5.3.3
Inland Water
5.3.3.1
The water quality monitoring results at stations
in Tin Shui Wai Nullah, TSR1 and TSR2, are shown in Table 5.7. According to the EPD’s
publication “River Water Quality in Hong Kong in 2020”, Tin Shui Wai Nullah had an overall WQO compliance of 86% in
2020 (Figure 14). The upstream
monitoring station (TSR2) and downstream station (TSR1) maintained “Good” and
“Fair” WQI respectively in 2020. In terms of E. coli, the two river monitoring stations of Tin Shui Wai Nullah recorded
“Very High” (TSR1) and “High” E. coli levels, mainly due to discharges from livestock farms, surface runoffs
from unsewered villages as well as expedient connections in old districts.
Table 5.7 Summary Statistic of 2020 River Water Quality of Tin Shui Wai
Nullah
|
|
Tin
Shui Wai Nullah
|
WPCO
WQO
|
|
Parameters
|
TSR1
|
TSR2
|
(in
inland waters)
|
Dissolved Oxygen (mg/L)
|
5.9
(2.5 - 7.2)
|
9.2
(8.1 - 10.3)
|
Waste
discharges shall not cause the level of dissolved oxygen to be less than 4 mg/L
|
pH
|
7.6
(6.9 - 8.2)
|
8.5
(7.5 – 9.0)
|
The pH of the
water should be within the range of 6.0-9.0
|
Suspended solids (mg/L)
|
9.7
(2.0 - 28.0)
|
5.4
(2.5 - 250.0)
|
Waste
discharges shall not cause the annual median of suspended solids to exceed 20
mg/L
|
5-day Biochemical Oxygen
Demand (mg/L)
|
5.6
(4.5 - 21.0)
|
2.0
(1.6 - 2.7)
|
Waste
discharges shall not cause the 5-day biochemical oxygen demand to exceed 5
mg/L
|
Chemical Oxygen Demand
(mg/L)
|
15
(9 - 41)
|
6
(3 - 14)
|
Waste discharges shall not cause the chemical oxygen
demand to exceed 30 mg/L
|
|
|
<0.5
(<0.5
- <0.5)
|
<0.5
(<0.5 - <0.5)
|
Not available
|
E. coli (cfu/100mL)
|
260 000
(26 000 – 3 600 000)
|
21 000
(5 500 - 91 000)
|
Not available
|
Faecal Coliforms (cfu/100mL)
|
480 000
(77 000 - 4 100 000)
|
49 000
(20 000 - 170 000)
|
Not available
|
Ammonia-nitrogen (mg/L)
|
2.600
(0.970 - 5.000)
|
0.560
(0.230 - 1.800)
|
Not available
|
Nitrate-nitrogen (mg/L)
|
0.620
(0.006 - 1.200)
|
0.750
(0.650 - 2.200)
|
Not available
|
Total Kjeldahl Nitrogen (mg/L)
|
3.10
(1.70 - 6.00)
|
1.30
(0.38 - 2.50)
|
Not available
|
Ortho-phosphate Phosphorus (mg/L)
|
0.140
(0.015 - 0.370)
|
0.057
(0.009 - 0.120)
|
Not available
|
Total Phosphorus (mg/L)
|
0.31
(0.22 - 0.65)
|
0.10
(0.04 - 0.23)
|
Not available
|
Sulphide
(mg/L)
|
<0.02
(<0.02 - 0.06)
|
<0.02
(<0.02 - <0.02)
|
Not available
|
Aluminium
(µg/L)
|
71
(<50 - 125)
|
141
(<50 - 421)
|
Not available
|
Cadmium
(µg/L)
|
<0.1
(<0.1 - <0.1)
|
<0.1
(<0.1 - <0.1)
|
Not available
|
Chromium
(µg/L)
|
<1
(<1 - <1)
|
<1
(<1 - 1)
|
Not available
|
Copper
(µg/L)
|
5
(2 - 10)
|
<1
(<1 - 1)
|
Not available
|
Lead
(µg/L)
|
<1
(<1 - <1)
|
<1
(<1 - <1)
|
Not available
|
Zinc
(µg/L)
|
<10
(<10 - 25)
|
<10
(<10 - 19)
|
Not available
|
Flow
(m3/s)
|
NM
|
0.108
(0.050 - 0.370)
|
Not available
|
Notes:
1. Data source:
EPD River Water Quality in Hong Kong in 2020
2. Data
presented are in annual medians of monthly samples; except those for faecal
coliforms and E. coli which are in annual geometric means
3. Figures in
brackets are annual ranges
4. cfu – colony
forming unit
5. Values at or
below laboratory reporting limits are presented as laboratory reporting limits
6. Equal values
for annual median (or geometric means) and ranges indicate that all data are
the same as or below laboratory reporting limits
7. NM indicates
no measurement taken
5.3.3.2
As part of this
assessment, water quality surveys were conducted near the HSKEPP to supplement
the baseline water quality information.
The surveys were conducted as per a water quality survey plan submitted
to EPD. The location of survey point, SP1
(downstream Tin Shui Wai Nullah) is shown in Figure 5.2. Water quality surveys were
conducted in 4 consecutive non-rainy days.
On each sampling day, grab samples for laboratory analysis shall be
collected at three specific times, including (i) 10:00am, (ii) 1:00pm and (iii)
4:00pm. All samples were retrieved at
the mid-depth of the water column. The water
quality survey results are tabulated in Table 5.8 below and summarized in the following paragraph.
5.3.3.3
Water quality survey
were conducted from 2 July 2021 to 5 July 2021 respectively to supplement the
water quality data near the HSKEPP. High levels of BOD5 (ranged from 6-18 mg/L)
and low level of DO (ranged from 0.74-8.72 mg/L) are measured in SP1 where poor
water circulation and high level of nutrients NH3-N (ranged from
5.5-11 mg/L), TKN (ranged from 5.8-14 mg/L) was observed, which may indicate
that growth of biomass occurred due to high organic loading of the inland
waters. This could be attributed to sewage discharges into the inland water. E. coli ranged from 59000-300000 cfu/100mL.
The levels of SS are
considered satisfactory, as compared to the higher annual mean reported in TSR1
in River
Water Quality in Hong Kong in 2020.
Table 5.8 Water Quality Survey Results under this Study
|
Parameters
|
SP1
|
In-situ
measurement
|
|
Temperature
(°C)
|
31.3
(29.4 – 32.8)
|
Dissolved Oxygen
(mg/L)
|
2.74
(0.74 – 8.72)
|
Dissolved Oxygen
(% Saturation)
|
38.2
(9.8 - 122.1)
|
pH
|
7.8
(7.5 – 8.3)
|
Salinity
|
2.52
(0.74 – 3.59)
|
Flow Velocity
(m/s)
|
0.1
(0.0 - 0.2)
|
Laboratory Results
|
|
Total Suspended Solids
(mg/L)
|
6.0
(2.9 – 8.8)
|
5-day Biochemical Oxygen
Demand (BOD5)
(mg/L)
|
10
(6 - 18)
|
Chemical Oxygen Demand (COD)
(mg/L)
|
28
(22 - 34)
|
E. coli
(cfu/100mL)
|
120 000
(59 000 - 300 000)
|
Ammonia
(mg NH3-N/L)
|
8.0
(5.5 - 11.0)
|
Nitrate
(mg NO3--N/L)
|
0.114
(<0.002 - 0.948)
|
Nitrite
(mg NO2--N/L)
|
0.087
(<0.002 - 0.319)
|
|
Total Kjeldahl Nitrogen (TKN)
(mg N/L)
|
10.0
(5.8 - 14.0)
|
|
Total Phosphorus (TP)
(mg PO43--P/L)
|
0.49
(0.34 - 0.77)
|
5.4.1.1
Major WSRs identified in the North Western and
North Western Supplementary WCZs are listed below and their indicative
locations are given in Figure 5.1.
·
Bathing Beaches (B1-B3);
·
Tuen Mun Typhoon Shelter (T1);
·
Tai O Estuary (S1);
·
Marine Park (E17, P1-P7);
·
Cooling Water Inlets (C3-C4,
C7-C10, C12, C15, C18);
·
WSD Saltwater Intakes (C5, C6, C11,
C14, C19, C20);
·
Various Coral / Mangrove /
Seagrass / Horseshoe Crab Habitat (E2-E4, E6-E9, E16, E19-E23);
·
Site of Special Scientific
Interest (SSSI) (E5, E10, E11); and
·
Fishing / Spawning Grounds in
North Lantau (E18).
5.4.1.2
Key marine WSRs identified in the Deep Bay WCZ
(Inner Marine Subzone, Outer Marine Subzone, Mariculture Subzone and Yuen
Long& Kam Tin (Lower) Subzone) are listed below and their indicative
locations are shown in Figure 5.1 and Figure 5.2.
·
Mai Po Inner Deep Bay Ramsar
Site/ Inner Deep Bay SSSI (E27);
·
Mai Po Marshes (E26);
·
Oyster Culture Area (E24);
·
Cooling Water Inlets (C1, C2);
and
·
Various Coral / Mangrove /
Seagrass / Horseshoe Crab Habitat (E12-E15, E25, E28).
5.4.1.3
Major inland water bodies within 500m of the
site boundary of the Project were identified.
These inland WSRs are shown in Figure 5.3 are listed on below.
·
Upstream of Tin Shui Wai Nullah
(W1);
·
Downstream river of Yuen Tau
Shan (W2);
·
Natural watercourse (W3);
·
Watercourse near the Kong Sham
Western Highway (W4, W5);
·
Existing ponds adjacent to
HSKEPP; and
·
Mitigation pond under Deep Bay
Link Project (AEIAR-064/2002).
5.5.1
General
5.5.1.1
According to Clause 3.4.6.2
of the Study Brief, the Study Area for this water quality impact assessment
include areas within 500m from the boundary of the Project and would be
extended to include WSRs, such as the natural streams and nullah in the
vicinity of the Project that may be impacted. Potential water quality impact associated to
the construction and operation of the Project will be assessed in accordance with
Annex 6 – Criteria for Evaluating Water Pollution and Annex 14 – Guidelines for
Assessment of Water Pollution under the EIAO-TM.
5.5.2
Construction Phase
5.5.2.1
Potential water quality
impacts during construction and operation of the Project are related to
wastewater generated from land-based construction works. There will be neither dredging, nor
reclamation works, and all the works will be land-based. Hence, water quality modelling is not
proposed. Practical water pollution
control measures were recommended to mitigate identified water quality impacts.
Modelling Tools
5.5.3.1
Computer modelling was
used to assess the potential water quality impacts from sewage effluent
discharge during the operation phase of HSKEPP.
The hydrodynamic and water quality modelling
platforms were developed by Delft Hydraulics, namely the Delft3D-FLOW and
Delft3D-WAQ respectively.
5.5.3.2
Delft3D-FLOW is a
3-dimensional hydrodynamic simulation programme which calculates non-steady
flow and transport phenomena that result from tidal and meteorological forcing
on a curvilinear, boundary fitted grid.
5.5.3.3
Delft3D-WAQ is a
water quality model framework for numerical simulation of various physical,
biological and chemical processes in 3 dimensions. It solves the advection-diffusion-reaction
equation for a predefined computational grid and for a wide range of model
substances.
Model Grid Layout and Properties
5.5.3.4
The Delft3D Yuen Long (YL) Model adopted under
the EIA study for "Yuen Long Effluent Polishing Plant – Investigation,
Design and Construction" (hereafter YLEPP EIA Study) was adopted for
this Study. Appendix 5.2 shows the grid layout
and properties of the YL Model at the study area. As shown in Appendix 5.2, the grids were
refined by means of a domain decomposition technique to achieve fine grid sizes
near the Project. The YL Model covers
the Hong Kong western waters including the North Western, North Western
Supplementary, Western Buffer and Deep Bay Water Control Zones (WCZs) and the
adjacent Mainland waters including the Pearl River Estuary. The YL Model consists of 38,590 grid
cells. Grid size at the open waters is
less than 400m in general. The grid
cells near Mai Po Nature Reserve are about 70m.
The grid quality of the detailed model is generally good except in some
areas at or close to the land boundary.
In view of the small flow velocity at the land boundary, numerical
errors associated with the change of orthogonality should be small. Therefore, the closed grid cells at the
coastlines have been adjusted to form a grid line that is parallel to land
boundary (rather than keeping these closed grid cells orthogonal). Orthogonality at open grid cells has been
checked to be adequate. The grid
properties of detailed model grid including orthogonality, N-smoothness and
M-smoothness are shown in Appendix 5.2.
5.5.3.5
The YL Model is linked to the Update Model, which
was constructed, calibrated and verified under the project “CE42/97 Update
on Cumulative Water Quality and Hydrological Effect of Coastal Development and
Upgrading of Assessment Tool (Cumulative Study)”. Computations were first carried out using the
Update Model to provide open boundary conditions to the YL Model. The Update Model covers the whole HKSAR and
the adjacent Mainland waters including the discharges from Pearl River. The influence on hydrodynamics and water
quality in these outer regions would be fully incorporated into the YL Model.
5.5.3.6
The performance of the YL Model has already been
checked against with Western Harbour (WH) Model which adopted under the EIA
Study for “Cumulative Environmental Impact Assessment Study for the Three
Potential Nearshore Reclamation Sites in the Western Waters of Hong Kong -
Investigation” (CEIA Study). Model
results under YLEPP EIA Study. The
results of the actual simulation periods (with sufficient spin-up periods) for
water level, depth averaged flow speed, depth averaged flow directions,
salinity predicted by the two models have been compared at two indicator points
within the modelled area. The results of
momentary flows and accumulated flows have been compared at the selected cross
sections to check for the consistency.
Locations of the selected indicator points and cross sections are shown
in Appendix 5.2-04 and Appendix 5.2-17. Momentary flow represents the instantaneous
flow rate at a specific time in m3/s whereas accumulated flow
represents the total flow accumulated at a specific time in m3. The comparison plots are given in Appendix 5.2-05 to 5.2-15 and Appendix 5.2-19 to 5.2-25. The comparison plots indicated that the model
results predicted by both models were in general consistent with each other which
implied that the model settings of the YL Model as well as the nesting
procedures were carried out correctly.
There would have minor difference between the two model results which
was caused by the difference in grid resolutions between the two models. As the YL Model has relatively finer grid
resolution, it should have a more accurate representation of the bathymetry and
coastline in Deep Bay waters as compared to the WH Model and hence the
deviation is reasonable.
5.5.3.7
The bathymetry schematization of this model has
been updated based on the depth data from marine charts (Charts for Local
Vessels 2018) produced by the Hydrographic Office of Marine Department, with
incorporation of the projects affecting bathymetry (tabulated in Table 5.9). The hydrodynamic effect of the Contaminated
Mid Pit (CMP) at East Sha Chau and The Brothers has also been incorporated and
the final level at the CMP after capping was assumed in the modelling
scenarios.
5.5.3.8
For each modelling
scenario, the hydrodynamic simulations was performed for both dry and wet
seasons, and the simulation period covered a 15-day full spring-neap cycle
(excluding the spin-up period) for each of the dry and wet seasons. The hydrodynamic results of 15 days will then
be used repeatedly to drive the water quality simulations for at least a 15-day
full spring-neap cycle (excluding the spin-up period) for each dry and wet
season respectively. A spin-up period of
7 days and 45 days will be provided for hydrodynamic simulation and water
quality simulation respectively. Hence,
the hydrodynamic model simulation period will consist of a spin-up period of 7
days plus an actual simulation period of 15 days (total 22 days). Similarly, the water quality model simulation
period will consist of a spin-up period of 45 days plus an actual
simulation period of at least a complete year. In order to determine whether sufficient
spin-up period is provided for the simulation, a Spin-up Test was conducted by
repeating the model run for one more simulation period to check the spin-up
period is sufficient. The spin-up test results at EPD marine station (DM2 and
NM5) for dry and wet season are presented in Appendix 5.12. It was found that the results
of the two successive actual simulation and repeat model run were consistent
with each other, which indicated that the spin-up period was sufficient.
Other Model Setting and Model Parameters
5.5.3.9
The general settings of the model such as the approach
to the setup of boundary and initial conditions as well as the model
coefficients and parameters followed those adopted under the YLEPP EIA Study
(AEIAR-220/2019).
5.5.3.10
Major
factors that would affect the water quality simulated would be (i) the change
in pollution loading discharged to marine waters; and (ii) the change in
coastline configurations in different time horizons.
5.5.3.11
The recommended design capacity of the Project is 90,000
m3 per day was adopted for worst case assessment as it
represents the worst case in terms of the amount of Project flow under
operation phase.
5.5.3.12
The coastline configurations for construction
and operation phase will incorporate with the coastal developments due to the
major existing / planned projects that might potentially affect the
hydrodynamic regime and water quality in the study area. The planned reclamations as listed in Table 5.9 would be
completed and therefore would represent a worst case in terms of the tidal
flushing and assimilation capacity of the marine water. Table 5.9 shows the coastal development projects
incorporated in the coastline configurations for modelling.
Table 5.9 Projects Incorporated in
Modelling
|
Project
|
Source
of Information on Project Layout
|
|
Yuen Long South Effluent
Polishing Plant
|
EIA Study Brief for
“Yuen Long South Effluent Polishing Plant” (EIA Study Brief No.: ESB-313/2019)
|
|
Yuen Long Effluent Polishing Plant
|
EIA Report for
“Yuen Long Effluent Polishing Plant” (EIAO Register No.: AEIAR – 220/2019)
|
|
Development of
Integrated Waste Management Facilities (IWMF) Phase 1
|
EIA Report for
“Development of IWMF Phase 1” (EIAO Register No.: AEIAR – 163/2012)
|
|
Harbour Area
Treatment Scheme (HATS) Stage 2A
|
EIA Report for
“HATS Stage 2A” (EIAO Register No.: AEIAR – 121/2008)
|
|
Hong Kong – Zhuhai
– Macao Bridge (HZMB) Hong Kong Boundary Crossing Facilities (BCF)
|
EIA Report for
“HZMB Hong Kong BCF” (EIAO Register No.: AEIAR – 145/2009)
|
|
Hong Kong Link Road
(HKLR)
|
EIA Report for
“HZMB – Hong Kong Link Road” (EIAO Register No.: AEIAR – 144/2009)
|
|
New Contaminated
Mud Marine Disposal Facility (MDF) at Airport East / East Sha Chau Area
|
EIA Report for “New
Contaminated Mud MDF at Airport East / East Sha Chau Area” (EIAO Register
No.: AEIAR – 089/2005)
|
|
Expansion of Hong
Kong International Airport into a Three-Runway System (3RS)
|
EIA Report for
“3RS” (EIAO Register No.: AEIAR – 185/2014)
|
|
Sha Tin to Central
Link (SCL)
|
EIA Report for “SCL
Protection Works at Causeway Bay Typhoon Shelter: (EIAO Register No.: AEIAR –
159/2011), EIA Report for “SCL – Hung Hom to Admiralty Section” (EIAO
Register No.: AEIAR – 166/2012) and EIA Report for “SCL – Tai Wai to Hung Hom
Section” (EIAO Register No.: AEIAR – 167/2012)
|
|
Kai Tak Cruise
Terminal
|
EIA Report for
“Dredging, Works for Proposed Cruise Terminal at Kai Tak” (EIAO Register No.:
AEIAR – 115/2007)
|
|
Tuen Mun – Chek Lap
Kok Link (TM-CLKL)
|
EIA Report for
“TM-CLKL” (EIAO Register No.: AEIAR – 146/2009)
|
|
Tung Chung New Town
Extension (TCNTE)
|
EIA Report for
“TCNTE” (EIAO Register No.: AEIAR – 196/2016)
|
|
Contaminated Mid
Pit (CMP) at South Brothers
|
EIA Report for “New
Contaminated Mud Marine Disposal Facility at Airport East / East Sha Chau
Area” (EIAO Register No.: AEIAR-082/2004)
(Remark: The
hydrodynamic effect of the capped CMP will be incorporated into the
hydrodynamic model. The final level after
capping of the CMP is assumed in the model under all modelling scenarios)
|
|
CMP at East Sha
Chau
|
|
Sunny Bay
Reclamation
|
PWP Item No. 751CL
- Planning and Engineering Study on Sunny bay Reclamation
|
|
Road P1 (Tai Ho – Sunny Bay Section)
|
EIA Report for “Road P1 Advance Works at Yam O on Lantau Island” (EIAO Register No.: AEIAR-090/2005)
|
Background Pollution Loading
5.5.3.13
The pollution loading of the HSKEPP used for modelling
will be compiled with reference to the design flow and loads. The pollution loading of other background
discharges to the marine water will be based on the pollution loads complied for
the YL Model under the YLEPP EIA study for cumulative assessment.
5.5.3.14
In addition to the background pollution loading
extracted from the YLEPP EIA study, the following pollution loads in Table 5.10 from
concurrent EIA projects were also considered to assess the cumulative impacts.
Table 5.10 Pollution Loads within Deep Bay from
Concurrent EIA Projects
|
Project
|
Source
of Information on Project Layout
|
|
Yuen Long South Effluent Polishing Plant (EIA Study Brief No.:
ESB-313/2019)
|
A new Yuen Long
South Effluent Polishing Plant with a design tertiary treatment capacity of
65,000 m3/d
|
|
Yuen Long Effluent Polishing Plant (EIAO Register No.: AEIAR –
220/2019)
|
The existing Yuen
Long STW will be upgraded to Yuen Long Effluent Polishing Plant with a design
tertiary treatment capacity of 180,000 m3/d.
|
|
North East New
Territories New Development Areas (EIAO Register No.: AEIAR – 175/2013)
|
Increase population
of around 180,000 with sewage treated at the expanded Shek Wu Hui Effluent
Polishing Plant with a design tertiary treatment capacity of 190,000 m3/d.
|
|
Development of Lok
Ma Chau Loop
(EIAO Register No.:
AEIAR – 176/2013)
|
Development of Lok
Ma Chau Loop with sewage treated at the proposed Lok Ma Chau Sewage Treatment
Works with a design tertiary treatment capacity of 18,000 m3/d.
|
Modelling Scenarios
5.5.3.15
The following five assessment scenarios have
been evaluated in this study:
·
Scenario 1: Base Case – “Without Project” condition;
·
Scenario 2: Normal Operation of HSKEPP – normal operation of HSKEPP
(ADWF = 90,000 m3/day) with secondary plus treatment level discharge
to Urmston Road submarine outfall via NWNT tunnel;
·
Scenario 3: NWNT Tunnel Maintenance Discharge – 200,000 m3/day CEPT effluent from upgraded SWSTW and
90,000 m3/day secondary plus treated effluent from HSKEPP, all discharging
to Tin Shui Wai Nullah for 12 days when NWNT tunnel is under maintenance;
·
Scenario 4: Emergency
Discharge from HSKEPP – 2-hr emergency discharge (20,000 m3) of raw
sewage from HSKEPP under power / plant failure to Tin Shui Wai Nullah; and
·
Scenario 5 (Sensitivity
Test): Sensitivity Test of HSKEPP – normal
operation of HSKEPP (ADWF = 90,000 m3/day) with tertiary treatment
level discharge to Tin Shui Wai Nullah.
“Without Project” and
“With Project” conditions – Scenario 1 and 2
5.5.3.16
San Wai Sewage Treatment Works (SWSTW) is
designed to cater for the sewage in San Wai catchment. Under the approved
EIA study for “Upgrading and expansion of San Wai Sewage Treatment
Works and expansion of Ha Tsuen Sewage Pumping Station” completed in 2003, it was proposed to expand the
design flow of SWSTW to 246,000 m3/day and upgrade its treatment
level to Chemically Enhanced Primary Treatment (CEPT) plus UV
disinfection. This approved EIA
concluded that the proposed upgrading works would be environmentally
acceptable. However, during the later
detailed design stage under Agreement No. CE43/2007 (DS) (), the capacity of
SWSTW (Phase 1) has been revised to 200,000 m3/day. The treatment level of SWSTW would also be
upgraded to CEPT plus UV disinfection under Agreement No. CE43/2007 (DS). The construction of SWSTW (Phase 1) has been
substantially completed in March 2021.
The SWSTW has gone into the operation phase starting from that point.
5.5.3.17
Scenario 1 represents the base case condition
without commissioning of HSKEPP. Under
this situation, the Urmston Road submarine outfall would only receive the
treated effluent from the SWSTW. It is assumed
that the design capacity of the existing SWSTW will be expanded to 246,000 m3/day
and its treatment level will be upgraded to CEPT plus UV disinfection as proposed
in the approved EIA study for SWSTW.
5.5.3.18
Scenario 2 represents the “with Project”
condition with implementation of the proposed HSKEPP. The Urmston Road submarine outfall would
receive the treated effluents from the new HSKEPP and the
upgraded SWSTW. It is assumed that
the upgraded SWSTW (200,000 m3/day) proposed under Agreement no.
CE43/2007(DS) would provide CEPT plus UV disinfection, whilst the new HSKEPP
(90,000 m3/day) would employ secondary treatment plus UV
disinfection. With more advanced
treatment technology adopted in HSKEPP, BOD5, TN and E. Coli can be reduced by more than
60%. The commission of HSKEPP begins
from 2031 and it is envisaged to treat 90,000 m3/day in 2038. The effluent flow and qualities of the SWSTW
and HSKEPP are presented in Table 5.11.
Table 5.11 Assumed Effluent Qualities under
Normal Operation Scenarios
|
|
Scenario 1
|
Scenario 2
|
Scenario 5 (sensitivity test)
|
|
Discharge Location
|
Urmston Road Submarine Outfall
|
Urmston Road Submarine Outfall
|
Urmston Road Submarine Outfall
|
Tin Shui Wai Nullah
|
|
STW
|
Upgraded San Wai STW (1)
|
Upgraded San Wai STW (1)
|
HSKEPP
|
Upgraded San Wai STW (1)
|
HSKEPP
|
|
Effluent (m3/day)
|
246,000(5)
|
200,000(6)
|
90,000
|
200,000
|
90,000
|
|
Treatment Level
|
CEPT and UV Disinfection
|
CEPT and UV Disinfection
|
Secondary Plus
|
CEPT and UV Disinfection
|
Tertiary
|
|
5-day Biochemical Oxygen Demand (BOD5)
(2)
|
mg/L
|
100
|
100
|
20
|
100
|
10
|
|
Suspended Solids (SS) (2)
|
mg/L
|
55
|
55
|
30
|
55
|
10
|
|
Ammonia Nitrogen (NH3-N) (3)
|
mg/L
|
25
|
25
|
2
|
25
|
2
|
|
Total Nitrogen (TN) (3)
|
mg/L
|
33.8
|
33.8
|
10
|
33.8
|
10
|
|
Total Phosphorus (TP) (3)
|
mg/L
|
2.26
|
2.26
|
2.26
|
2.26
|
1
|
|
E.coli (4)
|
no./100mL
|
2.0 x 104
|
2.0 x 104
|
1000
|
2.0 x 104
|
100
|
Notes:
1. The CEPT
effluent quality is based on the assumptions adopted in the Appendix 5.2 of
approved EIA Report for “Hung Shui Kiu New Development Area”
2. Data are 95th percentile of effluent quality of respective treatment design
standards
3. Data are
annual average of effluent quality of respective treatment design standards
4. Data are
monthly geometric mean of effluent quality of respective treatment design
standards
5. Design flow
of upgraded SW STW assumed in the approved EIA for SW STW
6. Design flow of upgraded SW STW (Phase 1) proposed in HSKNDA
(AEIAR-203/2016)
Emergency Discharge due to Maintenance of NWNT Tunnel – Scenario 3
5.5.3.19
As advised by DSD, the longest substantial
emergency repair and maintenance works for NWNT tunnel and the Urmston Road
Submarine Outfall could be up to 12 days. The worst case of the emergency discharge
duration of 12 days would therefore be taken for assessing the NWNT tunnel
maintenance scenario. It was further
assumed that effluent from SWSTW (CEPT and disinfection) and HSKEPP (Secondary
plus level treatment) will be discharged via the proposed rising main for raw
sewage and emergency bypass pipe to Tin Shui Wai Nullah. The
period of the maintenance discharges simulated in the water quality models will
be during dry and wet seasons.
Emergency Discharge from HKSEPP – Scenario 4
5.5.3.20
Water quality modelling was carried out to evaluate
the impact from the discharge of untreated effluent during temporary failure of
power supply as well as other incidents such as pump or equipment failure.
5.5.3.21
Considering emergency situations, untreated
effluent would be discharged directly into the proposed
rising main for raw sewage and emergency bypass pipe to Tin Shui Wai Nullah. One common reason for system failure relates
to unstable power supply. A number of
design alternatives have been provided in Section 2.6 to minimize emergency
discharges, including the provision of duel power supply from CLP plus a
further backup supply from renewable energy, power outage is unlikely to occur for
HSKEPP. In case if unstable power and system hanged,
the standby units would serve the process and the system restarting time will
be less than 2 hours according to DSD’s normal practice. Interim bypass after the primary sewage treatment
(PST) will also be provided in case there is failure in downstream treatment
units to avoid raw sewage bypass.
5.5.3.22
To illustrate the worst possible water quality
effect under this emergency situation, the emergency discharge is assumed to
occur for a period of 2 hours with a total discharge volume of 20,000 m3. This emergency discharge scenario was
simulated for both dry and wet seasons.
The location of the HSKEPP emergency bypass pipe to Tin Shui Wai Nullah
is shown in Figure 5.2. The effluent flow and qualities of HSKEPP
assumed in the water quality modelling under the emergency situation are
tabulated in Table 5.12.
Table 5.12 Assumed Effluent Qualities under
Emergency Discharge Scenarios
|
|
Scenario
3
|
Scenario
4
|
|
Discharge
Location
|
Tin
Shui Wai Nullah
|
Tin
Shui Wai Nullah
|
|
Discharge
Duration
|
12
days
|
2
hr
|
|
STW
|
Upgraded
San Wai STW
|
HSKEPP
|
HSKEPP
|
|
Effluent
|
200,000 m3/day
|
90,000
m3/day
|
Total:
20,000 m3
|
|
Effluent
Type
|
CEPT
and UV Disinfection
|
Secondary
Plus
|
Raw
Sewage
|
|
5-day
Biochemical Oxygen Demand (BOD5)
|
mg/L
|
100
|
20
|
210
|
|
Suspended
Solids (SS)
|
mg/L
|
55
|
30
|
320
|
|
Ammonia
Nitrogen (NH3-N)
|
mg/L
|
25
|
2
|
30
|
|
Total
Nitrogen (TN)
|
mg/L
|
33.8
|
10
|
50
|
|
Total
Phosphorus (TP)
|
mg/L
|
2.26
|
2.26
|
7
|
|
E.coli
|
no./100mL
|
2.0
x 104
|
1,000
|
4.0
x 107
|
Sensitivity Test of
HSKEPP: HSKEPP with Tertiary Treatment Level discharge to Tin Shui Wai Nullah –
Scenario 5
5.5.3.23
A sensitivity test, namely Scenario 5, was
conducted to investigate the change in water quality impacts due to higher
effluent qualities of HSKEPP discharge to Tin Shui Wai Nullah. Similar to Scenario 2, it is assumed that the
design capacity of the HSKEPP will remain 90,000 m3/day. Under Scenario 5, the HSKEPP will employ
tertiary treatment level which will be discharged to Tin Shui Wai Nullah (Deep
Bay WCZ) via the proposed rising main (see Figure 5.2). The assumed tertiary effluent quality is
shown in Table 5.11. In comparison to secondary plus treatment proposed
in scenario 2, the tertiary treatment design significantly reduce the suspended
solids, BOD5, total phosphorus and E. coli in effluent.
Initial Dilution Model
5.5.3.24
Under Agreement
No. CE 2/2011 Hung Shui Kiu New Development Area Planning and Engineering Study – Investigation, an initial dilution
model was simulated to describe the characteristics of effluent plume in the
vicinity of the Urmston Road submarine outfall in various flow and ambient
conditions. The normal operation of
HSKEPP fits the designed discharge capacity of the Urmston Road submarine outfall
and the initial dilution model results are provided in Appendix 5.1. There will be no initial dilution model for
discharge to Tin Shui Wai Nullah as it is expected that the effluent will be
fully mixed with the waters in Tin Shui Wai Nullah and finally discharge to
Deep Bay. The characteristic of the
effluent discharge is therefore considered as river discharge and initial
dilution modelling (near field modelling) is therefore considered not
applicable for this discharge option.
Model Validation
5.5.3.25
The performance
of the YL Model has been checked against with WH Model results. The results of the actual simulation periods
(with sufficient spin-up periods) for water level, depth averaged flow speed,
depth averaged flow directions, salinity predicted by the two models have been compared
at two indicator points within the modelled area. The results of momentary flows and
accumulated flows have been compared at the selected cross section to check for
the consistency. Locations of the
selected indicator points and cross section are shown in Appendix 5.2-04. Momentary
flow represents the instantaneous flow rate at a specific time in m3/s
whereas accumulated flow represents the total flow accumulated at a specific
time in m3. The comparison
plots are given in Appendix 5.2-05
to 5.2-16. The comparison plots indicated that the model
results predicted by both models were in general consistent with each other
which implied that the model settings of the YL Model as well as the nesting
procedures were carried out correctly.
There would have minor difference between the two model results which
was caused by the difference in grid resolutions between the two models. As the YL Model has relatively finer grid
resolution, it should have a more accurate representation of the bathymetry and
coastline in Deep Bay waters as compared to the WH Model and hence the
deviation is considered reasonable.
5.6.1
Construction Phase
General Construction Activities
5.6.1.1
Wastewater generated
from construction activities, including general cleaning and polishing, wheel
washing, dust suppression and utility installation may contain high SS
concentrations. It may also contain a
certain amount of grease and oil. Potential
water quality impacts due to the wastewater discharge can be minimised if
construction and site management practices are implemented to ensure that
litter, fuels, and solvents do not enter public drainage systems. It is expected that if the good site practice
suggested in Section 5.7.1 are followed as far as
practicable, the potential water quality impacts associated with construction
activities would be minimal.
Construction Site Runoff
5.6.1.2
Surface runoff
generated from the construction site may contained increased loads of SS and
contaminants. Potential pollution
sources of site run-off may include:
·
Run-off and erosion of exposed bare soil and earth,
drainage channels, earth working areas and stockpiles;
·
Wash water from dust suppression sprays and wheel
washing facilities; and
·
Fuel, oil and lubricants from maintenance of
construction vehicles and equipment.
5.6.1.3
During rainstorms, site
run-off would wash away the soil particles on unpaved lands and areas with
topsoil exposed, if any. The run-off is
generally characterized by high concentrations of SS. Release of uncontrolled site run-off would
increase the SS levels and turbidity in the nearby streams. Site run-off may also wash away contaminated
soil particles and therefore cause water pollution.
5.6.1.4
Windblown dust would be
generated from exposed soil surfaces in the works areas. It is possible that windblown dust would fall
directly onto the nearby water bodies when a strong wind occurs. Dispersion of dust within the works areas may
increase the SS levels in surface runoff causing a potential impact to the
nearby sensitive receivers.
5.6.1.5
It is important that
proper site practice and good site management be followed to prevent run-off
with high level of SS from entering the surrounding waters. Best Management Practices (BMPs) in
controlling construction site discharges are recommended for this Project. With the implementation of BMPs to control
run-off and drainage from the construction site, disturbance of water bodies
would be avoided and deterioration in water quality would be minimal. Suggested measures to control construction
site run-off and drainage are described in Section 5.7.1.3.
Accidental Spillage of Chemicals
5.6.1.6
The use of chemicals
such as engine oil and lubricants, and their storage as waste materials has the
potential to impact water quality if spillage occurs and enters adjacent
streams. Waste oil may infiltrate into
the surface soil layer, or runoff into the nearby streams, increasing
hydrocarbon levels. The potential
impacts could however be mitigated by practical mitigation measures and good
site practices as described in Section 5.7.1.15.
Sewage Effluent from Construction Workforce
5.6.1.7
During the construction
of the Project, the workforce on site will generate sewage effluent, which is
characterized by high levels of BOD, ammonia and E. coli counts. According to Section
5.6.10 of the Construction Industry Council (CIC)’s publication “Reference
Materials – Construction Site Welfare, Health and Safety Measures”, the number of toilet facilities should be provided at a ratio of not
less than one for every 25 workers. Potential water quality impacts upon the
local drainage and fresh water system may arise from these sewage effluents, if
uncontrolled.
5.6.1.8
Temporary sewage
generation can be adequately treated by interim sewage treatment facilities,
such as portable chemical toilets. The
number of the chemical toilets required for the construction sites should be
subject to later detailed design, the capacity of the chemical toilets, and
contractor's site practices. A licensed
contractor should be employed to provide appropriate and adequate portable
toilets and be responsible for appropriate disposal and maintenance.
5.6.1.9
Provided that sewage is
not discharged directly into storm drains or inland waters adjacent to the
construction site, temporary sanitary facilities are used and properly
maintained, and mitigation measures as recommended in Section 5.7.1.18 are adopted as far as
practicable, it is unlikely that sewage generated from the site would have a
significant water quality impact.
Construction Works in Close Proximity
to Inland Water
5.6.1.10
Construction activities in close vicinity to the
inland water courses (a section of the Upstream of Tin Shui Wai Nullah is
within the Project area as shown in Figure 5.3) may impact water quality
due to the potential release of construction waste and wastewater. Construction waste and wastewater are
generally characterized by high SS concentration and elevated pH. The implementation of adequate construction
site drainage and Best Management Practices as described in Section 5.7.1.2 - 5.7.1.9 and provision of mitigation measures as
specified in ETWB TC (Works) No. 5/2005 “Protection of natural streams/ rivers
from adverse impacts arising from construction works” as detailed in Section 5.7.1.14, it is
anticipated that water quality impacts would be minimal.
Groundwater from
Contaminated Areas, Contaminated Site Run-off and Wastewater from Land
Decontamination
5.6.1.11
It
is identified that some of the construction works areas currently occupied as
open area storage, chemical storage area and warehouses would have land
contamination issues. Proper land
contamination remediation and mitigation measures are proposed in Section 7.
Any contaminated material disturbed, or material which comes into contact with
the contaminated material, has the potential to be washed with site run-off
into watercourses. Any wastewater
discharge from land decontamination processes could also adversely affect the
nearby water environment. Excavated
contaminated materials would be properly stored, housed and covered to avoid
generation of contaminated run-off. Open stockpiling of contaminated materials
will not be allowed. Any contaminated site run-off and wastewater from land
decontamination activities will be properly treated and disposed in compliance
with the requirements of the TM-DSS. Mitigation measures for contaminated site
run-off and wastewater from land decontamination are recommended in Section 5.7.1.20. With
proper implementation of the recommended mitigation measures, the potential
water quality impacts arising from the land decontamination works would be
minimised.
5.6.1.12
Groundwater
pumped out or from dewatering process during excavation works in the
contaminated areas would be potentially contaminated. Any contaminated
groundwater will be either properly treated or properly recharged into the
ground in compliance with the requirements of the TM-DSS. No direct discharge
of contaminated groundwater will be adopted. Mitigation measures and monitoring
requirements for contaminated groundwater discharge/ recharge are recommended
in Sections 5.7.1.21 to 5.7.1.22. With proper implementation of the
recommended mitigation measures, no unacceptable water quality would be
expected from the groundwater generated from contamination areas.
5.6.2.1
Potential water quality
impacts associated with the operation phase include:
·
Project effluent discharge;
·
Effluent discharge during maintenance of NWNT
tunnel;
·
Emergency effluent discharge;
·
Project effluent discharge under sensitivity test
·
Treated effluent reuse;
·
Transportation of organic waste;
·
Wastewater from Sludge Treatment;
·
Non-point source surface run-off from new
impervious areas; and
·
Chemical spillage from storage facilities.
Project Effluent Discharge
5.6.2.2
During the operation phase,
sewage discharge will be the major water pollution source. As mentioned in Section 2, the proposed HSKEPP (Scenario 2) will be designed to achieve a
treatment capacity of 90,000 m3/day in ADWF. The plant adopts secondary plus treatment
standard and the effluent will be discharged at Urmston Road submarine outfall
via NWNT tunnel. The proposed
scenario under normal operation (Scenario 2) assumed that the CEPT effluent of
200,000 m3 per day from San Wai STW and the secondary plus treated
effluent of 90,000 m3/day from the proposed
HSKEPP would be discharged at the Urmston Road submarine outfall. Table 5.13 summarized
the pollution loads to NWNT tunnel under normal operation of San Wai STW and
HSKEPP (Scenario 1, 2 and 5).
Table 5.13 Pollution Load to NWNT Tunnel under
Scenario 1, 2 and 5
|
Parameters
|
Unit
|
Scenario
1
|
Scenario
2
|
Scenario
5
|
|
BOD5
|
kg/d
|
24,600
|
21,800
|
20,000
|
|
SS
|
kg/d
|
13,530
|
13,700
|
11,000
|
|
NH3-N
|
kg/d
|
6,150
|
5,180
|
5,000
|
|
TN
|
kg/d
|
8,315
|
7,660
|
6,760
|
|
TP
|
kg/d
|
556
|
655
|
452
|
|
E. coli
|
count/d
|
4.92E+13
|
4.09E+13
|
4.00E+13
|
Notes: The
calculated pollution loads are based on the assumed effluent flow and qualities
of the SWSTW and HSKEPP as presented in Table 5.11.
5.6.2.3
As shown in Table 5.13, there
would be an increase in pollution loads for SS and TP discharge to NWNT tunnel
due to the proposed effluent discharge from HSKEPP (90,000 m3/day)
under Scenario 2 as compared to Scenario 1.
Under Scenario 5, as only 200,000 m3/day CEPT treated effluent
from SWSTW would be discharged to NWNT tunnel while 90,000 m3/day
tertiary treated effluent would be discharged to Tin Shui Wai Nullah, the
pollution loads discharge to NWNT tunnel under Scenario 5 would be lower than
the pollution loads as adopted under Scenario 1.
Overall Water Quality in North Western and North Western Supplementary
WCZs
5.6.2.4
North Western and North Western Supplementary
WCZs are open waters with good tidal flushing capacity and the level of
compliances in these two WCZs for key water quality parameters of concern
including DO and UIA (which two are important indicator for maintaining a
healthy ecosystem in marine water) was relatively good with reference to past
monitoring data presented in Table 5.5 as well as
the model predictions for both base case (Scenario 1) and normal operation of
HSLEPP (Scenario 2).
5.6.2.5
The water quality modelling results for dry
season and wet season are presented as contour plots in Appendix 5.3 and Appendix 5.4 respectively. The contour plots are presented for 10th
percentile depth-averaged dissolved oxygen (DO), depth-averaged 5-day
biochemical oxygen demand (BOD5), depth-averaged total inorganic
nitrogen (TIN), depth-averaged unionized ammonia (UIA), depth-averaged total
nitrogen, depth-averaged total phosphorus (TP), depth-averaged E.coli, depth-averaged suspended solids
(SS), maximum and mean sedimentation rates, and depth-averaged salinity in the
North Western WCZ. Each figure attached
in Appendix 5.3 and Appendix 5.4 contains two contour
plots for comparison. The upper plot shows the model output for base case
scenario (Scenario 1), whereas the lower plot shows the model outputs for
proposed scenario (Scenario 2). All
contour plots are presented as arithmetic averages except for the E.coli
levels which are geometric mean values and the DO levels which are 10th
percentile values. The water quality in
Inner Deep Bay was found not to be impacted by Scenario 2 hence the contour
plots are not presented nor discussed in this section.
5.6.2.6
Appendix 5.7
tabulates the model results of both baseline and proposed scenarios at
identified WSRs and observation points in ultimate year, dry season, and wet
season. These water quality parameters include
dissolved oxygen (DO), sedimentation rate, E. coli, salinity,
5-day biochemical oxygen demand (BOD5), total inorganic nitrogen
(TIN), total phosphorus (TP), total nitrogen (TN) and suspended solids
(SS). The results are presented as both
annual and seasonal arithmetic depth-averaged values except for the E.coli
levels (which are geometric mean depth-averaged values) and the DO levels
(which are presented as 10th percentile bottom value and 10th percentile depth-averaged
values) as well as the sedimentation rates (which are the maximum values over
the simulation period). The depth-averaged
salinity predicted at the WSRs and EPD Stations were compared to the base case
(Scenario 1) and the percentage changes are tabulated. The results provided for flushing water
intakes and cooling water intakes in this appendix are maximum values over the
simulation period except for the minimum DO.
These data are the results predicted in the middle water layer
where the seawater intake points are located. Only the annual predicted water
quality will be discussed in the subsequent sections.
Dissolved Oxygen
5.6.2.7
Under base case (Scenario 1) and normal
operation of HSKEPP (Scenario 2), full compliance with the marine WQO for 10th
percentile depth-averaged DO (> 4mg/L) was predicted in the North Western
WCZ for dry and wet season as shown in Appendix 5.3 and Appendix
5.4.
5.6.2.8
From Appendix 5.7, full compliance of
DO was predicted at all WSRs and EPD monitoring stations in the North Western and
North Western Supplementary WCZ. Full
compliance of minimum DO is predicted for all WSD saltwater intakes. The predicted 10th percentile
bottom DO and depth-averaged DO were to be 3.57-6.10 mg/L and 4.40-6.23 mg/L respectively
under Scenario 1 (base case). In the
normal operation of HSKEPP (Scenario 2), the 10th percentile bottom
DO and depth-averaged DO were to be 3.60-6.05 mg/L and 4.39-6.20 mg/L respectively. The maximum percentage change in bottom DO
and depth-averaged were -2.04% and -0.65%, the change in DO level due to
Scenario 2 is negligible.
Unionized Ammonia / Total Inorganic Nitrogen / Ammonia Nitrogen
5.6.2.9
From Appendix 5.7, the predicted depth-averaged
TIN at all the WSRs and EPD stations in the North Western and North Western
Supplementary WCZ under Scenario 1 and Scenario 2 failed to comply the WQO
except for E5, E21, E22, E23. The
non-compliance of TIN ranged from 0.52-0.76 mg/L for Scenario 1 and 0.52-0.76mg/L
for Scenario 2. As mentioned in Section
5.3.2.2,
the Study Area are subject to a high TIN level due to the background discharge
from Pearl River Estuary, with non-compliances for TIN recorded in this WCZ
even under the existing situations. Since the maximum percentage change in TIN
was predicted to be 0.01%, the change in TIN level due to Scenario 2 is negligible.
5.6.2.10
Full compliance in mid-depth NH3-N
was predicted in all WSD saltwater intakes and their predicted values were
0.237-0.311 mg/L under scenario 1, and 0.235-0.311mg/L in Scenario 2. Full
compliance of UIA was predicted in all WSRs and EPD Stations. The predicted UIA
value for Scenario 1 and Scenario 2 were both 0.006-0.013mg/L. The change in both NH3-N and UIA
is therefore negligible.
Total
Phosphorus
5.6.2.11
No WQO for total phosphorus is available for the
North Western WCZ and North Western Supplementary WCZ. The predicted depth-averaged mean TP
predicted under both scenarios as presented were less than 0.10 mg/L. The maximum change of TP level is 0.002mg/L (-6.1%), recorded at E25. Hence, changes in TP levels in the North
Western WCZ caused by Scenario 2 were considered to be negligible.
5.6.2.13
Under scenario 1
and scenario 2 simulated for this Study, non-compliance with the WQO for TIN
was predicted at 3 beaches (namely B1 – B3), Tuen Mun Typhoon Shelter (T1), Tai
O Estuary (S1) and various ecological resources (E2-E4, E6 -E11, E16
-E23). The number of WSRs with WQO non-compliances
for TIN was predicted to be the same between the two scenarios. No increase in TIN levels (caused by the
change in the effluent flow and quality of Urmston Road outfall) is predicted
at the WSRs in the North Western and North Western Supplementary WCZ. Similarly, there is no TP increase in the
Urmston Road outfall discharge predicted under the normal operation of the
Project. The occurrence of red tide from
the Project is therefore not expected.
5.6.2.14
TIN and TP were
not a critical factor for triggering red tide in the North Western and North
Western Supplementary waters as the existing red tide occurrence in these two
WCZs was low (despite the fact that the TIN and TP levels and the rates of TIN
non-compliances recorded in these areas were high). The low red tide occurrence in these two WCZs
was limited by other physical factors (e.g. the relatively strong tidal
flushing in these two WCZs would minimize the accumulation of algal biomass).
Suspended Solids
5.6.2.15
Full compliance of WQO for depth-averaged SS was
predicted in all WSRs and EPD Stations in the North Western and North Western
Supplementary WCZ. The maximum percentage
differences in the predicted annual SS levels in the all the North Western WCZ is
-1.54% (at E5), which comply well with the WQO for SS of no more than 30%
change from the ambient levels.
5.6.2.16
Full
compliance of WSD’s SS limit (≤ 10mg/L) is observed at all WSD
Saltwater Intakes expect for C11. At
C11, SS levels were predicted to be 10.75 and 10.77 mg/L under Scenario 1 and
Scenario 2, which both exceeded the target water quality objective. The
percentage change is +0.2% and the increase is considered not significant. Full compliance of CLP SS limit of ≤
700 mg/L for Castle Peak Power Station cooling water intake (C3). No unacceptable SS
impact is predicted for Scenario 2.
5-day Biochemical
Oxygen Demand
5.6.2.17
There is no WQO
available for BOD in the marine water of the North Western nor North Western
Supplementary WCZ. The predicted BOD levels
for WSRs and EPD Stations would be 0.3-3.0 mg/L for both Scenario 1 and
Scenario 2. The WSD has however
specified a target BOD objective for their saltwater water intakes (≤ 10mg/L). Full compliance is
observed for all WSD saltwater intakes in the North Western WCZ and their
predicted values were to be 0.87-2.15mg/L and 0.86-2.11mg/L for Scenario 1 and
Scenario 2 respectively. No unacceptable
BOD impact is predicted for Scenario 2.
Salinity
5.6.2.18
Full compliance of WQO for salinity was
predicted in all WSRs and EPD Stations in the North Western and North Western
Supplementary WCZ. The maximum change of
salinity levels caused by Scenario 2 (as compared to Scenario 1) was
approximately 0.3 %, which comply well with the WQO of no more than 10% change
from the background levels. The
salinity impact was negligible. No unacceptable salinity impact is predicted for Scenario 2.
E. coli
5.6.2.19
The depth-averaged geometric mean E.coli
levels predicted in the bathing beaches (secondary contact recreation zones)
along the coast of Tuen Mun and Tsuen Wan Districts comply with the target WQO
of 180 no. per 100 mL (for water sports activities) under both scenarios
(Scenario 1 and Scenario 2). The maximum
E. coli allowable level for WSD
saltwater intake is less than 20,000 no./100mL and all saltwater intakes in the
North Western WCZ fulfilled the criterion.
Maximum Sedimentation Rate
5.6.2.20
The maximum sedimentation rates predicted for
ecological resources in the North Western and North Western Supplementary WCZ under
both scenarios were less than 12 g/m2/day. As for corals sites, the
maximum sedimentation rate is 8 g/m2/day, which complied well with
the assessment criterion of 200 g/m2/day for ecological subtidal
habitat (refer to Section 5.2.9.1).
5.6.2.21
In summary, the model predicted that the TIN
level in the North Western and North Western Supplementary WCZs was high. The high TIN level was mainly contributed
from the Pearl River flow and other background discharges in the Study
Area. Other water quality parameters
(SS, salinity, DO, BOD, Sedimentation rate, E.
coli and TP) would not show any unacceptable water quality impact under the
normal operation of HSKEPP (Scenario 2).
Overall Water Quality in Deep Bay WCZs
5.6.2.23
Appendix
5.7 shows the predicted water quality for selected WSRs and EPD
Stations. Detail description of Appendix
5.7 can be found in Section 5.6.2.6. Only the annual predicted water quality will
be discussed in the subsequent sections.
Dissolved Oxygen
5.6.2.24
Full compliance of WQO for DO is predicted for
WSRs in the Deep Bay WCZ except for E28 and DM2. Non-compliance of WQO for depth-averaged DO (≥
4mg/L) is predicted for Mangrove along Shan Pui River (E28) for both Scenario 1
and Scenario 2 and their respective values are 2.87 mg/L and 2.85 mg/L. The normal operation of HSKEPP would reduce the
annual minimum depth-averaged DO by 0.7% at E28, the reduction is considered not significant. The predicted water quality at EPD monitoring
station DM2 in the Deep Bay meets the WQO for 10th percentile bottom
DO (≥ 2mg/L) but failed to comply 10th percentile
depth-averaged DO (≥ 4mg/L). At
DM2, the 10th percentile depth-averaged DO is predicted to be 3.89
mg/L in both Scenario 1 and Scenario 2. Overall speaking, no unacceptable
adverse impact in terms of DO is predicted during the normal operation of
HSKEPP.
Unionized Ammonia / Total Inorganic Nitrogen
5.6.2.25
As mentioned in Section
5.3.2.3, the Study Area is subject to a high TIN level due to
high nutrient background loading, with non-compliances for TIN recorded in this
WCZ even under the existing situations. The
predicted depth-averaged TIN at all the
ecological WSRs and EPD monitoring stations in the Deep Bay WCZ under Scenario
1 and Scenario 2 failed to comply with the WQO (≤ 0.7mg/L). Without HSKEPP (Scenario 1), the predicted
TIN levels at selected WSRs and EPD Stations would be 0.78-7.56 mg/L. The normal operation of HSKEPP (Scenario 2)
slightly reduce the TIN levels in selected WSRs and EPD Stations. In comparison to Scenario 1, the
depth-averaged TIN under the normal operation of HSKEPP (Scenario 2) is
predicted to be 0.77-7.56 mg/L. The
maximum percentage change was -1.3% (at E12), which the values in Scenario 1
and Scenario 2 were 0.78 mg/L and 0.77 mg/L respectively.
5.6.2.26
Similarly,
non-compliance in depth-averaged UIA (≤ 0.021mg/L) is predicted at
selected WSRs and EPD Stations in the Inner Marine Subzone, Mariculture Subzone
and Yuen Long & Kam Tin (Lower) Subzone within the Deep Bay WCZ (E26, E27,
E15, E24, E25, E28, DM1, DM2, DM3). Under
Scenario 1, the non-compliance UIA level ranged from 0.022-0.116mg/L and with
HSKEPP (Scenario 2), the UIA levels is predicted to be 0.022-0.116mg/L. The maximum percentage change in UIA was -7.1%
(0.001mg/L increment at DM4 from 0.014 mg/L in Scenario 1 and 0.013 mg/L in
Scenario 2). Hence, no adverse UIA and
TIN impact at the identified WSRs would be expected from the Project.
Total
Phosphorus
5.6.2.27
The WQO did not
specify the target for total phosphorus for the Deep Bay WCZ. The predicted depth-averaged TP predicted at
selected WSRs and EPD Stations under scenario 1 and Scenario 2 are 0.068-1.194
mg/L and 0.068-1.193 mg/L respectively. The
model predicted that the Project would reduce the TP levels at all selected
WSRs and EPD stations in Deep Bay WCZ.
The maximum percentage change was -0.26%, which the values in Scenario 1
and Scenario 2 at E25 were 0.766 mg/L and 0.764 mg/L respectively. Hence, no adverse water quality impact for TP
would therefore be expected.
5.6.2.28
Section 5.6.2.12 suggested the conditions to trigger growth of algal
biomass. Section 5.6.2.12 stated that with TIN level increase by 0.1 mg/L and TP
increase by 0.02 mg/L, the occurrence of red tide is favourable hence vice
versa. Among selected WSRs and EPD
Stations in the Deep Bay WCZ, TIN level increases are less than 0.1mg/L from
Scenario 1 to Scenario 2. Similarly, TP predicted
in all selected WSRs and EPD Stations did not show an increase of 0.02 mg/L
during normal operation of the Project (Scenario 2) when compared to the base case
(Scenario 1). The occurrence of red tide
from the Project is therefore not expected. TIN and TP were not a critical factor for
triggering red tide in the Deep Bay WCZ as the existing red tide occurrence was
low.
Suspended Solids
5.6.2.29
The maximum percentage differences in the
predicted SS levels at all selected WSRs and EPD Stations in the Deep Bay WCZ
caused by the proposed Project is 0.55%, which comply well with the WQO for SS
of no more than 30% change from the ambient levels.
5.6.2.30
Full compliance is observed in CLP Black Point
Power Station (C2) for suspended solids requirement of ≤ 700 mg/L.
5-day Biochemical Oxygen Demand
5.6.2.31
The BOD levels
predicted under Scenarios 1 and 2 are compared in Appendix
5.7. Operation of this Project under Scenario 2 would
slightly increase the depth-averaged BOD level in the selected WSRs and EPD
Stations within the Deep Bay WCZ. There
is no WQO available for BOD in the marine water in the Deep Bay WCZ. Non-compliance of BOD is predicted in E28
where the WQO for BOD (Yuen Long & Kam Tin (Lower) Subzone is present Under the base case (Scenario 1), the BOD
level predicted at E28 was 10.7 mg/L, as compared to the WSD’s target objective
of no more than 5 mg/L. (Yuen Long & Kam Tin (Lower) Subzone). With the implementation of this Project (under
Scenario 2), the BOD at E28 would be 10.8 mg/L, which was 0.9% higher than
Scenario 1, the
increase is considered not significant. Hence, the BOD impact due to the Project is
negligible.
Salinity
5.6.2.32
The difference in the predicted salinity levels
in the Deep Bay WCZ caused by Scenario 2 (as compared to Scenario 1) were also
negligible. The maximum change of
salinity levels caused by Scenario 2 (as compared to Scenario 1) were
approximately 1.0%, which comply well with the WQO of no more than 10% change
from the background levels.
E. coli
5.6.2.33
Full compliance
of E.
coli was predicted in WSRs and EPD
Stations in the Deep Bay WCZ except for E14 and E15. The geometric mean of E. coli at Mangrove along Shan Pui River (E28) complies with
the WQO target (≤ 1000) for Yuen Long & Kam Tin (Lower) Subzone. E. coli
exceedance was predicted in Mariculture Subzone where the geometric mean level
would be 11677 (E14) and 15885 (E15) no./100mL for Scenarios 1, as compared to
the WQO of no less than 610 no./100mL.
In Scenario 2, the geometric mean E. coli level would be 11706 and 15888 no./100mL at E14 and
E15 respectively. As the exceedance was
predicted under both Scenarios 1 (without Project scenario) and 2 (with Project
scenario) and the E. coli level
would be slightly increase by 0.25% (E14) and 0.02% (E15) under Scenario 5
(with Project scenario), the E. coli
exceedance at E14 and E15 was not caused by this Project. The high E. coli level could be attributed by background pollution
from rivers upstream. No adverse E. coli impact is predicted from the Project operation.
Maximum Sedimentation Rate
5.6.2.34
The maximum sedimentation rates predicted for
ecological WSRs in the Deep Bay WCZ under Scenario 1 and Scenario 2 were 18.99
and 18.79 g/m2/day respectively. As for corals sites, the maximum sedimentation
rate in Scenario 1 is 10.76 (E12) and 13.40 (E13) g/m2/day, and 10.64
(E12) and 13.17 (E13) g/m2/day in Scenario 2, which complied well
with the assessment criterion of 200 g/m2/day for ecological subtidal
habitat (refer to Section 5.2.9.1).
5.6.2.35
In summary, the
Inner Marine Subzone of Deep Bay WCZ and Yuen Long & Kam Tin (Lower)
Subzone was most affected by background nutrient pollution. The high TIN level was mainly
contributed from the Shenzhen River and other background discharges in the
Study Area.
In addition, the WQO non-compliance were predicted in the Mariculture
Subzone including E. coli, TIN
and UIA. The level of WQO compliances in
the Outer Marine Subzone of Deep Bay WCZ for the key parameters of concern
including DO and UIA (which are two important indicators for maintaining a
healthy ecosystem in marine water) was relatively good. The changes in water
quality due to operation of the Project under Scenario 2 would not lead to
adverse water quality impacts.
Effluent
Discharge During NWNT Tunnel Maintenance
5.6.2.36
During the maintenance
of NWNT tunnel that last for 12 days, it is assumed that the 200,000 m3/day
CEPT treated effluent from SWSTW and 90,000 m3/day secondary plus
treated effluent from HSKEPP would discharge into the Tin Shui Wai Nullah via
the proposed rising main for raw sewage and emergency bypass pipe.
5.6.2.37
The construction and design of the emergency
bypass pipe would be carried out in a separate agreement by CEDD. The emergency bypass pipe to Tin Shui Wai
Nullah and its associated environmental impact has been assessed in approved
EIA report ref. AEIAR-203/2016under the Project of Agreement No. CE 2/2011 (CE)
Hung Shui Kiu New Development Area Planning and Engineering Study –
Investigation.
5.6.2.38
Maintenance discharge would typically lower the
water quality of watercourses through an a period of released contaminants and
organic matters, however, most of the watercourses identified within the
assessment area are highly modified and exposed to high levels of disturbance,
and supports a low ecological diversity.
Hence, insignificant impact on water quality and ecology is expected as
emergency discharge would not result in long-term or unacceptable water quality
impact on nearby watercourses, mitigation ponds and other wetland habitats.
5.6.2.39
The time-series model
results for dry season and wet season (Scenario 3) are presented in Appendix 5.8 and Appendix 5.9 for depth-averaged dissolved oxygen (DO), depth-averaged
5-day biochemical oxygen demand (BOD5), depth-averaged total
inorganic nitrogen (TIN), depth-averaged unionized ammonia (UIA),
depth-averaged total nitrogen, depth-averaged total phosphorus (TP),
depth-averaged E.coli and
depth-averaged suspended solids (SS). As
the discharge is within the Inner Deep Bay, WSRs at proximity to the discharge
location which include Mai Po Marshes SSSI (E26), Mai Po Inner Deep Bay Ramsar
Site/ Inner Deep Bay SSSI (E27), Oyster Culture Area (E24) Mangroves in Inner
Deep Bay (E25) and Mangrove along Shan Pui River (E28) were selected to illustrate the
spatial changes of pollution elevations at WSRs both close to and further away
from the maintenance discharge point.
Each figure attached in Appendix
5.8 and Appendix
5.9 contains a comparison
for the certain water quality parameter under normal operation of HSKEPP
(Scenario 2) and NWNT maintenance (Scenario 3) at each selected WSR. All time-series plots are presented as
seasonal arithmetic averages except for the E.coli levels which are
geometric mean values. The water quality
in North Western and North Western Supplementary WCZ was found not to be
significantly impacted by Scenario 3 hence the time-series plots are not
presented nor discussed in this section.
5.6.2.40
Appendix 5.7 tabulates the model results of both normal operation
scenario (Scenario 2) and 12-day NWNT maintenance (Scenario 3) at identified
WSRs in Section
5.6.2.39 in ultimate year, dry season, and wet season. Detailed description of Appendix 5.7 can be referred to Section 5.6.2.6. Annual means
values in Appendix 5.7 is not be used for
assessing the impacts of the short-term maintenance discharge.
Time Series Results at Selected WSRs
5.6.2.41
Appendix
5.8 and Appendix
5.9 show the time-series model results
for dry and wet seasons at the selected WSRs.
These WSRs include Oyster Culture Area
(E24) Mangroves in Inner Deep Bay (E25), Mai Po Marshes SSSI (E26), Mai Po
Inner Deep Bay Ramsar Site/ Inner Deep Bay SSSI (E27) and Mangrove along Shan
Pui River (E28). Elevation on selected water quality parameters (BOD5, TIN,
UIA, TN, TP, SS) and DO depletion was observed at mainly E25, E26, E27 and E28
which located close to the maintenance discharge of HSKEPP. The elevated levels at these WSRs were
observed during the 12-day maintenance discharge period and remained for a
period after the end of maintenance discharge.
The elevated levels at these WSRs would be recovered to the normal
operation of HSKEPP (Scenario 2) of about 21 days after termination of the maintenance
discharge for parameters including DO, BOD, TIN, UIA and SS. The maintenance discharge has no observable
elevation on E. coli. The more distant WSRs – Oyster Culture Area (E24),
was found not to be elevated for water quality parameters during or after the maintenance
discharge event, hence the result is not discussed. The maximum value (except
minimum value for DO) provided in this section are obtained from the period of
33 days from the maintenance discharge (model simulation period from January 16
– February 18 in dry season and from May 31-July 2 in wet season), and the
water quality in Scenario 3 is compared to Scenario 2 (of the same period). Table 5.14 presented the maximum
percentage change due to maintenance discharge from HSKEPP.
Table 5.14 Maximum Percentage Change due to
Maintenance Discharge
|
WSRs
|
Parameters
(Depth Averaged)
|
|
DO
|
BOD
|
TIN
|
UIA
|
TN
|
TP
|
E. coli
|
SS
|
|
Dry Season
|
|
E24
|
-3.35%
|
0.68%
|
7.79%
|
15.43%
|
6.42%
|
6.12%
|
0.22%
|
0.94%
|
|
E25
|
-64.74%
|
26.69%
|
33.36%
|
67.87%
|
32.13%
|
25.89%
|
2.43%
|
11.39%
|
|
E26
|
-46.50%
|
16.67%
|
25.23%
|
51.87%
|
24.12%
|
19.46%
|
1.09%
|
8.39%
|
|
E27
|
-17.25%
|
6.24%
|
15.64%
|
25.45%
|
13.30%
|
13.44%
|
0.40%
|
4.00%
|
|
E28
|
-40.92%
|
16.54%
|
25.50%
|
53.57%
|
24.13%
|
19.74%
|
0.96%
|
8.45%
|
|
Wet Season
|
|
E24
|
-6.43%
|
0.53%
|
16.68%
|
28.71%
|
14.70%
|
6.12%
|
0.00%
|
0.00%
|
|
E25
|
-117.46%
|
42.56%
|
106.18%
|
201.18%
|
89.25%
|
46.46%
|
0.47%
|
6.36%
|
|
E26
|
-72.37%
|
27.34%
|
70.93%
|
142.43%
|
60.23%
|
27.04%
|
0.07%
|
3.96%
|
|
E27
|
-28.46%
|
8.05%
|
64.81%
|
106.79%
|
46.28%
|
27.67%
|
0.00%
|
0.66%
|
|
E28
|
-60.35%
|
26.11%
|
58.31%
|
124.71%
|
51.26%
|
21.29%
|
0.02%
|
4.72%
|
Notes: The
values in the above table refers to the percentage change between maintenance
discharge (Scenario 3) and normal operation of HKSEPP (Scenario 2) with maximum
elevation for each water quality parameters.
Hence, Maximum Percentage Change = max. of [(concentration under
Scenario 3) – (concentration under Scenario 2)] / (concentration under Scenario
2).
5.6.2.42
The greatest DO impact
was found at E25 which is located nearest to the discharge point. The DO level at E25 decreased dramatically
during the maintenance discharge period.
The lowest DO levels of 2.3 mg/L and 0 mg/L were predicted at E25 in dry
and wet seasons respectively and returned to almost the same as the normal
operation levels of around 4.8 mg/L and 4.0 mg/L in dry and wet seasons
respectively of about 21 days after termination of the maintenance discharge. The influences on water quality in terms of
the decrease in DO at E26, E27 and E28, which are located further away from the
discharge, are significantly smaller, if any.
5.6.2.43
During maintenance discharge
event in wet season, the minimum DO levels at E25, E26 and E28 were predicted
to be very low (ranged from 0 mg/L to 1.6 mg/L; see Appendix 5.9-01). In comparison with the DO
levels (Scenario 2) in wet season, the minimum DO levels predicted at E25, E26
and E28 were ranged from 2.8 mg/L to 4.0 mg/L, which is also considered to be
low (as compared to the WQO for DO). The
maintenance discharge event in dry season (see Appendix 5.8-01) is however considered to have relatively lower DO impact as compared
to the minimum DO levels at E25, E26 and E28 between Scenario 3 (ranged from
2.3 mg/L to 3.5 mg/L) and Scenario 2 (ranged from 3.9 mg/L to 4.8 mg/L). To avoid undesirable water quality impact in
Deep Bay, it is therefore recommended that the 12-day NWNT tunnel maintenance
should be taken place in dry season and should be avoided in wet season.
Unionized
Ammonia/ Total Inorganic Nitrogen/ Total Nitrogen
5.6.2.44
In terms of UIA, TIN
and TN, it is considered that the greatest impact would occur at E25 which is
located nearest to the discharge point.
The highest UIA levels predicted at E25 was almost 0.17 mg/L and 0.36
mg/L in dry and wet seasons respectively, as compared to the Scenario 2 values
of 0.16 mg/L for both dry and wet seasons.
The highest TIN levels predicted at E25 was almost 11.4 mg/L and 8.4
mg/L in dry and wet seasons respectively, as compared to the Scenario 2 values
of 9.6 mg/L and 5.5 mg/L in dry and wet seasons respectively. The highest TN levels predicted at E25 was
almost 14.5 mg/L and 10.3 mg/L in dry and wet seasons respectively, as compared
to the Scenario 2 values of 12.1 mg/L and 6.9 mg/L in dry and wet seasons
respectively.
5.6.2.45
The UIA, TIN and TN
levels returned to almost the same as the normal operation levels of about 21
days after the end of the maintenance period.
Although the UIA, TIN and TN levels could reach 0.36 mg/L, 11.4 mg/L and
14.5 mg/L respectively at E25, the relative impact is considered low as the normal
operation levels at this station were very high (0.16 mg/L, 9.6 mg/L and 12.1
mg/L for UIA, TIN and TN levels respectively).
Again, the predicted UIA, TIN and TN impacts were significantly lower,
if any, at the rest of the WSRs (E26, E27, E28) which are relatively further
away from the discharge locations.
5-day
Biochemical Oxygen Demand / Total Phosphorus/ Suspended Solids/ E. coli
5.6.2.46
In terms of BOD, TP, SS
and E. coli, the greatest impact was found at E25 under
Scenario 3. These impacts are however
considered small and would be restored within only a few days after termination
of the discharge period. The time-series
plots showed that the impacts for these parameters at E25 would be minimal
under the 12-day maintenance discharge scenario. The maintenance discharge would only cause
minimal increase in the BOD, TP, SS and E. coli levels at E25. It is therefore
deduced that the Mangroves in Inner Deep
Bay (E25) would not be adversely affected by the
Project during the NWNT tunnel maintenance period.
5.6.2.47
In summary, although
the maintenance discharge event would inevitably cause an increase in pollution
levels in Deep Bay, it is considered that the potential water quality impact to
the nearby WSRs is acceptable and reversible.
The model predicted that the elevated levels at the selected WSRs could
be recovered to the normal operation of HSKEPP (Scenario 2) of about 21 days
after termination of the maintenance discharge event. The more distant WSRs, including Oyster
Culture Area (E24) was found not to be elevated during or after the maintenance
discharge event. It is recommended that
the 12-day NWNT tunnel maintenance should be taken place in dry season and
should be avoided in wet season.
Emergency Effluent Discharge
5.6.2.48
During emergency
situations, such as loss of power supply at the HSKEPP, or mechanical faults /
equipment failures may occur at the proposed HSKEPP. In Scenario 4, it is assumed that an
emergency discharge from the HSKEPP would occur for a period of 2 hours and
discharge (total discharge volume: 20,000m3) into the Tin Shui Wai
Nullah via the proposed rising main for raw sewage and emergency bypass pipe in
case of power or plant failure.
5.6.2.49
The construction and design of the emergency
bypass pipe would be carried out in a separate agreement by CEDD. The emergency bypass pipe to Tin Shui Wai
Nullah and its associated environmental impact has been assessed in approved
EIA report ref. AEIAR-203/2016 under the Project of Agreement No. CE 2/2011
(CE) Hung Shui Kiu New Development Area Planning and Engineering Study –
Investigation.
5.6.2.50
Emergency discharge would typically lower the
water quality of watercourses through an acute spike of released contaminants
and organic matters, however, most of the watercourses identified within the
assessment area are highly modified and exposed to high levels of disturbance,
and supports a low ecological diversity.
Hence, insignificant impact on water quality and ecology is expected as
emergency discharge would not result in long-term or unacceptable water quality
impact on nearby watercourses, mitigation ponds and other wetland habitats.
5.6.2.51
To minimise the risk of
untreated effluent discharge due to emergency events, various contingency
measures will be provided to the HSKEPP, such as provision of standby unit for
all major equipment and back-up power for dual power supply. Details of the mitigation measures are
discussed in Section 5.7.2. With the above design provision as
contingency measures, the risk of failure of HSKEPP is considered to be
negligible.
5.6.2.52
The time-series model
results for dry season and wet season (Scenario 4) are presented in Appendix 5.10 and Appendix 5.11 for depth-averaged dissolved oxygen (DO), depth-averaged
5-day biochemical oxygen demand (BOD5), depth-averaged total
inorganic nitrogen (TIN), depth-averaged unionized ammonia (UIA),
depth-averaged total nitrogen, depth-averaged total phosphorus (TP), depth-averaged
E.coli and depth-averaged suspended solids
(SS). As the discharge is within the
Inner Deep Bay, WSRs at proximity to the discharge location which include Mai
Po Marshes SSSI (E26), Mai Po Inner Deep Bay Ramsar Site/ Inner Deep Bay SSSI
(E27), Oyster Culture Area (E24) Mangroves in Inner Deep Bay (E25) and Mangrove
along Shan Pui River (E28) were selected to illustrate the spatial changes of pollution elevations at WSRs both
close to and further away from the emergency discharge point. Each figure attached in Appendix
5.10
and Appendix 5.11
contains a comparison for the certain water quality parameter under normal
operation of HSKEPP (Scenario 2) and emergency (Scenario 4) at each selected WSR. All time-series plots are presented as
seasonal arithmetic averages except for the E.coli levels which are
geometric mean values. The water quality
in North Western waters was found not to be significantly impacted by
Scenario 4 hence the time-series plots are not presented nor discussed in this
section.
5.6.2.53
Appendix
5.7 tabulates the model results of both normal operation scenario
(Scenario 2) and 2-hour emergency discharge (Scenario 4) at identified WSRs in section 5.6.2.52 in ultimate
year, dry season, and wet season. Detailed
description of Appendix 5.7 can be referred to Section 5.6.2.6. Annual means
values in Appendix 5.7 are not be used for
assessing the impacts of the short-term emergency discharge.
Dissolved Oxygen
5.6.2.54
The impact of
short-term emergency discharge under Scenario 4 on DO levels among the selected
WSRs is insignificant in general. Biggest
DO impact were predicted at E25 and E26. According to Appendix 5.7, the predicted DO levels at E25 and E26 in dry season were 6.92 mg/L
and 6.02 mg/L during the emergency discharge scenario, as compared to
the predicted DO level of 6.95 mg/L and 6.06 mg/L under the normal operation
scenario (Scenario 2). The
percentage decrease in DO levels were 0.43% at E25 and 0.66% at E26. Full
compliance of DO was predicted at all the remaining WSRs under emergency
discharge scenario during dry season. In
wet season, non-compliance of DO is predicted for E26 and E28 for both Scenario
2 and Scenario 4. The DO levels at E26
and E28 in wet season were predicted to be 3.85mg/L and 2.85mg/L under Scenario
2, while in Scenario 4 the values were 3.84mg/L and 2.85mg/L respectively.
Unionized Ammonia / Total Inorganic Nitrogen
5.6.2.55
The impact of
short-term emergency discharge under Scenario 4 on UIA and TIN levels among the
selected WSRs is insignificant in general. The biggest TIN and UIA impacts due
to the short-term emergency discharge under Scenario 3 was observed at E26. The predicted TIN and UIA levels at E26
during dry season were 8.17 mg/L and 0.096 mg/L respectively, as compared to
8.15mg/L and 0.096mg/L predicted in Scenario 2.
The percentage increase in TIN is predicted to be 0.24% under Scenario
3. In wet season, the predicted TIN and
UIA levels at E26 were 4.83mg/L and 0.138mg/L, in comparison to 4.81mg/L and
0.137mg/L under Scenario 2, which is an increase of 0.42% and 0.73%
respectively. As mentioned previously,
the Study Area are subject to high nitrogen nutrient loads due to the
background discharge from region, with non-compliances for TIN and UIA at all
selected WSRs recorded in Deep Bay WCZ even under the baseline scenario
(Scenario 1).
Suspended Solids
5.6.2.56
The impact of
short-term emergency discharge under Scenario 4 on suspended solids among the
selected WSRs is insignificant in general.
The biggest SS impact due to the short-term emergency discharge under
Scenario 4 was observed at E26 during wet season with an increase of 0.32%, the
respective SS levels under Scenario 4 and Scenario 2 were 30.9 and 30.8mg/L. The predicted SS levels at all selected WSRs
comply to the WQO of no more than 30% change
from the ambient SS levels (as predicted under Scenario 1).
5-day Biochemical Oxygen Demand
5.6.2.57
The impact of
short-term emergency discharge under Scenario 4 on BOD among the selected WSRs
is insignificant in general, for both the dry and wet season. Non-compliance of BOD levels is recorded at
E28 for both Scenario 2, Scenario 4 as well as the baseline scenario (Scenario
1).
E. coli
5.6.2.58
The impact of
short-term emergency discharge under Scenario 4 on E coli among the selected WSRs is insignificant in general, for both the dry
and wet season. Full compliance of E coli is predicted at E28 for Scenario 4 and
Scenario 2.
Time Series Results at Selected WSRs
5.6.2.59
The time series plots
under the emergency discharge scenarios at selected WSRs are presented in Appendices 5.10 and 5.11. Slight elevation on selected
water quality parameters was observed at E25 which was close to the emergency
discharge of HSKEPP. The elevated levels
at these WSRs were observed right after the emergency discharge. The elevated levels at these WSRs would be
recovered to the normal operation of HSKEPP (Scenario 2) within 0.5 days for
most parameters except E. coli, which can also be recovered in a relatively short time of about 2 day
after termination of the emergency discharge.
The more distant WSRs in Inner Deep Bay such as Oyster Culture Area (E24),
Mai Po Marshes SSSI (E26), Mai Po Inner Deep Bay Ramsar Site/ Inner Deep Bay
SSSI (E27) and Mangrove along Shan Pui River (E28), was found not to be
elevated during or after the emergency discharge event. Table 5.15 presented the maximum percentage change due to
emergency discharge from HSKEPP.
Table 5.15 Maximum Percentage Change due to
Emergency Discharge
|
WSRs
|
Parameters
(Depth Averaged)
|
|
DO
|
BOD
|
TIN
|
UIA
|
TN
|
TP
|
E. coli
|
SS
|
|
Dry Season
|
|
E24
|
-0.08%
|
0.00%
|
0.11%
|
0.23%
|
0.08%
|
0.14%
|
0.02%
|
0.05%
|
|
E25
|
-1.29%
|
0.38%
|
0.46%
|
1.08%
|
0.43%
|
0.63%
|
101.25%
|
0.57%
|
|
E26
|
-0.97%
|
0.25%
|
0.36%
|
0.83%
|
0.35%
|
0.53%
|
12.01%
|
0.48%
|
|
E27
|
-0.40%
|
0.09%
|
0.20%
|
0.37%
|
0.18%
|
0.31%
|
0.58%
|
0.22%
|
|
E28
|
-0.95%
|
0.19%
|
0.35%
|
0.83%
|
0.34%
|
0.49%
|
12.21%
|
0.45%
|
|
Wet Season
|
|
E24
|
-0.08%
|
0.02%
|
0.18%
|
0.34%
|
0.14%
|
0.13%
|
0.00%
|
0.03%
|
|
E25
|
-3.28%
|
1.21%
|
1.19%
|
2.37%
|
1.15%
|
0.68%
|
68.76%
|
0.57%
|
|
E26
|
-2.12%
|
0.79%
|
0.99%
|
1.95%
|
0.93%
|
0.59%
|
40.73%
|
0.44%
|
|
E27
|
-0.64%
|
0.41%
|
0.94%
|
1.38%
|
0.73%
|
0.50%
|
0.61%
|
0.26%
|
|
E28
|
-1.73%
|
0.59%
|
0.85%
|
1.71%
|
0.78%
|
0.45%
|
11.52%
|
0.39%
|
Notes: The
values in the above table refers to the percentage change between emergency discharge
(Scenario 4) and normal operation of HSKEPP (Scenario 2) with maximum elevation
for each water quality parameters.
Hence, Maximum Percentage Change = max. of [(concentration under
Scenario 4) – (concentration under Scenario 2)] / (concentration under Scenario
2).
5.6.2.60
In summary, although
the emergency discharge would inevitably cause a spike in pollution levels in Inner
Deep Bay, it is considered that the potential water quality impact to the
nearby WSRs is acceptable, and the water quality could be recovered in 2 days. In addition, the chance of emergency
discharge event occurrence is rare with design measures in Section 2.6 were
adopted. Mitigation measures as stated
in Section
5.7.2 would also be
provided. Therefore, no adverse water
quality impact is expected.
Project Effluent Discharge Under
Sensitivity Test
5.6.2.61
As mentioned in Section 2, the sensitivity test (Scenario 5) act as a supplementary normal
operation scenario to predict the water quality impact when the 90,000 m3/day
tertiary treated effluent from HSKEPP discharge into Tin Shui Wai Nullah and
eventually into Inner Deep Bay. The sensitivity test (Scenario 5) under normal operation assumed
that the CEPT effluent of 200,000 m3 per day from San Wai STW HSKEPP
would be discharged at the Urmston Road submarine outfall while the tertiary
treated effluent of 90,000 m3/day from
HSKEPP will discharge into Tin Shui Wai Nullah via the proposed rising main.
5.6.2.62
The construction and design of the proposed
rising main would not be under the scope of the Project. Permanent
discharge in Tin Shui Wai Nullah would lower the water quality of watercourses
in a long run through increased pollution load. However, most of the
watercourses identified within the assessment area are highly modified and
exposed to high levels of disturbance, and supports a low ecological diversity. In addition, the water quality impacts were
minimized by the HSKEPP adopting the highest level of treatment (tertiary
treatment). Hence, the water quality and ecological impact associated to this
Scenario is at least, curtailed.
Overall Water Quality in North Western and North Western Supplementary
WCZs
5.6.2.63
North Western and North Western Supplementary
WCZs are open waters with good tidal flushing capacity. Since the sensitivity test (Scenario 5) discharge is in Inner
Deep Bay, little water quality impact is found in the North Western and North
Western Supplementary WCZ, hence contour plot for the North Western and North
Western Supplementary WCZ were not shown.
5.6.2.64
Appendix
5.7 tabulates the model results of both base case (Scenario 2) and
sensitivity test (Scenario 5) at identified WSRs in the North Western and North
Western Supplementary WCZs in ultimate year, dry season, and wet season. Detailed description of Appendix 5.7 can be
referred to Section 5.6.2.6. Only the annual predicted water quality will
be discussed in the subsequent sections.
Dissolved Oxygen
5.6.2.65
Full compliance of WQO for DO is predicted in
all WSRs and EPD Stations in the North Western and North Western Supplementary
WCZ. Full compliance of minimum DO at
all WSD saltwater intakes. The change in
the DO level caused by the change in the discharge location was predicted to be
negligible. Among all seawater intakes,
the percentage change in minimum DO ranges from -1.8% (at C10) to +0.47% (at C20)
when comparing the sensitivity test (Scenario 5) with base case (Scenario 1). The depth-averaged DO predicted at WSRs and
EPD Stations for Scenario 2 and Scenario 5 were 4.24-6.23 mg/L and 4.22-6.21
mg/L respectively. Scenario 5 in general cause a slight drop in depth-averaged
DO (compared with Scenario 1) for all ecological WSRs in the North Western and
North Western Supplementary WCZ. The
highest percentage decrement is found at Sha Chu and Lung Kwu Chau Marine Park
(P4), the DO levels predicted for Scenario 5 and Scenario 1 were 4.43 and 4.47
mg/L (-0.77%). The changes in DO
predicted for EPD stations did not exceed -0.4%, which is considered not
significant.
Unionized Ammonia / Total Inorganic Nitrogen / Ammonia Nitrogen
5.6.2.66
All
WSD saltwater intake in the North Western WCZ were predicted to meet the WSD
target objective for maximum mid-depth NH3-N. Full compliance of WQO for depth-averaged UIA was
predicted in all WSRs within the North Western WCZ.
5.6.2.67
Non-compliance
of WQO for depth-averaged TIN is predicted in all selected WSRs and EPD
Stations within the North
Western WCZ and North Western Supplementary WCZ except for E5, E21, E22 and E23. Under Scenario 1, the predicted annual TIN levels at all selected WSRs and EPD Stations were
ranged from 0.53-0.76
mg/L. For the tertiary treated effluent from HSKEPP discharge into Tin Shui Wai
Nullah (Scenario 5),
the predicted annual TIN levels at all selected WSRs
and EPD Stations would be slightly reduced to 0.52-0.75 mg/L.
The maximum percentage change was -2.9% (with 0.01mg/L reduction). The predicted non-compliance for TIN was
mainly contributed by the background pollution sources, which carries a high
loading of nitrogen nutrients (as compared to the WQO for TIN). Hence, no adverse TIN impact is predicted in
the North Western waters under Scenario
5.
Total Phosphorus
5.6.2.68
No WQO for total phosphorus available for the
North Western WCZ and North Western Supplementary WCZ. The predicted depth-averaged TP at WSRs and
EPD Stations in the North Western and North Western Supplementary WCZ under Scenario
1 and Scenario 5 were to be 0.033-0.063 mg/L and 0.030-0.062 mg/L. Improvement of TP is predicted under Scenario
5 and the largest TP reduction (7.36%) is predicted at E23. No adverse TP impact was predicted under
Scenario 5.
Suspended Solids
5.6.2.69
The maximum percentage differences in the
predicted annual SS levels in the all the North Western WCZ WSRs including
bathing beaches, typhoon shelter, Tai O Estuary, ecological resources and EPD
routine monitoring stations caused by change in the discharge location was -1.54%
(at E5), which comply well with the WQO for SS of no more than 30% change from
the ambient levels.
5.6.2.70
Full
compliance of annual
mid-depth maximum SS is found in all WSD Saltwater Intakes except for C11. At C11, the SS levels were 10.75 and 10.76
mg/L under Scenario 1 and Scenario 5, marginally exceeded the target water
quality objective (≤ 10mg/L). The SS
level change due to Scenario 5 at C11 is considered not significant. Other cooling water intake point in CLP power
plants fully comply to the limit of ≤ 700 mg/L. Slight improvement in SS (as compared to
Scenario 1) is predicted under Scenario 5 and the percentage reduction ranged
from 0.01% to 1.20%, which is considered no adverse SS impact.
5-day Biochemical Oxygen Demand
5.6.2.71
There is no WQO
available for BOD in the marine water of the North Western or North Western
Supplementary WCZ. Slight improvement in
BOD (compared to Scenario 1) was predicted under Scenario 5 and the percentage
reduction ranged from 0.22% to 2.49%. The
WSD has however specified a target BOD objective for their saltwater water
intakes (≤ 10mg/L).
Full compliance is observed for all WSD saltwater intakes in the North
Western WCZ.
Salinity
5.6.2.72
The differences
in the predicted salinity levels among selected WSRs and EPD Stations the North
Western waters caused by Scenario 5 were also negligible. The maximum change of salinity levels under
Scenario 5 was approximately 0.2%, which comply well with the WQO of no more
than 10% change from the background levels.
E. coli
5.6.2.73
The
depth-averaged geometric mean E.coli levels predicted in the bathing
beaches comply with the target WQO of 180 no. per 100 mL (for water sports
activities) under both scenarios (Scenario 1 and Scenario 5). The maximum E. coli allowable level for WSD saltwater intake is less than
20,000 no./100mL and all saltwater intakes in the North Western WCZ fulfilled
the criterion.
Maximum Sedimentation Rate
5.6.2.74
Full compliance
of maximum sedimentation rate was predicted in North Western and North Western
Supplementary WCZ. The maximum
sedimentation rate criterion assessment for ecological subtidal habitat (refer
to Section 5.2.9.1) is 200 g/m2/day. The maximum sedimentation rates predicted in corals WSRs in the
North Western WCZ under Scenario 1 and Scenario 5 were to be 6.72- 8.00 g/m2/day
and 6.66-7.93 g/m2/day. The
greatest percentage change observed in coral WSRs was -1.7%. No adverse sedimentation impact is predicted
under Scenario 5.
5.6.2.75
In summary, the model predicted that the TIN
level in the North Western and North Western Supplementary WCZs was high. The high TIN level was mainly contributed
from the Pearl River flow and other background discharges in the Study
Area. Other water quality parameters
(SS, salinity, DO, BOD, Sedimentation rate, E.
coli and TP) would not show any unacceptable water quality impact under the
sensitivity test with tertiary treated effluent from HSKEPP discharge into Tin
Shui Wai Nullah (Scenario 5).
Overall Water Quality in Deep
Bay WCZs
5.6.2.76
Deep Bay WCZ is a shallow semi-enclosed water
body with low water exchange rate and poor tidal flushing and therefore,
pollution loading in the Deep Bay WCZ (e.g. due to the polluted storm
discharges from Shenzhen River, Kam Tin River, Yuen Long Creek and Tin Shui Wai
Nullah) would tend to accumulate inside the bay and cannot be easily flushed
out.
5.6.2.77
The water quality modelling results for dry
season and wet season are presented as contour plots in Appendix 5.10 and Appendix 5.11 respectively. The contour plots are presented for 10th
percentile depth-averaged dissolved oxygen (DO), depth-averaged 5-day
biochemical oxygen demand (BOD5), depth-averaged total inorganic
nitrogen (TIN), depth-averaged unionized ammonia (UIA), depth-averaged total
nitrogen, depth-averaged total phosphorus (TP), depth-averaged E.coli, depth-averaged suspended solids
(SS), sedimentation rates and depth-averaged salinity in the Inner Deep
Bay. Each figure attached in Appendix 5.10 and Appendix 5.11 contains two
contour plots for comparison. The upper plot shows the model output for the
base case (Scenario 1), whereas the lower plot reflects the model output for
the sensitivity test (Scenario 5).
Baseline scenario (Scenario 1) assumed that the CEPT effluent of 246,000
m3 per day from upgraded San Wai STW would be discharged at the
Urmston Road outfall. The Sensitivity
test scenario (Scenario 5) is as described in Section 5.6.2.56.
5.6.2.78
Appendix
5.7 shows the predicted water quality for selected WSRs in the Deep Bay
WCZ. Detail description of Appendix
5.7 can be found in Section 5.6.2.6. Only the annual predicted water quality will
be discussed in the subsequent sections.
Dissolved Oxygen
5.6.2.79
Full compliance of WQO target for DO is
predicted at all WSRs and EPD Stations in the Deep Bay WCZ under scenario 5
except for E28 and DM2. Non-compliance
of WQO for 10th percentile depth-averaged minimum DO (≥ 4mg/L)
is predicted for Mangrove along Shan Pui River (E28) for both Scenario 1 and
Scenario 5 and their values were the same, 2.87 mg/L. The predicted water quality at EPD Stations DM2
meets the WQO for 10th percentile bottom DO (≥ 2mg/L) but fail
to comply the 10th percentile depth-averaged DO (≥
4mg/L). At DM2, the 10th
percentile depth-averaged DO is predicted to be 3.89 mg/L and 3.86 mg/L in
Scenario 1 and Scenario 5 respectively. The percentage reduction is 0.8%, which
is considered not significant.
Unionized Ammonia / Total Inorganic Nitrogen
5.6.2.80
As mentioned in Section
5.3.2.3, the Study Area are subject to a high TIN level due
to high nutrient background loading, with non-compliances for TIN recorded in
this WCZ even under the existing situations. The predicted depth-averaged
TIN at all WSRs and EPD Stations in the
Deep Bay WCZ under Scenario 5 and Scenario 1 failed to comply with the
WQO. Without HSKEPP (Scenario 1), the
predicted TIN levels at selected WSRs and EPD Stations were predicted to be
0.78-7.56 mg/L. Scenario 5 slightly
improved the predicted TIN and the range of TIN was to be 0.77-7.55 mg/L,
except for DM2. At DM2, the TIN level increase
from 4.11 mg/L in Scenario 1 to 4.12 mg/L in Scenario 5, the 0.2% increment is
not considered as significant. The
maximum percentage change was -1.3%, which is considered not significant. Similarly, non-compliance in depth-averaged
UIA (≤ 0.021mg/L) is predicted at selected WSRs and EPD Stations in the
Inner Marine Subzone, Mariculture Subzone and Yuen Long & Kam Tin (Lower)
Subzone within the Deep Bay WCZ (E26, E27, E15, E24, E25, E28, DM1, DM2,
DM3). Under Scenario 1, the non-compliance
UIA ranged from 0.022-0.116mg/L and under the sensitivity test (Scenario 5),
the UIA levels is predicted to be 0.022-0.117mg/L. UIA levels predicted under Scenario 5 were
slightly higher than the ones in Scenario 1.
The maximum percentage change was 2.8% (with 0.001mg/L increment), which
is considered not significant. Hence, no adverse UIA and TIN impact at the
selected WSRs would be expected from the Project.
Total Phosphorus
5.6.2.81
The WQO did not
specify the target for total phosphorus for the Deep Bay WCZ. The predicted depth-averaged TP predicted at
WSRs and EPD Stations under scenario 1 and Scenario 5 were 0.068-1.194 mg/L and
0.067-1.198 respectively. Maximum
increase in TP (1.18%) is predicted at E25, which is considered insignificant with
an absolute increment of 0.009mg/L, locating closest to the discharge
location.
5.6.2.82
Section 5.6.2.12 suggested the conditions to trigger growth of algal
biomass. Among selected WSRs and
EPD Stations in the Deep Bay WCZ, no increase of
0.1mg/L in TIN levels is predicted from Scenario 1 to Scenario 5. Similarly, TP predicted in all selected WSRs
and EPD Stations did not show an increase of 0.02 mg/L during normal operation
of the Project (Scenario 2) when compared to the baseline Scenario (Scenario
1). The occurrence of red tide from the
Project is therefore not expected. TIN
and TP were not a critical factor for triggering red tide in the Deep Bay WCZ
as the existing red tide occurrence was low.
Suspended Solids
5.6.2.83
The maximum percentage differences in the
predicted SS levels at all selected WSRs and EPD Stations in the Deep Bay WCZ
caused by the proposed Project is -2.32% (at E25), which comply well with the
WQO for SS of no more than 30% change from the ambient levels. Full compliance is
observed in CLP Black Point Power Station (C2) for suspended solids requirement
of ≤ 700 mg/L.
5-day Biochemical Oxygen Demand
5.6.2.84
Scenario 5 would
slightly increase the depth-averaged BOD level in the selected WSRs and EPD
Stations within the Deep Bay WCZ. There
is no WQO available for BOD in the marine water in the Deep Bay WCZ. Under the base case (Scenario 1), the BOD
level predicted at E28 was 10.7 mg/L, as compared to the WSD’s target objective
for Yuen Long & Kam Tin (Lower) Subzone of no more than 5 mg/L. Under Scenario 5, the BOD at E28 would also be
10.7 mg/L, hence, no adverse water quality impact is expected.
Salinity
5.6.2.85
The differences
in the predicted salinity levels among WSRs and EPD Stations in the Deep Bay
WCZ under Scenario 5 were also negligible.
The maximum change of salinity levels caused by Scenario 5 in comparison
to Scenario 1 was approximately 2.3% (at E26), which comply well with the WQO
of no more than 10% change from the background levels.
E. coli
5.6.2.86
The geometric
mean of E.
coli at Mangrove along Shan Pui River
(E28) complies with the WQO target (≤ 1000) for Yuen Long & Kam Tin
(Lower) Subzone. E. coli non-compliance was predicted in Mariculture Subzone
where the geometric mean level would be 11677 (E14) and 15885 (E15) no./100mL
for Scenarios 1, as compared to the WQO of less than 610 no./100mL. In Scenario 5, the geometric mean E. coli level would be 11643 and 15852 no./100mL at E14 and
E15 respectively. As the non-compliance
was predicted under both Scenarios 1 and 5 that the E. coli level would be slightly decrease by 0.29% (E14) and
0.20% (E15) under Scenario 5, the E. coli
exceedance at E14 and E15 was not caused by this Project. The high E. coli level could be attributed by background pollution
from rivers upstream. Therefore, no
adverse E.
coli impact is predicted from Scenario
5.
Maximum Sedimentation Rate
5.6.2.87
Full compliance
of maximum sedimentation rate was predicted in the Deep Bay WCZ. The maximum sedimentation rate criterion
assessment for ecological subtidal habitat (refer to Section 5.2.9.1) is 200 g/m2/day. The maximum sedimentation rates predicted in corals WSRs in the
Deep Bay WCZ under Scenario 1 and Scenario 5 were to be 10.76-13.40 g/m2/day
and 10.71-13.28 g/m2/day. The
maximum percentage change among coral WSRs was -0.90%. No adverse sedimentation impact is predicted
under Scenario 5.
5.6.2.88
In summary,
Scenario 5 would not cause unacceptable water quality impact in the Deep Bay
WCZ due to the tertiary treated effluent qualities from the HSKEPP. The model predicted that no additional
non-compliance in WQOs for DO, TIN, UIA, TN, TP, SS and E. coli levels at all selected WSRs and EPD monitoring
stations, which implies that no non-compliance of WQO or any criterion was
induced by Scenario 5 as compared to the baseline condition. The predicted salinity levels would comply
well within the WQO of no more than 10% change from the background levels (as
compared to Scenario 1). No unacceptable
water quality impact from Scenario 5 upon the WSRs would therefore be expected.
Treated Effluent Reuse
5.6.2.89
As stated in Section 2.5.1.9 to
2.5.1.11, reuse of treated effluent inside the HSKEPP
would be limited to non-potable uses within HSKEPP only for chemical
preparation, water supplement to deodorisation units and cleaning of treatment
equipment. All the treated effluent
reuse will be adopted within the proposed sewage treatment stream via automatic
system and well serving as part of sewage treatment process. There would not be any human contacts to the
reuse effluent during these processes and thus health impact consideration is
not anticipated.
5.6.2.90
With treated effluent
would be limited to internal non-potable uses and implementation of water
pollution preventive measures listed in Section 5.7.2.15, no adverse water
quality impacts are anticipated from reuse of treated effluent inside the HSKEPP.
Transportation of Organic Waste
5.6.2.91
As stated in Section 2.3.3, food or organic wastes
will be collected and pre-treated in the pre-treatment facilities outside HSKEPP
and transported to HSKEPP for co-digestion with sludge generated from sewage
treatment. Provided that the incoming food
or organic wastes would be pre-treated and delivered to the pre-treated food
waste reception facility at HSKEPP for co-digestion, no adverse water quality
impact is therefore expected. As the pre-treated
food waste reception facility would be enclosed, spills (if any) would be
contained and will not contaminate the surface runoff.
5.6.2.92
The contained
wastewater from the enclosed tankers will be collected at the pre-treated food waste
reception facility, conveyed to the food waste bunker and mixed with the
treated effluent of HSKEPP before being fed into the digesters. There would be no direct discharge of the
wastewater into surroundings.
Wastewater from Sludge Treatment
5.6.2.93
The HSKEPP would have
advanced sludge treatment capacity to cater its sludge and additional organic
waste as stated in Section 2.5.2. The sludge treatment process
of the HSKEPP would involve dewatering process and wastewater would be
generated from the sludge treatment process which may have potential to cause
water pollution. Nonetheless, wastewater
generated from the dewatering process will be fed back into the HSKEPP for
treatment before final disposal. There
would be no discharge of wastewater from the dewatering process and hence no
adverse water quality impact is expected.
Surface Runoff
5.6.2.94
Potential water quality
impact may also arise from surface runoff discharge of HSKEPP during operation
phase. The surface runoff may contain
small amount of suspended solids that may leads to water quality impacts to the
nearby receiving waters. However,
impacts upon water quality would be minimal provided that a proper drainage
system would be provided to receive surface runoff to the drainage system at
the planning and design stages.
5.6.2.95
According to the DSD
"Stormwater Drainage Manual (5th edition, 2018)", annual rainfall in Hong Kong is around 2,400 mm. However, the EPD study namely "Update on
Cumulative Water Quality and Hydrological Effect of Coastal Developments and
Upgrading of Assessment Tool (Update Study)” suggested that only rainfall events of sufficient intensity and volume
would give rise to runoff and that runoff percentage is about 44% and 82% for
dry and wet season, respectively.
Therefore, only 1,512 mm of 2,400 mm annual rainfall would be considered
as effective rainfall that would generate runoff (i.e. 1,512 mm = 2,400 mm ×
(82%+44%)/2).
5.6.2.96
The footprint of the HSKEPP
is about 5.2 hectares. With development
of the Project, there would be an increase in the total paved area. The additional paved area is about 3,220 m2. All the treatment units in HSKEPP will be
covered or enclosed to minimize the inflow of surface run-off from entering the
treatment processes. Assuming 0.9 as the
runoff coefficient, the non-point source pollution from surface run-off is
estimated to be 12 m3/day (= 0.9 × 1,512 mm/year × 3,220 m2). It is anticipated that with proper
implementation of best management practices as recommended in Section 5.7.2, no adverse water
quality impact from non-point source surface run-off is expected.
Chemical Spillage
5.6.2.97
A number of chemicals,
including ferric chloride and polymers, would be stored onsite and be used for
wastewater treatment process such as sludge conditioning / dewatering. Adverse water quality impacts can be
minimised by appropriate storage management and drainage system design as
recommended in Section 5.7.2.
5.7.1.1
Measures as listed
below are recommended to mitigate the potential water quality impacts from the land-based
construction works.
General Construction Activities and Construction Site Runoff
Construction Site Runoff
5.7.1.2
The site practices
outlined in ProPECC PN 1/94 “Construction Site
Drainage” should be followed as far as practicable to minimise surface run-off
and the chance of erosion. The following
measures are recommended to protect water quality, and when properly
implemented should be sufficient to adequately control site discharges so as to
avoid water quality impact.
5.7.1.4
Silt removal
facilities, channels and manholes should be maintained and the deposited silt
and grit should be removed regularly (as well as at the onset of and after each
rainstorm) to prevent overflows and localised flooding. Before disposal at the public fill reception
facilities, the deposited silt and grit should be solicited in such a way that
it can be contained and delivered by dump truck instead of tanker truck. Any practical options for the diversion and
realignment of drainage should comply with both engineering and environmental
requirements in order to provide adequate hydraulic capacity of all drains.
5.7.1.5
Construction works
should be programmed to minimise soil excavation in the wet season (i.e. April
to September). If soil excavation cannot
be avoided in these months or at any time of year when rainstorms are likely,
temporarily exposed slope surfaces should be covered e.g. by tarpaulin, and
temporary access roads should be protected by crushed stone or gravel, as
excavation proceeds. Intercepting
channels should be provided (e.g. along the crest / edge of excavation) to
prevent storm run-off from washing across exposed soil surfaces. Arrangements should always be in place in
such a way that adequate surface protection measures can be safely carried out
well before the arrival of rainstorm.
5.7.1.6
Earthworks final
surfaces should be well compacted and the subsequent permanent work or surface
protection should be carried out immediately after the final surfaces are
formed to prevent erosion caused by rainstorms.
Appropriate drainage like intercepting channels should be provided where
necessary.
5.7.1.7
Measures should be
taken to minimise the ingress of rainwater into trenches. If excavation of trenches in the wet season
is necessary, they should be dug and backfilled in short sections. Rainwater pumped out from trenches or
foundation excavations should be discharged into storm drains via silt removal
facilities
5.7.1.8
Construction materials
(e.g. aggregates, sand and fill material) on sites should be covered with
tarpaulin or similar fabric during rainstorms
Boring and Drilling Water
5.7.1.10
Water used in ground
boring and drilling for site investigation or rock / soil anchoring should as
far as practicable be re-circulated after sedimentation. When there is a need for final disposal, the
wastewater should be discharged into storm drains via silt removal facilities.
Wheel Washing Water
5.7.1.11
All vehicles and plants
should be cleaned before they leave a construction site to minimise the
deposition of earth, mud and debris on roads.
A wheel washing bay should be provided at every site exit if practicable
and washwater should have sand and silt settled out or removed before
discharging into storm drains. The
section of construction road between the wheel washing bay and the public road
should be paved with backfill to reduce vehicle tracking of soil and to prevent
site run-off from entering public road drains.
Rubbish and Litter
5.7.1.12
Good site practices
should be adopted to remove rubbish and litter from construction sites so as to
prevent the rubbish and litter from spreading from the site area. It is recommended to clean the construction
sites on a regular basis.
Effluent Discharge
5.7.1.13
There is a need to
apply to EPD for a discharge licence for discharge of effluent from the
construction site under the WPCO. The
discharge quality must meet the requirements specified in the discharge
licence. All the runoff and wastewater
generated from the works areas should be treated so that it satisfies all the
standards listed in the TM-DSS. The
beneficial uses of the treated effluent for other on-site activities such as
dust suppression, wheel washing and general cleaning etc., can minimise water
consumption and reduce the effluent discharge volume. If monitoring of the treated effluent quality
from the works areas is required during the construction phase of the Project,
the monitoring should be carried out in accordance with the relevant WPCO
licence.
Construction Works in Close Proximity to Inland Water
5.7.1.14
The practices outlined
in ETWB TC (Works) No. 5/2005 “Protection of
natural streams / rivers from adverse impacts arising from construction works” should also be
adopted where applicable to minimise the water quality impacts on natural
streams or surface water systems.
Relevant mitigation measures from the ETWB TC (Works) No. 5/2005 are
listed below:
·
Construction works close to the inland waters
should be carried out in the dry season as far as practicable where the flow in
the surface channel or stream is low.
·
The use of less or smaller construction plants
may be specified in areas close to the water courses to reduce the disturbance
to the surface water.
·
Temporary storage of materials (e.g.
equipment, chemicals and fuel) and temporary stockpile of construction
materials should be located well away from any water courses when carrying out
of the construction works.
·
Stockpiling of construction materials and
dusty materials should be covered and located away from any water courses.
·
Construction debris and spoil should be
covered up and / or disposed of as soon as possible to avoid being washed into
the nearby water receivers.
·
Proper shoring may need to be erected in order
to prevent soil or mud from slipping into the watercourses
Accidental Spillage of Chemicals
5.7.1.16
Any service shop and
maintenance facilities should be located on hard standings within a bunded
area, and sumps and oil interceptors should be provided. Maintenance of vehicles and equipment
involving activities with potential leakage and spillage should only be
undertaken within the areas appropriately equipped to control these discharges.
5.7.1.17
Disposal of chemical
wastes should be carried out in compliance with the Waste Disposal
Ordinance. The “Code of Practice
on the Packaging, Labelling and Storage of Chemical Wastes” published under the Waste Disposal Ordinance details the requirements
to deal with chemical wastes. General
requirements are given as follows:
·
Suitable containers should be used to hold the
chemical wastes to avoid leakage or spillage during storage, handling and
transport
·
Chemical waste containers should be suitably
labelled, to notify and warn the personnel who are handling the wastes to avoid
accidents
·
Storage area should be selected at a safe
location on site and adequate space should be allocated to the storage area.
Sewage Effluent from Construction Workforce
5.7.1.19
Notices should be
posted at conspicuous locations to remind the workers not to discharge any
sewage or wastewater into the surrounding environment. Regular environmental audit of the
construction site will provide an effective control of any malpractices and can
encourage continual improvement of environmental performance on site. It is anticipated that sewage generation
during the construction phase of the project would not cause water pollution
problem after undertaking all required measures.
Groundwater from Contaminated Areas, Contaminated Site Run-off and
Wastewater from Land Decontamination
5.7.1.20
Remediation
of contaminated land should be properly conducted following the recommendations
of Land Contamination Assessment in Section
8. Any excavated contaminated material and exposed
contaminated surface should be properly housed and covered to avoid generation
of contaminated run-off. Open stockpiling of contaminated materials should not
be allowed. Any contaminated run-off or
wastewater generated from the land decontamination processes should be properly
collected and diverted to wastewater treatment facilities (WTF). The WTF shall deploy suitable treatment
processes (e.g. oil interceptor / activated carbon) to reduce the pollution
level to an acceptable standard and remove any prohibited substances (such as
total petroleum hydrocarbon) to an undetectable range. All treated effluent
from the wastewater treatment system shall meet the requirements as stated in
TM-DSS and should be either discharged into the foul sewers or tankered away
for proper disposal.
Effluent Discharge During Maintenance of NWNT Tunnel
5.7.2.1
Given the sensitivity
of inner Deep Bay in term of water quality and ecology, a number of additional
mitigation measures are proposed under this Project to further minimize the
water quality impact during the maintenance of NWNT Tunnel. These measures are described as follows.
5.7.2.2
Relevant government
departments including EPD, WSD and AFCD as well as key stakeholders for
mariculture and fisheries in the Deep Bay WCZ should be informed of the NWNT
Tunnel maintenance event prior to any HSKEPP and San Wai STW maintenance
discharge.
5.7.2.3
It is also recommended
under this Project that any NWNT Tunnel maintenance period should be shortened
as far as possible and should be conducted during dry season as the ambient DO
levels in Inner Deep Bay is low in wet season in comparison to dry season.
Emergency Effluent Discharge
5.7.2.4
Given the sensitivity
of inner Deep Bay in term of water quality and ecology, extensive effort will
be expedited to avoid the occurrence for emergency discharge. In order to achieve this, the design of HSKEPP
will be cautiously reviewed to include additional provisions including as
follows:
·
Applied peaking factors for all major
treatment units and electrical and mechanical equipment to a void equipment
failure;
·
By-pass mechanism would be provided for both
coarse screens and fine screens in the inlet to avoid/minimize failure in
coarse/fine screens; Interim
by-pass would be provided after the PST to avoid raw sewage by-pass as much as
possible;
·
Standby unit for all major equipment would be provided
in case of unexpected breakdown of pumping and treatment facilities such that
the standby pumps and treatment facilities could take over and function to
replace the broken pumps; and
·
Back-up power for dual power supply would be provided
in case of power failure to sustain the function of pumping and treatment
facilities.
5.7.2.5
To provide a mechanism
to minimise the impact of emergency discharges and facilitate subsequent
management of any emergency, an Emergency Response Plan will be formulated
prior to commissioning of HSKEPP to set out the emergency response procedures
and actions to be followed in case of equipment or sewage treatment
failure. The plant operators of HSKEPP
should carry out necessary follow-up actions according to the procedures of the
contingency plan to minimise any impacts on the identified WSRs due to
emergency bypass. Regular maintenances
and inspections to all treatment units, penstocks and plant facilities are necessary
to maintain a good operation condition.
Best Stormwater Management Practices
5.7.2.6
Best Management
Practices (BMPs) for stormwater discharge are recommended to reduce stormwater
pollution arising from the Project.
Design Measures
5.7.2.7
Exposed surface shall
be avoided within the Project site to minimise soil erosion. The site shall be either hard paved or
covered by landscaping area and plantation where appropriate.
5.7.2.8
Green areas / tree /
shrub planting etc. should be introduced within the site as far as possible
including open space and along roadside amenity strips, which can help to
reduce soil erosion.
5.7.2.9
The existing
watercourses in adjacent to the Project site will be retained to maintain the
original flow path. The drainage system
will be designed to avoid any case of flooding based on the 1 in 50 year return
period.
Devices / Facilities to Control Pollution
5.7.2.10
Screening facilities
such as standard gully grating and trash grille, with spacing which is capable
of screening off large substances such as fallen leaves and rubbish should be provided
at the inlet of drainage system.
5.7.2.11
Road gullies with
standard design and silt traps and oil interceptors should be incorporated
during the detailed design to remove particles present in storm water runoff.
Administrative Measures
5.7.2.12
Good management measures
such as regular cleaning and sweeping of road surface / open areas is
suggested. The road surface / open area
cleaning should also be carried out prior to occurrence of rainstorm.
5.7.2.13
Manholes, as well as
storm water gullies, ditches provided among the development areas should be
regularly inspected and cleaned (e.g. monthly).
Additional inspection and cleansing should be carried out before
forecast heavy rainfall.
Chemical Spillage
5.7.2.14
Chemical should be
stored on site at bunded area and separate drainage system as appropriate
should be provided to avoid any spilled chemicals from entering the storm drain
in case of accidental spillage. Also,
adequate tools for cleanup of spilled chemicals should be stored on site and
appropriate training shall be provided to staffs to further prevent potential
adverse water quality impacts from happening.
Preventive Measures for Cross Contamination and Misuse of Reclaimed
Water
·
All pipes and fittings used for
the reclaimed water supply and associated distribution system should be purple
in color (exact color code to be reviewed) for distinguishing them from the
pipes and fittings used for 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;
·
Warning plate with sign 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
treated effluent 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; and
·
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 portable water.
5.8.1.1
With implementation of
the recommended mitigation measures in Section 5.7, it is therefore
expected no adverse residual water quality impact is expected in construction
and operation phases. 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.
5.9.1
Construction Phase
5.9.1.1
Based on the current
construction programme as mentioned in Section 2, the Project construction works are anticipated to commence in early
2027 with completion of the Project by 2031.
According to the latest information on the HSK/HT NDA development,
HSK/HT NDA Advance Work Phase 3 will tentatively be completed in 2029; whilst
Stage 2 Works will tentatively be commenced in 2024 and completed in 2032; and
the Reaming Remaining Phase Development will tentatively be commence in 2030
and completed in 2036. The construction
of the Project would likely interact with these projects, which may have
cumulative environmental impacts.
5.9.1.2
The construction works
for the concurrent projects as mentioned in Section 5.9.1.1 would involve
land-based construction works only and the potential water quality impact would
include construction site runoff and drainage from works areas, wastewater from
general construction activities, sewage generated by construction workforce,
accidental spillage of chemicals, contaminated groundwater and wastewater, and
construction works near watercourses.
With proper adoption of mitigation measures and good site practices such
as guidelines as given in ProPECC PN 1/94, potential water quality impact would
be minimized. No adverse water quality
impacts were anticipated.
5.9.1.3
As no significant water
quality impact was expected from the Project and the identified concurrent
projects during construction phase, no adverse cumulative water quality impacts
were hence anticipated.
5.9.2
Operation Phase
5.9.2.1
As stated in Section 5.5.3, cumulative impacts
from other projects have already been incorporated in the YL Model in terms of
coastline configuration, background pollution loadings, etc. and hence the
cumulative water quality impacts have already been assessed in Sections 5.6.2 and 5.7.2.
5.10.1
Construction Phase
5.10.1.1
The potential water
quality impact from the land-based construction works can be controlled by the
recommended mitigation measures. Water
quality monitoring at designated control and impact monitoring stations are
proposed to monitor any sub-standard water discharge into the nearby water
bodies. A WPCO license should be
obtained if there has construction drainage discharge. Self-monitoring and reporting should be
carried out for monitoring the construction drainage discharge in accordance
with the WPCO license.
5.10.2
Operation Phase
5.10.2.1
During operation phase,
water quality monitoring is recommended for the first year of normal operation
of HSKEPP. Monitoring of the treated
effluent quality will be governed by the WPCO license to ensure that the
effluent quality would comply with the design standards, which is under the
ambit of RO of EPD. Detailed environmental monitoring procedures are provided
in the standalone EM&A manual.
5.10.3
Maintenance of NWNT Tunnel
5.10.3.1
Water quality
monitoring is recommended in Inner Deep Bay for NWNT Tunnel maintenance during
both construction and operational phases of this Project. A five-month baseline
monitoring programme covering the period from November to March is proposed at
a frequency of twice per month to establish the baseline water quality
conditions at selected monitoring points. In case of NWNT Tunnel maintenance during the
operational phase of this Project, water quality in Deep Bay should be
monitored at a frequency of 3 times per week throughout the maintenance period
until the baseline water quality is restored or at least 1 month after
termination of the effluent bypass (whichever is longer). In case of emergency discharge during
operation phase of this Project, a follow-up water quality monitoring exercise
shall be commenced within 24 hours after the start of the emergency discharge
at all proposed monitoring stations.
5.11.1
Construction Phase
5.11.1.1
Minor water quality
impacts would be associated with land-based construction works. Water quality impacts may result from
wastewater generated from the general construction activities, construction
site runoff, construction works near inland watercourses, sewage effluent from
workforce and accidental chemical spillage.
The impacts could be mitigated and controlled by implementing the
recommended mitigation measures. No
adverse water quality impact from construction works for the HSKEPP is
anticipated. No adverse residual water
quality impacts are expected.
5.11.2
Operation Phase
5.11.2.1
Mathematical modelling
was conducted under this EIA to study the water quality impacts caused by the
potential change in the effluent flow and qualities from the proposed HSKEPP. The model results indicated that the proposed
HSKEPP would not impose adverse water quality impact into the North Western and
North Western Supplementary WCZ. The
model predicted that the proposed HSKEPP would have no significant impact on
the TIN, UIA, TN, TP, SS, DO and E.coli levels in the Deep Bay WCZ. The
model results showed that the predicted salinity levels at WSRs in North
Western, North Western Supplementary, Deep Bay WCZs would comply with the WQO
of no more than 10% change from the background levels. No unacceptable water quality impact from
normal operation of the proposed HSKEPP upon the receiving marine water would
therefore be expected.
5.11.2.2
During the NWNT tunnel
maintenance, it is unavoidable to result in worsen water quality (DO, BOD, TIN,
UIA, SS and E. coli) due to increase of pollution loading
discharging into the Deep Bay WCZ. In
order to further minimize the water quality impact, it is recommended under
this Project to schedule the NWNT tunnel maintenance during dry season. The water quality model predicted that the
pollution elevation in Deep Bay WCZ and the associated water quality recovery
period would be significantly reduced and minimized for the HSKEPP maintenance
discharge during dry season. An event
and action plan and a water quality monitoring programme (as presented in the
standalone EM&A Manual) is also proposed for the NWNT tunnel maintenance
events during both construction and operational phases to minimize the water
quality impacts.
5.11.2.3
For emergency discharge,
the model results indicated that elevated levels of key water quality
parameters would be recovered within 0.5-2.0 days after termination of the
emergency discharge for WSRs within Inner Deep Bay. The more distant WSR i.e. Oyster Culture Area
was found not to be affected by emergency discharge event. The occurrence of emergency discharge from
the proposed HSKEPP can be minimised by the implementation of appropriate
mitigation measures, including dual power supply and provision of standby facilities. An Emergency Response Plan will be formulated
prior to commissioning of HSKEPP to minimize the impact of emergency discharges
and facilitate subsequent management of the emergency.
5.11.2.4
Other water quality
impacts associated with the operation phase are identified as surface runoff
from paved areas and accidental spillage.
It is expected that these potential impacts can be prevented by adopting
recommended mitigation measures. No unacceptable
residual water quality impact is expected.