Expansion of Sha Tau Kok Sewage Treatment Works 5. WATER QUALITY IMPACT ASSESSMENT 5.1 Introduction 5.1.1 This Section describes the baseline conditions and potential impacts on marine water quality from the construction and operation of the Project at Sha Tau Kok (STK). Mathematical modelling was used to predict potential impacts to water quality, and the predictions were assessed with reference to the relevant environmental legislation, standards and tolerance criteria. 5.2 Legislation Requirement & Guidelines 5.2.1 The following relevant legislation and associated guidance are applicable to the evaluation of water quality impacts associated with the Project: a) Water Pollution Control Ordinance (WPCO); b) Technical Memorandum for Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal Waters (TM- ICW); c) Environmental Impact Assessment Ordinance (Cap. 499. S.16) and the Technical Memorandum on EIA Process (EIAO-TM), Annexes 6 and 14; and d) Practice Note for Professional Persons, Construction Site Drainage (ProPECC PN1/94). Water Pollution Control Ordinance 5.2.2 The Water Pollution Control Ordinance (WPCO) is the primary legislation for the control of water pollution and water quality in Hong Kong. Under the WPCO, Hong Kong waters are divided into 10 Water Control Zones (WCZs). Each WCZ has a designated set of statutory Water Quality Objectives (WQOs). 5.2.3 The Study Area will cover Mirs Bay WCZs. The applicable WQOs for the Mirs Bay WCZ are presented in Table 5.1 and used in the following water quality impact assessment. Table 5.1 Water Quality Objectives Applicable to the Mirs Bay WCZ Water Quality Objective Applicable Area A AESTHETIC APPEARANCE a) Waste discharges shall cause no objectionable odours or Whole zone discolouration of the water. b) Tarry residues, floating wood, articles made of glass, plastic, rubber Whole zone or of any other substances should be absent. c) Mineral oil should not be visible on the surface. Surfactants should Whole zone not give rise to lasting foam. d) There should be no recognisable sewage-derived debris Whole zone e) Floating, submerged and semi-submerged objects of a size likely to Whole zone interfere with the free movement of vessels, or cause damage to vessels, should be absent. f) Waste discharges shall not cause the water to contain substances Whole zone which settle to form objectionable deposits. B BACTERIA a) The level of Escherichia coli should not exceed 610 per 100 milligrams Secondary Contact per litre, calculated as the geometric mean of all samples collected in Recreation Subzone one calendar year. & Fish Culture Zones b) The level of Escherichia coli should be zero per 100 ml, calculated as Water Gathering the running median of the most recent 5 consecutive samples taken at Ground Subzones intervals of between 7 and 21 days. 5-1 B&V / ERM Expansion of Sha Tau Kok Sewage Treatment Works Water Quality Objective Applicable Area (c) The level of Escherichia coli should not exceed 1000 per 100 ml, Other inland waters calculated as the running median of the most recent 5 consecutive of the Zone samples taken at intervals of between 7 and 21 days. C COLOUR a) Waste discharges shall not cause the colour of water to exceed 30 Water Gathering Hazen units. Ground Subzones b) Waste discharges shall not cause the colour of water to exceed 50 Other inland waters Hazen units. of the Zone D DISSOLVED OXYGEN a) Waste discharges shall not cause the level of dissolved oxygen to fall Marine waters below 4 mg per litre for 90% of the sampling occasions during the excepting Fish year; values should be calculated as water column average. In Culture Subzones addition, the concentration of dissolved oxygen should not be less than 2 milligrams per litre within 2 metres of the seabed for 90% of the sampling occasions during the year. b) The dissolved oxygen level should not be less than 5 milligrams per Fish Culture litre for 90% of the sampling occasions during the year; values should Subzones be calculated as water column average (arithmetic mean of at least 3 measurements at 1 metre below surface, mid-depth and 1 metre above seabed). In addition, the concentration of dissolved oxygen should not be less than 2 milligrams per litre within 2 metres of the seabed for 90% of the sampling occasions during the year. c) Waste discharges shall not cause the level of dissolved oxygen to be Inland waters of the less than 4 milligrams per litre. Zone E pH a) The pH of the water should be within the range of 6.5 - 8.5 units. In Marine waters addition, waste discharges shall not cause the natural pH range to be extended by more than 0.2 units. b) Waste discharges shall not cause the pH of the water to exceed the Water Gathering range of 6.5-8.5 units. Ground Subzones c) The pH of the water should be within the range of 6.0 - 9.0 units. Other inland waters of the Zone F TEMPERATURE Waste discharges shall not cause the natural daily temperature range Whole zone to change by more than 2.0 °C. G SALINITY Waste discharges shall not cause the natural ambient salinity level to Whole zone change by more than 10%. H SUSPENDED SOLIDS a) Waste discharges shall neither cause the natural ambient level to be Marine waters raised by 30% nor give rise to accumulation of suspended solids which may adversely affect aquatic communities. b) Waste discharges shall not cause the annual median of suspended Water Gathering solids to exceed 20 milligrams per litre. Ground Subzones and Other inland waters of the Zone I AMMONIA The un-ionized ammoniacal nitrogen level should not be more than Whole zone 0.021 milligram per litre, calculated as the annual average (arithmetic mean). J NUTRIENTS 5-2 B&V / ERM Expansion of Sha Tau Kok Sewage Treatment Works Water Quality Objective Applicable Area a) Nutrients shall not be present in quantities sufficient to cause Whole zone excessive or nuisance growth of algae or other aquatic plants. b) Without limiting the generality of objective (a) above, the level of inorganic nitrogen should not exceed 0.3 milligram per litre, expressed as annual water column average (arithmetic mean of at least 3 measurements at 1m below surface, mid-depth and 1m above seabed). K 5-DAY BIOCHEMICAL OXYGEN DEMAND a) Waste discharges shall not cause the 5-day biochemical oxygen Water Gathering demand to exceed 3 milligrams per litre. Ground Subzones b) Waste discharges shall not cause the 5-day biochemical oxygen Other inland waters demand to exceed 5 milligrams per litre. of the Zone L CHEMICAL OXYGEN DEMAND a) Waste discharges shall not cause the chemical oxygen demand to Water Gathering exceed 15 milligrams per litre. Ground Subzones b) Waste discharges shall not cause the chemical oxygen demand to Other inland waters exceed 30 milligrams per litre. of the Zone M TOXINS a) Waste discharges shall not cause the toxins in water to attain such Whole zone levels as to produce significant toxic, carcinogenic, mutagenic or teratogenic effects in humans or fish or any other aquatic organisms, with due regard to biologically cumulative effects in food chains and to toxicant interactions with each other. b) Waste discharges shall not cause a risk to Whole Zone any beneficial Whole zone uses of the aquatic environment. Note: (a) Reference: Statement of Water Quality Objectives (Mirs Bay Water Control Zone) Technical Memorandum for Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal Waters (TM-ICW) 5.2.4 All discharges during both the construction and operation phases of the Project are required to comply with the TM-ICW issued under Section 21 of the WPCO. 5.2.5 The TM-ICW defines acceptable discharge limits to different types of receiving waters. Under the TM-ICW, effluents discharged into the drainage and sewerage systems, inshore and coastal waters of the WCZs are subject to pollutant concentration standards for specified discharge volumes. These are defined by the Environmental Protection Department (EPD) and are specified in licence conditions for any new discharge within a WCZ. 5.2.6 The TM-ICW also prohibits new effluent (1) within 100m of the boundaries of a gazetted beach in any direction, including rivers, streams and storm water drains, (2) within 200m of the seaward boundaries of a marine fish culture zone or a site of special scientific interest, and within 100m of the landward boundaries (3) in any typhoon shelter, (4) in any marina and (5) within 100m of a seawater intake point. As shown in Figure 5.1, there is no gazetted beach, site of special scientific interest, typhoon shelter, marina and existing seawater intake within 100m from the proposed submarine outfall. The nearest fish culture zone at Sha Tau Kok is more than 500 m away. However, the outfall of the existing STKSTW, which would also be used by the temporary sewage treatment plant (TSTP), is already located within 500 m from the Sha Tau Kok Fish Culture Zone. Environmental Impact Assessment Ordinance (Cap. 499. S.16) and the Technical Memorandum on EIA Process (EIAO-TM), Annexes 6 and 14 5-3 B&V / ERM Expansion of Sha Tau Kok Sewage Treatment Works 5.2.7 Annexes 6 and 14 of the EIAO-TM provide general guidelines and criteria to be used in assessing water quality impacts. 5.2.8 The EIAO-TM recognizes that, in the application of the above water quality criteria, it may not be possible to achieve the WQO at the point of discharge as there are areas which are subjected to greater impacts (which are termed by the EPD as the mixing zones) where the initial dilution of the discharge takes place.