Annexes 6 and 14 of the Technical Memorandum on Environmental Impact Assessment Process (EIAO-TM), issued under Section 16 of the Environmental Impact Assessment Ordinance (EIAO), outline the criteria and guidelines for assessing water quality impacts.

5.2.2 Water Pollution Control Ordinance

The Water Pollution Control Ordinance (WPCO), and its subsidiary legislation, provides the main statutory framework for the protection and control of water quality in Hong Kong. Water quality is controlled through a set of Water Quality Objectives (WQOs) defined for each of the ten demarcated Water Control Zones (WCZs) in Hong Kong. The WQOs determine the water quality that should be achieved and maintained to support beneficial uses such as marine waters, inland waters, bathing beach subzones, secondary contact recreation subzones and fish culture subzones.

In accordance with Section 3.4.5.2 of the EIA Study Brief (ESB-289/2015), consideration has been given to the requirements of the Deep Bay WCZ. The respective WQOs for this WCZ are presented in Table 5.1.

Table 5.1: Water Quality Objectives for Deep Bay WCZ

Water Quality Parameters	Objectives	Deep Bay WCZ Subzone
Offensive Odour, Tints	Not to be present	Whole zone
Colour	Not to exceed 30 Hazen units	Yuen Long & Kam Tin (Upper) Subzone, Beas Subzone, Indus Subzone, Ganges Subzone and Water Gathering Ground Subzones
Colour	Not to exceed 50 Hazen units	Yuen Long & Kam Tin (Lower) Subzone and other inland waters
Visible foam, oil scum, litter	Not to be present	Whole zone
Escherichia coli (E. coli)	Not to exceed 610 per 100mL, calculated as the geometric mean of the all samples taken in one calendar year	Secondary Contact Recreation Subzone and Maricultural Subzone
	Should be zero per 100mL, calculated as the running median of the most recent 5 consecutive samples taken at intervals between 7 and 21 days	Yuen Long & Kam Tin (Upper) Subzone, Beas Subzone, Indus Subzone, Ganges Subzone and Water Gathering Ground Subzones
	Not to exceed 1,000 per 100mL, calculated as the running median of the most recent 5 consecutive samples taken at intervals between 7 and 21 days	Yuen Long & Kam Tin (Lower) Subzone and other inland waters
	Not to exceed 180 per 100mL, calculated as the geometric mean of all samples collected from March to October inclusive in one calendar year. Samples should be taken at least 3 times in a calendar month at intervals of between 3 and 14 days	Yung Long Bathing Beach Subzone
Dissolved Oxygen (DO)	Not less than 4.0mg/L^-1	Yuen Long & Kam Tin (Upper and Lower) Subzones, Beas Subzone, Indus Subzone, Ganges Subzone, Water Gathering Ground Subzones and other inland waters of the Zone
- depth-averaged	Not less than 4.0mg/L^-1 for 90% of samples	Outer Marine Subzone except Mariculture Subzone
- within 2m of the seabed	Not less than 2.0mg/L^-1 for 90% of samples	Outer Marine Subzone except Mariculture Subzone
- 1m below the surface	Not less than 4.0mg/L^-1 for 90%of the sampling occasions during the year	Inner Marine Subzone except Mariculture Subzone
- 1m below the surface	Not less than 5.0mg/L^-1 for 90%of the sampling occasions during the year	Mariculture Subzone
pH	To be in the range of 6.5 - 8.5, change due to waste discharge not to exceed 0.2	Marine waters excepting Yung Long Bathing Beach Subzone
	Not to exceed the range of 6.5 – 8.5 due to waste discharge	Yuen Long & Kam Tin (Upper and Lower) Subzones, Beas Subzone, Indus Subzone, Ganges Subzone and Water Gathering Ground Subzones
	To be in the range of 6.0 - 9.0	Other inland waters
	To be in the range of 6.0 – 9.0 for 95% of samples. Waste discharge shall not cause the natural pH range to be extended by more than 0.5 units	Yung Long Bathing Beach Subzone
Salinity	Change due to waste discharge not to exceed 10% of ambient	Whole zone
Temperature	Change due to waste discharge not to exceed 2°C	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
Suspended solids	Annual median not to exceed 20mg/L^-1 due to waste discharges	Yuen Long & Kam Tin (Upper and Lower) Subzones, Beas Subzone, Ganges Subzone, Indus Subzone, Water Gathering Ground Subzones and other inland waters
Unionized Ammonia (Ammoniacal nitrogen)	Annual mean not to exceed 0.021mg/L^-1 as unionised form	Whole zone
Nutrients	Nutrients shall not be present in quantities sufficient to cause excessive or nuisance growth of algae or other aquatic plants	Inner and Outer Marine Subzone
	Without limiting the generality of objective (a) above, the level of inorganic nitrogen should not exceed 0.7mg/L^-1, expressed as an annual mean.	Inner Marine Subzone
	Without limiting the generality of objective (a) above, the level of inorganic nitrogen should not exceed 0.5mg/L^-1, expressed as annual water column average (arithmetic mean of at least 2 measurements 1m below surface and 1m above seabed).	Outer Marine Subzone
5-Day Biochemical Oxygen Demand (BOD₅)	Not to exceed 3mg/L^-1	Yuen Long & Kam Tin (Upper) Subzone, Beas Subzone, Indus Subzone, Ganges Subzone and Water Gathering Ground Subzones
5-Day Biochemical Oxygen Demand (BOD₅)	Not to exceed 5mg/L^-1	Yuen Long & Kam Tin (Lower) Subzone and other inland waters
Chemical Oxygen Demand (COD)	Not to exceed 15mg/L^-1	Yuen Long & Kam Tin (Upper) Subzone, Beas Subzone, Indus Subzone, Ganges Subzone and Water Gathering Ground Subzones
Chemical Oxygen Demand (COD)	Not to exceed 30mg/L^-1	Yuen Long & Kam Tin (Lower) Subzone and other inland waters
Toxic substances	Should not attain such levels as to produce significant toxic, carcinogenic, mutagenic or teratogenic effects in humans, fish or any other aquatic organisms.	Whole zone
Toxic substances	Waste Discharge should not cause a risk to any beneficial use of the aquatic environment.	Whole zone
Phenol	Not to be present to produce a specific odour, or in concentration greater than 0.05mg/L^-1 as C₆H₅OH	Yung Long Bathing Beach Subzone
Turbidity	Not to reduce light transmission substantially from normal level due to waste discharges	Yung Long Bathing Beach Subzone

Source: Statement of Water Quality Objectives (Deep Bay Water Control Zone). Water Pollution Control Ordinance (Cap. 358R), 1997.

Table 5.2: Key Water Quality Objectives for inland waters in Yuen Long & Kam Tin (Lower) Subzone of Deep Bay Water Control Zones

Min. DO (mg/L)	pH range	Max. BOD₅ (mg/L)	Max. COD (mg/L)	Max. Annual Median SS (mg/L)	Max. E. coli (CFU/100ml)
≥ 4	≥ 6.5 and ≤ 8.5	≤ 5	≤ 30	≤ 20	≤ 1,000

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

5.2.3 Technical Memorandum on Standards for Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal Waters

Effluent discharges are controlled under the WPCO. The Technical Memorandum on Standards for Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal Waters (TM-DSS) sets limits for effluent discharges under Cap. 358AK. Specific limits are set for different WCZs, effluent flow rates and discharges to surface waters, coastal waters and sewers.

The proposed development is located adjacent to Group C (pond fish culture) Inland Waters. The standards for effluents discharged into Group C inland waters are provided in Table 5.3. The key inland water WQOs associated with the WCZs are provided in Table 5.2.

Table 5.3: Standards for effluents discharged into Group C inland waters (All units in mg/L unless otherwise stated; all figures are upper limits unless otherwise indicated)

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

Source: Table abstract from Technical Memorandum on Effluent Standards (EPD, 1991).

5.2.4 Town Planning Board Guidelines No. 12C

Effluent treatment is required prior to discharge into the water courses in the Deep Bay Area, in order to meet the criteria of “no net gain” in pollution load as specified in the Town Planning Board Guidelines No. 12C. The underlying principle is to protect the important habitats and wildlife of the Deep Bay region.

5.2.5 Practice Note for Professional Persons on Construction Site Drainage

Environmental Protection Department (EPD) issued a practice note for professional persons on the handling and disposal of construction site discharges. The Practice Note for Professional Persons on Construction Site Drainage (ProPECC Note PN 1/94) provides good practice guidelines to manage the various types of discharge from a construction site. Practices outlined in ProPECC Note PN 1/94 should be followed as far as possible during construction to minimize the potential water quality impacts from construction site drainage.

5.3 Assessment Area, Water Sensitive Receivers and Background Conditions

5.3.1 Assessment Area and Water Sensitive Receivers

In accordance with Section 3.4.5 of the EIA Study Brief, the study area for the water quality impact assessment comprises the area 500m from the Project boundary, the Deep Bay WCZ and sensitive receivers in the vicinity of the Project site.

The Project site is located within the Comprehensive Development at Wo Shang Wai (EP-31/2008/D) (WSW Development). The Project Area lies within two Drainage Basins (Basin 9 the North District & Basin 10 the Yuen Long Basin) under different Drainage Master Plan Studies (DMP). The direction of water flow in the water ditches and drainage channels are basically from south to north discharging to the Shenzhen River in the Deep Bay WCZ.

Aside from the Wetland Restoration Area (WRA) within the planned WSW Development, there are fishponds and other ponds located primarily to the north and west of the Project site, as well as some water ditches and drainage channels, which are potential water sensitive receivers (WSRs). These are presented in Figure 5.1.

5.3.2 Background Water Quality Conditions

The existing water pollution sources may include runoff and wastewater from adjacent aquaculture (fish pond) activities at the northern boundary of the planned WSW Development and the disposal of domestic sewage (septic tank systems) from the adjacent unsewered developments. Effluent from fish pond activities may contain high nutrient (e.g. ammonia nitrogen) loading, while domestic sewage contains high suspended solids and E. coli levels.

5.3.2.1 River Water Quality

The EPD River Water Quality Report summarizes the results collected from monthly river water quality monitoring at 82 stations in 35 inland watercourses. A total of 48 parameters have been measured including physical and aggregate properties, flow, aggregate organics, nutrients, inorganic constituents, faecal bacteria, metals and pigments. The health of the rivers is rated by a Water Quality Index (WQI) based on the dissolved oxygen, 5-day Biochemical Oxygen Demand (BOD₅) and ammonia-nitrogen concentration. The river water quality at the Deep Bay WCZ has been used as reference for the baseline river water quality for this Study.

The routine river water quality monitoring data collected by EPD between 2011 and 2015 has been reviewed for the nearest monitoring location to Wo Shang Wai at Fairview Park Nullah. This nullah is a short concrete channel within the Fairview Park residential development, which shares the same drainage basin as the southern part of the Project Area (see Table 5.4 for the summary of findings).

Table 5.4: Summary of River Water Quality at Nearby Fairview Park Nullah in the Deep Bay Water Control Zones between 2011 and 2015 (Source: River Water Quality in Hong Kong in 2011 – 2015 (EPD))

Parameters	EPD River Monitoring WQO	2011	2012	2013	2014	2015
pH	6.0 – 9.0	7.8 (7.1 – 8.9)	7.5 (7.3 – 8.5)	7.6 (7.2 – 8.3)	8.0 (7.1 – 8.8)	7.5 (7.2 – 8.8)
BOD₅ (mg/L)	≤ 5 (Max)	11 (4 – 20)	5 (3 – 14)	6 (2 – 18)	8 (3 – 23)	6 (4 – 20)
COD (mg/L)	≤ 30 (Max)	30 (13 – 46)	22 (12 – 54)	26 (14 – 50)	26 (10 – 39)	20 (13 – 45)
SS (mg/L)	≤ 20 (Annual Median)	29 (6 – 49)	26 (11 – 56)	15 (6 – 41)	22 (4 – 590)	30 (7 – 47)
DO (mg/L)	≥ 4 (Min)	9.2 (3.9– 17.2)	5.5 (3.8 – 11.7)	5.7 (2.9 – 10.1)	6.5 (4.8 – 10.5)	6.3 (5.4 – 14.1)
E. coli (cfu/100mL)	≤ 1000 (Max)	18,000 (4,200 – 97,000)	16,000 (2,800 – 330,000)	55,000 (3,500 – 2,500,000)	28,000 (1,900 – 210,000)	29,000 (8,500 – 200,000)
Ammonia-nitrogen (mg/L)	--	4.55 (0.74 – 5.60)	4.15 (2.30 – 6.60)	3.95 (0.71 – 5.50)	0.94 (0.45 – 6.90)	2.55 (0.33 – 3.20)
Nitrate-nitrogen (mg/L)	--	0.56 (0.10 – 1.00)	0.96 (0.35 – 1.30)	0.82 (<0.01 – 1.20)	0.39 (0.11 – 1.10)	0.82 (0.36 – 1.80)
Aluminium (µg/L)	(a) Waste discharges shall not cause the toxins in water to attain such levels as to produce significant toxic carcinogenic, mutagenic or teratogenic effects in humans, 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 any beneficial uses of the aquatic environment.	170 (60– 400)	240 (110 – 470)	185 (110 – 390)	180 (98 – 3,147)	250 (153 – 405)
Cadmium (µg/L)	<0.1 (<0.1 – <0.1)	<0.1 (<0.1 – <0.1)	<0.1 (<0.1 – 1.0)	<0.1 (<0.1 – 0.9)	<0.1 (<0.1 – 1.3)
Chromium (µg/L)	<1 (<1 – 2)	1 (<1 – 3)	<1 (<1 – 2)	1 (<1 – 7)	2 (<1 – 3)
Copper (µg/L)	3 (2 – 5)	3 (2 – 7)	3 (1 – 5)	5 (2 – 73)	4 (3 – 5)
Lead (µg/L)	2 (<1 – 4)	2 (1 – 7)	2 (1 – 4)	3 (<1 – 86)	2 (1 – 3)
Zinc (µg/L)	35 (20 – 170)	30 (10 – 120)	50 (20 – 138)	35 (17 – 436)	37 (23 – 50)

Parameters

EPD River Monitoring WQO

2011

2012

2013

2014

2015

6.0 – 9.0

7.8

(7.1 – 8.9)

7.5

(7.3 – 8.5)

7.6

(7.2 – 8.3)

8.0

(7.1 – 8.8)

7.5

(7.2 – 8.8)

BOD₅ (mg/L)

≤ 5 (Max)

(4 – 20)

(3 – 14)

(2 – 18)

(3 – 23)

(4 – 20)

COD (mg/L)

≤ 30 (Max)

(13 – 46)

(12 – 54)

(14 – 50)

(10 – 39)

(13 – 45)

SS (mg/L)

≤ 20 (Annual Median)

(6 – 49)

(11 – 56)

(6 – 41)

(4 – 590)

(7 – 47)

DO (mg/L)

≥ 4 (Min)

9.2

(3.9– 17.2)

5.5

(3.8 – 11.7)

5.7

(2.9 – 10.1)

6.5

(4.8 – 10.5)

6.3

(5.4 – 14.1)

E. coli

(cfu/100mL)

≤ 1000 (Max)

18,000

(4,200 – 97,000)

16,000

(2,800 –

330,000)

55,000

(3,500 – 2,500,000)

28,000

(1,900 – 210,000)

29,000

(8,500 – 200,000)

Ammonia-nitrogen (mg/L)

4.55

(0.74 – 5.60)

4.15

(2.30 – 6.60)

3.95

(0.71 – 5.50)

0.94

(0.45 – 6.90)

2.55

(0.33 – 3.20)

Nitrate-nitrogen (mg/L)

0.56

(0.10 – 1.00)

0.96

(0.35 – 1.30)

0.82

(<0.01 – 1.20)

0.39

(0.11 – 1.10)

0.82

(0.36 – 1.80)

Aluminium (µg/L)

(a) Waste discharges shall not cause the toxins in water to attain such levels as to produce significant toxic carcinogenic, mutagenic or teratogenic effects in humans, 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 any beneficial uses of the aquatic environment.

170

(60– 400)

240

(110 – 470)

185

(110 – 390)

180

(98 – 3,147)

250

(153 – 405)

Cadmium (µg/L)

<0.1

(<0.1 – <0.1)

<0.1

(<0.1 – <0.1)

<0.1

(<0.1 – 1.0)

<0.1

(<0.1 – 0.9)

<0.1

(<0.1 – 1.3)

Chromium (µg/L)

(<1 – 2)

(<1 – 3)

(<1 – 2)

(<1 – 7)

(<1 – 3)

Copper (µg/L)

(2 – 5)

(2 – 7)

(1 – 5)

(2 – 73)

(3 – 5)

Lead (µg/L)

(<1 – 4)

(1 – 7)

(1 – 4)

(<1 – 86)

(1 – 3)

Zinc (µg/L)

(20 – 170)

(10 – 120)

(20 – 138)

(17 – 436)

(23 – 50)

*Note: 1. WQO follows for river monitoring stations in the Northwestern New Territories (EPD, 2015)
2. Data presented are in annual medians of monthly samples, except those for E. coli which are in annual geometric means.
3. Figures in brackets are annual ranges.
4. Figures in bold show non-compliance to WQO.

It is observed that Fairview Park Nullah achieved 62% compliance with river WQOs in 2015 when compared to 58% compliance in 2014 (EPD, 2015). The recorded maximum levels of chemical oxygen demand, minimum levels of dissolved oxygen, annual median of biochemical oxygen demand and the annual median suspended solids typically exceed the river WQOs, while high ammonia nitrogen and E. coli counts further indicate poor water quality in the Fairview Park Nullah. This is considered to be a clear indication of the negative impact of the discharge from existing unsewered villages and the remaining livestock farms to the river water quality.

5.3.2.2 Other Inland Water Quality

Baseline water quality monitoring and construction phase impact water quality monitoring has been undertaken under the EM&A programme for the approved EIA study for the planned WSW Development Project. The sampling locations include the drainage channel which flows to the Mai Po Ramsar Site (MP1 and MP2); the fishpond at the north of the planned WSW Development (MP3); and the drainage channel along fishponds area (MP4, MP5 to MP6). Figure 5.1 shows the baseline water quality sampling locations. The water quality at monitoring stations MP3 to MP6 are summarised in Table 5.5 and Table 5.6.

Table 5.5: Summary of Baseline Water Quality Monitoring Results of the Planned WSW Development (2009)

Parameter	WQO under WPCO	MP1	MP2	MP3	MP4	MP5	MP6
pH	6.5 – 8.5	7.3	7.4	8.2	7.3	7.3	7.4
pH	6.5 – 8.5	(7.1 – 7.5)	(7.3 – 7.7)	(7.7 – 8.6)	(7.1 – 7.6)	(7.1 – 7.5)	(7.2 – 7.5)
BOD₅ (mg/L)	≤ 5	3	2.8	10.8	4.2	5	4.3
BOD₅ (mg/L)	≤ 5	(<2 – 5.5)	(<2 – 5.0)	(5.0 – 16.5)	(<2 – 8.5)	(<2 – 10.5)	(<2 – 9.5)
SS (mg/L)	≤ 20 (Annual Median)	104	60	50	37	48	54
SS (mg/L)	≤ 20 (Annual Median)	(13 – 316)	(18 – 219)	(35 – 66)	(24 – 54)	(28 – 70)	(24 – 76)
DO (mg/L)	≥ 4	2.71	4.57	8.92	5.69	5.52	6.2
DO (mg/L)	≥ 4	(1.15 – 4.65)	(0.85 – 7.10)	(6.60 – 11.90)	(3.80 – 10.05)	(3.80 – 8.60)	(4.50 – 9.10)
Turbidity (NTU)	--	79	46	53	42	56	65
Turbidity (NTU)	--	(10 – 178)	(17 – 171)	(39 – 68)	(23 – 65)	(26 – 84)	(33 – 96)

Source: Proposed Comprehensive Development at Wo Shang Wai, Yuen Long, Baseline Monitoring Report, 2010.

Notes: 1. WQO follows for Yuen Long & Kam Tin (Lower) Subzone, Statement of Water Quality Objectives (Deep Bay Water Control Zone), WPCO (Cap. 358);
2. Values in brackets are sample ranges, where applicable. Values in bold type are exceedances of the WQOs.

Table 5.6: Summary of Impact Water Quality Monitoring Results of the Planned WSW Development (2015)

Parameters	WQO under WPCO	MP3	MP4	MP5	MP6
pH	6.5 – 8.5	7.5	7.4	7.4	7.4
pH	6.5 – 8.5	(7.3 - 7.7)	(7.2 - 7.7)	(7.2 - 7.6)	(7.2 - 7.6)
BOD₅ (mg/L)	≤ 5	8.1	4.4	6.5	6.1
BOD₅ (mg/L)	≤ 5	(2.0 - 33.0)	(2.0 - 26.0)	(2.0 - 32.5)	(2.0 - 39.0)
SS (mg/L)	≤ 20 (Annual Median)	30	20	26	26
SS (mg/L)	≤ 20 (Annual Median)	(6 - 86)	(2 - 116)	(2 - 69)	(4 - 90)
DO (mg/L)	≥ 4	7.6	7.4	7.5	7.2
DO (mg/L)	≥ 4	(6.5 - 9.5)	(6.3 - 9.1)	(6.4 - 8.7)	(6.0 - 8.7)
Turbidity (NTU)	--	34	21	29	27
Turbidity (NTU)	--	(5 - 77)	(1 - 51)	(8 - 59)	(7 - 59)

Source: Proposed Comprehensive Development at Wo Shang Wai, Yuen Long, Project Website http://www.woshangwai-ema.com/eng/EM&A_D_WQ.htm (accessed in April 2016)

Compared to the results of Year 2009, monitoring data in 2015 shows an improvement in the average and minimum DO levels except at MP3 whereby the average DO level is lower in 2015 compared to the baseline. However, the annual range in 2015 is largely the same as baseline range. There has been a marked improvement in both Turbidity and SS at all stations, and pH has stayed largely the same as baseline range except at MP3 which shows a notable decrease. Conversely, BOD has improved on average at MP3 but has increased in all the other stations compared to baseline.

5.4 Assessment Methodology

The water quality impact assessment has been carried out in accordance with Appendix D of the EIA Study Brief and the guidelines specified in Annex 6 and 14 of the EIAO-TM.

Potential water pollution sources that may be generated by the Project during construction, operation and decommissioning phase have been identified. The potential impacts to nearby water sensitive receivers have been evaluated, and where necessary, appropriate mitigation measures have been recommended to reduce any identified adverse impacts on water quality.

5.5 Identification and Evaluation of Water Quality Impacts

5.5.1 Construction Phase

The main construction elements of the Project broadly include site formation, excavation and foundation works, general building and structure works, and installation of sewerage pipes. Potential sources of construction-related water quality impacts include:

● Construction site runoff

● Accidental spillage of chemicals

● Sewage from construction workforce

● General construction activities

5.5.1.1 Construction Site Runoff

Activities within a construction site can generate surface runoff which may contain high levels of suspended solids and contaminants. Such surface runoff can pollute watercourses and lead to deteriorations in water quality if left uncontrolled.

The key sources of pollution to surface water runoff from the construction site include:

● Runoff from exposed earth surfaces within the construction site, particularly during inclement weather

● Erosion of uncovered stockpiles

● Release of bentonite slurries, concrete washing and wastewater from other grouting activities

● Dewatering associated with excavation and/or piling activities

● Wastewater from dust suppression spraying and wheel wash facilities

Uncontrolled discharges from a construction site have the potential to adversely impact nearby stormwater drains, but can be managed through good site practices and provision of appropriate site drainage management facilities. The good site practices outlined in ProPECC Note PN1/94 will be implemented to control site runoff and drainage during construction phase. Precautionary measures relating to rainstorms as stated in Appendix A2 of ProPECC Note PN1/94 will also implemented to avoid water pollution due to site runoff during inclement weather.

5.5.1.2 Accidental Spillage of Chemicals

Chemicals stored on-site during construction phase such as petroleum, oil and grease, lubricants and solvents may be accidentally spilt or leaked at the construction site. If left unattended, such spilt / leaked chemicals may enter the site drainage system and adversely impact stormwater drains outside the site. To avoid and minimise such potential impacts, appropriate site storage and bunding of chemicals should be implemented as part of good site practice, and measures should be taken to clean up any spilt / leaked chemicals immediately.

5.5.1.3 Sewage from Construction Workforce

Domestic sewage will be generated by the construction workforce during construction phase. Release of untreated sewage into the surrounding environment can adversely impact the aquatic environment by causing pollution / eutrophication of nearby water courses, odour nuisance and subsequent deterioration in aquatic biodiversity. On-site portable toilets should be provided, maintained and regularly transported off-site for proper disposal to avoid sewage discharge into the surrounding environment.

5.5.1.4 General Construction Activities

Inadequate or improper storage of construction materials and waste has the potential to impact water quality through release of construction debris (such as packaging and construction materials) and general refuse (waste food containers, paper, bottles and cans) into the construction site drainage system. Implementation of good site management practices and provision of adequate waste receptacles will be adopted to mitigate the potential impacts of general construction activities on water quality.

5.5.2 Operation Phase

5.5.2.1 Onsite Sewage Treatment Plant (STP)

Sewage generated by the WSW Development is proposed to be handled by an onsite STP until the Government trunk sewer is available. The onsite STP will adopt Membrane Bioreactor (MBR) technology and the effluent from the STP will be fully reused at the WSW Development for toilet flushing and irrigation of landscaped areas. Once the Government trunk sewer is available, the onsite STP will be decommissioned but will not be demolished. The basis and assumptions in relation to the STP design and sewerage network, reclaimed water demanded, as well as reclaimed water storage tank have been presented in the Sewerage Impact Assessment Report (SIA) in Appendix 2.1.

The design population for the onsite STP is 1,245 persons based on development with 400 residential units. The calculated average dry weather flow (ADWF) and peak flow would be 482 m³/d and 32 l/s respectively. The proposed onsite sewerage network and design parameters are presented in the SIA attached in Appendix 2.1, and the key information is reproduced in Figure 5.2 and Appendix 5.1.

The on-site sewage treatment facility will be designed generally in accordance with EPD’s “Guidelines for the Design of Small Sewage Treatment Plant”. Considering the high sensitivity of the development site, the onsite STP is proposed to be designed with capacity to handle a peak flow of three times of ADWF (i.e. 1,446 m³/d). Any surplus flow would be equalised in an equalisation tank with 4 hours retention time at such flows (i.e. minimum 241 m³).

The estimated total quantity of sludge generated in the proposed STP is 13 m³/day. The sludge generated from the on-site STP will be properly collected and stored on-site. The collected sludge will be transported to the designated landfill site by designated sewage tankers for disposal. Details of the sludge management are presented in Chapter 6.

5.5.2.2 Potential Operation Phase Water Quality Impacts

During operation phase, the key potential water quality impacts are associated with operation of the onsite STP for treatment of the sewage effluent generated by the WSW Development, and subsequent use of the reclaimed water. The key sources of pollution during operation phase include:

● Reclaimed water use onsite

● Discharge of reclaimed water to Deep Bay WCZ

● Emergency discharge of untreated sewage effluent from the onsite STP

5.5.2.3 Reclaimed Water Use Onsite

Reclaimed water from the onsite STP is proposed to be fully used onsite as flushing and irrigation water. The water demands for toilet flushing and landscape irrigation have been estimated in the SIA in Appendix 2.1 based on the Layout Plan of Landscape Area of the WSW project. The relevant information is reproduced in Appendix 5.2.

The estimated toilet flushing and landscape irrigation water demand is 78 m³/d and 509 m³/d respectively. The reclaimed water from the onsite STP (482 m³/d) can thus be fully utilised within the site, and there would be no offsite discharge of reclaimed water under normal operation.

To enable the reclaimed water to be safely used, the onsite STP will be designed to meet stringent treatment standards. The treatment standards adopted are based on the reuse water quality standards recommended in the “Water Supplies Department Inter-departmental Working Group on the Implementation of Reclaimed Water Supply in Sheung Shui and Fanling” for non-potable uses. The Water Supplies Department's reclaimed water standards as summarized in Table 5.7 will be adopted by this Project.

Table 5.7: WSD Reuse Water Quality Standard for Non-Potable Uses

Water Quality Parameter	Unit	WSD Criteria (Irrigation & Non-Potable Uses)
pH	-	6-9
Turbidity	NTU	≤ 5
Total Suspended Solids	mg/l	≤ 5
BOD₅	mg/l	≤ 10
E. coli	cfu/100ml	Non-detectable
Total Residual Chlorine	mg/l	≥ 1 (out of treatment system) ≥ 0.2 (at point of use)
Dissolved Oxygen (DO)	mg/l	≥ 2
Colour	Hazen Unit	≤ 20
Threshold Odour Number (TON)	TON	≤ 100
Ammonia nitrogen	mg/l	≤ 1
Synthetic detergents	mg/l	≤ 5

Note: The water quality standards for all parameters shall be applied at the point of use, unless otherwise specified

The MBR process proposed for the onsite STP is proven technology that has been applied in other projects in Hong Kong, and this process coupled with the proposed post-process ultraviolet disinfection and chlorine dosing (for E. coli removal) is capable of treating effluents to the water quality standards specified in Table 5.7. In addition, preventive measures for avoiding cross-contamination and mis-use of the reclaimed water will be put in place.

With sewage effluent treated to the WSD reclaimed water standards specified in Table 5.7 and implementation of water pollution preventive measures, no adverse water quality impacts are anticipated from use of reclaimed water in the Project site.

5.5.2.4 Discharge of Reclaimed Water to Deep Bay WCZ

In cases of potential adverse weather conditions (e.g. successive heavy rainy days), or maintenance of landscape areas, the supply of reclaimed water may temporarily exceed demand. This may result in either direct (surplus discharge) or indirect / non-point (as excessive irrigation water) overflow of reclaimed water into the stormwater drainage system and eventually into Deep Bay.

To avoid this, an onsite reclaimed water storage tank (which does not form part of the storage volume for regular operation of the onsite STP) is proposed which would temporarily store any excessive reclaimed water. The reclaimed water would be progressively consumed in the following periods or used in case of shortage of irrigation water. Any further excessive reclaimed water will be tanked away to public Sewage Treatment Works to prevent overflow of reclaimed water.

The sizing of the buffer tank has taken into account extreme weather condition so as to minimise the likelihood and frequency of excess reclaimed water requiring tanking away. It is considered that no irrigation would be required when the soil is saturated at the landscaping area, thus irrigation would normally stop when daily rainfall depth reaches 15 mm (which is equivalent to the average daily irrigation rate of 10 l/m²/d assuming a runoff coefficient of 0.35). Past rainfall records from a nearby weather station (Au Tau Automatic Weather Station from 2004 to 2013) show that successive rainfall events with daily rainfall depth over 15 mm mostly occurred over 2 or 3 successive days, accounting for over 90% of the total number of adverse weather events.

Based on the above, a minimum capacity of 1,180 m³ reclaimed water storage tank, which equates to three days of net reclaimed water available for irrigation (i.e. ADWF of 482 m³ minus 78 m³ for flushing demand and 13 m³ for sludge disposal) is proposed to temporarily store the excessive reclaimed water.

With provision of the buffer tank to cater for most of the successive extreme rainfall events, the likelihood and frequency of excess reclaimed water requiring tanking away can be minimized.

5.5.2.5 Emergency Discharge of Untreated Sewage Effluent from the Onsite STP

During emergency situations, such as loss of power supply at the onsite STP, or mechanical faults / equipment failures, untreated sewage effluent may overflow and cause potential impacts at downstream WSRs. With the ‘no net increase of pollution load’ requirement as stipulated in the Town Planning Board Guideline, any discharge of sewage leading to a net increase in pollution load is not environmentally acceptable. To minimise the risk of untreated sewage effluent discharge due to emergency events, a number of contingencies will be provided at the onsite STP, such as equalisation tank, dual or standby power supply, standby sewage treatment units, flow sensors and alarm systems. As a last resort and in case operation of the onsite STP cannot be resumed after all these contingency measures have been exhausted, any surplus raw sewage will be tanked away to the public STW. The estimated worst case quantity of 20 m³/hr of raw sewage (based on the design average dry weather flow of 482 m³/day) can be readily tanked away by 3-4 tankers per hour (with capacity of 5-6 m³ per tanker). With these contingency measures in place, the risk of untreated sewage effluent discharge to Deep Bay WCZ due to emergency events is considered to be negligible.

5.5.3 Decommissioning Phase

The onsite STP is proposed as an interim measure to handle the sewage effluents generated by the WSW Development, until the Government trunk sewer is implemented. Once all the sewerage is connected to the permanent Government sewer, the onsite STP would be decommissioned. It is currently proposed that decommissioning would involve only isolation of the sewerage connections to the onsite STP and retirement of the treatment units after the connection to the Government sewer has been established, while the STP building and other structural components would be retained in place.

Arrangements would be made between the Estate Manager and Drainage Services Department (DSD) for activation, transfer and/or testing and commissioning of the relevant sewerage connections to the Government sewer where applicable, and the onsite STP would not be decommissioned until the sewerage connection to the Government sewer has satisfactorily completed these arrangements. Any wastewaters generated from the decommissioning process (e.g. cleaning out of the treatment and storage units) and any residual untreated sewage or reclaimed water would be pumped out and tanked away to the public STW for offsite treatment and disposal.

With these proposed arrangements, no adverse water quality impacts are expected during decommissioning phase.

5.6 Mitigation of Adverse Water Quality Impacts

5.6.1 Construction Phase

5.6.1.1 Construction Site Runoff

Construction site runoff and wastewater shall be collected and diverted to the temporary drainage system installed by the Contractor for treatment (to remove sediment) prior to discharge to the existing stormwater system. The Contractor shall obtain a discharge licence from EPD under the WPCO for all discharges from the site and shall ensure such discharges meet the requirements of the TM-DSS.

Good site practices outlined in ProPECC Note PN1/94 should also be adopted to minimise runoff from construction works areas. The following measures are recommended, but are not exhaustive, and other relevant measures listed in ProPECC Note PN1/94 should be implemented as necessary to minimise the impacts of construction on downstream water quality:

● Temporary site drainage facilities shall be designed and implemented by the Contractor prior to commencement of construction to convey surface runoff to storm drains. The design of the silt/ sand removal traps and sediment basins shall follow the design in ProPECC Note PN1/94;

● Perimeter cut-off drains shall be installed in advance of any excavation and site formation works to convey site runoff from the works areas to the silt removal facilities;

● Runoff into the excavation areas during rainstorm events shall be minimised as far as practicable. Any wastewater pumped out of the excavation areas shall be treated to remove suspended solids prior to discharge;

● Maintenance and inspection of the drainage system and sediment removal facilities should be carried out regularly to remove any sediment and blockages, especially when rainstorms are forecast;

● Final surface levels should be compacted and final surface protections installed to prevent erosion by rainstorms;

● Open stockpiles of material should be covered on site with waterproof layers such as tarpaulin.

● The wheels of all vehicles and plant should be cleaned before leaving the works areas. The washwater should be treated to remove any suspended sediment;

● Surface water from concrete batching areas and the rest of the site should be separated as far as possible. Wastewater from any concrete batching plant (if required) shall be treated to the required standards including pH adjustment and settlement of suspended sediments before discharging to stormwater drains; and

● Manholes (including those constructed as part of the Project) should be adequately covered and temporarily sealed at all times.

Precautionary measures relating to inclement weather outlined in Appendix A2 of ProPECC Note PN1/94 should also be adopted to prevent water pollution due to site runoff. The following measures are recommended, but are not exhaustive, and other relevant measures listed in Appendix A2 of ProPECC Note PN1/94 should be implemented as necessary to minimise the impacts of construction on downstream water quality:

● Silt removal facilities, channels and manholes should be maintained and deposited silt and grit should be removed regularly;

● Temporarily exposed slope surfaces should be covered;

● Temporary access road should be protected by crushed stone or gravel;

● Intercepting channels should be provided to prevent storm runoff from washing across exposed soil surfaces; and

● Trenches should be dug and backfilled in short sections. Measures should be taken to minimize the ingress of rainwater into trenches.

5.6.1.2 Accidental Spillage of Chemicals

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. The Contractor should register as a chemical waste producer if chemicals are to be generated from site. Off-site disposal of chemical waste should only be carried out in accordance with the requirements of the WDO.

In addition, the following measures shall be observed:

● The labelling and storage of chemicals should be in accordance with the “Code of Practice on the Packaging, Labelling and Storage of Chemical Wastes” and maintained at all times by the Contractor;

● Oils and fuels should only be stored in designated areas which have appropriate pollution prevention control facilities such as oil and grease traps and petrol interceptors;

● The maintenance of vehicles should only be undertaken in areas of the site served by these pollution prevention measures; and

● All fuel tanks and storage areas should be locked and located on sealed areas of the site, within bunded areas with a capacity equal to 110% of the storage capacity of the largest container. The bund should be drained of surface water after each rainfall event.

5.6.1.3 Sewage from Construction Workforce

Portable toilets shall be provided throughout construction phase and shall be regularly maintained, collected and disposed by a licensed waste collector.

5.6.1.4 General Construction Activities

Mitigation measures to be adopted for general construction activities including the following:

● Construction waste, debris and refuse generated onsite should be stored in designated areas and properly contained.

● Waste materials should be regularly removed offsite.

● Stockpiles of construction materials such as cement and excavated material should be covered when not in use.

With the adoption of these good site practices, no adverse water quality impacts are anticipated.

5.6.2 Operation Phase

5.6.2.1 Reclaimed Water Use Onsite

For use of reclaimed water, the following measures shall apply:

● The onsite STP shall comprise MBR technology with post-process disinfection via ultraviolet (UV) treatment and chlorine dosing; and

● Sewage effluent shall be treated to meet the Water Supplies Department's reclaimed water standards as summarized in Table 5.7. Samples of reclaimed water shall be taken regularly and tested by a HOKLAS or other internationally accredited laboratory to ensure the effluent quality meets the required reuse standard.

In addition, preventive measures for cross-contamination and mis-use of reclaimed water shall include the following:

Engineering Measures

● Water to be supplied for potable use, toilet flushing and irrigation should be stored in three different tanks in different colours and clearly labelled;

● All pipes and fittings used for the reclaimed water supply and associated distribution system should be purple in colour (exact colour code to be reviewed) for distinguishing them from the pipes and fittings used for the fresh water supply and its distribution systems;

● Regular checking/inspections of the reclaimed water supply and associated distribution systems should be carried out to identify any possible cross connection to the fresh water supply and distribution system. Non-toxic dye may be adopted in the checking/inspections;

● Non-return valves should be installed on both the inlet pipes feed from reclaimed water storage tank and WSD’s supply mains, to the toilet flushing and irrigation waters storage tanks; and

● All precaution measures should be clearly stated in the O&M manual of the STP, toilet flushing and irrigation systems.

Management Measures

● Warning plate with sign and letter “NOT FOR POTABLE USE 不能飲用” would be shown on the toilet flushing and irrigation water storage tanks, and tagged on all accessible water taps supplying reclaimed water if any within the developments, notifying the staff, visitors and the public at large that reclaimed water is being used and is not suitable for drinking;

● All water taps of reclaimed water at communal areas, if any should be locked in order to avoid mis-use of reclaimed water for other non-planned use;

● Proper signage, promotion and training workshops will be provided periodically to all management and operation staffs of the Development, as well as future land owners on the proper use of reclaimed water and potable water; and

● All precaution measures should be clearly stated in the management manual of the Development.

5.6.2.2 Discharge of Reclaimed Water to Deep Bay WCZ

For prevention of discharge / overflow of reclaimed water, the following measures shall be incorporated as part of the onsite STP:

● Provision of a minimum capacity of 1,180 m³ reclaimed water storage tank to store excessive reclaimed water in case of emergency (e.g. extreme adverse weather) or maintenance of landscape area;

● Reclaimed water storage tank will be partitioned into several compartment to allow partial shut-down of the tank for maintenance;

● The operation of the project will maintain the reclaimed water demands for toilet flushing and landscape irrigation as detailed in the SIA in Appendix 2.1 to ensure the reclaimed water can be totally used. A minimum of 50,850m² landscape areas within the development will be maintained (as committed in the Town Planning Board application for this development) using reclaimed water for irrigation; and

● Level sensors connected with alarm signalizing system will be installed to keep monitoring on storage volume of reclaimed water to avoid overflow of reclaimed water. The warning signal will be automatically generated and sent to the Estate Manager when the flow in the tank reaches a pre-set level so as to allow the Estate Manager sufficient time (e.g. 1 day) to arrange and mobilize tanker service to tank away the excessive reclaimed water with 1-day reclaimed water storage capacity reserved as contingency.

As a last resort and when irrigation is stopped due to continuous adverse weather or prolonged suspension of irrigation or flushing water supply systems for maintenance / repairing, any further excessive reclaimed water shall be tanked away to the public STW for offsite treatment and disposal.

For prevention of excessive irrigation leading to surface runoff, the following measures shall be adopted:

● A pre-set semi-automatic control irrigation system with underground drip pipes would be installed in the private garden and managed by the Deed of Mutual Covenant manager to ensure that reclaimed water would be used properly for irrigation.

● Installation of flow meters to monitor the irrigation water demand, with daily cut-off limits applied to prevent excessive irrigation using the reclaimed water.

All the recommended measures for collection, treatment and disposal to ensure no net increase in Pollution to Deep Bay shall be incorporated in the Project contract document.

5.6.2.3 Emergency Discharge of Untreated Sewage Effluent from the Onsite STP

For prevention of emergency discharge / overflow / surface runoff of untreated sewage effluent, the following measures shall be incorporated as part of the onsite STP:

● Regular test, maintenance and replacement of membranes or equipment will be carried out according to the recommendations from manufacturers to lower the chances of facilities breakdown;

● Provision of equalization tank to store three times of ADWF for a period of 4 hours (i.e. minimum 241 m³);

● Provision of emergency storage tank to store the overflow of raw sewage with a capacity of approximate 130 m³ based on the latest information (actual size to be confirmed in the detailed design stage).

● Dual or standby power supply;

● Standby unit for major equipment to allow for partial shut down for maintenance; and

● Installation of flow measurement and level sensors connected with alarm signalizing system to keep monitoring on inflow rate to avoid sewage overflow.

In case operation of the STP cannot be resumed after all the above mitigation measures have been exhausted, raw sewage shall be tanked away to the public STW for offsite treatment and disposal.

Details of these and other specific contingency measures shall be documented in a contingency plan to be prepared by the operator of the STP. The contingency plan shall cover situations when the reclaimed water cannot meet the proposed criteria as well as situations when the STP is out of service, and shall be implemented throughout operation of the onsite STP.

5.6.3 Decommissioning Phase

The onsite STP shall not be decommissioned until the sewerage connection to the Government sewer has been fully established and implemented. Any residual wastewaters generated from cleaning out and decommissioning of the treatment and storage units and any residual reclaimed water would be pumped out and tanked away to the public STW for offsite treatment and disposal. The water pollution preventive measures as detailed in Section 5.6.1 are also applicable and shall be implemented for decommissioning activities.

5.7 Evaluation of Cumulative and Residual Impact

Proposed Comprehensive Development at Wo Shang Wai, Yuen Long

The Project will be constructed and operated concurrently with the WSW Development. Both projects have in place mitigation measures to prevent polluted discharges to the surrounding environment during construction phase, and during operation phase, this Project will form and be managed and maintained as part of the WSW Development.

With implementation of the recommended design and mitigation measures, no adverse cumulative or residual impacts are expected during construction, operation and decommissioning of the Project.

Hong Kong Section of Guangzhou – Shenzhen – Hong Kong Express Rail Link

The construction site for the Mai Po Ventilation Building is adjacent to the Project site to the northeast. The construction of the Ventilation Building is largely completed with few further construction activities expected. This project has in place mitigation measures to prevent polluted discharges to the surrounding environment. Therefore, no adverse cumulative or residual impacts are expected.

5.8 Environmental Monitoring and Audit

During construction phase, regular site audits shall be conducted to check implementation of the recommended mitigation measures.

During operation phase, the management, maintenance and operation of the onsite STP shall follow the O&M Manual for the onsite STP and the Management Manual of the Development, which shall cover the mitigation measures specified in this EIA report. An EM&A program will be implemented according to the EM&A manual. A water quality monitoring programme is proposed to monitor compliance with the reclaimed water use criteria.

5.9 Conclusion

This Section has identified the potential water quality impacts associated with construction, operation and decommissioning of the onsite STP. For construction phase, potential water quality impacts (mainly land-based) including construction site runoff, accidental spillage of chemicals, sewage from the construction workforce and general construction activities can be readily mitigated with implementation of environmental best practices for construction site management as well as water pollution preventive and mitigation measures. For operation phase, the interim sewage treatment plant will employ Membrane Bioreactor technology to treat the sewage generated from the WSW development and the effluent will be further polished by disinfection to meet the WSD water reuse standards. With treatment to the required WSD reuse standards and the full use of the reclaimed water at the WSW Development, alongside preventive and contingency measures for avoidance and minimisation of treated or untreated effluent discharge to Deep Bay, the Project would comply with the ‘no net increase in pollution load’ requirement. In addition, preventive measures for cross-contamination and mis-use of reclaimed water will be implemented through engineering and management measures. Decommissioning of the onsite STP would not occur until all sewage from the WSW Development is fully diverted to the permanent Government sewers. With implementation of these recommended measures, no adverse water quality impacts would arise due to the Project.