6 WATER POLLUTION

6.1 Introduction

6.1.1.1 This section presents an assessment of the potential water quality impacts associated with the construction and operation phases of the proposed STF.  Recommendations for mitigation measures have been made, where necessary, to reduce the identified water quality impacts to an acceptable level.

6.2 Environmental Legislation and Standards

6.2.1 Water Pollution Control Ordinance (WPCO)

6.2.1.1 The Water Pollution Control Ordinance (Cap. 358) is the major legislation relating to the protection and control of water quality in Hong Kong.  According to the Ordinance and its subsidiary legislation, Hong Kong waters are divided into ten water control zones (WCZ).  Corresponding statements of Water Quality Objectives (WQO) are stipulated for different water regimes (marine waters, inland waters, bathing beaches subzones, secondary contact recreation subzones and fish culture subzones) in the WCZ based on their beneficial uses.  The study area is located within the Deep Bay WCZ and the corresponding WQO are listed in Table 6.1.

Table 6.1      Summary of Water Quality Objectives for Coastal Waters of Deep Bay Water Control Zone

Parameter	Objective	Part(s) of Zone
E. coli	annual geometric mean not to exceed 610 cfu/100 ml	secondary contact recreation subzone mariculture subzone
Dissolved Oxygen within 2 m of bottom	not less than 2 mg/L for 90% samples	outer marine subzone except mariculture subzone
Depth averaged dissolved oxygen	not less than 4 mg/L for 90% samples
Dissolved Oxygen at 1 m below surface	not less than 4 mg/L for 90% samples	inner marine subzone except mariculture subzone
pH value	within the range 6.5 to 8.5; change due to waste discharge not to exceed 0.2	marine waters
Salinity	change due to waste discharge not to exceed 10% of natural ambient level	whole zone
Temperature	change due to waste discharge not to exceed 2^oC	whole zone
Suspended solids	waste discharge not to raise the natural ambient level by 30%, nor cause the accumulation of suspended solids which may adversely affect aquatic communities	marine waters
Nutrients	not to be present in quantities that cause excessive growth of  algae or other aquatic plants	Inner and outer marine subzones
	annual mean depth-averaged inorganic nitrogen not to exceed 0.5 mg/L	Outer marine subzone
	annual mean depth-averaged inorganic nitrogen not to exceed 0.7 mg/L	Inner marine subzone
Unionized Ammonia	annual average should not exceed 0.021 mg/L	whole zone

6.2.2 Technical Memorandum

6.2.2.1 Besides setting the WQOs, the WPCO controls effluent discharging into the WCZs through a licensing system.  A Technical Memorandum (TM) on Standards for Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal Waters has been issued under the WPCO which gives guidance on the permissible effluent discharges based on the type of receiving waters (foul sewers, storm water drains, inland and coastal waters).  The limits control the physical, chemical and microbial quality of effluents.  Any effluent discharges during the construction and operational stages should comply with the standards for effluents discharged into the coastal waters of the Deep Bay WCZ, as shown in Table 8 of the TM.

6.2.2.2 In addition to the provisions of the TM, the ‘No Net Increase Requirement in Pollution Loading’ aims to provide protection to the inland and marine water quality of the Deep Bay WCZ.  The policy requires that developments within the Deep Bay catchment areas do not result in an increase in pollution loads to the inland and marine waters.

6.2.3 Practice Notes

6.2.3.1 A practice note (PN) for professional persons has been issued by the EPD to provide environmental guidelines for handling and disposal of construction site discharges.  The ProPECC PN 1/94 “Construction Site Drainage” provides good practice guidelines for dealing with various types of discharge from a construction site.  Practices outlined in the PN should be followed as far as possible during construction to minimize the water quality impact due to construction site drainage.

6.3 Baseline Conditions

6.3.1 Inland Waters

6.3.1.1 The construction of the STF and the associated access road has the potential to affect the inland watercourse of Tsang Kok stream within the Deep Bay WCZ.  There is one EPD routine water quality monitoring station (DB8) along the Tsang Kok Stream (see Figure 6.1).  A summary of the published monitoring data (in 2006) for this station is presented in Table 6.2.

Table 6.2      Summary of Water Quality Monitoring Results for Tsang Kok Stream in 2006

Parameter	Unit	DB8
Dissolved oxygen	mg/L	9.5 (7.4 – 10.9)
pH	-	7.9 (6.5 – 9.0)
Suspended solids	mg/L	4 (2 – 88)
5-day Biochemical Oxygen Demand	mg/L	1 (1 – 2)
Chemical Oxygen Demand	mg/L	4 (2 – 9)
Oil & grease	mg/L	0.5 (0.5 – 0.5)
Faecal coliforms	cfu/100mL	5400 (1900 – 17000)
E.coli	cfu/100mL	310 (31 – 3600)
Ammonia-nitrogen	mg/L	0.02 (0.01 – 0.08)
Nitrate-nitrogen	mg/L	0.25 (0.09 – 0.98)
Total Kjeldahl nitrogen, SP	mg/L	0.12 (0.06 – 0.35)
Ortho-phosphate	mg/L	0.01 (0.01 – 0.02)
Total phosphorus, SP	mg/L	0.02 (0.02 – 0.03)
Sulphide, SP	mg/L	0.02 (0.02 – 0.08)
Aluminium	µg/L	120 (50 – 290)
Cadmium	µg/L	0.1 (0.1 – 0.1)
Chromium	µg/L	1 (1 – 1)
Copper	µg/L	1 (1 – 2)
Lead	µg/L	1 (1 – 20)
Zinc	µg/L	10 (10 – 20)
Flow	L/s	NM

Notes:        

1. Data presented are in annual medians of monthly samples, except those for faecal coliforms and E.coli which are in annual geometric means. Figures in brackets are annual ranges.

2. SP – soluble and particulate fractions (i.e. total value).

3. NM indicates no measurement taken

6.3.1.2 River water quality monitoring data for the year 2006 at station DB8 showed the overall compliance rate of the Tsang Kok stream with the WQOs was 100%, with full compliance with the WQOs of pH, suspended solids, dissolved oxygen, COD and BOD₅.  The water quality of this minor stream was reported to be excellent and free from point source pollution.

6.3.1.3 A description of the physical characteristics of the stream course is presented under the ecological section in Section 7.

6.3.2 Marine Waters

6.3.2.1 The EPD water quality monitoring station DM4 in the Deep Bay WCZ is the nearest monitoring station in the vicinity of the Project area (see Figure 6.1).  A summary of the published monitoring data (in 2006) for this station is presented in Table 6.3.

Table 6.3      Summary Statistics of Marine Water Quality in the Deep Bay WCZ at Station DM4

Parameter	EPD Monitoring Station DM4	Water Quality Objectives (in marine waters)
Temperature (^oC)	24.7 (19.2 – 29.3)	Natural daily level ± 2 ^oC
Salinity (ppt)	23.9 (14.2 – 30.8)	natural ambient level ± 10 %
Dissolved Oxygen (DO) (mg L^-1)	5.7 (4.0 – 6.6)	³ 4 mg L^-1
Dissolved Oxygen (DO) – Bottom (mg L^-1)	5.6 (3.8 – 6.7)	³ 2 mg L^-1
DO (% saturation)	78 (57 – 85)	-
DO –Bottom (% saturation)	77 (55 – 88)	-
pH value	7.6 (7.2 – 8.1)	6.5 - 8.5 (± 0.2 from natural range)
Secchi disc (m)	0.5 (0.2 – 1.2)	-
Turbidity (NTU)	19.5 (11.0 – 33.0)	-
SS (mg L^-1)	13.5 (3.0 – 35.1)	£ natural ambient level + 30%
Silica (as SiO2) (mg L^-1)	2.8 (0.7 – 6.3)	-
BOD5 (mg L^-1)	0.9 (0.3 – 1.5)	-
Nitrite Nitrogen (mg L^-1)	0.138 (0.096 – 0.215)	-
Nitrate Nitrogen (mg L^-1)	0.54 (0.25 – 0.99)	-
Ammonia Nitrogen (mg L^-1)	0.41 (0.19 – 0.76)	-
Unionised Ammonia (mg L^-1)	0.009 (0.002 – 0.016)	£ 0.021 mg L^-1
Total Inorganic Nitrogen (mg L^-1)	1.10 (0.80 – 1.40)	£  0.5 mg L^-1
Total Kjeldahl Nitrogen (mg L^-1)	0.64 (0.43 – 0.96)	-
Total Nitrogen (mg L^-1)	1.30 (0.96 – 1.69)	-
Orthophosphate Phosphorus (mg L^-1)	0.06 (0.02 – 0.09)	-
Total Phosphorus (mg L^-1)	0.08 (0.05 – 0.11)	-
Chlorophyll-a (µg L^-1)	3.84 (1.0 – 13.0)	-
E. coli (cfu per 100 mL)	740 (150 – 2900)	< 610 cfu per 100 mL
Faecal Coliform (cfu per 100 mL)	1400 (270 – 4100)	-

Notes:

1. Data source: EPD (2006). Marine Water Quality In Hong Kong in 2006.

2. Except as specified, data presented are depth-averaged data.

3. Data presented are arithmetic means except for E. coli and faecal coliforms that are geometric means.

4. Data enclosed in brackets indicate ranges.

6.3.2.2 According to the “2006 Marine Water Quality in Hong Kong”, water quality monitoring data for station DM4 met the WQOs for dissolved oxygen (bottom and depth-averaged) and unionized ammonia.  Non-compliance was recorded with the WQO for total inorganic nitrogen which was reported to be the result of a persistent nutrient pollution problem in Deep Bay.  An increase in the level of E.coli (annual geometric mean) was reported with non-compliance with the WQO.

6.4 Water Sensitive Receivers

6.4.1.1 Two moderate sized streams are located near to the proposed Project site at the eastern part of the ash lagoon area and discharge into a tidal channel to the east of the ash lagoon area (refer to Figure 7.1).   The lower reaches of stream W1 are routed through a man-made, tidally influenced channel to the south of the ash lagoons. Although the substrate of this channel is natural, the banks have been lined with geo-textile matting.  The second stream (stream W2) drains into the tidal channel from the southeast.  The section of stream flowing through the existing WENT Landfill site has been wholly channelized with concrete.

6.4.1.2 Apart from the coastal waters of Deep Bay, no other marine sensitive receivers within the vicinity of the Project area were identified.  The STF will adopt a “zero-discharge” scheme and no process effluent will be discharged into Deep Bay during the operational phase. 

6.5 Assessment Methodology

6.5.1.1 The Assessment Area as specified in the EIA Study Brief covers an area within 300m of the Project site boundary, and all relevant water sensitive receivers downstream of any emergency bypass from the proposed sewage treatment works within STF, including the existing cooling water system of Black Point Power Station.

6.5.1.2 The water sensitive receivers that may be affected by the various construction activities for the STF were identified.  Potential sources of water quality impact that may arise during the construction phase of the Project were described.  This task included identifying pollutants from point discharges and non-point sources to surface water run-off.  All the identified sources of potential water quality impact were then evaluated and their impact significance determined.  The need for mitigation measures to reduce any identified adverse impacts on water quality to acceptable levels was determined.

6.5.1.3 The assessment of operational stage water quality impacts addressed the following potential areas of concern: any proposal for on-site wastewater treatment plant(s); analysis on the adequacy of existing and future sewerage infrastructure; any proposal for upgrading or providing effective sewerage infrastructure; possible emission of microbes associated with the transportation, storage and handling of dewatered sewage sludge into surrounding waterbodies; and any thermal discharge from cooling water system. 

6.6 Identification and Evaluation of Environmental Impacts

6.6.1 Construction Phase

6.6.1.1 Potential sources of water quality impact associated with the construction phase of the proposed STF have been identified and include:

l Drainage and construction site runoff during site formation and foundation piling;

l General construction activities;

l Sewage effluent produced by on-site workforce; and

l Release of PFA leachate from ash lagoon into the aquatic environment.

Drainage and Construction Site Runoff

6.6.1.2 Runoff from the construction works area may contain increased loads of sediments, other suspended solids and contaminants.  Potential sources of pollution from site drainage include:

l Runoff and erosion from exposed soil surfaces, earth working areas and stockpiles;

l Release of grouting and cement materials with rain wash;

l Wash water from dust suppression sprays; and

l Fuel and lubricants from maintenance of construction vehicles and mechanical equipment.

6.6.1.3 Sediment laden runoff during site formation works, if uncontrolled, may carry pollutants (adsorbed onto the particle surfaces) into the nearby stream and coastal waters.

6.6.1.4 As a good site practice, mitigation measures should be implemented to control construction site runoff and drainage from the works areas, and to prevent runoff and drainage water with high levels of suspended solids from entering the nearby water bodies.  With the implementation of adequate construction site drainage and provision of sediment removal facilities as described in Section 6.7.2.1, it is anticipated that unacceptable water quality impacts would not arise.  The construction site drainage would be collected by the temporary drainage system installed by the Contractor and then treated on-site before discharging into the sea via silt removal facilities.  Water pumped out from foundation piling would also be discharged into the sea via silt removal facilities.  The Contractor would be required to obtain a license from EPD for discharge to the coastal waters.

General Construction Activities

6.6.1.5 On-site construction activities may cause water pollution as shown below:

l Uncontrolled discharge of debris and rubbish such as packaging, construction materials and refuse; and

l Spillages of liquids stored on-site, such as oil, diesel and solvents etc, are likely to result in water quality impacts if they enter water bodies.

6.6.1.6 Good construction and site management practices should be implemented, as detailed in Section 6.7.2.2 and Section 6.7.2.3, to ensure that litter, fuels and solvents do not enter the nearby stream and coastal waters.   

Sewage Effluent

6.6.1.7 Domestic sewage would be generated from the workforce during the construction phase.  However, this sewage can be adequately treated by interim sewage treatment facilities, such as portable chemical toilets, which can be installed within the construction site.  It is unlikely that sewage generated from the site would have a significant water quality impact, provided that sewage is not discharged directly to the stream or the public drainage system, and chemical toilets are used and properly maintained.

Release of PFA Leachate from Ash Lagoon into the Aquatic Environment

6.6.1.8 The proposed STF will be located in the eastern part of the existing ash lagoon area.  The ash lagoons were constructed in the mid- to late 1980’s and were divided by bunds into the East, Middle and West Lagoons.  Since 1989, the lagoons have been used for the storage of PFA, a by-product of the coal-burning. PFA is a fine, grey powder formed from the rock particles contained within coal, consisting mainly of silica, alumina and iron oxide. 

6.6.1.9 The Middle and West Ash Lagoons are still used for the storage of PFA.  In 1997, China Light and Power (CLP) began to use the Middle Lagoon as part of its water collection and conservation system.  The ash lagoon area is underlain by marine deposits which consist of fine grained material.  Alluvium is present underneath the marine deposits.  Depths of alluvium may vary from approximately 4.0 to 19.0m.  The layer of alluvium is underlain by completely decomposed granite (CDG) with depths ranging from approximately 3.5 to 15.2m.[1]

6.6.1.10 There is liner, which was constructed of cementitious materials, at the bottom of the ash lagoons.  However, in view of the nature of the liner, it is highly likely that this layer would have already been cracked / broken under the loadings imposed by the existing ash at the lagoons.

6.6.1.11 The marine deposits in the sea wall location have been removed prior to the sea wall construction. In order to prevent leakage of PFA leachate through the sea wall to Deep Bay, filter layers are laid underneath the amour stone on the inner face of the sea wall.  On the seaward side of the outer sea wall, armour stone and wave wall are provided to resist the storm effects.

6.6.1.12 During construction phase of the Project, piling would be applied for foundation construction.  The piles would penetrate through the base of the East Lagoon to the hard granite bedrock to support the facility and the soil layer underneath the lagoon would be disturbed.  However, the piling activities are unlikely to cause significant changes in geological structure of the lagoon site.  The present of piles would restrict the movement of groundwater in the soil layer.  The liner underneath the East Lagoon would have already been creaked / broken.  The underground conditions at the base on the East Lagoon are expected to be rather stable.  Leakage of PFA leachate through the base of the East Lagoon to Deep Bay after the pile construction, if any, would not be much different from the existing condition.

6.6.1.13 To evaluate the potential impacts of the PFA leachate to the nearby aquatic environment, the chemical characteristics of the PFA leachate and chemical toxicity data for aquatic life have been reviewed.  The PFA leaching trial using seawater was conducted by Scott Wilson Kirkpatrick (1991)[2].  The leaching trial result showed that the metals contents in the PFA varied with the type of coal and the length of PFA aging.  Only low concentrations of potential contaminants were leached into seawater solution.  The contaminants with the greatest tendency to leach into solution were found to be cadmium, chromium and aluminium.  Fresh PFA tended to leach more metals compared to the lagooned PFA and was more variable among various coal types.  Results from the lagooned PFA showed smaller variations and metal leaching was more consistent.

6.6.1.14 Table 6.4 shows the concentrations of different parameters from the lagooned PFA leaching trials.  The major heavy metals released from the lagooned PFA were aluminium and chromium, with maximum concentrations of 900 and 300 mg/l respectively.  The maximum cadmium concentration measured in the leaching trials was 4 mg/l.  There was an uncertainty of the actual concentration of copper and nickel released from lagooned because of the high reporting limits.  The analytical instrument for the seawater solution in the leaching trials was only available to detect copper concentration higher than 75mg/l and nickel higher than 25mg/l.

Table 6.4      Comparison of Leaching Trial Results with the Background Levels and USEPA Water Quality Standards

Parameter	Leaching Trial Results (mg/l)	Background Concentration (mg/l) ^Note 1	USEPA Water Quality Standard for Saltwater (mg/l)
Aluminium	900	132	n/a
Chromium	300	1.5	50 (210) ^Note 2
Cadmium	4	< 0.05	9.3
Copper	<75	< 5	2.9
Zinc	30	6	86
Nickel	<25	< 5	8.3
Iron	20	145	n/a
Lead	6	0.9	8.5
Manganese	3	17.5	n/a
Selenium	14	< 1	71
Arsenic	3	1.2	36

Notes:

1. The background concentrations were based on the results measured around Black Point and Tap Shek Kok abstracted from Scientific Series, Chemical Analysis Report 20/91

2. The value of 50 mg/l represents the standard for Chromium (VI) in saltwater and there is no standard for Chromium (III) in saltwater.  The criterion for Chromium (III) in freshwater is 210 mg/l.

6.6.1.15 Table 6.4 also provided the concentration of trace metals measured around Black Point and Tap Shek Kok in 1991 as background concentration, as well as USEPA Water Quality Standards for Saltwater.  Concentrations for aluminium, iron and manganese are not available in the USEPA standard.  Comparison result of the leaching trial of these three parameters with the background concentration presented that both iron and manganese were below the background concentration, whiles aluminium concentration is about 7 times higher than background concentration.  When diluted by the ambient seawater, the aluminium concentration would be indistinguishable from the background level within a short distance from the release point.

6.6.1.16 To compare other parameters with the USEPA standard, the concentrations of all metals except chromium are below the USEPA standards.  In the event that release of PFA leachate occurs, the potential water quality would be low.  In fact, the ash would remain in the East Lagoon and there would be no off-site disposal of ash into the aquatic environment.

6.6.1.17 A monitoring programme was conducted by CLP between 1987 and 1988 to monitor water quality at the location outside the Middle Lagoon.  The monitoring result is shown in Table 6.5, indicating that there were no significant difference between the trace metal results measured outside the Middle Lagoon and at oyster buoy and farm in Deep Bay.  To compare these monitoring results with the background concentrations of trace metals measured around Black Point and Tap Shek Kok measured in 1991, no evidence shows that the operation of the Tsang Tsui Lagoons has caused adverse water quality impacts in the vicinity of the lagoon site.

Table 6.5      Results of CLP monitoring programme between 1987 and 1988

Parameters	Monitored Average Concentrations (mg/l)		Background Concentration (mg/l)
	At Location Immediately Outside the Middle Lagoon (Sep 1988 – Jan 1989)	At Oyster Buoy and Farm in Deep Bay (Jun 1987 – Nov 1987)
Cadmium	0.09	0.41	< 0.05
Copper	3.6	2.1	< 5
Lead	2.3	1	0.9
Zinc	7	23	6
Arsenic	< 5	5.1	1.2
Selenium	< 10	No data available	< 1

6.6.1.18 Chemical toxicity data for aquatic life have been reviewed in order to evaluate the potential impacts of the PFA leachate.  High concentrations of heavy metals can be detrimental to aquatic life.  The effects of high concentration of metals may cause the changes in tissues, growth rates, blood chemistry, behaviour and reproduction of aquatic organisms.  Fish can excrete excess heavy metals but bivalves cannot regulate excess heavy metals resulting in metal accumulation in the tissues.

6.6.1.19 There are no relevant aquatic life criteria in Hong Kong.  The USEPA Aquatic Life Criteria (estuarine/coastal), which provide a general guide to assess the potential risk to the environment in the presence of excess metal, is applied to compare with the leaching trial results (as shown in Table 6.6).  The parameters of aluminium, chromium, iron and manganese are not available in the USEPA estuarine/coastal Aquatic Life Criteria.  Except the uncertainty due to the high reporting limits for copper and nickel, most of the listed heavy metal concentrations are lower than the criteria.

Table 6.6      Comparison of the Leaching Trial Results with the USEPA Aquatic Life Criteria

Parameter	Leaching Trial Results (mg/l)	USEPA Aquatic Life Criteria (mg/l)
Aluminium	900	-
Chromium	300	-
Cadmium	4	8
Copper	<75	2.9
Zinc	30	76.6
Nickel	<25	7.1
Iron	20	-
Lead	6	5.8
Manganese	3	-
Selenium	14	71
Arsenic	3	50

6.6.1.20 The water quality guidelines for general saltwater aquaculture uses adopted in the New Zealand Guidelines for Fresh and Marine Water Quality (Table 6.7) and the UK Water Quality Standards for the Protection of Saltwater Life (Table 6.8) are also applied to compare with the leaching trial results.  Concentrations of aluminium, chromium, copper, iron and selenium are higher than the New Zealand Guidelines, while concentrations of chromium, cadmium and copper are higher than the UK Standards.

6.6.1.21 Dilution for these metals could lower the concentrations to meet the New Zealand Guidelines and the UK Standards.  The estimated dilution rates are shown in Table 6.7 and Table 6.8.  In order to meet the New Zealand Guidelines, the highest dilution rate would be >90 for aluminium.  Chromium and copper would require a dilution rate of >15 and iron and selenium require a dilution rate of >2.  To meet the requirements of the UK Standards, the highest dilution rate is >20 for chromium, while required dilution rate for cadmium and copper are 1.6 and 20 respectively.  The nearest oyster beds at Pak Nai are approximately 3 km away from the lagoons.  The require dilutions are likely to be achieved for pollutants in the moving tidal current travelling for such a long distance.  It is anticipated that the potential impacts to the nearby oyster beds would be insignificant.

Table 6.7      Comparison of the Leaching Trial Results with the New Zealand Guidelines for Fresh and Marine Water Quality

Parameter	Leaching Trial Results (mg/l)	New Zealand Water Quality Guidelines for the Inorganic Chemicals (mg/l)	Required Dilution to Meet the Guidelines
Aluminium	900	< 10	> 90
Chromium	300	< 20	> 15
Cadmium	4	< 5	-
Copper	<75	< 5	> 15
Zinc	30	< 100	-
Nickel	<25	< 100	-
Iron	20	< 10	> 2
Lead	6	< 20	-
Manganese	3	< 100	-
Selenium	14	< 10	> 2
Arsenic	3	< 30	-

Source:  Australian and New Zealand Guidelines to Fresh and Marine Water Quality – Volume 1 (July 1999)

Table 6.8      Comparison of the Leaching Trial Results with the UK Water Quality Standards for the Protection of Saltwater Life

Parameter	Leaching Trial Results (mg/l)	UK Water Quality Standards (mg/l)	Required Dilution to Meet the Standards
Aluminium	900	-	-
Chromium	300	15	>20
Cadmium	4	2.5	>1.6
Copper	<75	5	>15
Zinc	30	40	-
Nickel	<25	30	-
Iron	20	1000	-
Lead	6	25	-
Manganese	3	-	-
Selenium	14	-	-
Arsenic	3	25	-

6.6.1.22 Table 6.9 lists the chemical toxicity data for aquatic life.  LC₅₀ (concentration at which 50% mortality occurs) of the heavy metals for the species that could be found in Deep Bay are present.  Based on the available data of the LC₅₀, exposure of polychaete worm to aluminium of 405 mg/L for 96 hours would cause 50% mortality.  The maximum concentration of aluminium (900 mg/L) detected in the leaching trials is higher than the reference concentration.  A dilution rate of 3 times of the initial concentration would reduce the maximum concentration of aluminium to around 300mg/L.  It is also observed that exposure of mytilus edulis to zinc of 30 mg/L for 14 days would cause 50% mortality.  A dilution rate of 3 times of the initial concentration of zinc (30 mg/L) is required to reduce the maximum concentration to around 10 mg/L.  This low dilution rate is likely to be achieved in a moving water environment.  The potential impact due to high concentration of aluminium would be insignificant.  The concentrations of other parameters from the leaching trials are much lower than the corresponding LC₅₀ concentrations.

6.6.1.23 As the leakage through the base of the East Lagoon would not be significant, the PFA leachate in the East Lagoon is unlikely to cause unacceptable impact on the aquatic environment from an ecotoxicological point of view.  The site conditions of East Lagoon during construction and operational phases would not be much different from the existing conditions.  As the ash would remain in the East Lagoon and would not be disposed of, detailed ecotoxicological assessment and additional toxicity test are considered not necessary.

Table 6.9         Chemical Toxicity Data for Aquatic Life

Parameter	Leaching Trial Results (mg/l)	Crassostrea gigas (Pacific Oyster)		Mytilus edulis (Common Bay Mussels)		Oryzias laptipes (Medala, high-eyes)		Scylla serrata (Green Crab)		Crangon crangon (Common Shrimps)		Artermia salina (Brine Shrimps)		Amphiphods		Polychaete Worm
		LC50	Conc. (mg/L)	LC50	Conc. (mg/L)	LC50	Conc. (mg/L)	LC50	Conc. (mg/L)	LC50	Conc. (mg/L)	LC50	Conc. (mg/L)	LC50	Conc. (mg/L)	LC50	Conc. (mg/L)
Aluminium	900	48h	1000000	-	-	-	-	-	-	-	-	3d	3100	-	-	96h	405
Chromium	300	-	-	-	-	96h	120000	-	-	48h	100000	24h 48h	5300 3540	-	-	96h	1000
Cadmium	4	4h 96h	85 19500	96h	960	48h	560000	-	-	96h	460	24h 48h	3100 1540	4d	14.5	10d 28d	83 39
Copper	<75	14h 96h	100 560	10d	45	24h 48h	610 410	-	-	48h	10000	24h 48h	800 440	-	-	4d 28d	77 44
Zinc	30	4d	100	14d	10	24h	20000	-	-	48h	100000	24h 48h	4460 1700	-	-	28d	350
Nickel	<25	-	-	-	-	-	-	-	-	48h	100000	48h	162985	-	-	7d 10d	7700 16090
Iron	20	-	-	-	-	24h	18500	-	-	48h	33000	-	-	-	-	-	-
Lead	6	-	-	105h 150h	5000 500	24h 48h	350000 205000	-	-	96h	63000	24h 48h	10000 5010	-	-	96h 28d	7660 1430
Manganese	3	-	-	-	-	24h	1000000	-	-	48h	3300	-	-	-	-	-	-
Selenium	14	-	-	-	-	-	-	24h 72h	68000 33000	-	-	-	-	-	-	-	-
Arsenic	3	21d	10	-	-	-	-	-	-	96h 192h	96000 70000	24h	1.3 umol/L	-	-	96h	7400

Note:

The media type of the underlined data is freshwater and the media type is marine water for the other data.

Source of information: Ecotox Database System

6.6.2 Operation Phase

6.6.2.1 Potential sources of water quality impact associated with the operation of the proposed STF have been identified and include:

l Wastewater generated from the operation of the STF;

l Discharge of saline water from the proposed desalination plant; and

l Possible emission of microbes associated with the transportation, storage and handling of dewatered sewage sludge.

Wastewater from the STF Operation

6.6.2.2 During the operation of the STF, wastewater would be generated from sanitary facilities used by plant personnel, commercial activities and sludge treatment activities in the STF plant.  The commercial activities in the STF mainly include the restaurant trade as there would be a canteen in the future development of STF. In addition, the source of wastewater from sludge treatment activities is mainly washdown water including plant washdown, flushing, and vehicle washing etc. Cooling water used for cooling the incinerator flue gas before entrance to the baghouse filter would be completely recycled within the incineration system.  Water used in the boiler in heat recovery system would be completely recycled within the incineration system or vaporized.  Thus there would not be any discharge of water from the facility.

6.6.2.3 There would be 60 employees working at the STF and the average flow of domestic wastewater generated would be approximately 13.8 m³ per day.  Moreover, the washdown water generated from the sludge treatment activities is estimated to be approximately 11.0 m³ per day.  Therefore, the total average sewage flow generated during the operation of the STF would be approximately 24.8 m³ per day.

6.6.2.4 According to the above estimation, an on-site wastewater treatment plant will be provided with a maximum capacity of 100 m³ per day.  All wastewater generated at the facility will be discharged to the on-site wastewater treatment plant and treated by the process of Membrane Bioreactor (MBR).  In case of emergency situation in which sewage needs to by-pass the treatment system, the sewage will be conveyed to a sewage holding tank with a storage capacity of 2-day average flow.

6.6.2.5 The treated effluent from the wastewater treatment plant would be reused for plant washdown, uncontrolled landscape irrigation, vehicular washing, toilet flushing and groundskeeping in the STF development, and there would be no wastewater effluent discharged to the coastal waters of Deep Bay.  Table 6.10 shows the summary of wastewater generated and reused in STF.

6.6.2.6 The water quality standards for the reclaimed water are set out based on other local effluent reuse schemes.  In Hong Kong, effluent reuse has been tried out in the Ngong Ping STW in Lantau Island and the Shek Wu Hui STW in North District. 

Table 6.10    Summary of Wastewater Generated and Reused in STF

	Quantity (m^3/d)
Wastewater Generated from STF
Employees activities	13.8
Sludge treatment activities	11.0
Total	24.8
Reclaimed Water Demand from STF
Landscape Irrigation ^Note 1	144.0
Plant washdown	11.0
Toilet flushing, vehicle washing and groundskeeping	4.3
Total	159.3

Note:

1.     The amount is estimated based on the approximate total landscape area of 12,000 m² and an assumption of evaporation rate of 12L/d/m².

6.6.2.7 The water quality standards set for the reuse of the Ngong Ping treated effluent (the Limiting Standards) were agreed in 2002.  The intended key usages were toilet flushing and controlled irrigation.  The parameters were then set taking into account of the tertiary treatment standard prescribed for the Ngong Ping STW, the standards adopted previously for salt water supply for flushing purpose in Hong Kong, as well as making reference to other overseas’ relevant standards.  This set of Limiting Standard was hence adopted for the design and construction of Ngong Ping STW.  As part of the 2002 study, it was further agreed that, in case if human contact with the treated effluent is expected (e.g. unrestricted irrigation), then it would be necessary for the turbidity to be tightened from 10 to 5 NTU, and also for the E. coli standard to be tightened from 100 cfu/100 ml to non-detectable level.

6.6.2.8 Apart from the Ngong Ping project, an additional trial was undertaken by EPD in 2004 as the Shek Wu Hui STW Demonstration Scheme which generated treated effluent for non-potable uses including toilet flushing, unrestricted irrigation, as well as ornamental water features such as fish pond.  As compared to the Ngong Ping scheme, the Shek Wu Hui STW Demonstration Scheme adopted a somewhat similar but more stringent set of criteria (the Adopted Criteria) making reference to the generic USEPA reuse standards as well as the “Title-22: California Water Recycling Criteria” established by the California Department of Health Services.  These standards are designed to provide the highest possible degree of treatment under varying circumstances.  For example, the turbidity requirement is as low as 2 NTU and the fecal coliform bacterial level is required to be non-detectable.

6.6.2.9 Table 6.11 summarizes the effluent reuse standards for the Ngong Ping project, the Shek Wu Hui STW Demonstration Scheme, USEPA and the proposed standards for this project.  This project adopts the effluent reuse standards for the Shek Wu Hui STW Demonstration Scheme.

Table 6.11    Summary of Effluent Reuse Standards for Local Effluent Reuse Schemes, USEPA and This Project

Parameter	Local Effluent Reuse Scheme		USEPA	This Project
	Ngong Ping	SWHSTW
pH	-	6-8	6 – 9	6-8
BOD (mg/L)	< 10	< 10	< 10	< 10
Turbidity (NTU)	< 10 ^Note1	< 2	< 2	< 2
TSS (mg/L)	< 10	-	-	-
Total Coliform/ 100ml	<100 ^Note1	Non detectable	Non detectable	Non detectable
Chlorine Residual (mg/L)	> 0.5	> 1	> 1	> 1

Note:

1. Turbidity and E-coli level were suggested to be lowered to <5 NTU and non detectable respectively in case if use for unrestricted irrigation were required i.e. human contact with the treated effluent and/or aerosol is likely.

Discharge of Saline Water from Desalination Plant

6.6.2.10 Approximately 1,000 m³/day of saline water would be discharged from the proposed desalination plant for domestic consumption, process water requirement and plant washdown, flushing, vehicle washing, groundskeeping and landscape irrigation for STF daily operation.  The characteristics of the saline water discharge are summarized below:

l pH will be raised from feed of 8.0 - 8.5 to reject of 8.2 - 8.8;

l Salinity will be raised from feed of 30,000 - 35,000 mg/L to reject of 50,000 - 58,333 mg/L;

l Suspended solids (SS) will be raised 1.7 times of feedwater (seawater); and

l Biochemical oxygen demand (BOD) will be concentrated 1.7 times of feedwater (seawater).

6.6.2.11 Given that the discharge would comprise concentrated saline water only, at a concentration of about 1.7 times of the feedwater (seawater), with a low discharge volume, adverse impacts on water quality would not be expected.  A comparison of the characteristics of the saline water discharge with the standards for effluents discharged into the coastal waters of Deep Bay Water Control Zone is given in Table 6.12 below.  

Table 6.12    Comparison of Saline Water Discharge from Desalination Plant with Effluent Discharge Standard

Parameter	Saline Water ^Note 1	Discharge Standard ^Note 2 & 3	Compliance with Discharge Standard
pH	< 8.8	6 – 9	Yes
Temperature (^oC)	25 – 35	45	Yes
Suspended solids (mg/L)	20.1	25	Yes
BOD 5-day (mg/L)	1.9	10	Yes

Note:

1. Feedwater assumed to have a pH of 8.1, SS of 11.8 mg/L and BOD of 1.1 mg/L, as based on water quality monitoring results of EPD monitoring station DM4 for the year 2004

2. Discharge standard for flow rate of >1000 and  1500 m3/day

3. The effluent discharge standards do not specify a standard for salinity

Possible Emission of Microbes Associated with Transportation, Storage and Handling of Dewatered Sewage Sludge

6.6.2.12 The possible emission of microbes associated with the transportation, storage and handling of dewatered sewage sludge into surrounding waterbodies has been addressed in Section 4 under the health risk assessment associated with the STF operation.  The following features have been incorporated into the design of the STF to prevent microbes entering the surrounding water bodies during the transportation, storage and handling process:

l Dewatered sludge would be transported in water-tight ISO-type shipping containers from the SCISTW and SHWSTW by marine vessel to the WENT Landfill pier;

l Dewatered sludge from the other regional STW would be transported by road in water-tight containers or skips, or in truck mounted container trucks, to the STF; and

l A drainage system would be provided at the receiving area of the STF to collect drainage water during cleaning of the floor area.  The drainage water would be routed to the on-site wastewater treatment plant.

6.6.2.13 As discussed in Section 4.3, the following “risk control measures” identified as existing/expected safeguards during the operation of the STF are also applicable as water pollution prevention measures:

l Detection device/alarm should be installed to prevent overfilling of temporary sludge storage tank;

l Frequent and sufficient maintenance should be provided for the drainage system of STF; and 

l Multiple outlets in drainage system should be designed and provided to reduce the likelihood of drainage blockage. 

6.7 Mitigation Measures

6.7.1 Introduction

6.7.1.1 Proposed mitigation measures for containing and minimizing water quality impacts are summarised below.

6.7.2 Construction Phase

Construction Site Run-off and Drainage

6.7.2.1 The site practices outlined in ProPECC PN 1/94 “Construction Site Drainage” should be followed as far as practicable in order to minimise surface runoff and the chance of erosion.  These practices include the following items:-

l At the start of site establishment, internal drainage works and erosion and sedimentation control facilities should be implemented.  Channels, earth bunds or sand bag barriers should be provided on site to direct stormwater to silt removal facilities.  The detailed design and installation of the temporary on-site drainage system should be undertaken by the contractor prior to the commencement of construction;

l Before commencing any site formation work, all sewer and drainage connections should be sealed to prevent debris, soil, sand etc. from entering public sewers/drains;

l Boundaries of earthworks should be surrounded by dykes or embankments for flood protection, as necessary; 

l Sand/silt removal facilities such as sand traps, silt traps and sediment basins should be provided to remove sand/silt particles from runoff to meet the standards of the Technical Memorandum under the Water Pollution Control Ordinance.  The design of silt removal facilities should be based on the guidelines provided in ProPECC PN 1/94.  All drainage facilities and erosion and sediment control structures should be inspected monthly and maintained to ensure proper and efficient operation at all times and particularly during rainstorms;

l Water pumped out from foundation piles must be discharged into silt removal facilities;

l During rainstorms, exposed slope/soil surfaces should be covered by a tarpaulin or other means, as far as practicable.  Other measures that need to be implemented before, during and after rainstorms are summarized in ProPECC PN 1/94;

l Exposed soil areas should be minimized to reduce potential for increased siltation and contamination of runoff;

l Earthwork final surfaces should be well compacted and subsequent permanent work or surface protection should be immediately performed;

l Open stockpiles of construction materials or construction wastes on-site should be covered with tarpaulin or similar fabric during rainstorms; and

l All vehicles should be cleaned before leaving the works area to ensure no earth, mud and debris is deposited on roads.  An adequately designed and sited wheel washing bay should be provided at every site exit.  The wheel washing facility should be designed to minimize the intake of surface water (rainwater).  Wash-water should have sand and silt settled out and removed at least on a weekly basis to ensure the continued efficiency of the process.

General Construction Activities

6.7.2.2 Debris and refuse generated on-site should be collected, handled and disposed of properly to avoid entering the nearby water bodies and public drainage system.  Stockpiles of cement and other construction materials should be kept covered when not being used.

6.7.2.3 Oils and fuels should only be used and stored in designated areas which have pollution prevention facilities.  To prevent spillage of fuels and solvents to nearby water bodies and public drains, all fuel tanks and storage areas should be provided with locks and be sited on sealed areas, within bunds of a capacity equal to 110% of the storage capacity of the largest tank.  The bund should be drained of rainwater after a rain event.

Sewage from Construction Workforce

6.7.2.4 Temporary sanitary facilities, such as portable chemical toilets, should be employed on-site where necessary to handle sewage from the workforce.  A licensed contractor would be responsible for appropriate disposal and maintenance of these facilities.

Release of PFA Leachate from Ash Lagoon into the Aquatic Environment

6.7.2.5 The past monitoring data showed that the water quality at the location outside the ash lagoon area was not affected by the PFA filling operation.  Although the liner installed at the base of the lagoons would have been creaked / broken, the low permeability values of the marine deposits and alluvium underneath the PFA layer would limit the seepage of PFA leachate.  The foundation construction of the STF is not likely to accelerate the release of PFA leachate through the base of the lagoon site.

6.7.2.6 Environmental monitoring and audit (EM&A) should be included to ensure that the foundation construction would not cause unacceptable release of PFA leachate into the Deep Bay waters.  The parameters to be measured should include the heavy metals such as cadmium, chromium and aluminium, which have the greatest tendency to leach from the lagooned PFA into the seawater.  Details of the measurement requirements are presented in the EM&A manual

6.7.3 Operation Phase

Receiving Area in the STF

6.7.3.1 An adequate number of drains should be provided at the receiving area of the STF with drain piping for draining and cleaning all areas of the floor.  The floors should be adequately sloped to floor drains for collection of drainage water during cleaning.  Such floor drains should contain no traps.  The drainage water should be routed to an outdoor vented and trapped manhole for connection to the on-site wastewater treatment plant.

Wash Down Facilities at STF

6.7.3.2 Frequent and sufficient maintenance should be provided for the drainage system, and multiple outlets in the drainage system should be designed and provided to reduce the likelihood of drainage blockage. 

Temporary Sludge Storage at STF

6.7.3.3 A detection device/alarm should be installed to prevent overfilling of temporary sludge storage tank.

6.8 Residual Environmental Impacts

6.8.1.1 With the full implementation of the recommended mitigation measures for the construction and operation phases of the proposed Project, no unacceptable residual impacts on water quality are expected.  It is recommended that regular audit of the implementation of the recommended mitigation measures at the work areas be carried out during the construction phase.  No wastewater effluent will be discharged into the Deep Bay WCZ during the operation of the STF and hence no residual water quality impact is envisaged.

6.9 Environmental Monitoring and Audit

6.9.1.1 To ensure no adverse water quality impact to the nearby stream due to the discharge of surface runoff and drainage from the works areas, water quality monitoring of the Tsang Kok stream is recommended during site formation and the widening of the access road.  Marine water quality monitoring is also recommended during foundation pilling of the STF to ensure that the foundation construction would not cause an unacceptable release of PFA leachate into the Deep Bay waters.  Details of the recommended water quality monitoring parameters to be measured and monitoring locations are provided in the stand-alone EM&A Manual for the Project.  It is also recommended that regular site inspections be undertaken to inspect the construction activities and works areas in order to ensure the recommended mitigation measures are properly implemented.

6.9.1.2 Monitoring of the discharge quality of effluent from land-based construction sites should be conducted by the Contractor.  A detailed effluent sampling programme for water quality control during the construction phase should be submitted to EPD for approval prior to commencement of the construction works. 

6.9.1.3 Monitoring of water quality would not be required during the operation phase of the STF as there would be no wastewater effluent discharge from the on-site wastewater treatment plant.

6.10 Conclusion

6.10.1.1 The potential sources of water quality impact arising during the construction phase of the Project include construction site runoff and drainage, wastewater generated from general construction activities and sewerage from the workforce.  With implementation of the recommended mitigation measures and site practices outlined in ProPECC PN 1/94, no unacceptable residual impacts on water quality are expected.

6.10.1.2 During the operation phase of the Project, wastewater will be generated from sanitary facilities used by plant personnel, commercial activities and sludge treatment activities in the STF.  An on-site wastewater treatment plant will be provided.  All generated wastewater will be discharged to the on-site wastewater treatment plant and treated by the process of MBR.  The treated effluent from the wastewater treatment plant will be reused in the STF and there would be no wastewater effluent discharged to the coastal waters of Deep Bay.

6.10.1.3 Saline water would be discharged from the proposed desalination plant in a low discharge rate.  The saline water discharged from the desalination plant will comply with the standards for effluents discharging into the coastal waters of Deep Bay Water Control Zone.  Adverse impacts on water quality would not be expected. 

6.10.1.4 To prevent potential emission of microbes during transportation, storage and handling of dewatered sewage sludge into surrounding waterbodies, proper design of the STF will be conducted and the recommended “risk control measures” will be implemented.  No unacceptable water quality impacts are expected.

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[1] Environmental Impact Key Issue Report on Tsang Tsui PFA Lagoon prepared by L. G. Mouchel and Partners (Asia) in association with others.

[2] Scott Wilson Kirkpatrick (1991). Privatisation of SENT Landfill – Results of PFA Leaching Trials.