5.1.1.1 The upgrading and expansion of San Wai STW will expand the treatment capacity from 164,000 m3/d to 246,000 m3/d. The preliminary treatment method currently adopted at San Wai STW will be upgraded to enhance the removal of pollutants in the sewage. Different treatment options are considered in this study. Centralised disinfection will be applied to disinfect the effluent from San Wai STW and effluent after secondary treatment from Yuen Long STW. Discharge of effluent will be through the NWNT effluent tunnel to the outfall at Urmston Road.
5.1.1.2 An emergency bypass culvert connecting San Wai STW to Tin Shui Wai Drainage Channel is proposed to control sewage discharge in an emergency event when the NWNT effluent tunnel is not in operation. Effluent will be discharged to Deep Bay, which is an ecological sensitive area.
5.1.1.3 This section aims to assess the potential water quality impacts associated with the proposed Project. Assessment results for different treatment options and emergency discharge cases are presented in this section.
5.2 Environmental Legislation, Policies, Plans, Standards and Criteria
5.2.1.1 The relevant legislation and guidelines that are applicable to the
water quality impact assessment in this Study include:
· Water Pollution Control Ordinance (WPCO) Chapter 358 (as amended by the Water
Pollution Control (Amendment) Ordinance 1990 and 1993);
· Water Pollution Control (General) Regulations (as amended by the Water
Pollution Control (General) (Amendment) Regulations 1990 and 1994);
· Environmental Impact Assessment Ordinance (EIAO) (Cap. 499), Technical
Memorandum on Environmental Impact Assessment Process (EIAO-TM), Annexes 6 and
14;
· Standards for effluents discharged into drainage and sewerage systems, inland
and coastal waters; and
· The Practice Note for Professional Persons (ProPECC Note PN1/94) on
Construction Site Drainage.
5.2.1.2 The Water Pollution Control Ordinance (WPCO) (Cap. 358) sets forth the schedules of Water Quality Objectives (WQO) for each Water Control Zone (WCZ) and standards for effluents discharged into drainage and sewerage systems, inland and coastal waters.
5.2.1.3 The WCZs applicable to this study include the Deep Bay WCZ and North Western WCZ. The appointed days for the Deep Bay WCZ and North Western WCZ were 1 December 1990 and 1 April 1992 respectively. WQOs for these WCZs form the criteria for assessing the water quality impacts in this Study. The marine WQOs for the Deep Bay and North Western WCZs are presented in Table 5.1.
Table
5.1 Marine Water Quality Objectives for the
Deep Bay and North Western Water Control Zones
|
Objective |
Deep
Bay WCZ |
North
Western WCZ |
|
Dissolved
Oxygen |
·
Within
2 m of bottom: > 2 mg/L for 90% samples ·
Depth
averaged: > 4 mg/L for 90% samples ·
Rest
of water column: > 4 mg/L |
·
Within
2 m of bottom: > 2 mg/L for 90% samples ·
Depth
averaged: > 4 mg/L for 90% samples ·
Rest
of water column: > 4 mg/L |
|
E.
coli |
<
610 per 100 mL (annual geometric mean) |
<
610 per 100mL (annual geometric mean) Bathing
Beach Subzones: <180
per 100mL (geometric mean of all samples collected from March to October
exclusive) |
|
pH
|
6.5
– 8.5 and change due to waste discharge < 0.2 |
6.5
– 8.5 and change due to waste discharge < 0.2 |
|
Salinity |
Change
due to waste discharge < 10% of natural ambient level |
Change
due to waste discharge < 10% of natural ambient level |
|
Temperature |
Change
due to waste discharge < 2 oC |
Change
due to waste discharge < 2 oC |
|
Suspended
Solids |
<
30% increase in the natural ambient level or not to cause the accumulation
of suspended solids which may adversely affect aquatic communities |
<
30% increase in the natural ambient level or not to cause the accumulation
of suspended solids which may adversely affect aquatic communities |
|
Toxicants |
Not
to be present at levels producing significant toxic effect |
Not
to be present at levels producing significant toxic effect |
|
Un-ionized
ammonia |
<
0.021 mg/L (annual mean) |
<
0.021 mg/L (annual mean) |
|
Inorganic
Nitrogen |
·
Inner
sub-zone: < 0.7 mg/L (annual mean depth average) ·
Outer
sub-zone: < 0.5 mg/L (annual mean depth average) |
<
0.5 mg/L (annual mean depth average) |
Source:
Marine Water Quality in Hong Kong in 1999 by EPD
5.2.1.4 Annex 6 of the EIAO-TM sets forth general and project-specific criteria for evaluation of water quality pollution. Annex 14 of the EIAO-TM provides guidelines for assessing water pollution. It describes the components of an aquatic system that are subject to water pollution impact: water quality, hydrology, bottom sediments and ecology; as well as beneficial uses sensitive to water pollution. Assessment should rely on the concept of assimilative capacity of the receiving water body and water quality objectives. Assessment methodology should include the identification of impact causing factors, determination of impact boundary, any necessary baseline study, impact prediction and assessment, recommendation of mitigation measures, and monitoring. Annex 14 of the EIAO-TM also provides guidelines for assessing impacts from project-specific activities.
5.2.1.5 Discharges of effluents are subject to control under the WPCO. Effluent discharge standards are defined in the Technical Memorandum on Standards for Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal Waters (TM) issued under section 21 of the WPCO. Standards for effluents discharged into the coastal waters of Deep Bay WCZ and into the inshore waters of the North Western WCZ are shown in Tables 5.2 and 5.3 respectively.
5.2.1.6 Guidelines for the handling and disposal of construction discharges are provided in ProPECC Note PN1/94 on Construction Site Drainage. Methods to prevent water pollution from the construction activities are recommended in the guidelines and are considered relevant to the present study. The types of discharges from construction sites outlined in the guidance note include:
· surface run-off;
· groundwater;
· boring and drilling water;
· wastewater from concrete batching and precast concrete casting;
· wheel washing water;
· bentonite slurries;
· water for testing and sterilisation of water retaining structures and water
pipes
· wastewater from building construction;
· acid cleaning, etching and pickling wastewater; and
· wastewater from site facilities.
Table
5.2 Standards for Effluents Discharged into
the Coastal Waters of Deep Bay Water Control Zone
|
Flow
rate (m3/day) |
£10 |
>10
and £200 |
>200
and £400 |
>400
and £600 |
>600
and £800 |
>800
and £1000 |
>1000
and £1500 |
>1500
and £2000 |
>2000
and £3000 |
>3000
and £4000 |
|
Determinant |
||||||||||
|
pH
(pH units) |
6
– 9 |
6
– 9 |
6
– 9 |
6
– 9 |
6
– 9 |
6
– 9 |
6
– 9 |
6
– 9 |
6
– 9 |
6
– 9 |
|
Temperature
(oC) |
45 |
45 |
45 |
45 |
45 |
45 |
45 |
45 |
45 |
45 |
|
Colour
(lovibond units) (25mm
cell length) |
1 |
1 |
1 |
1 |
1 |
1 |
1 |
1 |
1 |
1 |
|
Suspended
solids |
50 |
50 |
50 |
50 |
50 |
50 |
25 |
25 |
25 |
25 |
|
BOD |
20 |
20 |
20 |
20 |
20 |
20 |
10 |
10 |
10 |
10 |
|
COD |
80 |
80 |
80 |
80 |
80 |
80 |
50 |
50 |
50 |
50 |
|
Oil
& Grease |
20 |
20 |
20 |
20 |
20 |
20 |
10 |
10 |
10 |
10 |
|
Iron |
10 |
10 |
10 |
7 |
5 |
4 |
3 |
2 |
1 |
1 |
|
Boron |
5 |
4 |
3 |
2.5 |
2 |
1.6 |
1.1 |
0.8 |
0.5 |
0.4 |
|
Barium |
5 |
4 |
3 |
2.5 |
2 |
1.6 |
1.1 |
0.8 |
0.5 |
0.4 |
|
Mercury |
0.1 |
0.001 |
0.001 |
0.001 |
0.001 |
0.001 |
0.001 |
0.001 |
0.001 |
0.001 |
|
Cadmium |
0.1 |
0.001 |
0.001 |
0.001 |
0.001 |
0.001 |
0.001 |
0.001 |
0.001 |
0.001 |
|
Other
toxic metals individually |
1 |
0.5 |
0.5 |
0.5 |
0.4 |
0.4 |
0.25 |
0.2 |
0.15 |
0.1 |
|
Total
toxic metals |
2 |
1 |
1 |
1 |
0.8 |
0.8 |
0.5 |
0.4 |
0.3 |
0.2 |
|
Cyanide |
0.1 |
0.1 |
0.1 |
0.1 |
0.1 |
0.08 |
0.06 |
0.04 |
0.03 |
0.02 |
|
Phenols |
0.5 |
0.5 |
0.4 |
0.3 |
0.25 |
0.2 |
0.1 |
0.1 |
0.1 |
0.1 |
|
Sulphide |
5 |
5 |
5 |
5 |
5 |
5 |
2.5 |
2.5 |
1.5 |
1 |
|
Total
residual chlorine |
1 |
1 |
1 |
1 |
1 |
1 |
1 |
1 |
1 |
1 |
|
Total
nitrogen |
100 |
100 |
100 |
100 |
100 |
100 |
80 |
80 |
50 |
50 |
|
Total
phosphorus |
10 |
10 |
10 |
10 |
10 |
10 |
8 |
8 |
5 |
5 |
|
Surfactants
(total) |
15 |
15 |
15 |
15 |
15 |
15 |
10 |
10 |
10 |
10 |
|
E.
coli (count/100
mL) |
1000 |
1000 |
1000 |
1000 |
1000 |
1000 |
1000 |
1000 |
1000 |
1000 |
Notes: 1. All units in mg/L unless otherwise stated; and
2.
All figures are upper limits unless otherwise indicated.
Source: Technical Memorandum on Standards for Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal Waters, Table 8, Environmental Protection Department.
|
Flow
rate (m3/day) |
£10 |
>10
and £200 |
>200
and £400 |
>400
and £600 |
>600
and £800 |
>800
and £1000 |
>1000
and £1500 |
>1500
and £2000 |
>2000
and £3000 |
>3000
and £4000 |
|
Determinant |
||||||||||
|
pH
(pH units) |
6
– 9 |
6
– 9 |
6
– 9 |
6
– 9 |
6
– 9 |
6
– 9 |
6
– 9 |
6
– 9 |
6
– 9 |
6
– 9 |
|
Temperature
(oC) |
40 |
40 |
40 |
40 |
40 |
40 |
40 |
40 |
40 |
40 |
|
Colour
(lovibond units) (25mm
cell length) |
1 |
1 |
1 |
1 |
1 |
1 |
1 |
1 |
1 |
1 |
|
Suspended
solids |
50 |
30 |
30 |
30 |
30 |
30 |
30 |
30 |
30 |
30 |
|
BOD |
50 |
20 |
20 |
20 |
20 |
20 |
20 |
20 |
20 |
20 |
|
COD |
100 |
80 |
80 |
80 |
80 |
80 |
80 |
80 |
80 |
80 |
|
Oil
& Grease |
30 |
20 |
20 |
20 |
20 |
20 |
20 |
20 |
20 |
20 |
|
Iron |
15 |
10 |
10 |
7 |
5 |
4 |
3 |
2 |
1 |
1 |
|
Boron |
5 |
4 |
3 |
2 |
2 |
1.5 |
1.1 |
0.8 |
0.5 |
0.4 |
|
Barium |
5 |
4 |
3 |
2 |
2 |
1.5 |
1.1 |
0.8 |
0.5 |
0.4 |
|
Mercury |
0.1 |
0.001 |
0.001 |
0.001 |
0.001 |
0.001 |
0.001 |
0.001 |
0.001 |
0.001 |
|
Cadmium |
0.1 |
0.001 |
0.001 |
0.001 |
0.001 |
0.001 |
0.001 |
0.001 |
0.001 |
0.001 |
|
Other
toxic metals individually |
1 |
1 |
0.8 |
0.7 |
0.5 |
0.4 |
0.3 |
0.2 |
0.15 |
0.1 |
|
Total
toxic metals |
2 |
2 |
1.6 |
1.4 |
1 |
0.8 |
0.6 |
0.4 |
0.3 |
0.2 |
|
Cyanide |
0.2 |
0.1 |
0.1 |
0.1 |
0.1 |
0.1 |
0.05 |
0.05 |
0.03 |
0.02 |
|
Phenols |
0.5 |
0.5 |
0.5 |
0.3 |
0.25 |
0.2 |
0.1 |
0.1 |
0.1 |
0.1 |
|
Sulphide |
5 |
5 |
5 |
5 |
5 |
5 |
2.5 |
2.5 |
1.5 |
1 |
|
Total
residual chlorine |
1 |
1 |
1 |
1 |
1 |
1 |
1 |
1 |
1 |
1 |
|
Total
nitrogen |
100 |
100 |
80 |
80 |
80 |
80 |
50 |
50 |
50 |
50 |
|
Total
phosphorus |
10 |
10 |
8 |
8 |
8 |
8 |
5 |
5 |
5 |
5 |
|
Surfactants
(total) |
20 |
15 |
15 |
15 |
15 |
15 |
10 |
10 |
10 |
10 |
|
E.
coli (count/100
mL) |
1000 |
1000 |
1000 |
1000 |
1000 |
1000 |
1000 |
1000 |
1000 |
1000 |
Notes:1.
All units in mg/L unless otherwise stated; and
2.
All figures are upper limits unless otherwise indicated.
Source:
Technical Memorandum on Standards for
Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal
Waters, Table 10a, Environmental Protection Department.