2.3 Consideration of Alternative Design and Layout
2.4 Consideration of Alternative Construction Methods
2.5 Consideration of Alternative Work Sequences
2.6 Selection of Preferred Scenario
2.7 Details of Selected Construction Methods
2.1.1
Po
Toi O is located in the southern part of Sai Kung District, next to Clear Water Bay. Around the bay is a
small settlement called Po Toi O village, which is not served by public
sewerage. The location of Po Toi O can be found in Figure 1-1.
2.1.2
The
proposed sewerage Project comprises the following works:
i. Provision of village sewerage to the unsewered areas of Po Toi O. The works involve construction of about 800m of gravity sewers and 400m of rising mains;
ii. Construction of a local sewage treatment plant (STP) with Average Dry Weather Flow (ADWF) of about 139m3/day; and
iii. Construction of a submarine outfall of about 385m in length.
2.1.3 This is an environmental enhancement project that aims to improve environmental hygiene of the Po Toi O area.
Existing Conditions
Purpose, Objectives and Environmental Benefits
2.2.2
Provision
of proper sewerage system to unsewered villages is a general government policy.
Without centralized public sewage treatment facility, villagers have to
regularly desludge the STS to maintain the cleaning performance and to avoid
overflow of sewage. However, the desludging process may bring along potential
hygiene and odour issues. Under the Port Shelter Sewerage Master Plan (SMP), Drainage Services Department (DSD) has proposed sewerage works at Po Toi
O. In addition to improving local hygiene
conditions and removing associated odour problem, the provision of sewerage is
a preventive measure to avoid potential environmental problems due to
insufficient desludging or structural defect of the STS. This can also relieve
villagers’ burden to maintain their STS (e.g. desludging).
2.2.3
The proposed sewerage consists of a local sewage treatment plant (STP), gravity sewers, rising mains
and a submarine outfall. It is designed to collect and treat the sewage and
wastewater from the communities in the Po Toi O village for final disposal
offshore. In the Sai Kung District Council meetings over the past five years,
the residents of Po Toi O generally
welcomed the proposal.
Scenarios with and without the Project
Without the Project
2.2.4
As
mentioned in Sections 2.2.1 - 2.2.2, wastewater generated in Po Toi O is treated by
STS. Hygiene and associated odour concerns may arise during desludging of the
STS. Also, ineffective treatment or
overflow of sewage from unmaintained STS may result in
leakage of raw or partially treated sewage into nearby water bodies
(e.g. sea and streams). Water pollution may affect villagers and the fish culture zone. These potential hygiene
and environmental problems cannot be fully rectified without proper collection
and treatment of sewage.
With the Project
2.2.5
Upon
completion of the Project, sewage from the villagers will be conveyed to Po Toi
O STP for centralized treatment and the treated effluent will be discharged through
a submarine outfall to a good dispersion point in Clearwater Bay. The environmental
hygiene in the Po Toi O area is expected to improve and the pollution risk due
to insufficient desludging will be eliminated. Also, villagers will be free
from routine maintenance of the STS.
Design and Layout of STP
2.3.1
Po
Toi O Sewage Treatment Works will adopt membrane bioreactor technology (MBR)
for sewage treatment. This technology allows a smaller footprint than that for
a conventional STP. To further minimize the STP footprint, most of the tanks
and equipment (e.g. equalization tank, MBR tank and screens) will be placed 10m
underground. This can also minimize emissions of odour generated from the treatment
processes. A single storey building of 5m high will be constructed above ground
to minimize landscape and visual impacts on the surrounding village and hilly
scenery. The layout of the STP is shown in Appendix 2.1.
Location of STP
2.3.2
In
order to identify a feasible location for placing the proposed STP, four
locations in the vicinity of Po Toi O were studied under Agreement No. CE 65/2006(DS). A plan showing the locations of
four studied sites (namely Sites 1 – 4) and the sensitive receivers in the
vicinity is provided in Figure 2-1.
·
Site 1 – Shore to the
North of Po Toi O Village
·
Site 2 – Within Po Toi O
Village in front of Hung Shing Temple
·
Site 3 – Shore to the
South of Po Toi O Village
·
Site 4 – Cut Slope to the
South of Po Toi O Village
2.3.3
All
four studied locations are considered based on the land requirement of the
proposed STP, and they are technically feasible from the drainage, sewerage and
structural points of view. They have also been reviewed in terms of accessibility,
environmental impacts, public concerns, and current zonings in OZP (zoning of
Po Toi O area is shown in Figure 10-2).
As given by Figure 10-2,
land of sufficient size in the proximity of the Po Toi O Village is either
zoned as Conservation Area or Costal Protection Area. A small piece of land
zoned as “Open Space” is sandwiched by Fairway Vista and Po Toi O Village.
However, it is not considered for placing the proposed STP due to the following
reasons: (1) This area is located on high altitude (+23.1 mPD). An extra
pumping station has to be constructed to pump the sewage from village houses at
lowland to the STP; (2) This area is surrounded by and is adjacent to
residential houses, and thus prone to adverse environmental constraints as Site 2 (unacceptable
noise and visual impacts plus odour nuisance in operational phase, see Section 2.3.5 for
details); (3) the size
of “Open Space” is too small to accommodate the proposed STP.
2.3.4
Site 1 is located at the shore to the north of Po Toi
O. Site 1 was not selected for the proposed STP due to two main reasons:
(1) inaccessibility for maintenance in operational phase and (2) objection from
village representatives due to high construction and operational environmental
impacts. Site 1 cannot be accessible by maintenance vehicles and hence maintenance
of the STP becomes infeasible. Also, this land is zoned as “Coastal Protection
Area”. In general, this zoning intends to conserve, protect and retain the
natural coastline and the sensitive coastal natural environment, and to deter against
development. Dredging and reclamation works required in site formation work for the
proposed STP will damage rocky shore lined with corals. In addition, this will cause
water deterioration that affects fish culture zone in Po Toi O bay, corals along
coastline and species of conservation importance amphioxus in the Po Toi O bay
channel seabed (see Figure 2-2).
Furthermore, the house behind Site 1 may be prone to odour nuisance in the
operational phase.
2.3.5
Site 2 is located at the residential area (Village Zone) of Po Toi O where
construction is easier due to less site constraints. However, Site 2 cannot
be accessed by maintenance vehicles, making maintenance of STP
infeasible. In addition, houses in the vicinity of Site 2 may be prone to
odour nuisance in the operational phase. Moreover, village representatives
objected the construction of the STP near their village houses, and in
particular, they strongly opposed the construction of the STP in front of
“Hung Shing Temple 洪聖宮”. In this connection, Site 2 was
not selected for the proposed STP.
2.3.6
Site 3 is located at the shore to the south of Po Toi
O Village. Similar to Site 1, this land is zoned as “Coastal Protection Area”
where development is generally against. Release of suspended solids from
dredging and reclamation works
for site formation for the proposed STP is also detrimental to the fish culture
zone and marine wildlife. In addition, village representatives also objected
construction of the proposed STP in Site 3 due to visual impact. In this
connection, Site 3 was not selected for the proposed STP.
2.3.7
Site 4 is located at the slope above Po Toi O Chuen
Road and to the east of Site 3. This land is zoned as “Conservation Area”. This
zoning intends to protect and retain the existing natural landscape, ecological
or topographical features of the area for conservation, educational and
research purposes and to separate natural environment such as Country Park from
adverse effects of development. Nevertheless, Site 4 is furthest away from residence in Po Toi O and thus
bringing the least odour concern during operation of the STP. In addition, as
the Site is located uphill, construction works are less likely to cause severe
water pollution like Site 1 and Site 3 do. The village representatives supported
the proposed STP to be located at this location. In view of the above, Site 4 was
selected for the proposed STP.
2.3.8
The
STP can either be constructed on the slope next to Po Toi O Chuen Road or
further uphill. The latter option is not preferred due to a number of
disadvantages. Further upslope is a natural, undisturbed shrubland. Establishment
of an access road for maintenance vehicles will bring substantial vegetation
clearance. Also, sewage has to be pumped uphill to the STP and this incurs
additional operational cost and is not environmentally friendly in the long
run. On the other hand, part of the slope next to Po Toi O Chuen Road is a cut
slope, which is a disturbed habitat. By placing the STP on the cut slope, maintenance
vehicles could easily gain access the site and thus vegetation clearance can be
minimized. In addition, the cut slope is at a lower altitude and less
energy will be required for pumping.
2.3.9
On
the cut slope, the STP was primarily designed next to a seasonal stream W2 in
the Project Profile (Figure 2-1 refers). It was
later shifted southward to create a buffer from W2 and also from the residences
as this reduces potential odour impact and operational noise concerns. In
addition, the STP is kept at a reasonable distance from the village so that the
rising mains and gravity sewers will not be unnecessarily long. After the above
amendment, this was adopted as the preferred Site in this EIA (Figure 1-1
refers).
Summary of Site
Selection for STP
2.3.10
Building
the STP in the village area will cause odour nuisance, unacceptable noise and
visual impacts on the nearby residence. The narrow strip of coastal protection
area has to be reclaimed to provide sufficient land to accommodate a STP. The
construction process will inevitably destroy the coastal habitat and bring
significant elevation in suspended solids in water, affecting fish culture zone
and marine wildlife (e.g. corals and amphioxus).
2.3.11
Construction
of STP within conservation area is the only viable option as ecological impact
can be minimized by choosing an already disturbed habitat (a cut slope).
Without marine works, water quality, marine ecological and fisheries impacts
can be avoided. Noise and odour nuisances are reduced by locating the STP away
from village area. View of the STP from residence can be partially screened by
embedding the plant in rock slope.
2.3.12
A
comparison table showing the impacts of the four sites is shown in Table 2-1.
Table 2-1: Summary of Site Selection Process for Sewage
Treatment Plant (Undesirable items are underlined)
|
Impact |
Site 1 |
Site 2 |
Site 3 |
Site 4 (Preferred Site) |
|
Zoning in OZP |
Coastal Protection Area |
Village Zone |
Coastal Protection Area |
Conservation Area |
|
Operation and Maintenance consideration |
Unacceptable. There
is no vehicle access to Site 1. Maintenance vehicles cannot gain access to the
STP. Maintenance activities such as sludge removal, replacement/ repair of
E&M parts become infeasible.
Site 1 is subject to tidal effect especially during typhoon, which would pose
a threat to the E&M equipment of the STP and thus risk plant breakdown. |
Unacceptable. There
is no vehicle access to Site 2. Maintenance vehicles cannot gain access to the
STP. Maintenance activities such as sludge removal, replacement/ repair of
E&M parts become infeasible. |
Undesirable. Maintenance
vehicles can
gain access to
Site 3 via Po Toi O Chuen Road. Maintenance activities such as sludge
removal, replacement/ repair of E&M parts become feasible. However, Site 3 is subject to tidal effect especially during typhoon, which would
pose a threat to the E&M equipment of the STP and thus risk plant breakdown. |
Acceptable. Maintenance vehicles can gain access to Site 4 via Po
Toi O Chuen Road. Maintenance works are feasible. |
|
Ecological & Water Quality Impacts |
Considerable impact. As dredging and
reclamation are required, impacts on water quality, marine ecology and coral
communities are expected. Furthermore,
existing coastline will be broken. |
Acceptable impact. The STP will be built on
developed
land. |
Considerable impact. As dredging and
reclamation are required, impacts on water quality, marine ecology and coral
communities are expected. Furthermore,
existing coastline will be broken. |
Acceptable impact. Shrubland habitat and one
common plant species of conservation importance will be lost (Gnetum luofuense, a climbing plant that has low transplantation
success due to difficulty in making root ball on rock slope and its twining
nature). However, impacts on water quality, coastline, marine ecology
and coral communities can be avoided. |
|
Air
& Noise
Impacts |
Unacceptable noise impact
and prone to odour nuisance as Site 1 is near existing residential houses. |
Unacceptable noise impact
and prone to odour nuisance as Site 2 is near existing residential houses. |
Insignificant odour nuisance
and noise impact due to sufficient
distance
from residence. |
Insignificant odour nuisance
and noise impact due to sufficient
distance
from residences. |
|
Visual Impact |
Unacceptable adverse
visual impact. Site 1 breaks the
existing coastline and block the seaview of residential houses behind and in
Fairway Vista. |
Unacceptable adverse
visual impact. Site 2 is sandwiched between
residential houses and in front of “洪聖宮”. |
Unacceptable adverse
visual impact. Site 3 breaks the
existing coastline and causes moderate visual impact on the residential houses near Site 3. |
Acceptable visual impact.
Site 4
is mainly embedded in existing slope, causing minimal impact on existing residential
houses. |
|
Local Opinion |
Objected by village
representatives due to its visual impact at seashore. |
Strongly objected by
village representatives due to its visual impact on “洪聖宮” and proximity to residential houses. |
Objected by village
representatives due to its visual impact at seashore and its close proximity
to residential houses. |
Supported by village
representatives. |
|
Construction Cost and Difficulties |
High. Involve excavation of
rock mass under the seashore, dredging
and reclamation works. |
Lowest. Involve excavation of
rock mass underground. |
High. Involve excavation of
rock mass under the seashore and dredging and reclamation works. |
High. Involve excavation of
rock slope and rock mass underground, and provision of retaining structure. |
|
Fisheries Impact |
Unacceptable. 111m to the existing
fish culture zone. Would cause adverse impact during the dredging works |
Minimal impact as
the STP is on developed land. |
Unacceptable. 86m to the existing fish
culture zone. Would cause adverse impact during the dredging works |
Minimal impact as
the STP is on slope. |
Need of
Submarine Sewage Outfall and Dredging
2.3.13
According to the Technical Memorandum of the Water
Pollution Control Ordinance, no new effluent will be allowed within 200m of the
seaward boundaries of a marine fish culture zone, or within 100m of the
landward boundaries.
2.3.14
For the discharge of treated effluent from the
sewage treatment plant, a 200m long pipeline was initially proposed to be laid
along the rocky shore to discharge treated effluent near shore (see Figure
2-2). However, local residents strongly objected
any construction works along the natural shoreline during the consultation
process. Furthermore, as there is a large natural slope along the shoreline,
substantial slope stabilization works will be required for construction and
future maintenance of the proposed pipeline. Therefore, the treated effluent has to be discharged by
a submarine outfall and diffuser.
2.3.15
Although this alternative will
involve minor dredging, no slope stabilization works are required and this is more
acceptable to local residents. Also, ecological impact will be minimized by
placing the
diffuser at outer Po Toi O bay to
avoid Amphioxus habitat as explained in Section 2.3.21. Minor dredging
is required for installing the diffuser at seabed. The water quality impact
related to dredging will be minimized by fully enclosing the dredging area
within cofferdam.
2.3.16
The submarine outfall directs effluent from rising
mains on land to the diffuser in seabed. The alignment of submarine outfall
must go straight towards the proposed diffuser location. Curved alignment will
increase resistance to effluent flow, prolonging the time for discharge. This
may also cause settlement of particles in effluent at turning point. Also, unnecessary
curvature will increase construction difficulty. Thus straight horizontal
alignment is adopted.
2.3.17
The submarine outfall will be constructed by Horizontal
Directional Drill (HDD)
as explained in Section 2.4.6. The
drill head will penetrate the ground at the coast, drill underneath the seabed
and emerge from the seabed at the proposed diffuser location. By adopting this
construction method, no dredging is required along the submarine outfall
alignment. Water deterioration, impacts on marine ecology and fisheries as well
as generation of marine sediment due to dredging operation will be
significantly minimized. The works will involve a dredging barge stationed next
to the cofferdam. With localized dredging operation, disturbance to marine
traffic and safety can be minimized.
2.3.18
Three diffuser location options are formulated and
illustrated in Figure
2-2:
·
Option 1 – Near rocky shore (Design in Project Profile)
·
Option 2 – Mouth of Po Toi O Bay
·
Option 3 – Outer Po Toi O Bay
2.3.19
For Option 1, the
diffuser location was designed based on the shortest permissible distance from
Po Toi O Fish Culture Zone (200m). It is located right next to the rocky shore where corals were later
recorded during the course of EIA Study (see Figure
2-2 for
location of corals). Construction works will likely damage the rocky shore and
the corals. Also, the water depth was far too shallow (about 3m) for the
installation of the diffuser as it may be damaged by boats cruising or
anchoring in the waterway.
2.3.20
For Option 2,
the diffuser is located at the mouth of Po Toi O Bay, about 141m away from Option
1, and is about 226 m away from Po Toi O Fish Culture Zone. Coral surveys
confirmed that no coral was recorded away from shore in waters near the central
part of Po Toi O Bay channel. As the water current is expected to be higher at
the outer Po Toi O channel with a much larger water body in Clearwater Bay, the
dispersion of the treated effluent should be much improved, reducing potential
water quality, marine ecological and fisheries impacts. Furthermore, greater
water depth (about 5m) would provide sufficient clearance for marine vessel
movements. However,
subsequent benthic survey recorded species of conservation importance,
Amphioxus, at the diffuser location.
2.3.21
For
Option 3, the diffuser is located at the outer part of Po Toi O Bay,
about 239m and 110m away from Options 1 and 2
respectively, and is about
326 m away from Po Toi O Fish Culture Zone. Additional
benthic survey did not
record any Amphioxus or other species of
conservation importance (e.g. coral) at this location. This new diffuser option
is even further away from the Po Toi O bay and the mariculture zone. The water
is over 10m deep, further improving navigation safety and effluent dispersion. The starting point of submarine
outfall is slightly shifted to the east when compared with Options 1 and 2 as a
larger works area for placing HDD equipment was found necessary.
Table 2-2: Summary of Site Selection Process
for Diffuser
|
|
Option 1 |
Option 2 |
Option 3 |
|
Location |
Diffuser near
rocky shore within PTO channel |
Diffuser at the
mouth of PTO bay |
Diffuser at the
outer part of PTO bay |
|
Water Depth |
3m |
5m |
10m |
|
Length of SO |
192m |
285m |
385m |
|
Benefits |
Lower cost due to
shorter SO |
Smaller impact on
coral |
Smaller impact on
coral and amphioxus |
|
Better water
dispersion |
Better water
dispersion |
||
|
Better marine
traffic safety |
Better marine
traffic safety |
||
|
Dis-benefits |
Impact on coral |
Impact on
amphioxus |
Highest cost due
to longer SO and deeper water |
|
Shallow water
depth affecting marine traffic safety |
Higher cost due
to longer SO |
* STP: Sewage Treatment Plant, SO: Submarine Outfall
Gravity
Sewer and Rising Mains Alignment
2.3.23
The
gravity sewer and rising mains will be laid along the footpath between
residences. The rising mains will eventually be connected to the submarine
outfall at the rocky shore. Therefore, the alignment of the pipelines will be
determined by the selected STP and residence locations.
Duration of Effluent Discharge
2.3.24
In
response to the worry raised by the locals on drifting of treated effluent back
to the Po Toi O bay under flood tide condition, an option on effluent discharge
only during ebb tide was considered. Due to the difficulty in operation with
varying tidal periods, the discharge was assumed to occur every 12.4 hours, the
approximate tidal period, under the control of a timer. The treated effluent
would be stored inside the plant between discharges.
2.3.25
However,
this method was considered not cost-effective or practical due to the following
considerations:
(1)
The
water quality model showed that the water quality would be similar to that
under normal discharge scenario, i.e. continuous discharge right after
treatment.
(2)
E. coli in effluent may propagate during storage. Additional
disinfection would be required before discharge, incurring extra operation
cost. Also, the contact time may not be sufficient to ensure that
the concentration can meet the discharge standard.
(3)
The
tide can be irregular at times, resulting in insufficient tidal flow.
Emergency Plant Breakdown
2.3.26
In
case the sewage treatment plant (STP) breakdowns due to power or equipment
failure, the following actions were considered:
·
Option 1: Discharge of untreated sewage along a channel
next to the STP directly into the bay
·
Option 2: Discharge of untreated sewage through the
submarine outfall
·
Option 3: Removal of untreated sewage by tankers to other
STP
2.3.27
For the first two options, the major water quality impact
during emergency discharge is elevation of E.
coli concentration.
2.3.28
Option 1 is a typical
provision in sewage treatment plants in Hong Kong by quickly removing sewage
before overflowing inside the plants. However, the discharge location is
located within 200m of the seaward boundaries of Po Toi O Fish Culture
Zone, which does not satisfy TM of WPCO mentioned
in Section 2.3.13. Also, water quality model
results found that E. coli in sewage
would be trapped within the Po Toi O bay. Although the impact is temporary,
significant elevation in bacterial content in water may contaminate the fish in
Po Toi O fish culture zone.
2.3.29
Option 2 should have smaller impact than
Option 1 has as the discharge point is further away from
fish culture zone. Stronger water current and deeper water depth at the mouth
of Po Toi O bay also allow better dispersion and dilution of sewage. However,
this option is highly undesirable as the pipe is only
280mm in diameter and has a sagging profile. Particles in the raw sewage would
settle and accumulate at the lowest point if raw sewage was to be discharged
through the pipe. Desilting this pressurized
main would be very difficult due to its inaccessibility at the landward side. On the other hand, constructing
another emergency submarine disposal pipe of
similar length (385m) would be expensive and not cost
effective, not to mention that the said problem remains unresolved.
2.3.30
Option 3 is practical and chosen as the preferred option. As the average dry weather flow (ADWF) of PTO STP would
be small (about 139m3/day),
it is possible to deploy tankers to transport away the sewage to Tseung Kwan
O Preliminary Treatment Works (TKO PTW) (or other nearby STW) in case of plant failure.
2.3.31
Emergency storage of 4-hour ADWF (23.19m3)
will be provided in the PTO STP. In case of plant failure, three 12m3
sewage tankers will be called in to transport the sewage from PTO STP to TKO
PTW. Each tanker will take different travel routes to reduce the risk of delay
due to traffic jam. The tanker arrangement is as follows:
Table 2-3: Tanker Away Arrangement
|
Tanker |
Source |
Potential Route |
Arrival Time |
Duty |
|
A |
DSD’s tanker which stationed at Sai Kung Sewage Treatment Works |
Sai Kung STW |
1.5 hours from plant failure |
Transport sewage from PTO STP to TKO PTW |
|
B |
DSD Sewage Treatment Division Term Contractor, required to arrive in 2
hours in contract |
Shatin STW* |
2.5 hours from plant failure |
Transport sewage from PTO STP to TKO PTW |
|
C |
DSD District Term Contractors (any 1 of the 3 districts), required to arrive in 1.5
hours in contract |
Kwun Tong Preliminary
Treatment Works* |
2.5 hours from plant failure |
Standby at PTO STP, to be on duty if Tanker A/B breaks down or encounters
traffic delay |
* Typical location where term contractor’s tankers are stationed
2.3.32
The distance between PTO STP and TKO PTW is about
12.1km or 18-minute travel distance. Including sewage loading and unloading
time, each tanker is assumed to take 2 hours round trip. Tankers A and B will
work in shift to continuously remove sewage from PTO STP. If one of the tankers
fails to arrive at PTO STP on time, Tanker C will come in to ensure that at
least two tankers will be operating. An operation drill prior to future
operation will be conducted to confirm that the time estimates are achievable at peak hours.
2.3.33
Appendix 5.5 shows the
fluctuation of sewage volume in PTO STP in case plant failure occurs during
peak sewage flow (6pm). With continuous removal of sewage by tankers in
rotation, the highest quantity stored in the plant will be 17.24m3,
which is well below the emergency storage capacity (23.19m3). With
about 6m3 storage buffer, the chance
of having sewage volume exceeding the storage
capacity is very low. No overflow of sewage from the PTO STP is anticipated.
2.3.34
Each tanker will deliver 12m3 sewage from
PTO STP to TKO PTW. Based on DSD’s past experience, it takes 15 minutes
to unload all sewage, the average flow rate will be 12m3/15
minutes/60 seconds = 0.013m3/s, which is far below the design
capacity of TKO PTW (5.55m3/s) [1]. No overloading of
TKO PTW is anticipated.
·
Delivery of an emergency
generator to PTO STP within 4 hours from plant failure
·
Provision of dual power
by CLP;
·
Provision of a supervisory
control and data acquisition system (SCADA), which signals to the operation and
maintenance personnel for emergency attendance in case of plant failure;
·
Provision of a standby
pump and screen at the PTOSTW;
·
Provision of emergency
storage of 4-hr ADWF sewage retention time;
·
Arrangement of tankers
for removing incoming sewage to other sewage treatment plants for treatment continuously
to ensure sufficient buffer for emergency storage.
2.3.36
Based on these provisions, emergency discharge of raw sewage is not expected, and thus no adverse impact on water
quality, marine ecology or fisheries due to emergency discharge is anticipated.
Capacity of the Sewage Treatment Plant
2.3.37
The
sewage treatment plant was planned to treat the sewage from both Po Toi O and
Tai Wong Kung in Project Profile stage. Nevertheless, since establishing a pipeline
system to cover Tai Wong Kung is not cost effective, Tai Wong Kung will not be
served by this Project. Therefore, the
average dry weather flow (ADWF) of the STP decreases from 220m3 in the Project Profile to
about 139 m3
per day in the current design.
Sewage Treatment Plant
2.4.1
The
proposed sewage treatment plant will be constructed on a cut slope with hard
bed rock. Rock breaking by hydraulic breakers and stabilization by bored piling
are common construction methods. Drilling of bored piles has the advantage of
less noise and vibration generation. Driven piles are not adopted due to existence
of rock layer. Blasting of bed rock is not considered due to safety concern.
Gravity Sewer and Rising Mains
2.4.2
Gravity
sewer and rising mains will be laid along the footpath in Po Toi O. Open trench
excavation involves excavation, pipe laying and backfilling, the most common
method for pipe laying in shallow ground layer. As there are different existing
underground utilities located along the footpath in Po Toi O, trenchless method
is considered risky for pipe laying in shallow ground layer.
2.4.3
As
mentioned in Section 2.3.1, all sewage treatment processes
will be conducted in the basement level. Sewage will flow along the gravity
sewers to the basement of STP. Between Po Toi O Village and STP, the gravity
sewers will be deep underground. Open trench method requires extensive
excavation works and cause disturbance to the existing footpath and
carriageway, and therefore this is not recommended. On the other hand, trenchless
method will be adopted in this section to drill through the hard rock layer.
Submarine Outfall
2.4.4 Submarine outfall conveys treated effluent from the rising mains in Po Toi O Village to the diffuser at the outer part of Po Toi O Bay. Two construction methods have been considered:
·
Option 1: Open dredging
·
Option 2: Horizontal
Directional Drill
2.4.5 For Option 1, the traditional construction method will use open dredging to create a trench for pipe laying and then backfill. Dredging will release marine sediment within Po Toi O bay. The semi-enclosed geography will hinder dispersion of suspended solid and cause water quality impact on the fish culture zone, coral and amphioxus within the bay. Contaminants in marine sediment may also dissolve in water. Furthermore, occupation of dredging barge and tugboat at the narrow channel at the mouth of Po Toi O bay will affect movement of fishing vessels and boats.
2.4.6
Option 2 Horizontal Directional Drilling (HDD) involves a drill head that penetrates underground from the shore until reaching the
location of diffuser. No dredging or
disturbance of surface sediment is required, minimizing water quality,
marine ecology, fisheries and marine traffic impacts. Therefore, this option is selected as the preferred method.
2.4.7
The following table summarizes the benefits and dis-benefits of HDD and
dredging for submarine outfall construction.
Table 2-4: Summary of Selection Process for
Construction Works of Submarine Outfall
|
Option 1 |
Option 2 |
|
|
Method |
Open Dredging |
HDD |
|
Benefits |
Lower cost |
No disturbance
of seabed surface and thus no release of SS |
|
Faster
Programme |
No disturbance to
FCZ, coral or amphioxus |
|
|
No obstruction
of marine traffic and thus safer |
||
|
Dis-benefits |
Adverse water
quality deterioration, fisheries and marine ecological impacts |
Higher cost |
|
Affect marine
traffic along Po Toi O channel and also navigation safety |
Longer
programme |
|
|
Impact Duration |
Throughout
dredging works and effect of water pollution will last until SS get diluted
and settled |
Not anticipated |
|
Impact Severity |
High |
Low |
Diffuser
2.4.8
At
the end of the submarine outfall on the seaward side, dredging of 1,200m3
marine sediment is required for installation of the diffuser. Two construction
methods have been considered:
·
Option 1: Open Dredging
·
Option 2: Dredging within
Fully Enclosed Cofferdam
2.4.9
Option 1 involves traditional open dredging,
which may result in elevation in SS and sedimentation rate at the FCZ, corals
and amphioxus. This option is not chosen given the sensitivity of these
sensitive receivers to water deterioration.
2.4.10
Option 2 involves dredging within fully
enclosed cofferdam by a dredger barge anchored outside. No release of sediment
into marine waters outside the cofferdam is expected. Only
minor disturbance of seabed is required during erection of the cofferdam.
2.4.11
The following table summarizes the benefits and dis-benefits of open and
enclosed dredging for diffuser construction.
Table 2-5: Summary of Selection Process for
Construction Works of Diffuser
|
Option 1 |
Option 2 (Preferred Option) |
|
|
Method |
Open Dredging |
Dredging within Fully Enclosed Cofferdam |
|
Benefits |
Lower cost |
Minor disturbance of seabed during cofferdam erection.
No release of SS during dredging |
|
Faster
Programme |
No disturbance
to FCZ, coral or amphioxus during dredging |
|
|
Dis-benefits |
Moderate water
quality, fisheries and marine ecological impacts |
Higher cost |
|
Longer
programme |
||
|
Impact Duration |
Throughout
dredging works and effect of water pollution will last until SS get diluted
and settled |
Short term release of SS during cofferdam erection. No water
quality impact is expected during dredging |
|
Impact Severity |
High |
Low |
2.5.1 Construction work will be divided into three major parts:
(1) sewage treatment plant
(STP),
(2) gravity sewers and
rising mains and
(3) submarine outfall and
diffuser installation.
2.5.2 The construction works are expected to commence in mid-2017 for completion in 2021 with one more year for defect correction. The proposed programme, which may be subject to amendment, is listed in Appendix 2.2.
2.5.3 After site clearance, the construction works for STP will commence. To minimize cumulative noise impact, pipe laying will be carried out after completion of the major noise emission works (i.e. excavation of STP). Due to topography of the area, pipe laying will progress from Po Toi O Village near the coast to Fairway Vista uphill. There will be three concurrent work zones, each of about 100m apart to reduce cumulative noise impact. The submarine outfall will be constructed near completion of civil works on the STP, and this will be followed by the diffuser installation.
Selected Site and Construction Method
2.6.1
In
summary, selection of the locations and construction methods for the sewage
treatment plant, submarine outfall and diffuser have considered the following
constraints:
·
Current zonings in OZP
·
Accessibility
·
Distances from sensitive
receivers, e.g. village houses, fish culture zone
·
Presence of corals,
amphioxus or other species of conservation importance
·
Effluent dispersion
effect
·
Marine traffic safety
·
Public concerns
2.6.2
The
preferred scenario is summarized below:
Table 2-6: Summary of Preferred Scenario
|
Component |
Location |
Existing
Condition |
Construction
Method |
|
Sewage
Treatment Plant (STP) |
Cut Slope next
to Po Toi O Chuen Road |
Shrubland |
Excavation and |
|
Gravity Sewers/
Rising Mains |
Between STP and
Po Toi O Village |
Cut slope/
Paved road |
Trenchless/ |
|
Po Toi O
Village |
Concrete paved
footpath |
Open Trench
Excavation |
|
|
Submarine
Outfall |
Along Po Toi O
Bay channel |
Below seabed |
Horizontal
Directional Drill (HDD) |
|
Diffuser |
Outer PTO Bay |
Seabed with no
record of corals/amphioxus |
Dredging in fully
enclosed cofferdam |
2.6.3
The
preferred scenario has the following environmental benefits comparing to other
options:
·
Avoid destruction of
coral-lined rocky shore and a major amphioxus habitat by shifting the diffuser
location away from shore and in seabed without amphioxus
·
Minimize terrestrial ecological
impact by placing the STP on a cut slope, which is an ecologically less
sensitive habitat
·
Minimize visual impact by
embedding the STP into existing cut slope
·
Minimize water quality,
marine ecological and fisheries impact by (1) having no dredging works for
constructing submarine outfall, and (2) confining dredging works for diffuser
within four-side enclosed cofferdam
·
Minimize odour and noise
nuisance by keeping sufficient buffer distance between the STP and village
houses (>59m)
2.6.4
In addition to the benefits mentioned above, the following measures will
also be implemented to minimize environmental impacts:
·
minimize the footprint
and mass of STP by constructing most of the STP underground
·
minimize noise
disturbance to nearby noise sensitive receivers by installing noisy plant
equipment underground
·
provide appropriate
façade treatment (e.g. adopting recessive colours on STP wall that blend in with
the natural tones of the landscape backdrop), compensatory tree planting and
vertical greening
Normal Operation
2.6.5
Sewage
from Po Toi O will be collected through the gravity sewer to Po Toi O STP for MBR treatment.
Treated effluent will be pumped through the submarine outfall underground and
discharged at a diffuser at the outer part of Po Toi O Bay. The ADWF is designed
at 139m3/day.
Emergency Plant Breakdown during Operation
2.6.6
The
PTO STP cannot operate in case of power or equipment failure. The SCADA system in
the PTO STP will signal to the operation and maintenance personnel for
emergency attendance. Standby pumps and screens will be provided at the PTO STP. According to the
performance pledge of CLP, electricity provision will be restored within 2
hours after fault outage. Also, DSD’s future term
contractor should be able to deliver an emergency generator to the STP within 4 hours in case of plant
failure.
2.6.7
If
the plant cannot be restored immediately, the in-coming sewage will be
temporarily stored at a capacity of 4-hour ADWF volume. Tankers will be
deployed to continuously transport the sewage to Tseung Kwan O Preliminary
Treatment Works (or other nearby STW) for treatment. This
can continuously maintain sufficient buffer for emergency storage (details available in Sections 2.3.30 - 2.3.35). The above measures
are the most appropriate and practical mitigation measures in case of
plant/power failure considering the project scale, risk of emergency condition, construction
difficulties and cost. Emergency discharge of untreated sewage is not expected.
2.7.1
Details of selected construction method from Section 2.4 are shown below:
Sewage Treatment Plant
2.7.2
The
proposed STP is one-storey high, with the top level at about 5m above ground.
The basement is about 10m below ground comprising two levels. A retaining structure
will be constructed to reinforce the slope behind the proposed STP.
2.7.3
During
the construction, a temporary platform will be erected on the existing slope
for the construction of a bored pile wall. Then the slope will be cut down to
the level of the Po Toi O Chuen Road. Temporary lateral support will be
constructed and the land will be further excavated to make room for the STP.
2.7.4
After the STP is built, E&M and building services installations
will commence. Run-in and run-out for sludge tank will also be constructed. All
pumps and plant equipment will be placed inside the building.
Gravity Sewers and Rising Mains
2.7.5
The
proposed gravity sewers and rising mains will be constructed by open trench
excavation and trenchless method.
2.7.6
For
gravity sewers or rising mains to be placed less than or equal to 3.5m deep underground, open trench excavation will be
adopted. The works area will be surrounded by temporary barriers. Trenches
will be excavated by hand held tools and lateral support will be provided to
prevent collapse. After pumping out groundwater, gravity sewers and/or rising mains will be placed in
segments. The trench will be backfilled with the excavated soil and the works area
will be reinstated.
2.7.7
For
gravity sewers to be placed deeper than 3.5m underground, trenchless method will be adopted.
The works area will be surrounded by temporary barriers. Jacking pit and
receiving pits will be excavated and the sewer will be installed by heading
method. The pits will be backfilled with excavated soil and the works area
will be reinstated.
2.7.8
Drilling with chemical agent will be considered as
far as practicable in the construction phase according to local situation.
Submarine Outfall & Diffuser Installation
2.7.9
Horizontal directional drilling (HDD) will be used
to construct submarine outfall. On the landward side, a 7m deep entry pit will be excavated
within sheet piles at the rocky shore above the high water mark. On the
seaward side, a 10m x 50m fully enclosed cofferdam will be constructed around receiving
pit by sheetpiles.
2.7.10
During the HDD process, the HDD drill rig will be inserted
into the entry pit and drill through the rock layer below the sea. A small drill head will be used to
form a small pilot hole on the first run. The drill head will emerge from the sea bottom at
the receiving pit within the cofferdam. Pre-reaming process will be carried
out using a larger drill head until the designed hole size is formed. A sleeve of 600 mm diameter
will be installed encasing the rising mains, followed by connection and installation
of the pre-casted diffuser (<5m2 in area). When the works are completed, the
entry pit and receiving pit will be backfilled to the original level. The sheet
piles will then be extracted. Rocks that are temporarily removed for HDD works
will be laid back onto the rocky shore for reinstatement.
2.7.11
Water protection measures are implemented during HDD
and dredging inside cofferdam. Bentonite will be used as drilling liquid in HDD. Cuttings in bentonite will be
extracted out for disposal and the
reconditioned bentonite will be reused for drilling process. No wastewater will be generated.
2.7.12
After erection of cofferdam, the water inside will be pumped out and stored in a
sedimentation tank of the barge for settling the suspended solids. The water
will be treated up to the requirement in the Wastewater Discharge License
for discharge. The marine sediment will be dredged by a dredger barge anchored outside the cofferdam. The removed
sediment will be stored in a sealed compartment of the dredger
barge. No release of marine sediment outside the cofferdam is anticipated. The dredger barge, tug boat and barge for material
storage will occupy around 1,020m2 of sea area.
2.8.1
Before
the commencement of this EIA Study, the Sai Kung District Council and local residents
had been consulted on the proposal for providing sewerage system to the un-sewered Po Toi O area.
Although residents of Po Toi O generally welcomed the proposal during the Sai
Kung District Council meetings over the past five years, they were worried
about the secondary impact of the treated effluent. Environmental concerns and
their solutions are summarized in Table
2-7.
2.8.2
During
the course of this EIA Study, a public consultation was conducted to present
the preferred layout to local communities, including residents from Po Toi O
Village and Fairway Vista, mariculturists and fishermen, Sai Kung District
Council and Green Groups. In general, all stakeholders welcomed the Project as
it can bring benefits to the community. However, environmental concerns (e.g.
water quality impact, odour impact, noise impact) were also brought out.
Environmental concerns and their solutions are summarized in Table 2-7.
Table 2-7 Summary of Environmental Concerns with
Solutions
|
|
Environmental Concerns |
Solutions |
|
(a)
|
Potential adverse impact of the effluent
on the Clear Water Bay First Beach and Second Beach |
The effluent will be discharged at outer Po Toi O Bay with deep water
depth (10m). Great depth and fast water flow result in sufficient dispersion
and dilution of effluent. According to Chapter 5 – Water Quality Impact Assessment, the water
quality in Clear Water Bay First Beach and Second Beaches will only have
negligible changes. No adverse impact is anticipated. |
|
(b)
|
Impact of the treatment plant on
the fishery industry in Po Toi O |
The effluent will be discharged at outer Po Toi O Bay, 326m away from
Po Toi O Fish Culture Zone. In case of plant failure, sewage will be tankered
away for treatment. No discharge of untreated sewage is
anticipated that may result in significant deterioration of water quality in the
FCZ. According to Chapter 8 – Fisheries Impact Assessment, normal operation of the STP will only
bring insignificant impact on fisheries (including culture fishery in Po Toi
O). |
|
(c)
|
E. coli and micro-organisms would be trapped in the sack-like Po Toi O bay |
The effluent will be discharged at outer Po Toi O Bay via a 385m long submarine
outfall, such that the great depth and fast water flow can provide sufficient dispersion
and dilution of the treated effluent already meeting the treatment standard.
No discharge of untreated sewage will be permitted in case of plant failure.
According to Chapter 5 –
Water Quality Impact Assessment, no
substantial change in E. coli is
expected during normal operation of the STP. |
|
(d)
|
Water quality impact from dredging works
and effluent discharge on fish culture zone |
Construction of the submarine outfall by
Horizontal Directional Drilling eliminates the need for dredging, except for
the diffuser location. At the diffuser location, since all dredging
and backfilling works will be conducted within a fully enclosed, water-tight
cofferdam, no water quality impact is expected due to these works. According to Chapter 5 – Water Quality Impact Assessment, once the Project is in operational
phase (i.e. discharging treated effluent), no substantial change in the water
quality around Po Toi O Fish Culture Zone is expected. |
|
(e)
|
Landscape and visual impact due to
construction of sewage treatment plant |
The STP is designed with a small building mass
by constructing most of the structure underground. This can minimize
construction work site. The STP will have non-reflective finishes and
recessive colours that blend in with natural tones of landscape backdrop. Tree
and shrub planting and vertical greening will be provided to screen the building.
These can minimize landscape and visual impact in operational phase. According to Chapter 10 – Landscape and Visual Impact Assessment, the impact
will be acceptable in construction phase if mitigation measures are
implemented properly, such as erecting decorative mesh screens or
construction hoardings and/or temporary noise barriers around works areas in
visually unobtrusive colours. |
|
(f)
|
Odour impact from public sewer and
operation of sewage treatment plant |
Public sewer will have U-shaped water trap for
preventing odour leak. During operation of STP, sludge removal will be
conducted within STP building, which is equipped with filters with over 99.5%
odour removal efficiency. According to Chapter
3 – Air Quality Impact Assessment, odour
impact is anticipated to be acceptable in operation phase. |
|
(g)
|
Noise generated from pumps during operation |
Most plant
equipment will be placed underground and enclosed within the sewage treatment
plant. According to Chapter 4 –
Noise Impact Assessment, operational
noise impact is anticipated to be acceptable. |
|
(h)
|
Ecological impact on coral communities |
The location of diffuser will be installed away
from the coast grown with corals. Dredging works in construction phase will
be fully enclosed within cofferdam, preventing leakage of suspended solids
that may affect coral communities. According to Chapter 7 – Marine Ecology Impact Assessment no direct impact on
coral communities is anticipated. |
2.9.1
There
may be two potential concurrent projects in the area, i.e.
·
Roundabout near the
minibus terminal;
·
Fish culture zone (FCZ)
dredging and relocation
2.9.2
During
the course of the study, CEDD has confirmed that there was no programme for the
fish culture zone dredging and relocation. There was also no information on
when the roundabout would be constructed.
Environmental Enhancement
Project by Nature
2.10.1
The proposed sewerage works
in Po Toi O is an environmental enhancement project by nature. It aims to
improve environmental hygiene in the Po Toi
O area through replacement of
septic tanks/soakaway systems (STS) in Po
Toi O area by public sewerage system. Sewage will be properly collected,
treated by MBR up to effluent discharge standard of Port Shelter Water Control
Zone and discharged via submarine outfall at a diffuser installed at seabed in
outer Po Toi O Bay.
Various Green
Measures in the Project (Mainly by Avoidance and Minimization of Potential
Impacts)
2.10.2
Po
Toi O has scenic seashore and is embraced by hilly terrain overgrown with
shrubs and trees. Village settlements along the shore and fish farming in the bay give Po
Toi O a unique rural
landscape. This EIA Study has identified and fully considered the potential
constraints imposed by this special environ, and has developed a number of green
initiatives at this planning stage. In addition to improving environmental hygiene
in the Po Toi O area, this Project targets to achieve environmental
friendliness in sewerage, construction and operation designs. The green measures
and the associated environmental
benefits of this Project are summarized as follows:
Green
Measures in Sewerage System Design
·
Minimization of loss of ecologically sensitive
habitats by locating the STP on a cut slope (disturbed
shrubland) and laying gravity sewers and rising mains along existing footpath
as far as possible (developed area);
·
Avoidance of destruction of coral-lined rocky shore
and major amphioxus habitat by shifting
the treated effluent discharge location over 100m away from the coral-lined shore
and species of conservation importance (amphioxus);
·
Minimization of fisheries impact by discharging
the treated effluent at about 326m away from fish culture zone;
·
Maximizing effluent dispersion by placing the diffuser on seabed at >10m depth
and at outer Po Toi O bay where current flow is fast;
·
Minimization of the footprint of the STP by constructing
most of the STP structure underground and adopting a less land demanding sewage
treatment technology (MBR);
·
Minimization of odour and noise nuisance by keeping sufficient buffer distance between the
STP and village houses;
·
Minimization of visual impact by embedding the STP into existing cut slope;
·
Maintenance of visual entity with surrounding
undisturbed landscape by provision
of appropriate façade treatment (e.g. adopting recessive colours on STP wall
that blend with the natural tones of the landscape backdrop), compensatory tree
planting and vertical greening;
·
Avoidance of impact on built heritage resources (e.g. Grade 3 historic building Hung Sing Temple)
by laying gravity sewers and rising mains away from their footprint;
Green
Measures in Construction Design
·
Avoidance of open dredging to minimize water quality, marine ecological and
fisheries impact by adopting (1) horizontal directional drilling for submarine
outfall below seabed and (2) enclosed dredging within cofferdam for diffuser;
·
Avoidance of concurrent construction activities by designing separate work programme for noisy
works (carrying out rock breaking works for STP prior to laying of gravity sewer
and rising mains and drilling for submarine outfall). Also, adopt three
workfronts during pipe laying in village, each of short section and at least
100m away from each other.
Green
Measures in Operation Design
·
Improvement of environmental hygiene and removal of
associated odour nuisance in the Po Toi O area by eliminating the need of desludging STS;
·
Avoidance of emergency discharge of raw sewage into
waterbodies by a number of designed arrangements in case of
emergency plant failure, such as notification to operator by SCADA system,
provision of standby equipment in the STP, provision of sufficient capacity of
emergency storage of incoming sewage and provision of tankers for continuous removal
of sewage to other STP for treatment.
·
Minimization of odour nuisance from STP by provision of odour removal system of 99.5%
efficiency in the STP;
·
Minimization of noise disturbance from STP by storing noisy plant equipment underground.
[1] Maunsell Consultants
Asia Ltd. 2005. Further Development of Tseung Kwan O Feasibility Study
(Application No. EIA-111/2015) Ch. 6 Sewerage and Sewage Treatment. [online]
Available at: http://www.epd.gov.hk/eia/register/report/eiareport/eia_1112005/HTML/EIA/HTML/Volume%201/Sec_6%20Sewerage.htm.
Last Accessed: 23 March 2016.