 Figures 4A11 and 4A12 of Appendix 5A show the predicted surface layer UIA patterns from the Update and Fine Grid models during high water spring tide and low water neap tide. The UIA pattern during low water neap tide were very similar while for the high water spring tide, there were slight differences in the Deep Bay and north-western regions, with the Fine Grid model predicted higher values. However, both models had produced similar low levels in the southern and eastern regions. Overall, the model was in reasonably agreement.

5.6.46 In comparison with the data recorded at EPD monitoring stations, the predicted UIA levels in the southern waters were generally in good agreement with field data, with values ranged between 0.003 and 0.006 mg/L. In the Deep Bay region, there were slight differences with field data, but were in acceptable range.

Wet Season

Salinity

5.6.47 Figures 4A13 and 4A14 of Appendix 5A show the predicted surface layer salinity patterns from the Update and Fine Grid models during high water spring tide and low water neap tide. In general, the two models had reproduced very similar salinity patterns in all regions. The predicted salinity levels, for both models, were low in the region influenced by the Pearl River fresh water discharges. Steep salinity gradient was predicted in the region further south of Lantau and Hong Kong Island. Overall, the predicted salinity patterns from the Update and Fine Grid models were in good agreement.

5.6.48 In comparison with the data recorded at EPD monitoring stations, the predicted salinity levels matched well with field data in the Deep Bay region. However, both the Update and Fine Grid models had underestimated the salinity levels in the regions south of Lantau and Hong Kong Island.

Dissolved Oxygen (DO)

5.6.49 Figures 4A15 and 4A16 of Appendix 5A show the predicted surface layer DO levels from the Update and Fine Grid models during high water spring tide and low water neap tide. The predicted DO patterns for the two models were in good agreement. Slightly higher DO was predicted in the Deep Bay region during low water neap tide but the levels were similar during spring tide. Meanwhile, at southern Lantau and Lamma Islands regions, the Fine Grid model had predicted slightly highly DO levels during spring tide.

5.6.50 In comparison with the data recorded at EPD monitoring stations, the two models agreed reasonable well with the field data in the inner Deep Bay area. However, for other areas, both models had underestimated the DO levels. Between the area of Lantau and Lamma, the predicted DO levels in the Fine Grid model were closer to the field data when compared with the Update Model during spring tide. The differences between the predicted DO levels and field data may be caused by the adoption of uniform wind speed across the entire modelling domain as discussed in Section 5.6.39.

Suspended Solids (SS)

5.6.51 Figures 4A17 and 4A18 of Appendix 5A show the predicted surface layer SS patterns from the Update and Fine Grid models during high water spring tide and low water neap tide. In general, the SS patterns predicted from the two models were quite similar. The high SS gradient due to the Pearl River Estuary in the Update Model was reproduced in the Fine Grid model. The predicted SS levels for the Fine Grid model in Deep Bay and in the north-western regions were slightly higher. The differences would probably due to the refinement of grid in the region. Comparing with EPD data, however, the Fine Grid model seemed to perform better around the Deep Bay region.

5.6.52 In comparison with the data recorded at EPD monitoring stations, the predicted salinity levels from the models were in reasonable agreement with field data in most of the region. In Deep Outer Bay area, the predicted SS levels from the two models were higher than the field data.

E. coli

5.6.53 Figures 4A19 and 4A20 of Appendix 5A show the predicted surface layer E. coli patterns from the Update and Fine Grid models during high water spring tide and low water neap tide. Similar E. coli patterns were produced in the two models. The Fine Grid model had successfully reproduced the high E. coli gradient due to the Pearl River Estuary and the elevated levels near stormwater and sewage outfalls. Meanwhile, low levels were predicted in the two models in other area, with E. coli concentration of less than 10 count/100ml.

5.6.54 In comparison with EPD monitoring stations, the predicted E. coli levels from the models were in good agreement with most of the field data.

Total Inorganic Nitrogen (TIN)

5.6.55 Figures 4A21 and 4A22 of Appendix 5A show the predicted surface layer TIN concentration for high water spring tide and low water neap tide. The two models show to have similar TIN gradient due to the Pearl River Estuary. Lower TIN levels were predicted in the southern and eastern region. From EPD stations, the Fine Grid model seemed to perform better in that region.

Unionised Ammonia Nitrogen (UIA)

5.6.56 Figures 4A23 and 4A24 of Appendix 5A show the predicted surface layer UIA patterns from the Update and Fine Grid models during high water spring tide and low water neap tide. The UIA patterns of both models were similar. Both models showed to have high UIA levels in Deep Bay and Victoria Harbour while most of the other areas, the levels were between 0.003 and 0.006 mg/L. Overall, the model was in good agreement with the Update Model.

5.6.57 In comparison with the data recorded at EPD monitoring stations, the predicted UIA levels in the whole region were comparable with field data.

Conclusion

5.6.58 The predicted results from the Fine Grid model agreed reasonably well with the Update Model for most of the parameters. Some deviations with the DO field data were observed which probably due to the issue discussed in the previous paragraph.

5.6.59 In conclusion, the Fine Grid model has been successfully verified. It is considered that it can be used for hydrodynamic and water quality simulations in the present Study.

Model Performance - Tai O Detailed Model

Coastline and Bathymetry

5.6.60 The coastline configuration as shown in Drawing No. 23400/EN/016 was used for the Tai O detailed model. The bathymetry data obtained from the Tai O Bay survey carried out by EGS in 1998 as reported in the 'Tai O Sheltered Boat Anchorage EIA' as well as the bathymetry information from the Fine Grid model were used as a base. The bathymetry used is shown in Drawing No. 23400/EN/061. The reference level of the Tai O detailed model was Principal Datum Hong Kong and the depth data were relative to this datum.

5.6.61 The flow aggregation and meteorological forcing used in the Tai O detailed model were the same as that for the Fine Grid model.

Initial and boundary conditions

5.6.62 The Tai O detailed model was linked to the Fine Grid model. Hydrodynamic computations were first carried out using the Fine Grid model to generate the open boundary conditions for the detailed model. Water levels and three-dimensional velocities generated by the Fine Grid model were defined at the open boundaries of the detailed model. In order to transfer information from the Fine Grid model to the detailed model thoroughly, the open boundaries were divided into small segments. There were in total three open boundaries for the Tai O detailed model, namely northern, western and southern respectively. Each of these 3 open boundaries was divided evenly in length into small segments. The number of small segments in each open boundary was 4, 5 and 2 for northern, western and southern respectively (11 segments in total). The 9 segments on the northern and western boundaries were defined as three-dimensional velocities while the remaining 2 segments on the southern boundary were defined as water levels.

5.6.63 In order to start the water quality model run from a more realistic condition, a spin-up period of one full spring/neap cycles was adopted for the first simulation period. After performing the spin-up, the influence on initial condition would be subsided and would not affect the concentrations of the simulated parameters. The computed water quality condition at end of the first simulation period was used as the initial condition for the second simulation period.

Modelling Substances

5.6.64 The key water quality parameters/processes covered in the model are the same as those covered by the Fine Grid model as presented in Tables 5.11, Table 5.12 and 5.14.

Simulation Periods

5.6.65 The simulation periods covered by the Tai O detailed model were the same as those covered by the Fine Grid model as presented in Table 5.13.

Pollution Loading

5.6.66 Pollution loads used to confirm the model performance represents the existing condition which follow the information from the Outlying Islands SMP Stage 2 Review Study and Tai O Sheltered Boat Anchorage EIA. The main pollution sources for the existing condition include the unsewered area, treated sewage input, urban runoff.

5.6.67 Pollution loading from the unsewered area at Tai O (Table 5.15) was estimated according to the assumption that only 900 out of 2,465 number of Tai O population are connected to trunk sewer for discharge to the Kau San Tei STW. These figures were extracted from the Outlying Island SMP Stage 2 Review Study Final Report issued in February 2002. It was assumed that the pollution loading in Table 5.15 would spread out evenly along the coastline of the village areas at Tai O creek and Tai O Bay.

Table 5.15 Estimated Pollution Loading Delivered to Tai O Bay from the Unsewered Areas for Present Condition

Contaminant	Untreated Sewage Concentration	Contaminant Load Generated per Person	Contaminant Load generated by Total Tai O Population	Loading from Unsewered Areas
SS	220 mg/L	0.0407 kg/day	100.33 kg/day	63.70 kg/day
BOD₅	220 mg/L	0.0407 kg/day	100.33 kg/day	63.70 kg/day
TN	40 mg/L	0.0074 kg/day	18.24 kg/day	11.58 kg/day
Total Organic Nitrogen (TON)	15 mg/L	0.002775 kg/day	6.84 kg/day	4.34 kg/day
NH₄	25 mg/L	0.004625 kg/day	11.40 kg/day	7.24 kg/day
TP	8 mg/L	0.00148 kg/day	3.65 kg/day	2.32 kg/day
Organic-P	3 mg/L	0.000555 kg/day	1.37 kg/day	0.87 kg/day
Inorganic-P	5 mg/L	0.000925 kg/day	2.28 kg/day	1.45 kg/day
Total Coliform	1.00E+08 no./100 mL	1.85E+11 no./day	4.56E+14 no,/day	2.90E+14 no./day

Note: The pollution loading was calculated using the raw untreated wastewater concentrations (medium strength) quoted by Metcalf and Eddy (1991) and the sewage generation rate of 185 L per person per day.

5.6.68 The Kau San Tei STW to the north of Tai O was also a source of pollutants to the local environment. The STW provides primary treatment of the sewage received. The estimated pollution loading is given in Table 5.16. Estimation for BOD, SS, Ammonia and E. coli loading was based on the daily flow and effluent concentrations presented in Table 7.5.2 of the Outlying Island SMP Stage 2 Review Study Final Report. Estimation of pollution loading for other parameters for which information on the effluent quality is not available, was based on the pollution load data provided in Table 5.15. Pollution loading discharged to the STW was derived by subtracting the "pollution loading lost to Tai O waters" from the "pollution loading generated by the entire Tai O population". Appropriate treatment removal given in Table 5.17 was applied to obtain the pollution loading discharged from the STW. The treated effluent would be discharged to the marine environment via a 440m submarine outfall to the north-west of the STW (outfall co-ordinates 813798.71 Northing, 803409.04 Easting).

Table 5.16 Estimated Pollution Loading from the Kau San Tei Sewage Treatment Works to the Marine Environment via Sea Outfall

Contaminant	Loading (All units in kg/day unless otherwise stated)
SS	85.05
BOD₅	126.36
TN	6.66
TON	2.50
NH₃-N	38.88
TP	1.33
Organic-P	0.50
Inorganic Phosphorus (Inorganic-P)	0.83
E. coli	4.01E+14 (count/day)

Table 5.17 Treatment Efficiency for Selected Parameters

Types of Treatment Plant	Org-N	Ortho-P	TP
Primary Treatment	15%	0%	15%

Source: Cumulative Effect Study Final Pollution Loading Inventory Report

5.6.69 The pollution loading from urban runoff was directly extracted from the approved EIA report for Tai O Sheltered Boat Anchorage Development and was estimated according to the measured pollution concentrations in urban runoff generated in Tin Shui Wai under the EPD Storm Pollution Study in 1997 as well as using the assumption that Tai O has an urban area of 15.75 ha which receives annual rainfall of 2.2143 m (Table 5.18). It was assumed that the loading from urban runoff would be evenly distributed amongst the local streams and rivers within the assessment area.

Table 5.18 Estimated Pollution Loading from Tai O Urban Runoff

Contaminant	Loading (kg/day)
SS	347.8
BOD₅	14.8
COD	124.2
TP	0.62
Sol. P	0.21
TKN	2.86
NO₃ + NO₂	1.46

Model Performance - Hydrodynamics

5.6.70 In order to ensure the suitability of the Tai O detailed model for evaluation of the water quality impact, confirmation of the model performance has been carried out. This was done by comparing the results of the Tai O detailed model with those of the calibrated Fine Grid model.
Velocity Fields

5.6.71 Figures B1 to B4 and B9 to B12 attached in Appendix 5B illustrate the comparison between the velocity fields simulated by the Tai O detailed models and those predicted by the Fine Grid model. Table 5.19 gives the time used for these comparison plots.

Table 5.19 Times used for Model Output

Tide State	Spring	Neap
Wet Season
Mid Ebb	10 August 1998 03:00	29 July 1998 17:00
Mid Flood	07 August 1998 07:00	30 July 1996 10:00
Dry Season
Mid Ebb	12 February 1998 13:00	19 February 1998 17:00
Mid Ebb	09 February 1998 17:00	18 February 1998 23:00

5.6.72 The velocity fields simulated by the detailed model agreed closely with those simulated by the Fine Grid model. However, the local model had much higher grid resolutions than that of the Fine Grid model which provided a more detailed description of the tidal circulation in the modelling area.
Tidal Levels and Currents at Monitoring Points

5.6.73 The water levels, salinity concentrations, current speeds and directions were predicted at 3 selected check points, namely x, y and z respectively, which are shown in Drawing No. 23400/EN/060.
5.6.74 The comparison of water levels between the local model results and the Fine Grid model results is illustrated in Figures B5 and B13 of Appendix 5B. There was a very good agreement between water levels at all the monitoring stations.

5.6.75 The comparison of the corresponding current speeds and directions at monitoring points is illustrated in Figures B7, B8, B15 and B16 of Appendix 5B. The current speeds and directions simulated by the Tai O detailed model agreed well with those predicted by the Fine Grid model at the 2 offshore stations (x and y). Relatively larger discrepancies were found at the inshore station z. It should be noted that the bathymetry in the inshore area of the Tai O detailed model has been updated with reference to the EGS 1999 survey data. The local model also adopted a much more representative coastline configuration. The difference in coastline and bathymetry information between the local model and the Fine Grid model was most likely the reason for a poorer reproduction of the current directions in the inshore area. It is considered that the bathymetry and coastline information for the Tai O detailed model is more representative that that of the Fine Grid regional model.

5.6.76 The salinity comparison is shown in Figures B6 and B14 of Appendix 5B. The salinity levels simulated by local model had largely the same patterns as those simulated by the Fine Grid model. The discrepancies were most significant at the inshore station z for the wet season. The wet season salinity at station z of the local model was in general slightly higher as compared to that of the Fine Grid model. Nevertheless, the salinity levels predicted by the detailed model were still considered comparable to that by the Fine Grid model and were in a reasonable range.

Conclusion

5.6.77 The results produced by the local model agreed fully with those of the Fine Grid model. Although some discrepancies in current and salinity levels was noted at the inshore station z, both sets of model results still agreed reasonably well with each other in terms of both patterns and magnitude. The discrepancy was likely to be due to a more accurate representation of the bathymetry and coastline by the Tai O detailed model than by the Fine Grid model.

Model Performance - Water Quality

5.6.78 The surface layer contour outputs produced by the Tai O detailed model for the existing condition are compared to the field data collected from the PPFS and Tai O Sheltered Boat Anchorage Study for DO, SS, E. coli, TIN, BOD and Salinity. The comparison plots for dry and wet season are attached in Appendices 5C and 5D respectively. The units of the figures presented in Appendices 5C and 5D are mg/L for all parameters except for E. coli in count/100ml and salinity in ppt. The contour outputs produced by the Fine Grid model are also included in the figures for comparison. The contour plots for dry season are presented for high water and low water whereas the contour plots for wet season are presented for spring ebb and neap flood.

Dissolved Oxygen (DO)

5.6.79 Figures 4C1a and 4C1b of Appendix 5C give the dry season contour comparison plots for DO. It can be seen from Figure 4C1a that the DO levels predicted by the detailed model at high water was relatively lower as compared to the field data collected from the Tai O Boat Sheltered Anchorage Study but the predicted DO results were still considered in a reasonable range. The model results however agreed very well with the field data at the low tide.

5.6.80 Figures 4D1a and 4D1b of Appendix 5D show the DO comparison plots for wet season. The DO results predicted by the local model agreed reasonably well with the field data collected under the PPFS study.

Suspended Solids (SS)

5.6.81 Figures 4C2a and 4C2b of Appendix 5C give the dry season contour comparison plots for SS. At both high water and low water, the SS results produced by the local detailed model agreed reasonably well with the measured field data. As shown in Figure 4C2b, the measured SS concentration at the Southeast corner (Station 7) of Tai O Bay was significant higher than the model results. As discussed in Section 5.6.6, the high measured SS concentration was likely due to boat disturbance to the seabed during the sampling event.

5.6.82 Figures 4D2a and 4D2b of Appendix 5D give the SS comparison plots for wet season. It is considered that the results predicted by the detailed model were in a reasonable range as compared to the field data collected from the PPFS for both spring ebb and neap flood tides.

E. coli

5.6.83 Figures 4C3a and 4C3b of Appendix 5C give the dry season contour comparison plots for E. coli. It can be seen that relatively high E. coli levels were predicted by the local model at the Tai O creek which agreed well with the field data. Similarly, Figures 4D3a and 4D3b of Appendix 5D showed that the results produced by the detailed model agreed very well with the field data for the wet season.

Total Inorganic Nitrogen (TIN)

5.6.84 Figures 4C4a and 4C4b of Appendix 5C give the comparison plots for dry season whilst Figures 4D1a and 4D1b of Appendix 5D present the model results for wet season. It was found that the TIN results predicted by the detailed model were in general relatively higher in comparison with the field data for the dry season. On the other hand, the wet season model results were in general relatively lower as compared to the field data. The predicted results and the field results were however in the same order of magnitude and the predicted results are considered in a reasonable range and were comparable to the field data.

Biochemical Oxygen Demand (BOD)

5.6.85 Figures 4C5a and 4C5b of Appendix 5C give the comparison plots for dry season whilst Figures 4D5a and 4D5b of Appendix 5D present the model results for wet season. The BOD results predicted by the detailed model agreed very well with the field data for both dry and wet season.

Salinity

5.6.86 Figures 4C6a and 4C6b of Appendix 5C give the comparison plots for dry season whilst Figures 4D6a and 4D6b of Appendix 5D present the model results for wet season. It can be seen in these figures that the salinity levels predicted by the detailed model were within the ranges of the measured data

Conclusion

5.6.87 The results predicted by the detailed model revealed that water in Tai O Bay had relatively low levels of TIN, BOD but had relatively high levels of E. coli within the inner part of Tai O Bay and Tai O Creek which agreed very well with the field investigation results as discussed in Sections 5.6.1 to 5.6.9 of baseline condition. The results predicted by the detailed model also agreed reasonably well with the field data for DO, SS and Salinity.

5.6.88 It was found that there were differences in the predicted concentration patterns between the detailed model and the Fine Grid model. The discrepancy was likely to be due to a more accurate representation of the pollution loading, bathymetry and coastline by the Tai O detailed model than by the Fine Grid model.
5.6.89 It is recommended that the Tai O detailed model can be used to assess the water quality impact on the Tai O waters for the present study.

Impact Assessment

5.6.90 Modelling has been carried out for the two scenarios, namely baseline scenario and export effluent scenario.

Baseline Scenario

Coastline and Bathymetry

5.6.91 The coastline configuration as shown in Drawing No. 23400/EN/016 was used for the baseline scenario. The bathymetry data obtained from the Tai O Bay survey carried out by EGS in 1998 as reported in the 'Tai O Sheltered Boat Anchorage EIA' as well as the bathymetry information from the Fine Grid model were used as a base. The baseline scenario also took into account the Tai O Sheltered Boat Anchorage Development as this development will be commissioned before the construction of the effluent export pipeline according to the available schedule from CED. The reduced development scheme as attached in Appendix 5E was adopted for modelling. The planned breakwater for the Tai O Sheltered Boat Anchorage was included in the model and was represented by a thin dam (see the 1st drawing in both Appendices 5F and 5G). The bathymetry was further updated at the planned anchorage area and its associated access channels to reflect the future sea bed condition. The bathymetry used for the baseline model is shown in Drawing No. 23400/EN/017.

Pollution Loading

5.6.92 Pollution loads used to confirm the model performance were also adopted for the baseline modelling. These pollution loads represent the existing condition which include the sewage input from unsewered area and Kau San Tei STW as well as the urban runoff (Tables 5.15, 5.16 and 5.18).

5.6.93 It should be noted that the Kau San Tei STW will be upgraded to CEPT plus disinfection under the 'Outlying Island Sewerage Stage 2'. The associated construction works will commence in late 2007 for commissioning in 2011. According to the latest information provided by EPD, construction works for extension/upgrading of the existing village sewerage at Tai O will commence in around the period from 2009 to 2011 to tie-in with the STW upgrading programme. The pollution loading adopted for the baseline modelling (Tables 5.15, 5.16 and 5.18) covered the interim period before the completion of the proposed Tai O village sewerage works and STW upgrading works to represent the worst case condition where the Tai O waters still receive a considerable amount of pollution load from unsewered areas. As no solid information can be available at this stage for estimation of the future pollution loading discharged into Tai O creek and Tai O bay, using the pollution loading for the existing condition is considered as a conservative approach.

5.6.94 The baseline scenario also took into account the direct discharge of pollutants from the boat anchorage development which was not considered during the assessment of the model performance (Section 5.6.60 to 5.6.69). Pollution load under Scenario 2 presented in Table 5.15 of the Tai O Sheltered Boat Anchorage EIA Report was adopted, assuming that there are 22 boats large enough to be used for residential purposes and the resulted pollution load discharges are summarised in Table 5.20.

Table 5.20 Estimated Pollution Load Delivered to Tai O Sheltered Boat Anchorage Directly from Vessels

Contaminant	Load per person (kg/day unless indicated)	Pollution Load (kg/day unless indicated)
SS	0.02	1.79
BOD₅	0.02	1.79
TN	0.0037	0.33
TON	0.0014	0.12
NH₃-N	0.0023	0.20
TP	0.0007	0.07
Organic-P	0.0003	0.02
Inorganic Phosphorus (Inorganic-P)	0.0005	0.04
Total Coliform	9.25E+10 count/day	8.14E+12 count/day

Other Model Set-up

5.6.95 The simulation period, flow aggregation, modelling substances and meteorological forcing as well as the set-up of boundary and initial condition used to confirm the model performance as described in Sections 5.6.60 to 5.6.65 were also applied for the baseline modelling.

Export Effluent Scenario

5.6.96 The set up of the Tai O detailed model for the effluent scenario was identical to that of the baseline scenario as discussed in Section 5.6.91 to 5.6.95 except that we have put the Ngong Ping export effluent discharge near the bus terminus at the western mouth of Tai O creek. The loading used for the Ngong Ping export effluent was based on the minimum required discharge standards as specified in Table 1.2 (Section 1). The average flow on peak days of 34.21 L/s (Table 1.1) was adopted for estimation of the pollution loading from the Ngong Ping STW. This flow rate was applied in the model uniformly for the whole simulation periods. It should be noted that the flow of 34.21 L/s adopted in the model is a very conservative assumption representing the peak flow on weekends and holidays. For the majority of the year (on normal weekdays), the estimated flow would only be 17.64 L/s. The flow of 34.21 L/s was used to estimate the pollution loading for input to the water quality model. In the hydrodynamic model runs, this effluent flow was not input into the model as the flow is considered small in comparison with the flow of Tai O creek and would unlikely have any significant hydrodynamic effect on the receiving water bodies. This is also considered a conservative approach because dilution effects on the pollutant levels of the receiving bodies could be induced by incorporating the flow input into the hydrodynamic models.

Potential Bacteria Re-growth Over the Effluent Transport Route

5.6.97 Photochemical damage caused by UV may be repaired by some organisms. Studies show that the amount of cell damage and subsequent repair is directly related to the UV dose. The amount of repair will also depend on the dose (intensity) of photo reactivating light. For low dose the resulting minimal damage can be more readily repaired than for high dose where the number of damaged sites is greater (Lindenauer et al., 1994)

5.6.98 There are two repair mechanisms: photoreactivation and dark repair. Report on Disinfection Pilot Plant Study under the Outlying Islands Sewerage - Stage 1 Phase 1 states that:
"Photoreactivation is a two step process involving the formation of an enzyme dimer complex. This stage of reaction does not require light. The next stage requires absorption of light energy (wavelength range 310 to 490 mm) to convert the enzyme dimers into thymine monomers, thus resulting in a reversal of the photochemical damage.

Dark repair does not require light energy. It is thought to be an enzyme repair process involving the excision of dimers and may be similar to the repair of cell damage caused by non-photochemical agents. Dimer formation in cytosine is repaired by this mechanism."

5.6.99 Under the Outlying Islands Pilot Plant Study, experiments were carried out to test the potential bacteria re-growth after UV disinfection. Samples after UV treatment were exposed to both dark and light conditions. Samples under dark condition would simulate the time when the wastewater is present in the pipeline before being discharged. Samples were then exposed to light at the receiving water temperature. Samples were then analysed for E. coli. The results showed that the increases in E. coli due to both dark repair and photo repair were not significant and should be well offset by natural die off. Details of the pilot study can be referred to Report on Disinfection Pilot Plant Study March 2000 issued under the Outlying Island Project. As such, it was assumed in this study that there would not be significant changes in the bacteria levels in the treated effluent between the discharge point at Ngong Ping STW and the discharge point at the Tai O seafront.

Impact Evaluation

5.6.100 Tables 5.21 and 5.22 present the predicted dry and wet season water quality results at sensitive receivers. Locations of sensitive receivers can be referred to Drawing No. 23400/EN/001. The water quality results for both the baseline scenario and the export effluent scenario are included in the table for comparison.
Secondary Recreation Subzones - Indicator Points 4a & 4b

5.6.101 The effluent from Ngong Ping STW will be of high quality and the pollution loading discharged from Ngong Ping STW would be small as compared to the background pollution loading from unsewered areas, treated sewage input, urban runoff and discharges from the boat anchorage development. It is believed that any impact due to the Ngong Ping effluent discharge would be localised. It is therefore not anticipated that the proposed discharge from the project would result in any adverse impact on the secondary recreational subzones on both sides of outer Tai O Bay which are some 1000m away from the discharge point. This is supported by the water quality modelling results as shown in Tables 5.21 and 5.22. All the predicted values for salinity, DO, SS, E. coli, TIN, UIA and BOD at Stations 4a and 4b complied very well with the WQO for both dry and wet seasons. The increases in the predicted pollutant concentrations caused by the export effluent were less than 0.5% at the monitoring points inside Tai O bay for both dry and wet seasons.

Planned Mangrove Stands at the New Salt Pans and Existing Mangrove at Tai O Creek -Indicator Points 1 & 2

5.6.102 The existing mangrove habitats at the Tai O creek currently receive a considerable amount of sewage discharges from unsewered developments. It is not expected that the additional input from the Ngong Ping STW would cause any adverse impact on the existing mangroves at Tai O Creek as well as the planned mangrove stands at the new salt pans, as the natural pollution tolerance (or pollution exclusion) displayed by mangroves is well documented. Recent studies of mangroves in Hong Kong and the Futien Nature Reserve in Shenzhen firmly conclude that the mangrove habitats are not adversely affected by high pollution loads, including concentrated sewage effluent (Tam et al, 1995; Tam & Wong, 1995; Wong et al, 1995; Wong, 1996(a) and 1996(b)). There is also considerable evidence that mangroves are unaffected by dramatic changes in salinity due to the nature of the inter-tidal habitat in which they grow (i.e. salinity levels can fluctuate from freshwater (0 parts per thousand (ppt)) to sea water (34 ppt)).

5.6.103 Nevertheless, a comprehensive modelling was performed to evaluate the water quality impact. The modelling results presented in Tables 5.21 and 5.22 showed that the predicted pollutant concentrations for the effluent export scenario at the Tai O creek for all parameters are considered low except for E. coli. Relatively high E. coli levels were predicted at Tai O creek for both wet and dry seasons. This agreed with the recent field data collected at Tai O creek under the PPFS and Tai O Sheltered Boat Anchorage Study. As discussed in Sections 5.6.2 to 5.6.9, the high E. coli levels at Tai O creek were contributed by the pollution input from existing unsewered developments. The increases in pollutant levels caused by the additional effluent from Ngong Ping were minimal for both dry and wet seasons as shown in Tables 5.21 and 5.22. It is concluded that the existing and future mangrove habitat within the study area would not be adversely affected by the pollution input during the operational phase of the Project due to the reasons discussed in Sections 5.6.102.

Future Tai O Sheltered Boat Anchorage - Indicator Point 3

5.6.104 As shown in Tables 5.21 and 5.22, the predicted values for salinity, DO, SS, E. coli, TIN, UIA and BOD at Station 3 complied very well with the WQO for the effluent export scenario. The differences in concentrations between the baseline and effluent scenarios for all parameters are very small for both dry and wet season (<2%).

Table 5.21 Predicted Pollutant Concentrations at Indicator Points for Dry Season

Parameter		Indicator Point (refer to Drawing No. 23400/EN/001)
		2 – Existing Mangrove at Tai O Creek			3 - Tai O Boat Anchorage Development			4a – Secondary Contact Recreational Subzone			4b – Secondary Contact Recreational Subzone
		Baseline	Effluent Export	% Difference	Baseline	Effluent Export	% Difference	Baseline	Effluent Export	% Difference	Baseline	Effluent Export	% Difference
Salinity (ppt)	Average	31.51	31.60	0.29%	31.98	31.93	-0.16%	31.81	31.77	-0.13%	32.22	32.15	-0.22%
	Minimum	31.24	31.24		31.74	31.70		31.16	31.16		31.65	31.63
	Maximum	32.03	32.05		32.44	32.34		32.81	32.75		32.67	32.61
Depth-averaged DO (mg/L)	10%ile	6.68	6.69	0.15%	6.45	6.56	1.71%	6.56	6.56	0.00%	6.49	6.46	-0.46%
	Minimum	6.56	6.56		6.51	6.51		6.44	6.44		6.42	6.42
	Maximum	8.17	8.21		6.68	6.68		6.54	6.54		6.66	6.66
Bottom DO (mg/L)	10%ile	6.43	6.43	0.00%	6.45	6.45	0.00%	6.43	6.43	0.00%	6.43	6.43	0.00%
	Minimum	6.35	6.35		6.33	6.33		6.37	6.37		6.35	6.35
	Maximum	7.95	7.97		6.64	6.64		6.52	6.52		6.64	6.64
SS (mg/L)	Average	16.36	16.39	0.18%	14.93	14.94	0.07%	13.70	13.70	0.00%	15.84	15.84	0.00%
	Minimum	13.98	14.00		13.78	13.79		11.37	11.37		12.51	12.51
	Maximum	18.39	18.46		16.99	17.00		18.25	18.25		18.35	18.36
BOD₅ (mg/L)	Average	1.42	1.43	0.70%	0.62	0.62	0.00%	0.55	0.55	0.00%	0.55	0.55	0.00%
	Minimum	0.76	0.77		0.52	0.52		0.41	0.41		0.45	0.45
	Maximum	2.38	2.42		0.67	0.68		0.62	0.62		0.63	0.63
E. coli (count/100mL)	Geometric Mean	6727	6729	0.03%	520	520	0.00%	219	219	0.00%	69	69	0.00%
	Minimum	3700	3700		189	189		31	31		11	11
	Maximum	13700	13700		1110	1110		1367	1367		298	298
UIA (mg/L)	Average	0.0041	0.0041	0.00%	0.0049	0.0049	0.00%	0.0055	0.0055	0.00%	0.0049	0.0049	0.00%
	Minimum	0.0027	0.0027		0.0046	0.0046		0.0048	0.0048		0.0044	0.0044
	Maximum	0.0054	0.0054		0.0052	0.0052		0.0059	0.0059		0.0056	0.0056
TIN (mg/L)	Average	0.3555	0.3603	1.35%	0.3636	0.3679	1.18%	0.3908	0.3909	0.03%	0.3521	0.3533	0.34%
	Minimum	0.2873	0.2916		0.3331	0.3366		0.3087	0.3087		0.3178	0.3186
	Maximum	0.4075	0.4127		0.3820	0.3875		0.4314	0.4314		0.4065	0.4066

Table 5.22 Predicted Pollutant Concentrations at Indicator Points for Wet Season

Parameter		Indicator Point (refer to Drawing No. 23400/EN/001)
		2 – Existing Mangrove at Tai O Creek			3 - Tai O Boat Anchorage Development			4a – Secondary Contact Recreational Subzone			4b – Secondary Contact Recreational Subzone
		Baseline	Effluent Export	% Difference	Baseline	Effluent Export	% Difference	Baseline	Effluent Export	% Difference	Baseline	Effluent Export	% Difference
Salinity (ppt)	Average	12.76	12.63	-1.02%	12.48	12.44	-0.32%	12.61	12.60	-0.08%	12.38	12.35	-0.24%
	Minimum	12.46	12.00		12.05	11.88		11.74	11.74		11.72	11.72
	Maximum	12.90	12.89		12.68	12.68		13.00	13.00		12.81	12.81
Depth-averaged DO (mg/L)	10%ile	6.37	6.39	0.31%	6.37	6.37	0.00%	5.09	5.09	0.00%	6.01	6.01	0.00%
	Minimum	5.71	5.72		5.80	5.80		5.05	5.05		5.43	5.43
	Maximum	8.60	8.66		8.50	8.51		6.77	6.77		9.20	9.20
Bottom DO (mg/L)	10%ile	5.78	5.79	0.17%	6.22	6.23	0.16%	4.99	4.99	0.00%	5.88	5.88	0.00%
	Minimum	5.42	5.43		5.76	5.76		4.86	4.86		5.25	5.25
	Maximum	8.46	8.49		8.16	8.17		6.02	6.02		8.76	8.77
SS (mg/L)	Average	17.38	17.42	0.23%	18.67	18.68	0.05%	17.66	17.66	0.00%	19.88	19.89	0.05%
	Minimum	15.55	15.58		16.67	16.69		15.22	15.22		15.64	15.65
	Maximum	19.77	19.80		22.19	22.20		23.77	23.77		27.14	27.14
BOD₅ (mg/L)	Average	2.55	2.56	0.39%	3.20	3.20	0.00%	1.77	1.77	0.00%	3.40	3.40	0.00%
	Minimum	1.40	1.40		1.77	1.78		0.80	0.80		1.16	1.16
	Maximum	4.16	4.18		5.35	5.36		4.68	4.68		6.46	6.46
E. coli (count/100mL)	Geometric Mean	3394	3397	0.09%	196	196	0.00%	103	103	0.00%	24	24	0.00%
	Minimum	1078	1079		24	24		8	8		3	3
	Maximum	6272	6279		633	633		511	511		122	122
UIA (mg/L)	Average	0.0069	0.0069	0.00%	0.0074	0.0074	0.00%	0.0079	0.0079	0.00%	0.0075	0.0075	0.00%
	Minimum	0.0063	0.0064		0.0066	0.0066		0.0065	0.0065		0.0064	0.0064
	Maximum	0.0080	0.0080		0.0090	0.0090		0.0110	0.0110		0.0097	0.0097
TIN (mg/L)	Average	0.2771	0.2808	1.34%	0.2866	0.2889	0.80%	0.3448	0.3448	0.00%	0.2906	0.2911	0.17%
	Minimum	0.2036	0.2096		0.2368	0.2379		0.3057	0.3058		0.2078	0.2082
	Maximum	0.3277	0.3307		0.3280	0.3306		0.3659	0.3659		0.3456	0.3457

Conclusion and Recommendation

5.6.105 The operational water quality modelling has taken into account the cumulative impacts from the existing background pollution discharges such as the pollution load from unsewered areas and the direct discharge of pollutants from the future boat anchorage development.

5.6.106 The predicted water quality results for the export effluent scenario complied very well with the WQO within the study area. Relatively high E. coli levels were predicted at the existing mangroves along the Tai O creek. The predicted relatively higher E. coli concentrations at Tai O creek were contributed by the pollution input from existing unsewered developments. No adverse impact on the mangrove habitats is anticipated due to their nature of pollution tolerance as discussed in Section 5.6.102. The apparent increases in the concentrations of some parameters in Tables 5.21 and 5.22 were based on the fact that the design peak flow was adopted for the entire simulation periods in the water quality modelling which was a very conservative approach as discussed in Section 5.6.96. Nevertheless, the increases in pollutant concentrations caused by the discharge of Ngong Ping export effluent as predicted by the model are considered small. Some examples are given in Appendices 5F and 5G to illustrate the comparison of concentration pattern between the baseline scenario and the export effluent scenario. These examples showed that the predicted concentration patterns for both scenarios are identical. The units of the figures presented in Appendices 5F and 5G are mg/L for all parameters except for E. coli in count/100ml and salinity in ppt.

5.6.107 The impact from the Ngong Ping export effluent is considered acceptable and no adverse effect would anticipated upon the sensitive receivers within the Tai O creek and Tai O bay during operational phase of the Project.

5.6.108 The predicted water quality results were based on the minimum effluent discharge standards as presented in Table 1.2. To ensure the effectiveness of the proposed treatment process, monitoring of effluent quality is recommended. It is proposed to monitor the effluent at the outlet chamber of the disinfection unit. Parameters to be monitored include pH, BOD, SS, nutrients and E. coli. Marine water quality monitoring is also recommended to verify the findings of the water quality modelling. A six-month baseline monitoring programme covering both dry and wet seasons is proposed at a frequency of twice per month to establish the baseline water quality conditions. It is suggested to start the impact monitoring 3 months after commissioning of Ngong Ping STW at a frequency of twice per month to determine whether there is any deterioration in water quality compared with the baseline measurements. The monitoring programme can be discontinued after one year if there is no obvious deterioration in water quality. Marine water quality such as turbidity, salinity, pH, DO, SS, TIN, NH₃-N, phosphate-phosphorus (PO₄-P) and E. coli should be monitored. It is recommended to set up 7 monitoring stations, 2 at Tai O creek, 1 at the Po Chue Tam outlet, 2 at inner Tai O Bay and 2 at outer Tai O Bay. Details of the monitoring programme are given in a separate Environmental Monitoring and Audit (EM&A) Manual.

5.7 Literature Cited

5.7.1 DSD, Agreement No. CE29/2001 Outlying Island Sewerage Stage 1 Phase 1 Ngong Ping Sewage Treatment Works and Sewerage, Project Profile.

5.7.2 DSD, Agreement No. CE29/2001 Outlying Island Sewerage Stage 1 Phase 1 Ngong Ping Sewage Treatment Works and Sewerage, Study Brief.

5.7.3 Maunsell Asia Consultants Limited. Outlying Island Sewerage - Stage 1 Phase 1 Package B - Siu Ho Wan STW Upgrading. Report on Disinfection Pilot Plant Study March 2000.

5.7.4 Montgomery Watson, Agreement No. 33/98 Outlying Islands Sewage Master Plan Stage 2 Review. Environmental Review of Ngong Ping Sewerage Project. April 2001.

5.7.5 Scott Wilson (Hong Kong) Limited. Agreement No. CE 41/98 Tai O Sheltered Boat Anchorage Environmental and Drainage Impact Assessment. Final Assessment Report. Prepared for Civil Engineering Department. May 2000.

5.7.6 Tam, N.F.Y, Li, S.H., Lan, C.Y., Chen, G.Z., Li, M.S. and Wong, Y.S. (1995). Nutrients and heavy metal contamination of plants and sediments in Futien mangrove forest. Hydrobiologia, 295: pp149-158.

5.7.7 Tam, N.F.Y. and Wong, Y.S. (1995). Mangrove soils as sinks for wastewater-borne pollutants. Hydrobiologia, 295: pp231-241.

5.7.8 Tam, N.F.Y. and Wong, Y.S. (1997). Ecological Study on Mangrove Stands in Hong Kong - Volumes I and V. Agriculture & Fisheries Department.

5.7.9 Wong, Y.S., Lan, C.Y., Chen, G.Z., Li, S.H., Chen, X.R. Liu, Z.P. and Tam, N.F.Y. (1995). Effect of wastewater discharge on nutrient contamination of mangrove soils and plants. Hydrobiologia, 295: pp243-254.

5.7.10 Wong, Y.S., Chen, G.Z., Ma, H. and Tam, N.F.Y. (1996(a)). Tolerance of Aegiceras corniculatum plants to synthetic sewage of different strength. Asia Pacific Conference on Science and Management of the Coastal Environment, 25-28 June 1996. Programme & Abstracts: pp279.

5.7.11 Wong, Y.S., Tam, N.F.Y., Lan, C.Y. and Chen, N.C. (1996(b)). Mangrove wetland ecosystems for wastewater treatment - Fieldwork and tide-tank experiments. Asia Pacific Conference on Science and Management of the Coastal Environment, 25-28 June 1996. Programme & Abstracts: pp76-77.

Flow rate (m3/day) Determinant	<10	>10 and <10	>100 and <500	>500 and <1000	>1000 and <2000
pH (pH units) Temperature (oC) Colour (lovibond units) (25mm cell length) Conductivity (µ s/cm at 20 oC) Suspended solids Dissolved oxygen BOD COD Oil & Grease Boron Barium Iron Arsenic Total chromium Mercury Cadmium Selenium Copper Lead Manganese Zinc Other toxic metals individually Total toxic metals Cyanide Phenols Hydrogen sulphide Sulphide Fluoride Sulphate Chloride Total reactive phosphorus Ammonia nitrogen Nitrate + nitrite nitrogen E. coli (count/100 ml)	6.5-8.5 35 1 1000 10 >4 10 50 1 2 2 2 0.05 0.05 0.001 0.001 0.01 0.2 0.1 0.5 1 0.1 0.3 0.05 0.1 0.05 0.2 1 800 800 1 1 15 <1	6.5-8.5 35 1 1000 10 >4 10 50 1 2 2 2 0.05 0.05 0.001 0.001 0.01 0.2 0.1 0.5 1 0.1 0.3 0.05 0.1 0.05 0.2 1 600 500 0.7 1 15 <1	6.5-8.5 30 1 1000 5 >4 5 20 1 1 1 1 0.05 0.05 0.001 0.001 0.01 0.2 0.1 0.5 1 0.1 0.2 0.05 0.1 0.05 0.1 1 500 500 0.7 1 15 <1	6.5-8.5 30 1 1000 5 >4 5 20 1 0.5 0.5 0.5 0.05 0.05 0.001 0.001 0.01 0.2 0.1 0.5 1 0.1 0.2 0.05 0.1 0.05 0.1 1 400 200 0.5 1 10 <1	6.5-8.5 30 1 1000 5 >4 5 20 1 0.5 0.5 0.5 0.05 0.05 0.001 0.001 0.01 0.1 0.1 0.5 1 0.1 0.15 0.02 0.1 0.05 0.1 0.5 200 200 0.5 0.5 10 <1