2
DESCRIPTION OF THE PROJECT
Project Requirements
2.1
The objective of this Project is to
abate the current water pollution problems at Sok Kwu Wan by constructing new
sewage collection, treatment and disposal facilities. This Project is currently undertaken by Drainage Services
Department under the Outlying Islands Sewerage Stage 1 Phase II Package J
project, as recommended in the Final Report of the Outlying Islands Sewerage
Master Plan (SMP) Study (Outlying Islands Sewerage Master Plan Final Report by
Montgomery Watson, 1994). At present,
the Project is in preliminary design stage.
2.2
The proposed works include a new
sewage collection system comprising gravity sewers, pumping stations and rising
mains to collect and transfer the sewage generated from the Sok Kwu Wan and the
surrounding areas to a new sewage treatment works (STW). The sewage will receive secondary treatment
including nitrogen removal and disinfection prior to disposal through a
submarine outfall to the entrance to of the Picnic
Bay.
2.3
Consultation with Islands District
Council, Lamma Area Committee and South Lamma Rural Committee were carried out
from February to April 2002 to solicit the public views and support on the
proposed sewerage facilities. The
scheme was generally supported and
concerns that were expressed by the public were addressed.
Project Location
2.4
The Project Area is located within
Picnic Bay (Sok Kwu Wan), at the eastern side of Lamma Island (see Figure
2.1). It covers the coastal area along
the southern part of Sok Kwu Wan stretching from Chung Mei to the west of Mo
Tat Wan. The Project Area is rural in
nature with the existing developments being predominantly village-type low-rise
buildings. It embodies two small
villages, Sok Kwu Wan Village and Chung Mei Village, with the former more
densely developed with residential flats, seafood restaurants and retail
outlets. As Sok Kwu Wan is easily
accessible from the Hong Kong Island by means of regular ferry and kai to
services, and pleasure crafts, this added convenience has attracted many
visitors, including tourists and diners to the Project Area during the holidays
and weekends. Also, there are a
gazetted fish culture zone (FCZ) at Picnic Bay and a bathing beach at Mo Tat
Wan in the Project Area.
Environmental Benefits of the Project
2.5
The Project represents a significant
step by the government to abate the water pollution problems at Sok Kwu
Wan. Water quality within Picnic Bay
has steadily deteriorated over the last decade, with the increase in the number
and size of seafood restaurants in Sok Kwu Wan Village and the expansion of the
fish culture business (Outlying Islands SMP Stage 2 Review Final Report by
Montgomery Watson, 2002). Water
circulation within the Bay is naturally very poor, leading to higher levels of
pollution and degradation of water quality. The water quality problem was
highlighted in the summer of 1999 when red tides were responsible for the death
of large numbers of fish in the culture zones within
Picnic Bay.
2.6
The major pollution source discharging
to the Bay is the unsewered village developments and restaurants. Houses and restaurants are currently served
by septic tank and soakaway systems.
Many of these systems are under unnecessary hydraulic capacity and
overflows are common. Grey water from
kitchens is often directed to the surface drainage system and then into the
marine waters without treatment.
2.7
The proposed sewerage system will
serve to collect and transfer the sewage arising from the existing villages
including restaurants to the proposed STW for a full secondary treatment with
nitrogen removal and disinfection and to the submarine outfall for
disposal. It will achieve the effect of
eliminating untreated or partially treated sewage discharges to the Picnic
Bay. The proposed submarine outfall
would dispose the treated effluent at a location of about 720 m from the coast near the mouth
of Picnic Bay. Based
on the SMP Review Final Report and
information obtained from Planning Department,sewage
generated from approximately 130830 residents, 6000 day visitors and restaurants in Sok Kwu Wan area (in 2016) (which
are equivalent to the ultimate population figures) as
well as sewage flows from Lo So Shing and Mo Tat
Wan, which are outside the Study Area and
would have total residents of
5 approximately 532
in 2016, would
be collected, treated and disposeal of from the proposed sewerage system, treatment facilities and submarine
outfall. This
represents the major benefit of
the Project, not only in the respect of water quality improvement to the Bay but also
to the local environment in the residential areas by eliminating unhealthy,
malodorous and potentially visually unpleasant surface discharges.
2.8
The sensitive receivers
within the Study Area are mainly villages, which are rural in nature with no major air
or noise pollution source. The
implementation of the Project may give rise to environmental impacts, such as
construction dust, noise and waste, on the receivers in the vicinity. Owing to the presence of the FCZ, potential
water quality
impacts from the construction of the proposed submarine outfall and emergency discharge
from the operation of the proposed pumping
stations and STW are of particular
concern. However, theseThe impacts from the construction of the project would be short-term,
being confined to the construction stage and could be controlled by
implementing appropriate mitigation measures.
The water quality impact
from the emergency discharge would be episodic and short-term and would be minimized
with sufficient preventive measures in place. Pumping
stations and treatment facilities would be required by the Project and hence
would permanently occupy some of the Study Area. Nevertheless, no areas of high ecological or landscape and visual
importance would be impacted. With appropriate design of the pumping
station and STW and incorporation of suitable mitigation measures, the
intrusion of new structures would be minimized.
Scope of the Project
2.9
The scope of this Project basically
follows the recommendations made under the SMP Study. Modifications have been
made taking into consideration of the updated population data and the increased
technical information, subsequent to the completion of the SMP Study. The main components of this Project includes
the follows (see Figure 2.5):
·
Construction construction of
approximately 1.4 km of gravity sewers;
·
trench excavation for installation of underground sewer
pipes along Chung Mei Village, Sok Kwu Wan Village and Mo Tat Road;
·
modification of the existing combined stormwater system
to separate drainage and sewerage systems;
·
construction of three new sewage pumping stations, P1a,
P1b and P2, with installed capacities (ADWF) of approximately 353 m3/day, 508 m3/day
and 879 m3/day, respectively;
·
construction of approximately 1.4 km rising main;
·
construction of a full secondary level sewage treatment
works (STW) with nitrogen removal and UV disinfection unit with an installed
capacity (ADWF) of 940 m3/day;
·
construction of a submarine effluent outfall of
approximately 750 765 m.
Consideration of Alternative Options
Option Selection in Sewerage Master Plan Study
2.10
In 1993, the Environmental Protection
Department (EPD) commissioned a SMP study of the Outlying Islands and was
completed in 1994. The objective of the
study was to develop a SMP for the control of water pollution with the
provision of adequate sewerage, sewage treatment and disposal facilities.
2.11
The SMP recommended the provision of
improvements to existing sewerage facilities and the implementation of major
extension of sewerage system to unsewered areas on the outlying islands
including Lantau Island, Cheung Chau, Lamma Island, Peng Chau and numerous
other small and lesser populated islands, to cope with expected future
development. The recommendations for
Sok Kwu Wan, Lamma Island were to be implemented as Package J, one of the 21
recommended packages to be carried out in 2 stages.
2.12
During the SMP study, various sewerage
system, treatment and disposal options for the Project were considered and they
were as follows:
·
Option 1 local
treatment (i.e. preliminary treatment) and discharge through a long outfall to
the entrance of the bay.
·
Option 2 on-site
treatment to produce a disinfected effluent (i.e. full secondary plus
disinfection) with disposal to inshore waters.
·
Option 3 export
to Yung Shue Wan sewerage network for treatment and discharge.
Figure
2.2 shows the above options extracted from the SMP Report.
2.13
Figure 2.2 shows the above options extracted from
the SMP Report.
2.14
The selected option was Option 1 and serves as the
basis for this Project. The discharge
zone of the outfall was proposed to be located such that the treated effluent
could be carried directly to the flow in the East Lamma Channel. The selected route to the acceptable
discharge zones was an on-land rising main to a location west of the village of
Mo Ta Wan and an outfall of up to about 600 m long. Another route which had been considered was a direct sea bed
line between 1 and 1.3 km long.
However, this route was not selected because of the higher construction
costs compared with the selected route ($7 to 9 million vs. $4 million).
2.15
Taking into account of the proximity
of the fish culture zoneFCZ with an
inshore discharge, Option 2 would require a high level of treatment and an
outfall of length up to 250m. The key
concern of this option was the water quality impact on the FCZ in the Bay
during both construction and operation.
Also, the area of land required by the full secondary treatment plus
disinfection under this option would be constrained by the availability of
land. Reclamation might also be
necessary for the STW site, which was bounded by the FCZ to the north and the
Refuse Transfer Station (RTS) to the west.
The dredging activities of the reclamation would likely have adverse
water quality impact on the FCZ. Hence,
this option is not preferable.
2.16
Option 3 would require extensive
lengths of pipework linking the Sok Kwu Wan catchment to the Yung Shue Wan
sewerage system. A series of pumping
stations would be necessary to overcome the significant difference in
levels. The operating cost associated
with this option would render it uneconomical and not preferable.
2.17
The selected sewage collection system
under the SMP is shown in Figure 2.4.
Recommended Option Based on the SMP Study
2.18
This option basically follows the
recommendations in the SMP study and developed taking into account of the
updated population data and other new information during the adoptive review
and subsequent phases of the Project.
Figures 2.3 to 2.4 illustrate various detailed options (STW site
locations, outfall alignment, pumping station locations and sewer alignments
etc.) considered for this Project.
Also, alternative sewage treatment processes have been identified and
assessed before its selection.
Consideration of alternative sites for the pumping stations and the STW,
alternative sewage treatment process, outfall alignment and sewer alignment are
detailed in the following sections.
Alternative Sites for STW
2.19
New site for the STW was proposed in
view of the need to reclaim land under the SMP option. An alternative STW site which is located at
an abandoned quarry further east of the Refuse Transfer Station (RTS) was
considered (Figure 2.3). Apart from the
need for site clearance to remove the loose rocks at the foot of the rock face
behind the site and slope stabilization works, a substantial portion of the
site is readily available. Hence,
reclamation would not require for this alternative site.
2.20
Potential environmental impacts
associated with the construction of the STW at the Reclamation Site (recommended
in SMP study) were expected, in particularly water quality impacts. The increase of suspended solids and
turbidity in the waters during the dredging work of the reclamation would
likely cause adverse water quality impacts on the nearby FCZ. Nevertheless, the alternative site would
eliminate the potential environmental impacts associated with the reclamation
as well as constraints to the operation of the RTS. Hence, the alternative STW site is preferable.
Alternative Sewage Treatment Processes
2.21
Five alternative treatment process
options were considered during the Adoptive Review and are as follows:
·
Option i This
option comprises the treatment process outlined in the SMP (Imhoff tank,
anaerobic filter, extended aeration, disinfection).
·
Option ii Similar
to Option i, but using a new model rotating biological contactor (RBC).
·
Option iii This
option comprises a very low-loaded extended aeration tank with disinfection.
·
Option iv A
variation to Option iii, membrane separators are installed in the aeration tank
to provide final settlement.
·
Option v This
option comprises a sequencing batch reactor (SBR) with disinfection.
2.22
After a screening process, 3 options
(Options ii, iii and v) were to be evaluated in greater detail in the
preliminary investigation stage. The
recommended option will be adopted and developed to preliminary design
standard. Option i was not considered
further taking into account of the close proximity of fish culture zones and
the strength of sewage wastes generated by the Catchment Area. Option iv was considered to be not
preferable for its lack of local operating experience. The remoteness of the location also hindered
the adoption of such complex technology.
2.23
The options to be studied further in the evaluation
process were: Option ii - RBC, Option iii – Low-loaded Extended Aeration
System, and Option v – SBR. The
evaluation of the treatment process options were based on cost and non-monetary
considerations including environmental impact, process performance, land
requirements and process complexity.
Having considered the presence of the FCZ in the vicinity, a high level
of treatment (i.e. full secondary treatment with disinfection) was considered necessary. Hence, Option v – sequencing batch reactor
(SBR) was selected. In addition, the
treatment process would include nitrogen removal in view of the high existing total inorganic
nitrogen (TIN) level in this area.
2.1
The options to be
studied further in the evaluation process were: Option ii - RBC, Option iii –
Low-loaded Extended Aeration System, and Option v – SBR. The evaluation of the treatment process
options were based on cost and non-monetary considerations including
environmental impact, process performance, land requirements and process
complexity. Having considered the
presence of the FCZ in the vicinity, a high level of treatment (i.e. full
secondary treatment with disinfection) was considered necessary. Hence, Option v – sequencing batch reactor
(SBR) was selected. In addition, the
treatment process would include denitrification in view of the high existing
total inorganic nitrogen (TIN) level in this area.
2.24
The recommended design of the STW
comprises fine screening, grit removal, SBR units and UV disinfection. In addition, sludge treatment will require
facilities for waste activated sludge holding/digestion, sludge conditioning,
mechanical de-watering of conditioned sludge and odour control. Chemicals to be used on site include ferric chloride,
lime and polymer. Both ferric chloride
and polymer in liquid form will be delivered and stored on site in drums with
lime in bags. Please refer to Appendix
2.2 for details of the recommended treatment method.
Alternative Outfall Alignment
2.25
Various discharge locations of the
proposed outfall were assessed in the SMP study (Refer to Sections
2.10-2.17). The recommended location
was selected based on environmental and engineering considerations. In order to avoid the potential impacts, the
outfall has been shifted further to Mo Tat Wan. In addition, the length of the outfall has been extended to 750
765
m from 600 m as recommended in the SMP such that a higher safety margin
would be available for the pollutants to properly disperse before reaching the
possible inshore sensitive receivers, in particular the FCZ.
2.26
Based on the selected discharge
location, various alignments of the outfall pipe have also been assessed. The key constraints for selecting the
proposed alignment were found to be the Sok Kwu Wan fish culture zone and Mo
Tat Wan beach (see Figure 2.3). It
should be noted that the width of trench excavation for the pipe installation
is about 33 m to construct a 3-m deep rubble mound protection, if open
excavation is proposed. This
requirement has been taken into account when considering the clearance from the
sensitive receivers for water quality impact assessment. Alternative
construction methods such as trenchless technology have been considered and
proposed to mitigate the potential water quality impacts onto the FCZ and Mo
Tat Wan beach (see Section on Construction Methods).
2.27
We have also considered the
feasibility of moving the discharge location further outwards of the Bay beyond
Mo Tat Wan. This will be a deviation from
the SMP recommendation. Also, this
option is unnecessary from an environmental standpoint and not preferable from
the engineering viewpoints because it increases significantly the length of the
rising main and submarine outfall, and hence the associated capital and O&M
costs.
Alternative Sites for Pumping Stations
2.28
Taking into account of the
encroachment into private lot and the LCSD playground by the SMP recommended
options of P1 and P2 respectively, alternative sites were considered for the
pumping stations.
2.29
The new P2 is located adjacent to the existing
soccer field to the north of the LCSD playground. It is set against the foot of the hill facing the sea and remote
from any domestic dwelling. As the
recommended site is located in an unallocated government land, no land
resumption is envisaged.
2.30
An alternative to the SMP P1 option
was proposed and named as P1a. Located
to the north of the SMP option, the P1a site is located in an unallocated
government land and therefore clear of any need for land resumption.
2.31
A new pumping station, P1b was introduced
to the west of Chung Mei stream. This
pumping station facilitates the conveyance of flow across the stream towards
the main Catchment Area en route to the STW for treatment.
2.32
In comparison with the SMP sites for
the pumping stations, the alternative sites are located further away from the
nearby sensitive receivers. Potential
environmental nuisances in terms of air quality and noise during construction
and operation of the pumping stations at the alternative sites would be less
significant compared to the SMP option.
2.33
Having considered the engineering and
environmental aspects, the alternative sites for the pumping stations were
considered preferable.
Alternative Sewer Alignment
2.34
Slight amendments have been made
to the SMP sewer alignment when compared with the Alternative alternative optionalignment. The major difference lies in the routing of
the trunk sewers in Sok Kwu Wan Village.
The new alignment runs along much of Sok Kwu Wan First Street with the
northern section of the trunk sewers following Sok Kwu Wan Second Street. The trunk sewers are located in Sok Kwu Wan
Second Street in the SMP alignment.
2.35
In comparison, Sok Kwu Wan Second
Street is much narrower than Sok Kwu Wan First Street and is also congested
with underground utilities. Working
areas of restaurants kitchens and shops are extended improperly into the street
whilst canopies of low headroom extended into the street are commonly
found. These site constraints have
rendered the SMP option unfavourable when compared with the Alternative alternative alignment.
Quarry Site Option
2.36
In view of the presence
of the FCZ near northern coast of Sok Kwu Wan, a Quarry Site Option which is to
transfer sewage flows from Sok Kwu Wan village to the North north East east Lamma Quarry Site (Potential
site for Harbour Area Treatment Scheme (HATS) facility) via a land based pumping main around Picnic
Bay has been considered based on the Outlying Island SMP Stage 2 Review – Final
Report (the review report) and other current available information and (Figure xx). If
the treatment plant were to be located at the Lamma Quarry site, the sewerage
conceptual design of flows draining from west to east within Sok Kwu Wan
village as recommended in the SMP study would be revised to direct village
flows from east
to west. Alternative pipe route across
the Picnic Bay may cut across the Fish Culture ZoneFCZ and is also cost prohibitive. The pipe route was not considered further.
Comparison between
Recommended Option and Quarry Site Option
Recommended Option
2.37
This option as described in the
above sections involves the
construction of 3 pumping stations (P1a, P1b and P2) and a STW. This option would transfer sewage flows from
the west of Sok Kwu Wan village to the STW located at a disused quarry to the east of Sok Kwu Wan
village.
2.38
This option would
require the construction of about 1.4 km sewerage pipeline along the south
coast of Sok Kwu Wan to the submarine outfall located to the west of Mo TakMo Tat Wan.
In comparison with the Quarry Site Option, shorter land pipeline would be required for
this option and therefore would reduce the potential risk of untreated sewage
leakage and the disturbance of vegetated areas due to the land acquisition
required for the pipeline reserve.
Since the pipeline on land
would generally be built along the existing footpath, the pipeline would be
relatively accessible.
2.39
The construction of the
sewerage pipeline would have the potential to induce air quality and noise
impacts on the sensitive receivers in Sok Kwu Wan village and island visitors. No air quality and noise impacts on the
sensitive receivers in Lo So Shing would be expected as they are located far
away from the worksites of this option.
Since no known archaeological site is identified within the worksites, potential direct impacts on archaeological resources would not
be expected.
2.40
Potential water quality
from emergency discharge of sewage at the proposed Pumping Station P1a and P1b
would be expected. Since the Pumping
Station P2 and STW are located in close
proximity to the gazetted FCZ, the engineering design would exclude the
emergency discharges at Pumping Station P2 and the STW. Hence, emergency discharges at these two
locations would not be allowed.
2.41
A long submarine
outfall is required for this option
in order to discharge the treated sewage to the East Lamma Channel. Owing to the proximity of the submarine
outfall pipe to the FCZ, construction of the outfall would
have potential water quality impact on the FCZ if open trench method were wasadopted. As confirmed, the construction of the
section of outfall pipeline near shore would use Horizontal Directional
Drilling (HDD) method that is a no-dig method in order to
reduce the impacts in relation to the submarine outfall construction.
Quarry Site Option
2.42
This option involves
the construction of at least two pumping stations (P1a and P1b) and a STW.
This option would transfer sewage flows from the west of Sok Kwu Wan
village to the STW located at a
disused quarry to the east of Sok Kwu Wan village. STW located at the
Lamma Quarry site.
2.43
It was estimated that
the total length of the sewerage pipeline for this option would belikely be double that
of the Recommended Option more than 6 km, which
is significantly longer than that proposed in the recommended option. The
substantial increase in the pipeline length would increase the potential risk
of untreated sewage leakage due to pipe bursting. In addition, due to the longer pipeline, larger vegetated areas
along the coast would likely be affected during
the construction of the pipeline. Also,
there would be considerable increase in energy requirement and O&M costs
due to the increased hydraulic head to overcome the pipe frictions along the
additional pipe lengths.
2.44
The section of pipeline
between Lo So
Shing and the Comprehensive Development Area (CDA) to the west of the Lamma
Quarry site would be along a very rocky section of coastline and hence access
for pipeline construction would be difficult and the main may need to be above
surface level unless
considerable rock excavation is undertaken.
Considerable rock excavation would likely give rise to adverse air
quality and noise impact to the nearby noise sensitive receivers at Lo So Shing
and island visitors. As a section of
the pipeline would likely
pass through the proposed CDA, it would be difficult to access this section of
pipeline for maintenance in the future.
2.45
Part of the pipeline
would be within the Lo So Shing Archaeological site, which is a known
archaeological site. Potential impact on the existing cultural heritagearchaeological resources would be expected during the
construction phase.
2.46
This option would
eliminate the need for Pumping Station P2 and would locate the STW at the Lamma
Quarry Site farther away from the FCZ compared to the STW site in the recommended
option. Nevertheless, similar to the
Recommended Option, twoPumping pumping Stations stations P1a and P1b would be required at the west end of Sok Kwu Wan (located at around 200 m not greater than 250m from the FCZ)) would
still be required for pumping sewage from Sok Kwu Wan from east to west and
around the bay to the STW at the Lamma Qquarry site.
A pumping station
(similar to P1b) would be required to pump the sewage collected at the eastern
side of Chung Mei Village
across the river and a pumping station (similar to P1a) would be required to
convey the collected sewage collected at Chung Mei Village to the alternative site
of the STW. Hence, potential water quality impact from
emergency discharge on the nearby FCZ would
still be expected.
2.47
This option may
eliminate the need for constructing a long submarine outfall in order to
discharge the treated sewage to the East Lamma Channel.
and the location of the
outfall would be located farther away from the FCZ compared to the Recommended
Option. Hence smaller water quality
impact on the nearby FCZ during construction phase would be expectedNevertheless, potential
water quality would be expected if open trench methodwas adopted.
Table 2.1 A Summary of
the Comparison Between the Recommended Option and Quarry Site Option
|
Recommended Option
|
Quarry Site Option
|
Engineering
Constraints
|
Pipeline
on Land
|
Approximately 1.4
km
|
> 6km
Probably
double
the length of that in Recommended Option
|
Submarine
Outfall
|
Required a long outfall of
about 750 m
|
Shorter lengths may be
sufficient.
|
Maintenance
|
Pipeline on land would be
relatively accessible.
|
Pipeline passing through
the CDA would likely be difficult to be accessed. Drainage reserve may be necessary.
|
Capital
Costs
|
Moderate
|
High
|
O&M
Costs
|
Moderate
|
High
|
Environmental Issues
|
|
Air
Quality
|
Construction dust impact
on receivers at Sok Kwu Wan village.
Odour emissions during
operation from pumping stations (P1a, P1b and P2) and STW.
|
Construction dust impact
on receivers at Sok Kwu Wan village and Lo So Shing.
Odour emissions during
operation from pumping stations (P1a and P1b) and STW.
|
Noise
|
Construction noise impacts
on Sok Kwu Wan village.
Operational noise impact
from pumping station (P1a, P1b and P2).
|
Construction noise impacts
on Sok Kwu Wan village and Lo So Shing.
Operational noise impact
from pumping station (P1a and P1b).
|
Water
Quality
|
Potential water quality
impact associated with dredging on the FCZ during the construction of the
submarine outfall pipeline off shore.
Inshore section of the outfall pipeline would be constructed using HDD
method and no dredging required for this section. With mitigation measures in place, potential water impact from
dredging would be mitigated to comply with WQO. Water quality impacts to the FCZ during construction phase
would be similar to the Quarry Site Option.
Potential water quality
impact to the FCZ from the discharge of treated sewage at a long submarine
outfall would not be expected during operation. WQO would likely be complied.
Similar impacts would be expected compared to the Quarry Site Option.
Emergency discharges would likely take
place at Pumping Stations P1a and P1b (the engineering design would not allow
emergency discharge at Pumping Station P2 and the STW). With the exclusion of emergency discharges
at Pumping Station P2 and STW, similar impacts would be expected compared to
the Quarry Site Option
Smaller risk of untreated
sewage leakage due to shorter pipeline (around 1.4 km) on
land.
|
Water quality impact from
dredging would be expected for the construction of submarine outfall. HDD method may be deployed to avoid
dredging operation. With mitigation
measures in place, potential water impact from dredging would be mitigated to
comply with WQO.
Potential water quality
impact on the
FCZ from the discharge of treated sewage at a shorter outfall would not be
expected during operation. WQO would
likely be complied.
Emergency discharges would likely take place at Pumping StationsP1a,
P1b and STW at the Lamma Quarry Site.
Greater risk of untreated
sewage leakage due to longer pipeline (> 6
km) on land.
|
Culture
Heritage
|
No direct impact on known
archaeological site.
|
Direct impact on Lo So
Shing Archaeological Site.
|
Waste
Management
|
Smaller amount of
excavated waste would likely be generated due to shorter pipeline.
Marine sediment would be
generated during dredging activities.
With the use of HDD, the amount of marine sediment from dredging would
be minimal.
|
Larger amount of excavated
waste would likely be generated due to longer pipeline.
Marine sediment would be
generated during dredging activities.
If HDD is adopted, the amount of marine sediment from dredging will be
minimal.
|
Ecology
|
Smaller vegetated
areas would likely be affected due to shorter pipeline.
Minor coastal area
would be affected during submarine outfall construction with the use of HDD.
|
Larger vegetated
areas would likely be affected due to longer pipeline.
Minor coastal area
would likely be affected during construction of the pipeline along the
coastline of the bay and the submarine outfall.
|
Landscape and
Visual
|
No significant landscape and
visual impact would be expected, except at locations ofthe pumping stations
and the STW.
|
No significant landscape
and visual impact would be expected, except at locations of the pumping stations
and the STW.
|
2.48
Based on the
environmental considerations, both the Recommended Option and the Quarry Site
Option would produce similar impacts on air quality, water quality, noise,and waste management and landscape and visual. The
recommended option is considered more acceptable in terms of noise, cultural heritage and ecological impacts. With respect to engineering constraints, the
Recommended Option is more preferable
due to shorter pipeline,the ssmaller risk of pipe bursting as a result of shorter
pipeline, more accessible for
maintenance and smaller capital and O&M costs. Overall, the Recommended Option is considered more preferable in
view of both the environmental
impacts and smaller engineering aspects.
Construction Methods
2.49
The Project consists of three pumping
stations, a STW, sewer network connecting village houses and restaurants, and a
submarine sewage outfall. The sewerage
layout has made full use of the road and footpath network between and within
the villages for the sewer main installations, abandoned quarry for the STW,
and open areas for the pumping stations.
Submarine Outfall
2.50
A key factor for deciding the
appropriate outfall pipe installation method is its potential impact on the fish
culture zonesFCZ and Mo Tat Wan Beach, in addition to the
cost and engineering considerations.
It is important to ensure that the potential water quality impacts, if
any will be controlled to the acceptable levels.
2.51
Owing to the close proximity of the
outfall alignment to the nearby gazetted FCZ, there are concerns regarding the
possible adverse impacts due to the dredging activities in particular the near
shore sections. No-dig method using
Horizontal Directional Drilling (HDD) has been proposed for the main outfall
pipe installation
near shore (approximately 480m from shore) to avoid the potential water
quality impacts arising from the dredging operation. The construction details of the HDD method are provided in
Appendix 2.1.
2.52
The offshore outfall pipe of approximately 240m and installation
of vertical riser pipes in the outfall diffuser zone will require open trench
dredging. No-dig pipe installation
method for this section will not be applicable. Adequate mitigation measures such as silt curtains and closed
dredging grabs will be adopted to minimize water pollution. Since the diffuser zonedredging area is would be about
300 m from the FCZ and 500 m from the secondary contact recreation subzone , the
potential water quality impact has been checked to be acceptable (see Section
on Water Quality).
Sewer Construction
2.53
The sewers, including trunk sewers and
branch sewers along village alleyways, will be installed by open
trenching. Owing to the limited space
in the alleyways and the congested utility services, installation
of the branch sewers will be carried out using hand-held tools. The construction of the trunk sewers will
use powered mechanical equipment and will involve the following activities:
·
Pavement breaking;
·
Installation of pipes and bedding;
·
Backfilling of soil materials; and
·
Reinstating road surface.
Pumping Station and STW
2.54
The construction of the pumping
stations and STW will involve the following activities:
Foundation
·
Excavation of soil material;
·
Formwork, reinforcement and fixing for underground
structure;
·
Concreting; and
·
Backfilling
Superstructural Works
·
Formwork, reinforcement and fixing for above ground
structure and
·
Concreting.
Works Programme
2.55
The key dates of the implementation
programme of this Project are as below:
Detailed
Design
|
September 2003 to June 2004
|
Tendering
|
July 2004 to
November 2004
|
Construction
|
December 2004 to
August 2007
|
Long-Term Treatment and Disposal Methods
2.56
The proposed facilities under this
Project have been reviewed in the recent Outlying Islands Sewerage Master Plan
SMP Stage 2 Review Study. The Review
Project was to review the adequacy of the existing and planned sewerage
improvement works recommended under the original SMP based on the latest
population and planning information.
The main recommendation under this SMP Review for Sok Kwu Wan was to
implement the proposed works under this Project as soon as possible. There was no recommended change to the
facilities to be implemented under this Project.
2.57
Potential long-term treatment and
disposal options for Sok Kwu Wan were evaluated in respect of the eventual
connection of flows to the Harbour Area Treatment SchemeHATS. The
options considered under the SMP Review Study are depicted in Figure 2.6. The implementation of this long-term method
will depend on the final selection of HATS options.
Related Projects
2.58
The Refuse Transfer Station (RTS) at
Sok Kwu Wan is currently in operation and would be involved during the
operation of the proposed STW. Both
sludge disposal options have recommended that the sludge generated from the
future STW be delivered to the RTS for transfer to Yung Shue Wan STW for
dewatering or subsequent disposal to landfill.
Public Consultations
2.59
Consultations with the South Lamma
Rural Committee, Lamma Area Committee and Island District Council were carried
out on 7th February 2002, 27th February 2002 and 8th
April 2002 respectively. The
consultations were successful in obtaining the support of the district council
and various committees. There were
concerns for possible impacts on the fish culture zone during construction of
the submarine outfall, and other environmental impacts including
air and noise nuisances
during the construction of the sewer alignment, visual impacts associated
with the pumping stations and sewage treatment works. Details of the
consultations are provided in Appendix 2.3.