6.1.1.1
This section presents a water quality impact assessment for
the construction and operation of the Project, including the sewerage and
sewage treatment implications and identifying the water quality issues,
assessing the potential impacts and recommending mitigation measures where
required.
6.2 Environmental Legislation and Standards
6.2.1
General
6.2.1.1
Reference to the Environmental Impact Assessment Ordinance (EIAO)
and the associated Environmental Impact Assessment Ordinance Technical
Memorandum (EIAO-TM) has been made for the assessment of potential water
quality impacts. Annexes 6 and 14
of the EIAO-TM set out the criteria and guidelines for evaluating water quality
impacts. The
following legislation is, also, relevant to the water quality assessment of
this Project:
·
Environmental Impact Assessment Ordinance (Cap 499); and
·
Water Pollution Control Ordinance (Cap 358).
6.2.1.2
Other relevant guidelines include:
·
Water Supplies Department (WSD) Water Quality Criteria;
·
Technical Memorandum on Standards for Effluents Discharged
into Drainage and Sewerage Systems, Inland and Coastal Waters;
·
Practice Note for Professional Persons on Construction Site
Drainage (ProPECC PN 1/94); and
·
Sewerage Manual (SM) and the Guidelines for Estimating
Sewage Flows for Sewage Infrastructure Planning (GESF).
6.2.2
Water Pollution Control
Ordinance
6.2.2.1
The Water Pollution Control Ordinance (WPCO) (Cap 358) enacted
in 1980 is the principal legislation to protect the water quality in
Table 6.1 Summary of WQOs for Eastern Buffer WCZ
|
Parameters |
WQOs |
Sub-Zone |
|
Offensive odour, tints |
Not to be present |
Whole zone |
|
Visible foam, oil scum, litter |
Not to be present |
Whole zone |
|
Colour |
Change due to waste discharges not to exceed
30 Hazen units |
Water gathering ground subzones |
|
Change due to waste discharges not to exceed 50 Hazen units |
Inland waters |
|
|
E. coli |
Not exceed 610 per 100 mL, calculated as the geometric mean of all samples collected in one
calendar year |
Inland waters |
|
Less than 1 per 100 mL, calculated as
the geometric mean of the most recent 5 consecutive samples taken at
intervals of between 7 and 21 days |
Water gathering ground subzones |
|
|
Not exceed 1000 per 100 ml, calculated as the
geometric mean of the most recent 5 consecutive samples taken at intervals of
between 7 and 21 days |
Inland waters |
|
|
Dissolved oxygen (DO) within 2 m of the seabed |
Not less than 2.0 mg/L for 90% of samples |
Marine waters |
|
Depth-averaged DO |
Not less than 4.0 mg/L for 90% of samples |
Marine waters |
|
Not less than 5.0 mg/L for 90% of
samples |
Fish culture subzones |
|
|
Not less than 4.0 mg/L |
Water gathering ground subzone and other
Inland waters |
|
|
pH |
To be in the range of 6.5-8.5, change due to waste discharges not to exceed 0.2 |
Marine waters |
|
To be in the range of 6.5-8.5 |
Water gathering
ground subzones |
|
|
To be in the range of 6.0-9.0 |
Inland waters |
|
|
Salinity |
Change due to waste discharges not to exceed
10% of ambient |
Whole zone |
|
Temperature |
Change due to human activity not to exceed 2°C |
Whole zone |
|
Suspended solids (SS) |
Not to raise the ambient level by 30% caused
by waste discharges and shall not affect aquatic communities |
Marine waters |
|
Change due to waste discharges not to exceed
20 mg/L of annual median |
Water gathering
ground subzones |
|
|
Change due to waste discharges not to exceed 25 mg/L of annual median |
Inland water |
|
|
Unionised ammonia (UIA) |
Annual mean not to exceed 0.021 mg/L as
unionised form |
Whole zone |
|
Nutrients |
Shall not cause excessive algal growth |
Marine waters |
|
Total inorganic nitrogen (TIN) |
Annual mean depth-averaged inorganic nitrogen
not to exceed 0.4 mg/L |
Marine waters |
|
5-day Biochemical Oxygen Demand (BOD5) |
Change due to waste discharges not to exceed 3
mg/L |
Water gathering ground subzones |
|
Change due to waste discharges not to exceed 5 mg/L |
Inland waters |
|
|
Chemical Oxygen Demand (COD) |
Change due to waste discharges not to exceed
15 mg/L |
Water gathering ground subzones |
|
Change due to waste discharges not to exceed 30 mg/L |
Inland waters |
|
|
Toxic substances |
Should not attain such levels as to produce
significant toxic effects in humans, fish or any other aquatic organisms |
Whole zone |
|
Waste discharges
should not cause a risk to any beneficial use of the aquatic environment |
Whole zone |
6.2.3
Water Supplies Department
(WSD) Water Quality Criteria
6.2.3.1
Besides the WQOs set under the WPCO, the WSD has also
specified a set of seawater quality objectives for water quality at their
flushing water intakes as shown in Table
6.2.
Table 6.2 WSD Standards at
|
Parameter (mg/L, unless otherwise stated) |
WSD Target Limit |
|
Colour (Hazen
Unit) |
< 20 |
|
Turbidity (NTU) |
< 10 |
|
Threshold Odour
Number (odour unit) |
< 100 |
|
Ammoniacal
Nitrogen |
< 1 |
|
Suspended Solids |
< 10 |
|
Dissolved Oxygen |
> 2 |
|
Biochemical
Oxygen Demand |
< 10 |
|
Synthetic Detergents |
< 5 |
|
E.coli (no. per 100 ml) |
< 20,000 |
6.2.4
Technical Memorandum on
Standards for Effluents Discharged into Drainage and Sewerage Systems, Inland
and Coastal Waters
6.2.4.1
Besides setting the WQOs, the WPCO controls effluent
discharge into the WCZs through a licensing system. The guidance on permissible effluent
discharges based on the type of receiving waters (foul sewers, inland/coastal/inshore/marine waters) is provided in
the Technical Memorandum on Standards for Effluents Discharged into Drainage
and Sewerage Systems, Inland and Coastal Waters. The limits given in this Technical
Memorandum cover the physical,
chemical and microbial quality of effluents. Any effluent discharge during the
construction and operational phases should comply with the standards for
effluents discharged into foul sewers or inland/inshore/marine waters of the Eastern
Buffer WCZ.
6.2.5
ProPECC Notes
6.2.5.1
The Practice Note for Professional Persons on Construction
Site Drainage (ProPECC PN 1/94) was issued by the EPD to provide environmental guidelines
for handling and disposal of construction site discharges. It provides good practice guidelines for
dealing with various types of discharge from a construction site. Practices as outlined in the ProPECC PN
1/94 should be followed during the construction phase in order to minimise the
water quality impact due to construction site drainage. Moreover, other ProPECC
Notes including the ProPECC PN 5/93 Drainage Plan (subject to Comment by the
Environmental Protection Department) would also be considered.
6.3.1.1
Since the implementation of the Stonecutter Island Sewage
Treatment Works (SCISTW) under the Harbour Area Treatment Scheme (HATS) Stage 1
in 2002 by which all sewage generated from Junk Bay (Tseung Kwan O), the
Kowloon Peninsula and east of Hong Kong Island (Chai Wan) was
diverted and
treated at the SCISTW, the water quality has improved markedly
with
increasing DO and decreasing nutrients and E.
coli levels. In 2010, the Eastern
Buffer WCZ achieved full compliance (100%) with the WQOs. The annual geometric mean E. coli level in the water in 2010 was below 50 colony
forming unit (cfu)
per 100mL. The EPD’s marine water quality
monitoring stations within the Eastern Buffer WCZ nearest to the Project
area include
EM1, EM2, EM3, ET1
(Chai Wan Typhoon Shelter) and ET2 (Aldrich Bay Typhoon Shelter). The monitoring data of these stations
are shown in Figure
6.1 and summarised
in Table 6.3.
Table 6.3 Marine
Water Quality for Eastern Buffer WCZ in 2010 at EM1, EM2, EM3, ET1 and ET2
|
Parameters |
EPD Monitoring
Station |
||||
|
EM1 |
EM2 |
EM3 |
ET1 |
ET2 |
|
|
Temperature (°C) |
22.4 (16.5-27.5) |
12 (16.4-27.7) |
22.2 (16.5-27.6) |
22.9 (17.6-28.7) |
22.4 (17.5-27.2) |
|
Salinity (ppt) |
32.6 (30.8-33.9) |
32.7 (30.9-33.9) |
32.9 (31.0-33.9) |
32.0 (30.7-33.1) |
32.2 (31.1-33.1) |
|
Dissolved
Oxygen (mg/L) |
6.3 (4.2-7.7) |
6.3 (4.5-8.0) |
6.4 (4.2-8.2) |
6.1 (4.2-7.7) |
4.9 (2.8-6.3) |
|
BOD5 (mg/L) |
0.7 (0.2-1.7) |
0.5 (0.1-1.0) |
0.5 (<0.1-0.8) |
1.1 (0.8-1.8) |
0.7 (0.5-0.9) |
|
SS (mg/L) |
3.2 (1.0-7.5) |
3.2 (1.3-7.7) |
3.1 (1.4-6.5) |
2.0 (1.2-2.9) |
1.8 (1.0-3.1) |
|
TIN (mg/L) |
0.13 (0.03-0.23) |
0.11 (0.03-0.22) |
0.09 (0.04-0.16) |
0.21 (0.16-0.27) |
0.26 (0.19-0.33) |
|
NH3-N
(mg/L) |
0.051 (0.012-0.101) |
0.041 (0.009-0.099) |
0.030 (0.012- 0.051) |
0.092 (0.046-0.167) |
0.125 (0.057-0.187) |
|
Chlorophyll-a (mg/L) |
4.8 (0.5-24.3) |
1.9 |
1.4 (0.4-3.9) |
6.2 (0.9-14.2) |
3.1 (0.4-11.6) |
|
E. coli (cfu/100mL) |
61 (7-1400) |
33 (2-1100) |
6 (1-110) |
190 (56-680) |
310 (41-1000) |
Notes: [1] Data presented are depth averaged (except as
specified) and are the annual arithmetic mean except for E. coli (geometric mean);
[2] Data
in brackets indicate ranges; [3] Underlined indicates occurrence of
non-compliance with that parameter of WQO.
6.4.1.1
The study area for the water quality impact assessment is
delineated to be within 300m from the proposed Project
site, which
would cover the relevant existing and potential water sensitive receivers
(WSRs) within approximately 5km from the Project site in the Eastern Buffer WCZ
as shown in Figure 6.1 and detailed in Table 6.4
below. Due to the highly urbanised
nature of the Eastern District of Hong Kong
Table 6.4 Water Sensitive Receivers within Eastern Buffer WCZ
|
WSR No. |
Descriptions |
|
WSR1 |
WSD Water |
|
WSR2 |
Shau Kei Wan Typhoon Shelter |
|
WSR3 |
Cooling Water Intake – Pamela Youde Nethersole
Eastern Hospital |
|
WSR4 |
Chai Wan Typhoon Shelter |
|
WSR5 |
WSD Water |
|
WSR6 |
|
|
WSR7 |
|
|
WSR8 |
Tung Lung Chau West – Corals |
|
WSR9 |
Tung Lung Chau Fish Culture Zone |
|
WSR10 |
Tung Lung Chau North – Corals |
|
WSR11 |
Tung Lung Chau South – Corals |
6.5.1.1
The criteria and guidelines for
assessing water quality impacts as stated in Annexes 6 and 14 of the EIAO-TM have
been followed.
6.5.1.2
Specific construction methods and
operational activities of the EMSD Hong Kong Workshop project have been reviewed
to identify if there is the potential to affect any of the adjacent marine
environment, drainage systems, groundwater hydrology and catchment types or
areas would be affected.
6.5.1.3
Potential pollution sources including
point discharges and non-point sources to surface water runoff, sewage and
polluted discharge generated from the Project have been identified. The
identified pollution sources have been evaluated to determine the significance
of impact.
6.5.1.4
The potential cumulative impacts
due to other related concurrent and planned projects activities or pollution
sources within the area around the alignment have been assessed and mitigation
measures proposed where required to ensure that any water quality impacts would
be controlled to acceptable levels.
The following concurrent projects are identified from a water quality
perspective:
·
Temporary Sand Depot at ~30m to the southeast of the Project
site;
·
Cement Works at
·
·
Headquarters and Bus Maintenance Depot in Chai Wan (EP-107/2001);
and
·
Open roads, e.g. Sheung On Street,
6.6 Construction Phase Impact Assessment
6.6.1
Identification of
Pollution Sources
6.6.1.1
As described in Section 3 of this
EIA report, the EMSD Hong Kong Workshop project comprises mainly the
construction of a land-based steel shed structure on a raft foundation. Upon site clearance, site formation
works will be carried out which will involve the excavation of soil for the
subsequent construction of a concrete slab and footings, erection of formwork
and cover shed, plant rooms, utilities and finally installation of building
services. As there
would be no marine construction works anticipated, potential water pollution
sources during the construction phase would originate from the land-based works
activities including excavation works for raft foundation, footings, concrete
slab, utilities work, etc. The main
water quality related issues will be to prevent erosion on-site and minimise
suspended solids (SS) being washed out into stormwater drains as construction site
runoff, as well as the need to control sewage wastewater, e.g. temporary
sewerage facilities, cementitious waters and general construction refuse.
6.6.1.2
It should be noted that the
proposed EMSD Hong Kong Workshop will not be provided with an automated
vehicles cleaning bay and the cleaning activities will be undertaken by hand.
6.6.2
Construction Site Runoff
6.6.2.1
Construction site runoff may
cause physical, biological and chemical effects. The physical effects include
potential blockage of drainage channels and increase of SS levels in the Eastern
Buffer WCZ. Local flooding may also
occur in heavy rainfall situations. The chemical and biological effects caused
by the construction runoff are highly dependent upon its SS levels and pH values. Runoff containing significant amounts of
concrete and cement-derived material may cause primary chemical effects e.g. increasing
turbidity and discoloration, elevation in pH, and accretion of solids. A number
of secondary effects may also result in toxic effects to water biota due to
elevated pH values, and reduced decay rates of faecal micro-organisms and
photosynthetic rate due to the decreased light penetration. Construction site runoff comprises:
·
Surface run-off may be contaminated and turbid water may
enter adjacent stormwater drainage system and marine area as excavated material
is delivered to ground surface;
·
Runoff and erosion from site surfaces, drainage channels,
earth working areas and stockpiles, release of concrete washing with
construction runoff and stormwater.
Effluent discharge from temporary site facilities should be controlled
to prevent direct discharge to the neighbouring drainage system, marine water
and storm drains. Such wastewater
may include wastewater resulting from dust suppression sprays and wheel washing
of site vehicles at site entrances; and
·
Fuel, oil, solvents and lubricants from maintenance of
construction machinery and equipment: The use of engine oil and lubricants, and
their storage as waste materials has the potential to create impacts on the
water quality of adjacent water courses if spillage occurs and enters
watercourses. Waste oil may
infiltrate into the surface soil layer, or run-off into local water courses,
increasing hydrocarbon levels.
6.6.2.2
Adoption of the relevant
guidelines and good site practices for the construction runoff would minimise
potential water quality impacts during the construction phase of the Project. As such, adverse water quality impacts
on the nearby WSRs within the Eastern Buffer WCZ as listed in Table 6.4 would not be
anticipated.
6.6.3
Accidental Chemical
Spillage
6.6.3.1
The on-site general construction
activities may cause potential water pollution due to any possible, unexpected and
accidental spillages of chemicals and/or liquids possibly stored on-site for
the construction works, e.g. oil, diesel and solvents, which could result in
water quality impacts if they enter the nearby stormwater drainage channels.
6.6.3.2
Implementation of good
construction and site management practices would be ensure that litter, fuels
and solvents do not enter the surrounding stormwater drains. As such, adverse water quality impacts
on the nearby WSRs within the Eastern Buffer WCZ as listed in Table 6.4 would not be
anticipated.
6.6.4
Sewage Effluent
6.6.4.1
Sewage effluents will arise from
the sanitary facilities provided for the on-site construction workforce. The
characteristics of sewage may include high levels of BOD5, ammonia
and E. coli. The associated impacts would include the
generation of rubbish and wastewater from eating areas, temporary sanitary
facilities and waste disposal areas.
Although such impact would be temporary, the additional population may
impose significant stress on the water quality in adjacent water bodies in the
absence of adequate mitigation.
Control of construction phase sewage could, therefore, be an issue and on-site
chemical toilets will be provided during construction. As such, adverse water quality impacts
would not be anticipated on the nearby WSRs within the Eastern Buffer WCZ as
listed in Table 6.4.
6.6.5
Cumulative Impacts
6.6.5.1
The known concurrent project
during the construction phase as described in Section 3 of this EIA report
would include the following where pollution sources may occur:
·
Proposed
6.6.5.2
As the interfacing works of this project
with those in the proposed Workshop would only involve land-based construction
works, provided that the proper mitigation measures recommended below will be
implemented by each project, adverse cumulative water quality impacts would not
be anticipated on the WSRs as listed in Table
6.4.
6.6.6
Construction Phase Mitigation
Measures
Construction
Site Run-off and General Construction Activities
6.6.6.1
In accordance with the Practice
Note for Professional Persons on Construction Site Drainage, Environmental
Protection Department, 1994 (ProPECC PN 1/94), construction phase mitigation
measures shall include the following:
·
At the establishment of works site, perimeter cut-off drains
to direct off-site water around the site should be constructed with internal
drainage works and erosion and sedimentation control facilities
implemented. Channels (both
temporary and permanent drainage pipes and culverts), earth bunds or sand bag barriers
should be provided to divert the stormwater to silt removal facilities. The design of the temporary on-site
drainage system will be undertaken by the contractor prior to the commencement
of construction;
·
Dikes or embankments for flood protection should be
implemented around the boundaries of earthwork areas. Temporary ditches should
be provided to facilitate the runoff discharge into an appropriate watercourse,
through a site/sediment trap. Sediment/silt traps should be incorporated in the
permanent drainage channels to enhance deposition rates;
·
The design of efficient silt removal facilities should be
based on the guidelines in Appendix A1 of ProPECC PN 1/94, which states that
the retention time for silt/sand traps should be 5 minutes under maximum flow
conditions. The sizes may vary
depending upon the flow rate, but for a flow rate of 0.1m3/s, a
sedimentation basin of 30m3 would be required and for a flow rate of
0.5m3/s the basin would be 150m3. The detailed design of
the sand/silt traps should be undertaken by the contractor prior to the
commencement of construction;
·
The construction works should be programmed to minimise
surface excavation works during rainy seasons (April to September). All exposed earth areas should be
completed and vegetated as soon as possible after the earthworks have been
completed, or alternatively, within 14 days of the cessation of earthworks
where practicable. If excavation of
soil cannot be avoided during the rainy season, or at any time of year when
rainstorms are likely, exposed slope surfaces should be covered by tarpaulin or
other means;
·
The overall slope of works sites should be kept to a minimum
to reduce the erosive potential of surface water flows, and all trafficked
areas and access roads should be protected by coarse stone ballast. An additional advantage accruing from
the use of crushed stone is the positive traction gained during the prolonged
periods of inclement weather and the reduction of surface sheet flows;
·
All drainage facilities and erosion and sediment control
structures should be regularly inspected and maintained to ensure their proper
and efficient operation at all times particularly following rainstorms. Deposited silts and grits should be removed
regularly and disposed of by spreading evenly over stable, vegetated areas;
·
Measures should be taken to minimise the ingress of site
drainage into excavations. If the
excavation of trenches in wet season is inevitable, they should be dug and
backfilled in short sections wherever practicable. The water pumped out from trenches or
foundation excavations should be discharged into storm drains via silt removal
facilities;
·
All open stockpiles of construction materials (for example,
aggregates, sand and fill material) should be covered with tarpaulin or similar
fabric during rainstorms. Measures
should be taken to prevent the washing away of construction materials, soil,
silt or debris into any drainage system;
·
Manholes (including newly constructed ones) should always be
adequately covered and temporarily sealed so as to prevent silt, construction
materials or debris being washed into the drainage system and storm runoff
being directed into foul sewers;
·
Precautions to be taken at any time of the year when
rainstorms are likely, actions to be taken when a rainstorm is imminent or
forecasted and during or after rainstorms, are summarised in Appendix A2 of
ProPECC PN 1/94. Particular
attention should be paid to the control of silty surface runoff during storm
events, especially for areas located near steep slopes;
·
All vehicles and plant should be cleaned before leaving a
construction site to ensure no earth, mud, debris and the like is deposited by
them on roads. An adequately
designed and sited wheel washing facilities should be provided at the exit of
every construction site where practicable.
Wash-water should have sand and silt settled out and removed at least on
a weekly basis to ensure the continued efficiency of the process. The section of access road leading to,
and exiting from, the wheel-washing bay to public roads should be paved with
sufficient backfall toward the wheel-washing bay to prevent vehicle tracking of
soil and silty water to public roads and drains;
·
Oil interceptors should be provided in the drainage system
downstream of any oil/fuel pollution sources. Oil interceptors should be emptied and
cleaned regularly to prevent the release of oil and grease into the storm water
drainage system after accidental spillage.
A bypass should be provided for oil interceptors to prevent flushing
during heavy rain;
·
The construction solid waste, debris and rubbish on-site
should be collected, handled and disposed of properly to avoid causing any
water quality impacts. The
requirements for solid waste management are detailed in Section 7 Waste
Management and Land Contamination of
this EIA report;
·
All fuel tanks and storage areas should be provided with
locks and sited on sealed areas, within bunds of a capacity equal to 110% of
the storage capacity of the largest tank to prevent spilled fuel oils from
reaching the nearby WSRs; and
·
By adopting the above mitigation measures with best
management practices it is anticipated that the impacts of construction site
runoff will be reduced to an acceptable level.
6.6.6.2
There is a need to apply to the 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 DSS-TM. Minimum
distances of 100m should be maintained between the discharge points of
construction site effluent and the existing seawater intakes. In addition, no new eff1uent discharges
in nearby Typhoon shelters should be allowed. 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 WPCO license.
Sewage Wastewater
6.6.6.3
Portable chemical toilets and
sewage holding tanks are recommended for the handling of the construction
sewage generated by the workforce.
A licensed contractor should be employed to provide appropriate and
adequate portable toilets and be responsible for appropriate disposal and
maintenance.
Accidental
Spillage of Chemicals
6.6.6.4
The contractor must register as a
chemical waste producer if chemical wastes would be produced from the
construction activities. The Waste Disposal Ordinance (Cap 354) and its
subsidiary regulations in particular the Waste Disposal (Chemical Waste)
(General) Regulation should be observed and complied with for control of
chemical wastes.
6.6.6.5
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 for leakage and spillage should only be
undertaken within the areas appropriately equipped to control these
discharges.
6.7 Operational Phase Impact Assessment
6.7.1
Identification of
Pollution Sources
6.7.1.1
During the operational phase,
there would be no direct discharge of wastewater into Eastern Buffer WCZ or any
other water bodies anticipated.
Therefore, quantitative water quality dispersion modelling would not be
necessary. Potential water
pollution sources during the operational phase would mainly include the sewage
from the workforce of the workshop.
6.7.2
Sewerage and Sewage
Treatment Implications
Existing, Committed and Planned
Sewerage Facilities
6.7.2.1
The proposed Workshop will be constructed in the form of a
low-rise roof shed structure with minimum clear headroom of 5.2m covering about
approximately 1,200m2 of the site as shown in Figures 3.2 and 3.3.
6.7.2.2
A desk study was carried out to identify the existing
sewerage facilities and the characteristics of the concerned sewerage
catchments within and in the vicinity of the proposed development.
6.7.2.3
An assessment of the Drainage Services Department (DSD)
sewerage record drawings has concluded that there are existing sewerage
networks in the vicinity of the proposed development for serving the concerned
sewerage catchments. The existing public sewers have pipe diameters ranging
from 225mm to 1,200mm running along Sheung On Street and
6.7.2.4
Apart from the above existing sewers, no committed or
planned sewerage facility was identified in the vicinity of the proposed
development.
Proposed Sewage Discharge from the
Development
6.7.2.5
Based on the design arrangement as
described in Section 3 of this EIA report, sewage from the proposed workshop is
anticipated to be generated mainly from the toilet and shower facilities in the
workshop and office, while potentially contaminated runoff is anticipated to be
generated from the cleaning activities of the vehicle cleansing bay to be
provided within the proposed development.
6.7.2.6
Since the proposed
development will be designed and operated in a similar manner to that of the existing
EMSD Hong Kong Workshop in Causeway Bay, the sewage flow have been estimated
with reference to the average monthly water consumption rate of this existing
EMSD Hong Kong Workshop.
6.7.2.7
The average monthly water consumption rate of the existing
EMSD Hong Kong Workshop between 2005 and 2011 generally ranged from 923m3
to 5,892m3, with the exception of an abnormally high consumption
rate in June 2010 due to a water leakage.
6.7.2.8
Based on the given average monthly water consumption rate of
6,000m3 (maximum), the
Average Dry Weather Flows (ADWF) from the proposed development has been estimated
to be 2.31 L/s. Apart from this flow
estimation with reference to the water consumption rate, the sewage flow from the proposed development has, also,
been assessed in accordance with the Unit Flow Factor (UFF), as recommended in
the Guidelines for Estimating Sewage Flows for Sewage Infrastructure Planning
(GESF). The recommended unit flow factors are tabulated below:
Table 6.5 Recommended Unit Flow Factor
|
Type of Property |
Unit |
Unit Flow (m3/d) |
|
Domestic |
|
|
|
Public rental |
Person |
0.190 |
|
Private R1 |
Person |
0.190 |
|
R2 |
Person |
0.270 |
|
R3 |
Person |
0.370 |
|
R4 |
Person |
0.370 |
|
Traditional Village |
Person |
0.150 |
|
|
Person |
0.270 |
|
Temporary and
Non-domestic |
Person |
0.150 |
|
Commercial* |
|
|
|
Commercial Employee |
Employee |
0.080 |
|
Commercial
Activities |
|
|
|
(a) Specific Trades |
|
|
|
·
J2 Electricity Gas &
Water |
Employee |
0.250 |
|
·
J3 Transport, Storage
& Communication |
Employee |
0.100 |
|
·
J4 Wholesale & Retail |
Employee |
0.200 |
|
·
J5 Import & Export |
Employee |
- |
|
·
J6 Finance, Insurance,
Real Estate & Business Services |
Employee |
- |
|
·
J7 Agriculture &
Fishing |
Employee |
- |
|
·
J8 Mining & Quarry |
Employee |
- |
|
·
J9 Construction |
Employee |
0.150 |
|
·
J10 Restaurants &
Hotels |
Employee |
1.500 |
|
·
J11 Community, Social
& Personal Services |
Employee |
0.200 |
|
·
J12 Public Administration |
Employee |
- |
|
(b) General –
Territorial Average |
Employee |
0.200 |
|
Industrial* |
|
|
|
Industrial Employee |
Employee |
0.080 |
|
Industrial
Activities |
|
|
|
·
Territorial Average |
Employee |
0.560 |
|
·
Tuen Mun, Tsueng Kwan O,
Yau Tong, Cheung Chau, Mui Wo |
Employee |
1.000 |
Note: * The
total unit flow generated from an employee in a particular trade is the sum of
the flows due to the employee and the unit flow factor for a particular trade
under consideration.
6.7.2.9
The unit flow factor of commercial activities “J3 Transport,
Storage & Communication” has been employed in the sewage flow estimations
for the proposed development. The evaluated Average Dry Weather Flows are
summarised in Table 6.6.
Table 6.6 Estimated ADWF (in accordance with GESF)
|
Location |
Design Population |
Unit Flow Factor (UFF), m3/d |
Average Dry Weather Flows (ADWF), m3/d |
|
New EMSD |
40 (maximum) |
0.180 (i.e. 0.080 +
0.100) |
7.20 (equal to 0.083 l/s) |
6.7.2.10
Therefore, it is envisaged that the flow estimations from
the average monthly water consumption rate of 2.31 L/s would provide the worst
case scenario in estimating the sewerage flow. For a conservative approach, this ADWF
of 2.31 L/s has been adopted for the sewerage assessment in this Project.
6.7.2.11
It should be noted that the estimated ADWF of 2.31 L/s or
200 m3/day was the actual average water consumption rate in the
existing EMSD Workshop in Causeway Bay in February 2005 with recorded
population of 193 in that period.
For a conservative approach, the above water consumption rate (i.e. ADWF
of 200 m3/day) was adopted for the sewerage assessment in the
capacity checking of the receiving sewerage infrastructure. It is anticipated
that the future sewage flow generated in this Project with a design population
of 40 during the operation phase would be much less than the above mentioned
ADWF of 200 m3/day. With the consideration of different scale/size,
the ADWF anticipated in this Project would be about 20% of that of the existing
Workshop in
6.7.2.12
The peak wet weather flows (PWWF) have been used in the
assessment of the hydraulic adequacy of the sewerage systems. Peaking factors,
including storm water allowances in accordance with the GESF, have been applied
to the ADWF to establish the peak flows, in order to provide a conservative
basis for the performance assessment of the sewerage facilities. The
recommended peaking factors are presented in Table 6.7.
Table 6.7 Recommended Peaking Factors for Sewers and Sewerage Facilities
|
Population |
Peaking Factor for Sewers* |
Population |
Peaking Factor for STW, PTW and PS * |
|
< 1,000 |
8.0 |
< 10,000 |
4.0 |
|
1,000 – 5,000 |
6.0 |
10,000 – 25,000 |
3.5 |
|
5,000 – 10,000 |
5.0 |
25,000 – 50,000 |
3.0 |
|
10,000 – 50,000 |
4.0 |
> 50,000 |
Max {3.9/N0.065,
2.4} |
|
> 50,000 |
Max {7.3/N0.15,
2.4} |
|
|
Notes: STW Sewage
Treatment Works
PTW Preliminary
Treatment Works
PS Pumping Stations
* denotes peaking factors
including stormwater allowance based on GESF
N denotes contributing
population / 1,000
Potential Sewerage Impact on Public
Sewers and Sewerage Facilities
6.7.2.13 A
new sewerage system, with pipe diameters ranging from 150mm to 225mm, will be
provided for collecting the sewage flows generated from the development. The preliminary layout of the
proposed sewerage works for the development is shown in Figure 6.2.
In order to avoid surface
runoff due to rainfall for entering the public sewerage system, separated
system (i.e. stormwater drainage system) will be provided for serving the uncovered
areas of the development. With the provision of this adequate stormwater
drainage system for serving the uncovered areas of the development, it is
anticipated that no surface runoff due to rainfall arising within the uncovered
areas of the site would enter the public sewerage system during the operational
phase.
6.7.2.14
The collected sewage flows from the development will be connected and discharged into
the existing 600mm diameter gravity sewer along Sheung On Street. The sewage
flows (PWWF) to be discharged
into the existing sewer is estimated to be 18.48 L/s (with peaking factor of
8.0 taken into account).
6.7.2.15
Comparing the PWWF of 18.48 L/s
from the development with the capacities of the existing 600mm diameter gravity sewer and the existing 2,100mm
diameter trunk sewers of 357.19 L/s and 9,777.62 L/s respectively, no
significant impact to the receiving sewerage networks and the associated
sewerage facilities arising from the proposed development is anticipated. The
hydraulic assessment for the existing sewerage systems is included in
the Appendix 6.1.
Agreement with Government
Authorities for Proposed Sewerage Discharge
6.7.2.16
The proposed
sewerage layout plans for connection
into the public sewerage system together
with the design arrangement of the proposed treatment facility for treating the
potential contaminated wastewater runoff from the Workshop shall be submitted
to the government authorities (including DSD and EPD) for approval in
the detailed design phase of this Project. Agreement from the authorities
should be sought prior to the commencement of the proposed sewerage connection
work.
6.7.2.17
As there would be potential contaminated discharge from the proposed vehicle cleansing bay in
the workshop, application for a discharge license pursuant to the WPCO should
be submitted to the EPD’s approval for the construction works of this
Project.
6.7.3
Cumulative Impacts
6.7.3.1
The known concurrent projects
during the operational phase of the project, as described in Section 3 of this
EIA report, would include the following where pollution sources may occur:
·
Proposed
·
·
Headquarters and Bus Maintenance Depot in Chai Wan
(EP-107/2001); and
·
Open roads, e.g. Sheung On Street,
6.7.3.2
As the interfacing operation of
these projects with those in the proposed Workshop would involve the provision
and implementation of individual mitigation measures and treatment facilities
for the treatment of any sewage wastewater arising from the respective project,
adverse cumulative water quality impacts would not be anticipated on the WSRs
as listed in Table 6.4.
6.7.4
Operational Phase Mitigation
Measures
6.7.4.1
All sewage arising from the Project should be collected and
diverted to the public sewerage system via proper connections to minimise water
quality impacts from the operation of the Project and ensure compliance with
Technical Memorandum on Standards for Effluents Discharged into Drainage and
Sewerage Systems, Inland and Coastal Waters under the WPCO.
6.7.4.2
In order to prevent the potential
contaminated wastewater from the proposed workshop from entering the existing
public sewerage systems, runoffs from the covered areas under the roof shed
including the vehicle cleansing bay and vehicle parking space will be properly
treated prior to the discharge into the sewerage system. The wastewater treatment facilities for the
development which comprised of petrol interceptor and sedimentation tank would
be designed using sedimentation process with a tentatively treatment capacity
of 1.5 m3/day. The treatment facilities only treat minor runoffs from
the covered areas under the roof shed including the vehicle cleansing bay and
vehicle parking space prior to discharging into the sewerage system. The
derivation of the 1.5 m3/day is shown below:
Cleansing Water System
·
Based on 5 no of cleansing draw off point provided in the
Workshop
·
Assuming 1 no. of cleansing of dia 15mm water tap during the
regular cleansing operation 2 times per day as 15 minutes period
·
Cleansing period = 15 minutes each time
·
Flow rate per tap
= 0.15 L/s
Water Consumption for Cleansing
Water System
·
Assume 2 times per day for operation
·
Operation time = 15 minutes
·
Flow rate of 1 no of
water point = 0.15 L/s
·
15 minutes x 0.15 L/s
x 60 sec x 2 times per day x
5 nos = 1,350 L/day or ~1.5 m3/day
6.7.4.3
The treated effluent for discharging into the public sewerage system
shall comply with the effluent standards as stated in the Technical Memorandum
on Standards for Effluents Discharged into Drainage and Sewerage Systems,
Inland and Coastal Waters under the WPCO. The wastewater treatment facilities
for the development will be designed to comply with the effluent discharge
standards and will be carried out in the detailed design stage of the
Project.
6.8.1.1
Adverse residual impacts during the construction and
operation phases of this Project would not be anticipated, provided that the
above mitigation measures are implemented, even though the operation would be
extended beyond July 2017 as planned.
6.9 Environmental Monitoring and Audit
6.9.1.1
The implementation of good construction works practices as
well as the various specific mitigation measures recommended above will be
important to prevent the pollution of marine water during the construction
phase. It is, therefore,
recommended that construction activities should be subject to a routine audit
programme throughout the construction period. Further details on the scope of this
audit are provided in the EM&A Manual.
No operational phase EM&A for water quality is considered required.
6.10.1.1
Potential water pollution sources have been identified as
construction site run-off, sewage from workforce, and potential risk of
chemical spillage. Mitigation measures including the implementation of the
construction site practices in accordance with the EPD’s ProPECC PN 1/94 Construction
Site Drainage, provision and management of portable toilets on-site, and
preventive measures to avoid accidental chemical spillages are recommended to
mitigate any adverse water quality impacts, based on which adverse residual
impacts would not be anticipated. Also,
there would be sewerage and sewage treatment implications during the operation
of this Project, but adverse water quality impact would not be anticipated with
the implementation of the recommended mitigation measures based on the findings
of this EIA study, even though the operation would be extended beyond July 2017
as planned.