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.