WATER
QUALITY
5.1.
This section assesses the potential
water quality impacts associated with the projectProject. Baseline information including the existing
water quality, hydrographic conditions of the study Study areaArea, projected
flow and pollution loads of the sewage treatment works (STW) were
collected. The collected data were used
for modelling of the initial mixing of sewage plumes. The potential impacts were assessed with reference to the
relevant environmental requirements.
5.2.
The USEPA Cornell Mixing Zone Expert
System (CORMIX) model was adopted to predict the initial dilution of sewage
plumes in the near-field region and the subsequent mixing zone. No far-field hydrodynamic modelling was
required as the effluent flow discharged from the proposed STW would be small.
5.3.
The assessment also identified the
environmental impacts arising from construction activities of the Project
and residual impacts.
Suitable mitigation measures were proposed to minimise these impacts.
5.4.
Legislation pertinent to water quality
impact assessment in this study includes:
·
Water Pollution Control Ordinance
(WPCO) Chapter 358 (as amended by the Water Pollution Control (Amendment)
Ordinance 1990 and 1993);
·
Water Pollution Control (General)
Regulations (as amended by the Water Pollution Control (General) Amended
Regulations 1990 and 1994);
·
Water Pollution Control (Appeal Board)
Regulations;
·
Technical Memorandum Standards for
Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal
Waters;
·
Annexes 6 and 14 of Technical
Memorandum on Environmental Impact Assessment Process (EIAO-TM).
5.5.
Under the Water Pollution Control
Ordinance (WPCO), Water Quality Objectives (WQOs) were established to achieve
the protection of the beneficial uses of water quality in Water Control Zones
(WCZs).
5.6.
The study area falls within the
gazetted Southern Water Control Zone (SWCZ).
The SWCZ supports a wide range of beneficial uses, including secondary
contact recreation, fish production and ship navigation. Of these sensitive uses, fish production is
of particular concern as there is a gazetted Fish Culture Zone (FCZ) within Picnic
Bay. Table 5.1 summarises the WQOs for
the SWCZ, including the specific requirements for secondary contact recreation
subzone and FCZ.
Table
5.1 Water Quality Objectives for
Marine Waters of Southern Water Control Zone
|
Parameter
|
Objective
|
Part(s)
of Zone
|
|
E. coli
|
annual geometric mean not to exceed 610/100 ml
|
secondary contact
recreation subzones; fish culture subzones
|
|
Dissolved
Oxygen within 2 m of bottom
|
not less than 2 mg/L for 90% samples
|
marine waters;
|
|
Depth averaged
Dissolved Oxygen
|
not less than 4 mg/L for 90% samples
not less than 5 mg/L for 90% samples
not less than 4mg/L at any point within the water column
|
marine waters; inland
waters
fish culture subzones
inland waters
|
|
pH value
|
within the range 6.5 to 8.5; change due to waste discharge not to
extend by 0.2
|
marine waters
except bathing beach subzones
|
|
Salinity
|
change due to waste discharge not to exceed 10% of
natural ambient level
|
whole zone
|
|
Temperature
|
change due to waste discharge not to exceed 2oC
|
whole zone
|
|
Suspended
solids
|
waste discharge not to raise the natural ambient level by 30%, nor
cause the accumulation of suspended solids which may adversely affect aquatic
communities
|
marine waters
|
|
Chemical Oxygen
Demand
|
not to exceed 30 mg/L
|
inland waters
of the WCZ
|
|
Aesthetic Appearance
|
(a) discharge should not cause objectionable odour
or discoloration
(b) no tarry residue, floating wood, articles made
of grass, plastic, rubber or any other substance
(c) Mineral oil not visible on the surface.
Surfactants should not give rise to a lasting foam.
(d) no recognizable sewage-derived debris
(e) no floating, submerged or semi-submerged
subjects likely to interfere with the free movement or damage of material
(f) not to contain substances which settle to form
objectionable deposits
|
whole zone
whole zone
|
|
Nutrients
|
not to be present in quantities that cause excessive growth of algae or other aquatic plants
annual mean depth average inorganic nitrogen not
to exceed 0.1 mg/L
|
marine waters
marine waters
|
|
5-Day
Biochemical Oxygen Demand
|
not to exceed 5 mg/L
|
inland waters
of the WCZ
|
|
Unionized
Ammonia
|
annual level should not exceed 0.021 mg/L
|
whole zone
|
|
Dangerous
Substances
|
not to be present at levels producing significant
toxic effect in humans, fish or any other aquatic organisms, with due regard
to biologically cumulative effects in food chains and to toxicant
interactions with each other
not to cause a risk to any beneficial use of the
aquatic environment
|
whole zone
whole zone
|
5.7.
Discharge from the STW into the
receiving waters is subject to control.
Submersible outfall with multiport diffusers is usually adopted for
sewage disposal. Effluent plumes would
be formed in the vicinity of the discharge points. Mixing of effluent with the ambient water achieves a rapid
initial dilution of pollutants in the mixing zone. It is specified in the mixing zone criteria of the EIAO-TM that
exceedance of water quality criteria in the mixing zone is allowed. At the boundary of the mixing zone, the requirements
of WQOs need to be met.
5.8.
Picnic Bay (Sok Kwu Wan) is located on
the south side of Lamma Island.
Hydrographic and water quality data were collected at Picnic Bay for the
wet and dry seasons. Oceanographic and
water quality measurements were made in the regions to the north, east and
south of Lamma Island covering the spring and neap tidal cycles. There were eight sampling positions for the
hydrological and water quality measurements (Figure 5.1). Wind speed and direction were measured at
Position 3 near the diffusers of the proposed submarine outfall. The surveys for the wet season were carried
out from 9-10 October 1998 for spring tide and 19-20 October 1998 for neap
tide. The collected data were used for
the preliminary assessment of water quality impacts.
5.9.
As the survey team carried out the wet
season measurements at locations different from the those
proposed by the Consultants, corrective hydrographic surveys for the wet season
were carried out from 16 March 1999 to 2 April 1999 and the
results were presented in the Hydrographic
Survey at Sok Kwu Wan, Final Report on Wet Season Surveys, Volumes I and
II. The dry season surveys for both
neap and spring tides were carried out from 29 November 1998 to 5 December 1998
and similarly, a corrective dry season survey was carried out from
1 February 1999 to 7 February 1999. The updated results were presented in the Hydrographic Survey at Sok Kwu Wan, Final
Report on Dry Season Surveys, Volumes I and II. The assessment results presented in this
report are based on the updated survey data.
5.10.
The dry and wet season surveys
included the measurements of drogue tracking, current speed and direction, wind
speed and direction, tide levels, water temperature, salinity, dissolved oxygen,
turbidity, conductivity and pH. The
collected water samples were analysed for suspended solids (SS), biochemical
oxygen demand (BOD), silica, ammonia nitrogen (NH3N), nitrate (NO3-N),
nitrite (NO2-N), total Kjeldahl nitrogen (TKN), total phosphorus (TP),
chlorophyll-a, and E. Coli. Appendix 5.1 provides the results of the hydrographic survey.
5.11.
For the wet season surveys, the
pollutant levels in the water of the study area were found to be low except the
total inorganic nitrogen (TIN) levels being relatively high (0.082 mg/L). The measured dissolved oxygen levels were
reasonably high (> 4 mg/L). During
ebb tide, the drogue tracking surveys (including surface float and mid-depth
float with vane 7.5m below sea surface) showed that floats released at the
proposed sewage outfall location (Position 3) were carried by the tide to the
east and then south past Ngai Tau along the East Lamma Channel. During flood tide, the floats were carried
by the tide westward into Picnic Bay approaching the shoreline of the quarry
and then to the north near Luk Chau.
Eddy currents were occasionally found near Luk Chau. The wet season surveys indicated that
vertical variations of salinity and temperature within the water column were
small.
5.12.
The dry season surveys showed that
pollutant levels in terms of suspended solids and BOD were low but the TIN
levels were high (0.229 mg/L) and exceeded the WQO limit of 0.1 mg/L. The seawater within the study area was well
oxygenated (> 4 mg/L). The drogue
tracking surveys during ebb tide showed that floats were carried by the tidal
currents to the east of Lamma Island round Ngai Tau and moved into the East
Lamma Channel. During flood tide,
floats were moved to the west and then north-passing around Luk Chau towards
the north of Lamma Island. Eddy
currents were found north of Luk Chau and within Luk Chau Wan. The temperature and salinity of the seawater
in the area were rather uniform. No
stratification of water was detected during the dry season surveys.
5.13.
The study area falls within the
SWCZ. This zone supports the beneficial
uses of commercial fisheries and ship navigation. Sensitive receivers include the gazetted Fish Culture Zone (FCZ)
at Picnic Bay and the secondary contact recreation subzone at Mo Tat Wan.
5.14.
As mentioned in Section 2, Horizontal
Directional Drilling (HDD) technique would be adopted to replace the
conventional open trench method, which involved intensive dredging, for the
construction of the near shore pipeline section of
the submarine outfalloutfall pipeline. Details of the HDD technique are provided in Appendix 2.1. By using the HDD technique, potential water
quality impact from the construction of the submarine outfall would be
minimised.
5.15.
As indicated in Appendix 2.1, the use
of the HDD technique could avoid dredging operation until at a location of
approximately 480m from the coast.
Minor dredging works would still be required for the construction of
approximately 240m outfall pipeline and the diffusers. The dredging area would be located at
aroundno less than 300m away from the gazetted FCZ
and 500m from the secondary contact recreation subzone (Figure 5.2). Sediment plume would be formed during the
dredging works, which would increase the suspended solid concentration in the
neighbouring water. Assessment of the
potential water quality impacts associated with the dredging works, including
potential contaminant release from the sediment, was presented in the following
section. Mitigation measures were
proposed to minimise these potential impacts.
5.16.
As the rate of the dredging works was
expected to be low and small scale (total dredging volume of around 26,000m3),
a near field model of sediment dispersion [R.E. Wilson 1979)]was adopted to
assess the impacts from suspended sediment plumes generated during dredging
activities.
5.17.
In this model, the depth-averaged
suspended sediment concentrations were calculated at varying distances from the
source of the sediment release. This
model was used to predict suspended sediment concentrations with distance from
the dredging works and hence determine at what distance the elevations in
suspended sediment concentrations would be acceptable according to the WQO. The WQO for suspended sediments for SWCZ
stated that the marine activities during the construction works must not cause
the natural ambient level to be raised by more than 30% nor give rise to
accumulation of suspended sediments.
5.18.
For the purpose of this assessment,
the ambient value of suspended sediments (SS) was represented by 90th
percentile of the reported concentrations.
EPD routine monitoring data (1998–2000) at monitoring station SM4, which
was the nearest station to the identified sensitive receivers, were used as the
source of the reported concentrations.
The ambient values and allowable increases in SS concentration under the
WQO for the identified sensitive receivers are given in Table 5.2
Table
5.2 Ambient and Allowable
Elevation in Suspended Sediment Concentrations (mg/L)
|
Sensitive Receiver
|
Ambient Suspended
Sediment (mg/L)
|
30% Elevation
(mg/L)
|
|
Wet Season
|
|
|
|
Gazetted FCZ at Picnic Bay/Mo Tat Wan
|
5.9
|
1.8
|
|
Dry Season
|
|
|
|
Gazetted FCZ at Picnic Bay/Mo Tat Wan
|
5.9
|
1.8
|
|
|
|
|
Notes:All values are depth-averaged
Effluent Discharge at Submarine Outfall
5.19.
The potential water quality impacts
associated with the submarine outfall discharge were assessed in the near-field
region and the subsequent mixing zone.
In the near-field region, the effluent plume would be dominated by the
initial discharge momentum, buoyancy and the ambient current. In the far-field region, the effluent plume
behaviour would be determined by the dispersion processes in the receiving
water. Hydrographic surveys for the wet
and dry seasons were carried out to record the baseline water quality
conditions in the study area. The
collected data revealed the quality of the existing aquatic environment and
served as a reference for the assessment of water quality.
5.20.
The assessment of water quality impacts
from the submarine outfall discharge was based on computer modelling of outfall
scenarios. Hydrographic surveys for the
dry and wet seasons provide input data for the modelling exercise. USEPA CORMIX2 was used to predict the
characteristics of effluent plume in the near-field region. The CORMIX2 was widely accepted for
modelling the multiport discharges under different ambient conditions. Extensive experimental and field data were
used to verify the model.
5.21.
Hydrodynamic modelling of water
quality in the far-field region was not required in this study due to the small
volume of the discharged effluent from the STW. As the CORMIX2 model also predicted the effluent plume behaviour
beyond the near-field region, further information on the initial dilution of
effluent plume in the subsequent mixing zone would also be obtained.
Emergency Discharge
5.22.
Since the proposed emergency outfall
of the pumping stations and STW would be located in the vicinity of the
gazetted FCZ, potential water quality impact from the emergency discharge at
the gazetted FCZ is anticipated. To
quantify the potential impact, CORMIX3 model was used to assess the emergency
discharge. The CORMIX3 model was used
to run for the emergency overflow from the three pumping stations and the STW.
5.23.
During the construction phase,
potential sources of water quality impacts were identified below:
·
Dredging works for the construction of
about 240m outfall pipeline and diffusers;
·
Construction site runoff and drainage;
·
General construction activities; and
·
Wastewater arising from workforce.
5.24.
As mentioned in Section 2, the HDD
technique would be adopted for the proposed submarine outfall
construction. With the adoption of this
technique, no dredging would be required until at a location of around 480m
from coastline. Minor dredging works
would be required for the construction of around 240m of the outfall
pipeline and the diffusers. The
dredging area would be located at aroundno less than 300m and 500m away from the
gazetted FCZ at Picnic Bay and the secondary contact recreation subzone at Mo
Tat Wan, respectively. The extent of the dredging works is illustrated in
Figure 5.2. In view of the vulnerable
nature of the FCZ, the suspended fine sediment released from the dredging
activities would pose adverse impact on the FCZ if no mitigation measures were
in place.
5.25.
Since the sewage pipe would be laid at
a depth of 10m below the existing seabed with the use of HDD technique, no
armoured rock would be required.
However, armoured rock for the section of pipeline and diffuser zone
constructed by dredging method would be required. The loss of fines from the laying of rock bedding in the open
trenches would be insignificant taking account of the coarse nature of rock
fill and the shallow water depth. No
sand fill would be used for the trench filling.
5.26.
During dredging, contaminants such as
heavy metals and other toxic substances would be released from sediment when
seabed was disturbed. The potential for
adverse effects on water quality through mobilization and release of
contaminants into the surrounding water column during dredging would depend on
the level of contamination of the marine sediment. As part of the project, a sediment sampling and testing survey
within the Study Area was undertaken to provide an indication of the sediment
quality for the purpose of this EIA Study.
5.27.
The HDD technique would involve
erecting a steel-working platform near the shore for setting up the horizontal
directional drilling system. Since the
setting up of the steel working platform and the drilling system is not
expected to cause significant disturbance to the seabed, adverse water quality
impact would not
be expected.
5.28.
The drilling process of the HDD
technique would require drilling fluid, which would be a mixture of water and
specialized additives. According to the
information provided by the Contractor, the drilling fluid (Bentonite fluid)
that would be used for the drilling process is environmentally friendly and without
dangerous contaminants, and is extensively used in Europe as well as America
for both HDD and water well applications(Appendix 5.1a). The used drilling fluid (slurry) would be
circulated back to the ground surface and collected in return pits for recycling
or disposal. Discharge of the slurry to
the bay would only occur when the drill breaks the ground at around 480m from
the coast. However, the discharge of
slurry would occur in the short term.
With the deployment of silt curtains around the exit area as stated in
Appendix 2.1, adverse water quality impact associated with the discharge of the
slurry during breaking the ground would not be anticipated.
5.29.
During land-based construction
activities, runoff and drainage from construction sites would be the major
sources of potential water quality impacts to the nearby water bodies. Site run-off and drainage would contain
increased loads of suspended solids and contaminants and would enter the
adjacent coastal waters, if uncontrolled.
In addition, sewage arising from the on-site construction workforce
would have the potential to cause water pollution if it was discharged directly
into adjacent waters without any appropriate treatment.
5.30.
During the operation phase, the
sewerage system would collect and treat the sewage from the Sok Kwu Wan
catchment area that had previously been chronically discharged, untreated or
only partially treated, directly into the surrounding water bodies, with
subsequent effects on the water quality of these water bodies. Hence, this Project was considered to have a
significant environmental benefit to this area in terms of improvements of
coastal water quality. Nevertheless,
effluent discharge from the STW would act as a point source of pollution, which
would have potential water quality impacts on the nearby sensitive
receivers.
5.31.
Operational failures of the pumping
stations and the STW would result in an overflow of untreated sewage. Since the proposed emergency outfalls of the
pumping stations and STW would be located close to the gazetted FCZ (Figure
5.3), emergency discharge of raw sewage at these outfalls would likely have
significant impacts on the FCZ.
Release of Suspended Sediment during Dredging
5.32.
The increase of suspended solids and
turbidity in the water column during the dredging activities would cause water
quality impacts to the neighbouring Mo Tat Wan and FCZ. The dredged volume was estimated to be approximately
26,000 m3. The dredging
would be undertaken using an open grab dredger, with a dredging rate of 365 m3
hr-1.
5.33.
With respect to rate of sediment loss
during dredging, two previous studies reviewed
world-wide data on loss rates from dredging operations and concluded that, for
grab dredgers working in areas with significant amounts of debris on the sea
bed, the sediment loss rate would be 25 kg m-3, while the loss rate
in areas where debris was less likely to hinder the operations would be 17kg m-3. Since there were no existing mooring buoy or
port facilities in the vicinity of the proposed dredging areas, it was unlikely
to be substantial quantities of debris on the seabed. The lower figure of 17 kg m-3 was therefore adopted
for this study and the loss rate was calculated to be 1.72 kg s-1.
5.34.
The following formula was used to
estimate the concentration of suspended sediment (SS) at a certain distance
from the source:
C(x) = q/(D*x*w*Öp)
Where C(x) = SS concentration at distance x from the
source
q = sediment loss rate = 1.72 kg s-1
D = water depth = 7 m
x = distance from source
w
= diffusion velocity = 0.01 m s-1
5.35.
The water depth of 7 m was the minimum
depth measured at the Study Area from the hydrographic survey. The diffusion velocity represented reduction
in the centre-line concentrations due to lateral spreading. As suggested by Wilson (1979), a diffusion
velocity of 0.01m s-1 was used for the calculation.
5.36.
Based on the above equation, the
elevation of SS concentrations at different distance from the discharging point
was predicted. The results are
presented in Table 5.3.
Table
5.3 Predicted Elevation in
Suspended Sediment Concentrations (Unmitigated Scenario)
|
Distance from Source
(m)
|
Concentration of
Suspended Sediment (mg/L)
|
|
100
|
138.9
|
|
150
|
92.6
|
|
200
|
69.5
|
|
250
|
55.6
|
|
300
|
46.3
|
|
350
|
39.7
|
|
400
|
34.7
|
|
450
|
30.9
|
|
500
|
27.8
|
|
550
|
25.3
|
|
600
|
23.2
|
|
650
|
21.4
|
|
700
|
19.8
|
5.37.
The closest distance of dredging area
to the gazetted FCZ at Picnic Bay and the secondary recreation subzone at Mo
Tat Wan were approximately 300m and 500m, respectively. The above table indicates that the SS
concentration at the gazetted FCZ and the secondary contact recreation subzone
would exceed the allowable elevation in SS concentrations given in Table 5.2. To mitigate the potential impact arising from
the dredging activities, appropriate water quality mitigation measures are
considered necessary and are discussed in Section 5.73 – 5.78.
Release of Contaminants during Dredging
5.38.
Since the sediment testing results
showed that marine sediments to be dredged for the proposed submarine outfall
were classified as Category L and no exceedance of the respective LCELs were
recorded (Section 6 refers), the potential impact of contaminants released from
the sediments would be of minimal. In
other words, the potential release of metals and organics from sediment into
the water column would not result in adverse impacts on water quality during
the dredging works.
5.39.
A comparison of the elutriate test
results for nutrients with the seawater sample from the site indicated that the
concentrations of NH3-N
in the elutriate samples (1.09 – 3.21 mg/L) were higher than the background
values (0.04 mg/L) recorded in the seawater sample. Therefore, this nutrient would likely be released from the
sediment into the marine waters when the seabed is disturbed during the
dredging activities.
5.40.
An assessment of nutrient release
during dredging operation were made in relation to the results of the predicted
elevation in SS concentrations and the sediment quality data for the Study Area. The predicted elevations of SS concentrations
for the unmitigated scenario at the sensitive receiver were used to calculate
the effects of these increased suspended sediment concentrations on NH3-N concentration. In the calculation, it was assumed that all
of the ammonia concentrations in the sediment were released to the water. These were conservative assumptions and
would likely result in an over-prediction of the potential impacts.
5.41.
In order to determine compliance with
the water quality criteria, the background water quality data at the sensitive
receivers was based on EPD’s monitoring data presented in Marine Water Quality in Hong Kong 2000. The calculated NH3-N
released from the sediment would result in a concentration of total ammonia in
the receiving waters that must be converted to unionised ammonia to compare
with the WQO of SWCZ (unionized ammonia: annual mean not to exceed 0.021 mg/L).
Based on the data collected at EPD monitoring station SM4 (temperature of 22.4oC,
pH 8.1, and salinity of 31.7 ppt), and the spreadsheet developed by the
Washington State Department of Ecology and adopted by the U.S. Environmental
Protection Agency (EPA) for water quality criteria calculations, it was
estimated that the unionised ammonia would constitute around 4.7% of the total
ammonia concentration. The result of
the analysis for NH3-N
is given in Tables 5.4 below.
Table
5.4 Calculation of Unionised
Ammoniacal Nitrogen Concentrations during Dredging
|
Sensitive Receiver
|
Elevation in SS
concentration
(mg/L)
|
NH3-N
in Sediment
(mg kg-1)(1)
|
Increase in Unionised
Ammonia
(mg/L)
|
Background Unionised
Ammonia
(mg/L)(2)
|
Total Unionised Ammonia
(mg/L)
|
|
Gazetted FCZ at Picnic Bay
|
46.3
|
39.1
|
8.5
x 10-5
|
0.002
|
0.0021
|
|
Secondary Contact Recreation Subzone at Mo
TakMo
Tat Wan
|
27.8
|
39.1
|
5.1
x 10-5
|
0.002
|
0.0021
|
Notes: (1) Maximum
concentration recorded from marine sediment testing at Sok Kwu Wan
(2) Taken from EPD routine water quality monitoring data (2000)
5.42.
The above table indicated that the
predicted unionised ammonia would be well below the WQO during the dredging
activities. Hence, no adverse water
quality impact in relation to unionised ammonia would be anticipated.
5.43.
During construction of the sewers,
pumping stations and STW, soil surfaces would be exposed and an elevated level
of suspended particles would be present in the surface run-off. Sediment laden runoff would carry pollutants
(adsorbed onto the particle surfaces) into the stormwater drainage system. Sources of water pollution include release
of cement materials with rain wash, wash water from dust suppression sprays,
and fuel, oil and lubricants from maintenance of construction vehicles and
mechanical equipment.
5.44.
Mitigation measures should be
implemented to control construction site runoff, and to minimise the chances of
introducing sediment and pollutants into the stormwater drainage system and the
receiving coastal waters.
General Construction Activities
5.45.
On-site construction activities would
cause water pollution from the following:
·
Uncontrolled discharge
of debris and rubbish such as packaging, construction waste and refuse.
·
Spillages of liquids
stored on-site, such as oil, diesel and solvents etc, would result in water
quality impacts if they entered adjacent drains or coastal waters.
5.46.
Good construction practices and site
management should be observed to ensure that litter, fuels and solvents would
not enter the nearby coastal waters and storm water drains.
5.47.
Domestic sewage would be generated
from the workforce during the construction phase. Sewage generated from the site would not have
adverse water quality impact if sewage was not discharged directly into
stormwater drains or coastal waters adjacent to the construction site and
temporary sanitary facilities, such as portable chemical toilets, were used
on-site and properly maintained.
Effluent Discharge
5.48.
Discharge from the STW would likely
influence water quality in the vicinity of the discharge location. The extent of water quality impacts on the
sensitive receivers relating to the changes of water quality would depend on
the initial dilution, spatial extent of the effluent plume and the assimilative
capacity of the receiving water. The
sensitive receivers in the study area, i.e. the gazetted FCZ at Picnic Bay and
the secondary contact recreation subzone at Mo Tat Wan, would be around 560 m
and 730 m from the discharge location of the proposed outfall (diffusers),
respectively. The outfall diffusers
would be located at approximately 750 m from the coast and would be
terminated near the entrance of the bay that provides better dispersion and
dilution.
5.49.
Computer modelling of the initial
mixing of the discharged effluent in the near-field region was carried out,
which provided the basis for evaluation of the receiving water quality. The potential impacts on the FCZ and the
secondary contact recreation subzone were assessed based on the modelling
results. The ambient water quality
conditions for the wet and dry seasons were considered in the modelling exercise.
Hydraulic Consideration of the Proposed Submarine
Outfall
5.50.
The length of the proposed submarine
outfall would be approximately 750 m extending from the shore near Mo Tat Wan
to the entrance of Picnic Bay. The
design flow, based on the population data for the study area, would be
940 m3/d. An outfall
pipe of 140 225 mm diameter would be used to transport
the effluent to the diffusers. The
effluent velocity in the outfall pipe (about 1 m/s, assuming a peaking
factor of 4) would be sufficiently high to avoid any deposition of fine solid
particles. To allow flexibility in the
design of the outfall, an outfall length of 600 m was adopted in the modelling
exercise and represented a conservative scenario with respect to the proposed
length of 750 m.
5.51.
Multiport diffusers would increase the
initial dilution of the discharged effluent in the receiving water. In view of the small discharge flow rate, an
alternating diffuser where the ports point vertically to upward would be
adopted. The port opening diameter
would be 100 mm and there would be 3 diffusers with one opening for
each diffuser head. To prevent seawater
intrusion into the submarine outfall during the periods of low flows, a
non-return valve will be installed at each port. Sufficient spacing between the diffuser
heads would be required so as to avoid merging of the rising plume. The distance between the diffuser heads
would at 15 m spacing. This
spacing would be larger than the water depth of 13 m. The feasibility of this arrangement would be
subject to engineering design and review.
The general outfall parameters are summarised as below:
Diffuser type : Unidirectional; discharge in the
vertical direction
No. of ports : 1 port per diffuser, 3 diffusers
Distance
from the shore : 600 m
Port
opening diameter : 100 mm
Water
depth : 13 m
Darcy-Weisbach
friction : 0.023
Factor
Effluent Quality
5.52.
The design flow was calculated based
on population data for the study area.
A peaking factor of 4 including storm water allowance was used to
estimate the peak flow rate from the STW.
The effluent discharge parameters (95th percentile value) for
the STW are:
Effluent
flow : 940 m3/d x 4 (peaking
factor) = 0.0435 m3/s
Effluent
density : 997 kg/m3
Effluent
BOD : 20 mg/L
Effluent
SS : 30 mg/L
Effluent
TKN : 40 mg/L
Effluent TIN : 12 mg/L (With denitrification
applied)
Effluent
NH3-N : 20 mg/L
Effluent
E. coli : 1500/100
mL
Modelling
Results
Modelling of the Effluent Plume - Wet Season
5.53.
The proposed location for submarine
outfall discharge would near the sampling Position 3 of the hydrographic
survey. The input environmental
parameters to the model for the wet season were based on the water quality data
collected at this position.
Current speed : 0.1 to 0.5 m/s
Seawater density : 1020.5 kg/m3 at surface
(High High Water);
1020.6 kg/m3 at
bottom
BOD (mg/L) : < 1
DO (mg/L) : 5.52
SS (mg/L) : 12
Total Kjeldahl
Nitrogen (TKN) (mg/L):0.25
Total Inorganic
Nitrogen (TIN) (mg/L):0.082
Ammonia Nitrogen
(NH3-N) (mg/L) : 0.04
E. coli (per 100 mL) : 22
5.54.
The modelling results for the wet
season conditions are presented in Table 5.5.
The effluent plume characteristics including its thickness, half-width
and initial dilutions were recorded at the end of the near-field region. Due to the strong ambient current and the
small discharge flow rate, the discharges from the diffuser ports were rapidly
deflected. The CORMIX2 model predicted
that, for ambient current of 0.1 m/s, the discharged effluent was fully mixed
over the entire layer depth at a distance of about 5 times the layer depth
downstream, i.e. 65 m, from the source within the near-field region. For ambient current up to 0.5 m/s, the near
field region extended further downstream to about 130 m from the source with a
thickness of 5.5 m.
Table
5.5 Modelling Results for the Wet
Season Conditions (Unstratified Conditions)
|
Description
|
|
|
0.1 m/s
|
0.5 m/s
|
|
Distance downstream (m)
|
65
|
130.2
|
|
Plume thickness (m)
|
13
|
5.48
|
|
Plume half-width (m)
|
15.86
|
17.27
|
|
Initial dilution
|
948.5
|
2175.5
|
|
BOD (mg/L), initial = 20
|
1
|
1
|
|
SS (mg/L), initial = 30
|
12
|
12
|
|
TKN (mg/L), initial = 40
|
0.29
|
0.27
|
|
TIN (mg/L), initial = 12
|
0.10
|
0.09
|
|
Ammonia Nitrogen (mg/L), initial = 20
|
0.061
|
0.049
|
|
E. coli (per 100 mL), initial = 1500
|
24
|
23
|
5.55.
As shown in Table 5.5, the initial
dilution for the case of ambient current at 0.5 m/s was much higher than that
of ambient current at 0.1 m/s. This
indicated that the ambient current played an important role in enhancing the
dilution of the discharged effluent. The rapid mixing of the discharged
effluent with the ambient water resulted in a high initial dilution. At the end of the near-field region, the
effluent parameters would become much lower concentrations when compared to
their initial concentrations. Figure
5.4 shows the increases of initial dilution with distance downstream from the
source in the near-field region and the subsequent mixing zone. Figure 5.4a shows the extent of the effluent
plume, in terms of TIN level from the diffusers.
5.56.
The above analysis for the wet season
conditions was based on the hydrographic surveys with no stratification. As Hong Kong marine waters west of Victoria
Harbour are expected to be stratified in density during wet season, EPD
monitoring data (Station SM4) in 1996 were used to represent the stratified
conditions. A linear density
distribution with the surface density of 1018.7 kg/m3 and bottom
density of 1021.1 kg/m3 was adopted in the modelling exercise. The modelling results are shown in Table 5.6. A plot of the effluent plume dilution is
shown in Figure 5.5.
Table
5.6 Modelling Results for the Wet
Season Conditions (Summer Stratified Conditions)
|
Description
|
|
|
0.1 m/s
|
0.5 m/s
|
|
Distance downstream (m)
|
6.17
|
24.74
|
|
Plume thickness (m)
|
1.73
|
0.74
|
|
Plume half-width (m)
|
16.08
|
15.83
|
|
Initial dilution
|
127.6
|
270.4
|
|
BOD (mg/L), initial = 20
|
1
|
1
|
|
SS (mg/L), initial = 30
|
12
|
12
|
|
TKN (mg/L), initial = 40
|
0.56
|
0.40
|
|
TIN (mg/L), initial = 12
|
0.18
|
0.13
|
|
Ammonia Nitrogen (mg/L), initial = 20
|
0.197
|
0.114
|
|
E. coli (per 100 mL), initial = 1500
|
38
|
28
|
5.57.
The effluent plume was deflected by
the strong ambient current and was influenced by the linear density
stratification. The initial dilutions
at the end of the near-field region for the two ambient current conditions were
comparatively lower than that of the unstratified wet season conditions.
Modelling of the
Effluent Plume - Dry Season
5.58.
The water quality data measured during
the dry season at Position 3 were used as inputs to the CORMIX model. The ambient currents for the dry season did
not have much difference to that of the wet season. In addition, there was no density or temperature stratification
in the waters. Environmental parameters
for the dry season surveys are:
Current speed : 0.05 to 0.5 m/s
Seawater density : 1021.0 kg/m3 at surface (High High Water);
1021.0
kg/m3 at bottom
BOD (mg/L) : < 1
DO (mg/L) : 6.3
SS (mg/L) : 8
TKN (mg/L) : 0.29
TIN (mg/L) : 0.229
NH3-N (mg/L) : 0.098
E. coli (per 100 mL) : 85
5.59.
The modelling results for the dry
season conditions are summarised in Table 5.7.
In the near-field region, the effluent plume in high ambient current of
0.5 m/s would experience full vertical mixing, while a vertical jet like
plume with slight deflection would be formed in low ambient current of
0.05 m/s. The initial dilutions
for the two ambient current conditions were high. Figure 5.6 shows the increases of initial dilution with the
downstream distance.
Table 5.7 Modelling
Results for the Dry Season Conditions
|
Description
|
|
|
0.05 m/s
|
0.5 m/s
|
|
Distance downstream (m)
|
14.94
|
65
|
|
Plume thickness (m)
|
8.98
|
13
|
|
Plume half-width (m)
|
29.89
|
15.04
|
|
Initial dilution
|
617.3
|
4494.7
|
|
BOD (mg/L), initial = 20
|
1
|
1
|
|
SS (mg/L), initial = 30
|
8
|
8
|
|
TKN (mg/L), initial = 40
|
0.36
|
0.30
|
|
TIN (mg/L), initial = 12
|
0.25
|
0.23
|
|
Ammonia Nitrogen (mg/L), initial = 20
|
0.130
|
0.102
|
|
E. coli (per 100 mL), initial = 1500
|
87
|
85
|
Evaluation of Potential Impact
5.60.
As mentioned in Section 5.13, two
sensitive receivers were identified in the vicinity of the proposed submarine
outfall, including the FCZ at Picnic Bay and the secondary contact recreation
subzone at Mo Tat Wan. Based on the
modelling results, the pollutant concentrations at these sensitive receivers
were predicted. The results are
presented in Table 5.8.
Table
5.8 Modelling Results at
Sensitive Receivers
|
Parameters
|
Predicted Concentration1
|
|
Wet Season Without Stratification
|
Wet Season With Stratification
|
Dry Season
|
|
0.1 m/s
|
0.5 m/s
|
0.1 m/s
|
0.5 m/s
|
0.05 m/s
|
0.5 m/s
|
|
At Gazetted FCZ
|
|
BOD (mg/L)
|
1
|
1
|
1
|
1
|
1
|
1
|
|
SS (mg/L)
|
12
|
12
|
12
|
12
|
8
|
8
|
|
TKN (mg/L)
|
0.278
|
0.257
|
0.333
|
0.268
|
0.333
|
0.297
|
|
TIN (mg/L)
|
0.09
|
0.08
|
0.11
|
0.09
|
0.24
|
0.23
|
|
NH3-N (mg/L)
|
0.054
|
0.043
|
0.081
|
0.049
|
0.120
|
0.101
|
|
Unionised Ammonia2
(mg/L)
|
0.003
|
0.002
|
0.004
|
0.002
|
0.006
|
0.005
|
|
E. coli (counts per
100 mL)
|
23
|
22
|
25
|
23
|
87
|
85
|
|
At Secondary Contact
Recreation Subzone at Mo Tat Wan
|
|
BOD (mg/L)
|
1
|
1
|
1
|
1
|
1
|
1
|
|
SS (mg/L)
|
12
|
12
|
12
|
12
|
8
|
8
|
|
TKN (mg/L)
|
0.275
|
0.256
|
0.317
|
0.263
|
0.330
|
0.296
|
|
TIN (mg/L)
|
0.09
|
0.08
|
0.10
|
0.09
|
0.24
|
0.23
|
|
NH3-N (mg/L)
|
0.052
|
0.043
|
0.073
|
0.046
|
0.118
|
0.101
|
|
Unionised Ammonia2
(mg/L)
|
0.002
|
0.002
|
0.003
|
0.002
|
0.006
|
0.005
|
|
E. coli (counts per
100 mL)
|
23
|
22
|
25
|
22
|
87
|
85
|
Note:
1 Background
level were included.
2 Unionised
ammonia concentration was calculated based on 4.7% of unionised ammonia
constituting the total ammonia concentration at 22.4oC, pH 8.1, and
salinity of 31.7ppt.
5.61.
The results shown in Table 5.8
indicated that the elevation in BOD, SS and E.
coli concentrations due to the effluent discharge at the submarine outfall
would be diluted and become negligible at the FCZ and the secondary contact
recreation subzone. The BOD levels
remained the same as the background levels and hence dissolved oxygen levels in
these sensitive receivers would not be changed. The resulting E. coli
concentrations were well below the WQO limit of 610 count/100 mL. The results also showed no exceedances of
the WQO limit of 0.021 mg/L for unionised ammonia. Hence, the WQOs, in terms of BOD, SS, unionised ammonia and E. coli would be satisfied.
5.62.
As indicated in Table 5.8, exceedances
of WQO limit for TIN were predicted during the wet season with stratification
and during dry season. However, the
exceedances were attributable to the high background TIN levels (0.08 mg/L for
wet season, 0.23 mg/L for dry season).
The background level for dry season had already exceeded the limit by
more than twice. Owing to the high
background TIN level, exceedance of the WQO limit for TIN would be unavoidable
even with a very small contribution from the treated effluent discharge at the
submarine outfall. According to the EPD
monitoring data at SM4, annual variations of TIN levels in 1998, 1999 and 2000
were in the range of 0.03-0.28 mg/L, 0.05-0.35 mg/L and 0.1-0.3 mg/L,
respectively. The predicted TIN values,
which varied from 0.08 - 0.24mg/L are within these annual ranges of TIN
level. Hence, the elevation of TIN from
the effluent discharge was not significant in contrast to annual variation of
TIN of at the Picnic Bay.
5.63.
The predicted TIN levels as shown in
Table 5.8 were considered to be over-estimated as they were calculated by
adding the elevated TIN levels directly to the measured background TIN
levels. The prediction was not taken
account of the beneficial effect of eliminating untreated or partially treated
sewage discharges to the Picnic Bay upon the operation of the proposed
pProject. The proposed system would collect and
transfer sewage from the Sok Kwu Wan area to the proposed STW for treatment and
would discharge the treated sewage at a long submarine outfall of around 750 m
from the coast. Having considered the
presence of the FCZ and the high background TIN level, the treatment process of
the STW would include denitrification in order to reduce the potential water
quality impact in relation to TIN .
With the denitrification applied, more than 50% reduction in TIN level
would be achieved after treatment. In
addition, the system would discharge the treated effluent through the submarine
outfall at a location of about 560m from the FCZ, instead of discharges of
untreated or partially treated sewage along the coastline of Sok Kwu Wan as the
existing situation. As such, the
potential water quality impact in relation toTIN on the sensitive receivers,
would not be expected to be worse than the existing conditions. In addition, the Project would improve the
water quality of Picnic Bay by eliminating untreated or partially treated
sewage discharges to the bay. Appendix
5.2 gives a calculation to demonstrate the difference in TIN level with and
without the Project. Based on the
calculation in Appendix 5.2, the predicted TIN levels (excluded the TIN level
contribution contribution from the discharges of untreated
or partially treated sewage) for dry season and wet season with stratification
have been plotted against the downstream distance from source and are shown in
Figures 5.7 and 5.8.
5.64.
Emergency discharge of raw sewage
would likely occur
as a result of the following occasions:
·
Operational failure of Pumping Stations
P1a, P1b or P2;
·
Operational failure of the STW (i.e.
complete failure of all Sequencing Batch Reactor (SBR) units installed in the
STW).
5.65.
In addition, discharge of treated
sewage at the emergency outfall of the STW would likely occur in the event of failure
of the effluent pumping station failure or blockage of the submarine outfall.
5.66.
To quantify the potential water
quality impact as a result of the emergency discharge, CORMIX3 model was run
for various scenarios. The parameters
given in Table 5.9 were input into the model.
Table
5.9 CORMIX Modelling Parameters
|
Parameters
|
P1a
|
P1b
|
P2
|
STW
|
|
Outfall/ Discharge channel
|
|
|
|
|
|
Effluent Density
|
997 kg/m3
|
|
Overflow Pipe
Diameter
|
0.225m
|
0.225m
|
0.3m
|
0.375m
|
|
Effluent Flow
(Cumulative)1
|
353 m3/d
|
507 m3/d
|
879 m3/d
|
940 m3/d
|
|
Effluent Flow
(Net)2
|
353 m3/d
|
154 m3/d
|
372 m3/d
|
61m3/d
|
|
Environmental Parameters
|
|
Water Depth near
Discharge Outlet (m)
|
2
|
2
|
2
|
2
|
|
Ambient Current
Speed
|
0.1 m/s
|
|
Ambient Water
Density
|
1020.6 kg/m3 (wet season) and 1021 kg/m3
(dry season)
|
|
Darcy-Weisbach
Friction Factor
|
0.023
|
|
Bottom Slope at
the mouth of the discharge
|
10o
|
10o
|
10o
|
10o
|
Note: 1 Combined
flow from pumping station(s) located upstream
2
For P1b
and P2, fFlow refers to flow from the catchment area of each
pumping stationP1b/STWP2 only. For P1a and STW, net flow includes incoming
flows from Lo So Shing and Mo Tat Wan respectively.
5.67.
The density difference between the
effluent and ambient receiving waters would affect the buoyancy of the sewage
plume. Since the effluent flows of the
three pumping stations would be very low, the sewage plumes would attach to the
shoreline. It would also be buoyant due
to the density difference between the sewage effluent and the ambient marine
water. The distances between the FCZ
and the pumping stations and STW are shown below.
·
Pumping Station P1a 230 m
·
Pumping Station P1b 200 m
·
Pumping Station P2 55 m
·
STW 55
m
5.68.
The strength of the raw sewage for the
emergency overflow are assumed as follows:
|
BOD
|
250 mg/L
|
|
Suspended Solids
(SS)
|
250 mg/L
|
|
E. coli
|
4.2 x 107
counts per 100 mL
|
|
NH3-N
|
25 mg/L
|
|
TIN
|
25 mg/L
|
|
TKN
|
|
5.69.
The treated effluent quality from the
STW was based on the parameters in Section 5.5052.
5.70.
The background levels were based on
the water quality data given in Sections 5.51 53 and 5.5658. The predicted water quality levels at the
FCZ are given in Table 5.10a for cumulative effluent flow and Table 5.10b for
net effluent flow.
Table
5.10a Predicted Water Quality at the
Fish Culture Zone (Cumulative Effluent Flow)
|
Parameter
|
Pollutant
Concentration at FCZ
|
|
P1a
|
P1b
|
P2
|
STW1
|
STW2
|
|
Distance from the FCZ (m)
|
230
|
200
|
55
|
55
|
55
|
|
Wet Season
|
|
SS (mg/L)
|
14
|
16
|
41
|
47
|
16
|
|
BOD (mg/L)
|
2
|
4
|
30
|
35
|
3
|
|
TKN (mg/L)
|
0.56
|
0.85
|
4.92
|
5.81
|
5.81
|
|
NH3-N (mg/L)
|
0.24
|
0.41
|
2.96
|
3.51
|
2.82
|
|
Unionized ammonia3 (mg/L)
|
0.016
|
0.028
|
0.198
|
0.235
|
0.189
|
|
TIN (mg/L)
|
0.28
|
0.45
|
3.00
|
3.55
|
2.86
|
|
E. coli (counts/100 mL)
|
3.31E+05
|
6.27E+05
|
4.90E+06
|
5.83E+06
|
2.30E+02
|
|
Dry Season
|
|
SS (mg/L)
|
10
|
12
|
37
|
43
|
12
|
|
BOD (mg/L)
|
2
|
4
|
30
|
35
|
3
|
|
TKN (mg/L)
|
0.60
|
0.89
|
4.96
|
5.85
|
5.85
|
|
NH3-N (mg/L)
|
0.29
|
0.47
|
3.02
|
3.57
|
2.88
|
|
Unionized ammonia3 (mg/L)
|
0.020
|
0.032
|
0.202
|
0.239
|
0.193
|
|
TIN (mg/L)
|
0.43
|
0.60
|
3.15
|
3.70
|
3.01
|
|
E. coli (counts/100 mL)
|
3.31E+05
|
6.27E+05
|
4.90E+06
|
5.83E+06
|
2.93E+02
|
|
|
|
|
|
|
|
Note:
1
Overflow of untreated sewage
2
Overflow of treated sewage
3
Unionised
ammonia concentration was calculated based on 4.7% of unionised ammonia
constituting the total ammonia concentration at 22.4oC, pH 8.1, and
salinity of 31.7ppt
Table 5.10b Predicted Water Quality at the Fish Culture
Zone (Net Effluent Flow)
|
Parameter
|
Pollutant
Concentration at FCZ
|
|
P1a
|
P1b
|
P2
|
STW1
|
P1a+P1b
|
|
Distance from the FCZ (m)
|
230
|
200
|
55
|
55
|
55
|
|
Wet Season
|
|
SS (mg/L)
|
14
|
13
|
35
|
17
|
15
|
|
BOD (mg/L)
|
2
|
2
|
24
|
5
|
4
|
|
TKN (mg/L)
|
0.56
|
0.43
|
3.95
|
0.98
|
0.74
|
|
NH3-N (mg/L)
|
0.24
|
0.15
|
2.35
|
0.49
|
0.35
|
|
Unionized ammonia2 (mg/L)
|
0.016
|
0.010
|
0.158
|
0.033
|
0.023
|
|
TIN (mg/L)
|
0.28
|
0.19
|
2.39
|
0.53
|
0.39
|
|
E. coli (counts/100 mL)
|
3.31E+05
|
1.88E+05
|
3.89E+06
|
7.64E+05
|
5.19E+05
|
|
Dry Season
|
|
SS (mg/L)
|
10
|
9
|
31
|
13
|
11
|
|
BOD (mg/L)
|
2
|
2
|
24
|
5
|
4
|
|
TKN (mg/L)
|
0.60
|
0.47
|
3.99
|
1.02
|
0.78
|
|
NH3-N (mg/L)
|
0.29
|
0.21
|
2.41
|
0.55
|
0.41
|
|
Unionized ammonia2 (mg/L)
|
0.020
|
0.014
|
0.162
|
0.037
|
0.027
|
|
TIN (mg/L)
|
0.43
|
0.34
|
2.54
|
0.68
|
0.54
|
|
E. coli (counts/100 mL)
|
3.31E+05
|
1.88E+05
|
3.89E+06
|
7.64E+05
|
5.19E+05
|
Note:
1
Overflow of untreated sewage
2
Unionised
ammonia concentration was calculated based on 4.7% of unionised ammonia
constituting the total ammonia concentration at 22.4oC, pH 8.1, and
salinity of 31.7ppt
5.71.
Table 5.10a shows that the water
quality at the sensitive receivers would exceed the WQOs, in terms of unionized
ammonia, TIN and E.coli during the emergency discharges from the pumping
stations and the STW. This indicated
that the emergency discharges would have adverse impacts on the FCZ,
particularly the discharges from the Pumping Station P2 and STW due to the proximity
of the discharge locations to the FCZ (about 55 m). The results also indicated that the water quality impacts arising
from the emergency discharge of treated effluent from the STW would cause less
significant impact compared to the discharge of untreated sewage, while
exceedances of WQO in terms of TIN and unionised ammonia would still be
predicted.
5.72.
Table 5.10b indicates the predicted
water quality at the FCZ based on the net effluent flow of the pumping stations
and STW. The results showed that
exceedances of WQOs in terms of TIN and E.
coli were still predicted for the discharges from all locations, whereas
exceedances of WQO for unionised ammonia were only predicted for the overflow
from the Pumping Station P2 and STW as well as the cumulative overflow from the
Pumping Station P1a and P1b. Since
unionised ammonia (free ammonia) is toxic to fish in high concentration
(>0.2 mg/L can cause fatalities in several species of fish according to
Chapter 24 in Chemistry for Environmental Engineering()),
compliance of WQO for unionised ammonia should be achieved as far as
possible. Hence, emergency discharges
at Pumping Station P2 and the STW would be undesirable. To minimize the possibility of emergency
discharge, extensive mitigation measures were recommended as detailed in
Sections 5.84-5.9092.
Dredging
5.73.
As discussed in Sections 5.30 32 – 5.3537, the
elevation in SS concentration due to the dredging works would exceed the
allowable elevation in SS concentrations at the gazetted FCZ and the secondary
contact recreation subzone. In order to
minimize the potential impact from the dredging works, the use of closed grab
dredger, 2-layer silt curtains and reduction of the dredging rate were
recommended.
5.74.
According to the Contaminated Spoil
Management Study (Mott
MacDonald, 1991, Table 6.12), the implementation of silt curtain around the
closed grab dredgers would reduce the dispersion of SS by about 75%. In order to ensure that 75% SS reduction
could be achieved, an additional silt curtain at around 50m from the dredging
area is recommended. Typical design and
arrangement of the 2-layer silt curtains is illustrated in Figures 5.9a and
5.9b. A dredging rate of 55 m3/hr
was also recommended in order to achieve the WQO.
5.75.
The elevation in SS concentrations
from the mitigated dredging works was predicted and the results are shown in
Table 5.11. Detailed calculation of the
mitigated SS concentration is given in Appendix 5.3.
Table
5.11 Predicted Elevation in
Suspended Sediment Concentrations (Mitigated)
|
Distance from Source (m)
|
Concentration of
Suspended Sediment (mg/L)
|
|
100
|
5.2
|
|
150
|
3.5
|
|
200
|
2.6
|
|
250
|
2.1
|
|
300
|
1.7
|
|
350
|
1.5
|
|
400
|
1.3
|
|
450
|
1.2
|
|
500
|
1.0
|
|
550
|
1.0
|
|
600
|
0.9
|
|
650
|
0.8
|
|
700
|
0.7
|
5.76.
As indicated in Table 5.11, the
elevation in SS concentrations from the mitigated dredging works would be
approximately 1.7 mg/L at the FCZ and 1 mg/L at the secondary contact
recreation subzone, and would comply with allowable SS concentration elevation
as in Table 5.2. It should be noted
that the calculation of the elevated SS concentrations was not taken account of
the tidal effect. To
further minimize the potential water quality impacts arising from the dredging
works, it was recommended that dredging operation should only be allowed during
ebb tide to ensure that sediment plume generated from the dredging works would
be transported away from the bay by tidal current.
5.77.
With the use of HDD technique for the
submarine outfall construction, only minor dredging works (total volume about
26,000 m3) would be required.
The duration of the dredging works would be short. Silt curtains would be installed around the
exist area of the pilot drill.
5.78.
In order to alleviate potential water
quality impacts from the construction of the Project, the following mitigation
measures should be implemented during the construction of the submarine
outfall:
·
Dredging should be
undertaken using closed grab dredgers with a maximumtotal
production rate of 55m3/hr;
·
Deployment of 2-layer
silt curtains with
the first layer enclosing the grab and the second layer at around 50m from the
dredging area while around the immediate dredging area while dredging
works are in progress;
·
Dredging
operation should be undertaken during ebb tide only;
·
all vessels should be sized such that adequate
clearance (i.e.
minimum clearance of 0.6m) is maintained between vessels and the sea bed
at all states of the tide to ensure that undue turbidity is not generated by
turbulence from vessel movement or propeller wash;
·
all pipe leakages should be repaired promptly and
plant should not be operated with leaking pipes;
·
excess material should be cleaned from the decks and
exposed fittings of barges before the vessel is moved;
·
adequate freeboard (i.e. minimum of 200m) should be maintained
on barges to ensure that decks are not washed by wave action;
·
all barges should be fitted with tight fitting seals
to their bottom openings to prevent leakage of material; and
·
loading of barges and hoppers should be controlled to
prevent splashing of dredged material to the surrounding water, and barges and
hoppers should not be filled to a level which would cause the overflow of
materials or sediment laden water during loading or transportation; and
·
the decks of all vessels should be kept tidy and free
of oil or other substances that might be accidentally or otherwise washed
overboard.
Construction Run-off and Drainage
5.79.
The Contractor should observe and
comply with the WPCO and the subsidiary regulations. The Contractor should follow the practices, and be responsible
for the design, construction, operation and maintenance of all the mitigation
measures as specified in ProPECC PN 1/94 “Construction Site Drainage”. The design of the mitigation measures should
be submitted by the Contractor
to the Engineer for approval. These
mitigation measures should include the following practices to minimise site
surface runoff and the chance of erosion, and also to retain and reduce any
suspended solids prior to discharge:
·
Provision of perimeter channels to intercept
storm-runoff from outside the site.
These should be constructed in advance of site formation works and
earthworks.
·
Works programmes should be designed to minimize works
areas at any one time, thus minimising exposed soil areas and reducing the
potential for increased siltation and runoff.
·
Sand/silt removal facilities such as sand traps, silt
traps and sediment basins should be provided to remove the sand/silt particles
from run-off. These facilities should
be properly and regularly maintained.
These facilities should be carefully planned to ensure that they would
be installed at appropriate locations to capture all surface water generated on
site.
·
Careful programming of the works to minimise soil
excavation works during rainy seasons.
·
Exposed soil surface should be protected by paving or
hydroseeding as soon as possible to reduce the potential of soil erosion.
·
Trench excavation should be avoided in the wet
season, and if necessary, these should be excavated and backfilled in short
sections.
·
Open stockpiles of construction materials on site
should be covered with tarpaulin or similar fabric.
General Construction Activities
5.80.
Debris and rubbish generated on-site
should be collected, handled and disposed of properly to avoid entering the
nearby coastal waters and stormwater drains.
All fuel tanks and storage areas should be provided with locks and be
sited on sealed areas, within bunds of a capacity equal to 110% of the storage
capacity of the largest tank. Open
drainage channels and culverts near the works areas should be covered to block
the entrance of large debris and refuse.
Wastewater Arising from Workforce
5.81.
Portable toilets should be provided by
the Contractors, where necessary, to handle sewage from the workforce. The Contractor should also be responsible
for waste disposal and maintenance practices.
Effluent at the Submarine Outfall
5.82.
Upon the operation of the Project, the
untreated or partially treated sewage arising from the existing villages
including restaurants that had previously been discharged into Picnic Bay would
be collected and transferred by the proposed sewerage system to the proposed
STW and submarine outfall for proper treatment and disposal. Hence, the Project would provide
environmental benefit to the area in light of the improvements in the water
quality of Picnic Bay.
5.83.
Although the TIN levels at the
sensitive receivers (as shown in Table 5.8) were predicted to exceed the WQO
during wet season stratified conditions and dry season, the exceedances were
attributable to the high background levels.
In addition, the prediction were considered to be over-estimated as they
were calculated by simply adding the elevated TIN values to the background
levels and were not taken account of the beneficial effect of eliminating
untreated or partially treated sewage discharges to the Picnic Bay upon the
operation of the proposed sewerage and treatment system (a full secondary
level treatment with nitrogen removal and UV disinfection). As mentioned in Section 5.63, the proposed
treatment process would remove more than 50% TIN from sewage and the proposed
system would discharge the treated effluent through the submarine outfall at a
location of about 560 m from the FCZ.
Hence, the submarine outfall discharge would not induce a water quality
impact worse than that of the existing conditions. In addition, the Project would improve water quality of Picnic
Bay by eliminating untreated or partially treated sewage discharges to the
bay. The calculation in
Appendix 5.2 indicates
that the elimination of untreated or partially treated sewage discharges during
the operation of the Project would reduce the pollution loading within the Sok
Kwu Wan area compared to the existing conditions. Therefore, it is considered that
the Project would provide environmental
benefit to the area by improving the water quality of Picnic
Bay.
5.1.
Emergency Discharge
5.84.
The emergency discharges from the
pumping stations and STW would be the consequence of pump failure, interruption
of the electrical power supply, SBR units failure or blockage of the submarine
outfall. Considering the presence of
the FCZ in the vicinity of the proposed emergency discharge locations, the
occurrence of emergency discharge would should be
minimised as far as possible. A number
of mitigation measures and contingencies as discussed below would be
implemented in order to reduce the possibility of emergency discharges. A flow chart showing implementation of the
mitigation measures and contingency measures in emergency is provided in Figure5.10.
5.1.
As a common
practice, would be provided for each of the three pumping
stations and the effluent pumping station of the STW. The provision of a standby pump would prevent the
build up of sewage at any single pumping station in case of the duty pump
failure and hence prevent emergency discharge. In order to protect against emergency discharge due to the
interruption of electrical power supply, the three pumping
stations and STW would be equipped with standby generators. All pumping stations would be provided with
2-hour temporary storage at ADWF, allowing time to activate the
emergency action plan. With the
provision of standby generators and pumps, and a 2-hour temporary storage, the
occurrence of emergency discharge would be remote.
5.85.
In common practices, either a standby pump or
standby generator and a 2-hour temporary storage at ADWF for allowing time to activate the emergency action
plan would be provided for a pumping station. In view of the presence of the FCZ in the vicinity,
both standby pump and standby generator would be provided for all the three pumping stations and the
effluent pumping station at the STW. The provision of a standby pump and
generator would prevent the build up of sewage at any singlethe pumping station in case of
the duty pump failure or/and the interruption of electrical power supply failure, and hence prevent
emergency discharge.
5.86.
Having considered the
presence of the FCZ in the vicinity of the emergency discharge points,
additional mitigation measures would be required. To further safeguard the FCZ against the emergency
discharge, the temporary storage of the pumping stations would be prolonged to
24 hours at net inflow (ADWF) by the provision of an offline storage tank
adjacent to each of the pumping stations.
The SBR units in the STW would act as storage tanks in case of emergency
to contain sewage. Based on DSD’s record, no more than 12 hoursduring the normal
hours was
required to resume the normal service of a
pumping station in case of failure. Table
5.12 shows the estimated time required for resuming the services of the pumpingstations during normal
hours and outside normal hours. With the provision of24-hour temporarystorage, DSD’s staff would have sufficient time to initiate the action plan
and resume the
service of the failed
pumping
station. Hence, emergency discharge from the pumping station would be unlikely to happen.
Table 5.12 Estimated Time Required for Resuming Normal
Services of Pumping Station in Emergency
|
Component
|
Emergency Occurs At
|
|
Normal Hour*#
|
Outside Normal Hour#
|
|
|
1
hr
|
1
hr
|
|
Travel Time by Public Ferry
|
3
hrs
|
10
hrs
|
|
Parts Replacement or Minor Repairs
|
4-
8 hrs
|
4 –
8 hrs
|
|
Total
|
8 –
12 hrs
|
15
–19 hrs
|
Note: # Under
normal weather condition.
* During normal service of public ferry.
5.87.
Owing to the close proximity of the
Pumping Station P2 and the STW to the FCZ, the emergency outfall at these
locations would be capped and hence no emergency discharge would be allowed without
permission. In order to
minimize the possibility of emergency discharge due to the failure of the SBR
units, an additional SBR unit would be installed at the STW. Hence, the STW would have three SBR units
instead of two units. Should one of the
three SBR units be broken down, the remaining two SBR units would still capable
of producing effluent of the design standard.
5.88.
To facilitate the automatically switch
on the standby generators and pumps, and automatically shutdown of the pumping
stations in case of emergency, telemetric devices would be installed at all
pumping stations to ensure that prompt action would be undertaken in an
emergency occasion.
5.89.
In addition to the above recommended
mitigation measures, the following contingency measures were recommended in
order to further reduce the risk of emergency discharge and minimize the
potential water quality impacts during an emergency discharge:
·
In case of operation failure of
Pumping Station P1a, the pumping station at Lo So Shing would be automatically
shutdown to stop inflow to P1a from Lo So Shing.
·
In case of the operation failure of
Pumping Station P1b, upstream pumping stations (P1a and Lo So Shing pumping
Pumping
stationStations) would be automatically shutdown to
stop any inflow to Pumping Station P1b.
·
In case that the Pumping Station P2 is
failed, upstream pumping stations (including Pumping Stations P1a and P1b, and
Lo So Shing pumping station) would be automatically shutdown to stop inflow to
Pumping Station P2. No emergency
discharge would be allowed.The sewage inflow
from catchment area of Pumping Station P2 would be stored in the 24-hour
offline temporary storage tank until normal service is resumed.
·
In the extremely unlikely conditions
that all 3 or 2 SBR units fail or the submarine outfall is blocked, all pumping
stations (including pumping stations at Mo TakMo Tat Wan and
Lo So Shing) would be automatically switched off to stop any inflows to the
downstream sewerage systems and the STW.
No further sewage from upstream pumping stations would be collected by
the STW during this occasion and hence no emergency discharge at the STW would
occur.
5.90.
It should be noted that the occurrence of emergency
discharge would be episodic and very short-term. With the implementation of the above-mentioned mitigation
measures and contingencies, risk ofthe possibility of an emergency overflow
occurring would be negligible and the potential
water quality impacts in the unlikely event that an overflow does occur would
be minimised. Compared with the
long-termed improvement of water quality of Picnic Bay from the Project and
given the recommended
measures
to prevent and minimise the emergency discharge, the impact would be minimal. The key aspects of the recommendations
are summarised as follows:
·
Standby pump at all pumping stations
and the STW in case of pump failure;
·
Standby generator at all pumping
stations in case of interruption of electrical power supply;
·
24-hour temporary storage for all
pumping stations in emergency;
·
Use of SBR units as storage tanks in
case STW failure;
·
No emergency discharge is allowed at
Pumping Station P2 and the STW;
·
Automatically shutdown the pumping
station at Lo So Shing in case of Pumping Station P1a failure;
·
Automatically shutdown the upstream
pumping stations (including
Pumping
Station P1a and Lo So Shing Pumping Station) in case of
Pumping Station P1b failure;
·
Automatically shutdown the upstream
pumping stations (including
Pumping
Station P1a and P1b, Lo So Shing Pumping Station) in case of
Pumping Station P2 failure;
·
Automatically shutdown all pumping
stations (including
Pumping
Station P1a, P1b and P2, Lo So Shing and Mo Tat Wan Pumping Stations) in case of
STW failure;
·
Implement a telemetry system to ensure
prompt action to be undertaken in an emergency occasion.
5.91.
The
design of Lo So Shing and Mo Tat Wan Pumping Stations are outside the scope
of this Project. It is recommended that
the design of these pumping stations should
be incorporatedthe above-mentioned measure of
automatically shutdown
in case of emergency.
5.92.
In
order to provide mechanism to minimize the possibility of emergency discharges,
a detailed emergency response plan should be
formulated to clearly state the response procedure in case of pumping stations or STW
failureandtraining on the
implementation of the response plan should be provided to the corresponding
staff. The response plan should also include an emergency call-out procedure to inform relevant government departments such as EPD and AFCD, and a list of contact
persons/parties
and their phone numbers in case of emergency
discharge. At this preliminary
design stage, details of the response plan and
exact manpower arrangement could not be
confirmed. A flow chart showing
the major components and
procedures of the emergency response plan
is provided in Appendix 5.4. The details of the plan based on the major
components showing in the flow chart (Appendix 5.4) should
be developed and approved by EPD/AFCD during the subsequent
detailed design stage.
It should be noted that the
occurrence of emergency discharge would be episodic and very short-termed. Compared with the long-termed improvement of
water quality of Picnic Bay from the Project and given the above mentioned measures
to prevent and minimise the emergency
discharge, the impact would be
minimial.
Residual Environmental Impacts
5.93.
With the implementation
of the recommended mitigation measures, no residual impact would be expected
during the construction phase.
5.94.
During the operation
phase, no residual impact
would be expected during normal operation of the proposed sewerage system and
STW. Although the TIN levels at the
sensitive receivers were predicted to exceed the WQO during wet season with
stratification and during dry season, the exceedances are attributable to the high
background levels. The background level
for dry season had already exceeded the limit by more than twice. Furthermore, the prediction are considered
to be over-estimated as they were predicted by simply adding the elevated TIN
levels from the
effluent to the background levels and were not taken account of the beneficial
effect of eliminating untreated or partially treated sewage discharges to the
Picnic Bay upon the operation of the Project.
The denitrification process in the STW would remove more than 50% of TIN from the sewage
and the treated effluent would be discharged at a location of more than 560m
from the FCZ. Hence, the operation of
the Project would not induce a water quality impact worse than that without the
implementation of
the Project. In fact, the proposed
project would improve the water quality of the Picnic Bay by eliminating the
discharge of untreated or partially treated sewage into the coastal area of the
bay.
5.95.
Unacceptable rResidual impacts
would occur during an emergency discharge occasion. Nevertheless, with the extensive mitigation measures and
contingencies applied, the risk occurrenceof emergency discharge would be negligible and
the residual impacts would be minimised in the unlikely
event that an overflow does occur would be minimised. Compared with the long-termed improvement of water
quality of Picnic Bay from the Project and given the recommended measures to
prevent and minimise the emergency discharge,Overall, no
unacceptable residual impact would be expected.
5.96.
The dredging works for the
construction of around 240 m submarine outfall pipeline and the diffuser section
zone
would be a main concern.
Environmental monitoring and auditing (EM&A) of marine water quality
was recommended during the construction phase. An EM&A program including
pre- and post-dredging monitoring for water quality, would be required to
ensure the implementation of the recommended water quality mitigation measures
during the construction works. Since
exceedances of TIN WQO limit were predicted during the operation of the
Project, water quality monitoring during the initial operation stage would be
required. Details of the EM&A
procedures are presented in a separate EM&A Manual.