3.        Environmental Impact Assessment

 

Introduction

 

3.1.       The EIA report has provided an assessment of the potential environmental impacts associated with the construction and operation of the Project.  The study has been undertaken in accordance with the Technical Memorandum on Environmental Impact Assessment Process (EIAO-TM) and the Study Brief.  The following key areas were addressed in the EIA Study:

 

·        Air quality impacts

·        Noise impacts

·        Water quality impacts

·        Sediment contamination

·        Solid waste

·        Ecological impacts

·        Fisheries impacts

·        Visual and landscape impacts

 

3.2.       The findings of the assessments for the above environmental issues are summarised in the following sections.  Cultural heritage aspects of the Project were addressed separately from the EIA Report. 

 

Air Quality

 

3.3.       Representative air sensitive receivers (ASRs) were identified in the EIA Study and are shown in Figure 3.1. 

 

3.4.       During construction phase, no adverse construction dust impact would be expected at all ASRs in the vicinity of the study area, except at ASR A24 (football court located next to the proposed Pumping Station P2).  In order to achieve the air quality objectives (AQO), it is recommended to install 2m high solid fences around the construction site of Pumping Station P2.  With the implementation of the mitigation measure, no exceedance of 1-hour TSP guideline level and 24-hour TSP AQO limit would be anticipated at the ASRs.  Notwithstanding this, the implementation of the Air Pollution Control (Construction Dust) Regulation and the inclusion of good site practices were also recommended.  

 

3.5.       During operation phase, all pumping stations, sequence batching reactor (SBR) feed pump station and sludge dewatering room would be all enclosed and the outlet air from these facilities would be properly treated by deodorization facilities with 99.5% odour removal efficiency before discharge via vent pipes.  With the implementation of all these measures, the predicted odour concentration at the representation ASRs would comply with the EIAO-TM criteria.  Hence, no unacceptable impacts were predicted.

 

Noise

 

3.6.       Representative noise sensitive receivers (NSRs) were identified and are shown in Figure 3.2. Construction noise impacts from the construction activities such as sewer alignment construction, pumping stations and STW construction, and the operation noise impacts from the pumping stations were identified as key environmental issues for this Project. The potential noise impacts arising from the Project have been assessed and evaluated.

 

3.7.       During the construction phase, the unmitigated noise levels at most of the NSRs would experience noise level exceeding the noise criteria stated in the EIAO-TM.  To alleviate the noise impact, noise mitigation measures including the adoption of good site practices, quieter equipment and restriction on the number of plant operating as well as other possible mitigation measures including erecting of noise screening structures and manual working were recommended.  With the recommended mitigation measures, no exceedance of the noise criteria was predicted at all NSRs, except short-term residual impact during sewer construction.  Having considered that the construction of the sewer is unavoidable as it is an integral element of the Project and the short duration of the sensitive receivers experiencing the impacts, the residual impacts were considered acceptable.

3.8.       During operation phase, no adverse noise impact arising from the operation of the pumping stations are predicted, except Pumping Station P1a.  To alleviate the noise impact, it is recommended to install a silencer at air discharge point of the deodouriser at Pumping Station P1a.   With the recommended mitigation measures applied, no exceedance of both daytime and nighttime noise criterion would be anticipated.

 

Water Quality

 

Construction Phase

 

3.9.       To minimize the potential water quality impacts from the submarine outfall construction, the Horizontal Directional Drilling (HDD) technique would be adopted.  By using the HDD technique, only minor dredging works would be required at a location of around 480m from the coast. 

 

3.10.   The sensitive receivers identified within the Study Area include the gazetted fish culture zone (FCZ) at Picnic Bay and secondary contact recreation subzone at Mo Tat Wan, which are located more than 300 m and 500 m from the proposed dredging works respectively.  With the use of closed grab dredger, 2-layer silt curtains and reduction in dredging rate, the elevations in suspended sediment (SS) concentration generated from the dredging activities at these sensitive receivers would comply with the Water Quality Objective (WQO). 

 

3.11.   The potential water quality impact arising from the release of contaminants from sediment disturbed during the dredging works was assessed.  It was predicted that, because of the low level of sediment contamination in the dredging area, adverse impacts arising from the release of heavy metals, nutrients, and organic compounds would not be expected. 

 

Operation Phase

 

3.12.   Potential impacts in relation to the discharge of treated effluent from the outfall for the wet and dry season have been assessed using USEPA CORMIX2 model.  The modelling results indicate that the predicted concentrations of SS, E. coli, and unionised ammonia would satisfy the Water Quality Objectives (WQOs), while the predicted total inorganic nitrogen (TIN) levels would exceed the WQO.  The exceedances of WQO for TIN were attributable to the high background TIN levels.  The background level for dry season had already exceeded the limit by more than twice.  Owing to the high background TIN level, exceedance of the WQO limit for TIN would be unavoidable even with a very small contribution from the treated effluent discharge at the submarine outfall.  According to the EPD monitoring data, the predicted TIN levels would fall within the background variations of TIN concentrations at Picnic Bay.    Furthermore, the prediction were considered to be over-estimated as they were predicted by simply adding the elevated TIN levels from the effluent to the background levels and were not taken account of the beneficial effect of eliminating untreated or partially treated sewage discharges to the Picnic Bay upon the operation of the Project.  The denitrification process in the STW would remove more than 50% of TIN from the sewage and the treated effluent would be discharged at a location of more than 560m from the FCZ.  Hence, the operation of the Project would not induce a water quality impact worse than that without the implementation of the Project.  In fact, the proposed project would improve the water quality of the Picnic Bay by eliminating the discharge of untreated or partially treated sewage into the coastal area of the bay. 

3.13.   The potential impact from emergency discharge due to the failure operation of the pumping stations and the STW has also been assessed using the USEPA CORMIX3 model.  In order to minimize the occurrence of emergency discharge as far as possible, the following mitigation measures and contingencies were recommended:

 

·        Standby pump at all pumping stations and the STW in case of duty pump failure;

·        Standby generator at all pumping stations in case of interruption of electrical power supply;

·        24-hour temporary storage for all pumping stations in emergency;

·        Use of Sequencing Batch Reactor (SBR) units as a storage tank in case of STW failure;

·        No emergency discharge is allowed at Pumping Station P2 and the STW;

·        Automatically shutdown the pumping station at Lo So Shing in case of Pumping Station P1a failure;

·        Automatically shutdown the upstream pumping stations (including Pumping Station P1a and Lo So Shing Pumping Station) in case of Pumping Station P1b failure;

·        Automatically shutdown the upstream pumping stations (including Pumping Stations P1a and P1b, and Lo So Shing Pumping Station) in case of Pumping Station P2 failure;

·        Automatically shutdown all pumping stations (including Pumping Stations P1a, P1b and P2, and Lo So Shing Pumping Station)in case of STW failure;

·        Implement a telemetry system to ensure prompt action to be undertaken in an emergency occasion.

 

3.14.   With the implementation of the above measures, an emergency overflow would be unlikely to occur and the potential water quality impacts would be minimized in the unlikely event that an overflow does occur.  Compared with the long-termed improvement of water quality of Picnic Bay from the Project and given the recommended measures minimise the possibility of the emergency discharge, the residual impact would be considered acceptable.

 

Sediment Contamination

 

3.15.   Based on the preliminary design, the total volume of dredged material was estimated to be approximately 26,000 m3.  A review of previous field investigation data on marine sediment quality reveals that Category H contaminated mud may be present within the boundary of the study area.  However, the available sediment data were of limited extent and collected about eight years ago, and hence could not fully represent the existing sediment quality of the proposed dredged areas of the present Project.  A preliminary sediment contamination survey was therefore conducted to provide more accurate and updated information on the baseline sediment quality of the proposed dredged areas such that recommendations on the appropriate disposal requirements of the dredged material can be made.  The findings of the sediment contamination survey indicate that the marine sediments to be dredged for the submarine outfall were classified as Category L.  The dredged sediment would therefore be suitable for open sea disposal. 

 

3.16.   The dredging works should be carried out in a controlled manner such that release of sediments into the marine environment would be minimized.  With the implementation of the recommended mitigation measures, residual water quality impacts would not be anticipated.

 

Solid Waste Management

 

3.17.   Wastes generated by the construction works would include workforce wastes, maintenance and chemical wastes, and construction and demolition material.  Provided that the identified waste arisings are handled, transported and disposed of using approved methods and that the recommended good site practices are adhered to, adverse environmental impacts would not be expected.

 

3.18.   Wastes generated during normal STW operations would include inlet screenings, grit from detritors and dewatered sludge.  It has been proposed that the sewage sludge generated from the STW would be dewatered on-site and the sludge cake would be delivered to Sok Kwu Wan refuse transfer station for disposal to nearby landfill site. 

 

Ecological Impact

 

3.19.   Construction and operation of the sewage treatment project in Sok Kwu Wan at Lamma might affect the terrestrial and marine environments. Based on site investigations there appeared to be no insurmountable ecological impacts arising from this project. The overall impacts of project construction and operation on ecology were predicted to be minor.

 

3.20.   Based on the nature of the project and the baseline conditions of the study area, the impacts of project construction and operation on terrestrial ecology were predicted to be minor. A small moist area of shrubland/grassland habitat near Pumping Station P2 which is used by Romer’s Tree Frog and thus of moderate to high ecological value would not be affected. Nevertheless, additional measures to control site runoff for protecting the tree frog site were proposed on a precautionary basis.Impacts on intertidal habitats would not occur on the sand flat but in a limited area on the rocky shore and thus were also predicted to be minor.  Hard corals of low ecological value (widely distributed in HK, small in size and having low coral cover (<5%)) were recorded in the shallow waters of the bay.  HDD technique would be used to avoid direct loss of coral colonies.  Construction of a submarine outfall would not involve any disturbance of the seabed until at a location of around 480m from coastline. The seabed loss area is of muddy substrate only.  No loss of seabed, which is available for hard coral colonization, would be constituted.  No hard corals would be directly impacted by the HDD or the dredging work, while silt curtains around the dredging site, use of closed grab dredger and reduction in dredging rate could prevent indirect impacts on corals.

 

3.21.   Based on results of water quality modelling, during the operational phase, discharge of the effluent would not adversely affect water quality in the Study Area. Hence no adverse impacts to marine ecology inducing coral communities in Sok Kwu Wan would be anticipated.

 

3.22.   Water quality impact arising from the emergency discharge of the pumping stations and STW are anticipated.  With the implementation of the mitigation measures as described in Water Quality Impact Assessment, the possibility of an emergency overflow occurring would and the potential water quality impacts would be minimized.

 

3.23.   With the implementation of the mitigation measures recommended, the impacts of project construction and operation on ecology are predicted to remain within acceptable bounds.  Due to the limited scale of the Project, low ecological value of the fauna and flora recorded, and the minor degree of impacts caused, no ecological monitoring and audit is proposed.

 

Fisheries Impact

 

3.24.   Baseline information on the cultured and capture fisheries resources and operations within the Study Area were gathered through desktop study and interviews with mariculturists.  These indicate that Sok Kwu Wan has a capture fishery of above average production for Hong Kong and is a nursery area for some commercially important species.  The most important and sensitive fisheries resource in Sok Kwu Wan is the gazetted Fish Culture Zone.  

 

3.25.   The potential impacts to fisheries sensitive receivers during the construction phase were identified as being the release of suspended sediment and release of ammonia associated with the dredging works required for the construction of about 240 m outfall pipeline and the diffuser section. These dredging works would be conducted no closer than 300m from the FCZ. With reference to Water Quality Impact Assessment in Section 5 of the EIA report, it was determined that the release of ammonia from sediments during the construction phase would comply with the WQO and would not have any toxic impact to fish in the Study Area.  In order to ensure environmental acceptability, it was determined that mitigation measures to prevent sediment plumes reaching the FCZ would be required and are the same as those recommended for water quality impacts. Potential impacts due to construction of the outfall pipeline would be reduced through the adoption of the HDD construction method. 

 

3.26.   The impact on fisheries sensitive receivers in the Study Area was also evaluated for the operational phase using water quality modelling results.  Based on these predictions, discharge of the effluent would not adversely affect water quality in the Study Area and hence no adverse impacts to fisheries sensitive receivers would be anticipated.

 

3.27.   The small increase in TIN levels owing to effluent discharge was identified as deserving special attention owing to its high background levels (often exceeding WQO) and the importance of nitrogen in mediating algal growth (and hence the potential for red tide formation).  However, according to the EPD monitoring data, the predicted TIN levels would fall within the background variations of TIN concentrations at Picnic Bay.  In addition, the proposed project would improve the water quality of the Picnic Bay by eliminating the discharge of untreated or partially treated sewage into the coastal area of the bay.  It is anticipated that the small contribution to TIN levels by effluent discharge would not significantly enhance algal growth, and the potential impact for inducing red tide in Sok Kwu Wan would not anticipated.

 

3.28.   Emergency discharge from the pumping station or STW would have the potential to lead to adverse impacts on the FCZ.   With the implementation of the mitigation measures as described in Water Quality Impact Assessment, the possibility of an emergency overflow occurring and the potential water quality impacts would be minimized. 

 

3.29.   In conclusion, with the implementation of the recommended mitigation measures, it would be expected that the implementation of the Project at Sok Kwu Wan would be environmentally acceptable for the fisheries perspective. 

 

Landscape and Visual Impacts

Landscape Impact Assessment

 

3.30.   In general, the Project would cause slight to moderate adverse landscape impact in the construction phase and negligible to slight/moderate adverse landscape impact in operation phase before the implementation of mitigation measures.  The impacts would be localised at the pipeline alignment along First Street, the pumping stations and STW.  They are small in scale and thus they will not cause serious landscape impact.  The following table shows the dimension of the pumping stations and STW above ground level.

 

 

Approximate Dimension (m)

Site Area

 

Length

Width

Height*

 

P1A

 

202.5m2

Pumping Station

11.85

4.0

300mm

14.22m2

Deodourizer

3

1

3

9m2

Standby Generator

5

4.5

3.5

70m2

Control Kiosk

2.6

1.5

1.5

5.85m2

P1B

 

241 m2

Pumping Station

11.85

4.0

300mm

14.22m2

Deodourizer

3

1

3

9m2

Standby Generator

5

4.5

3.5

78.75m2

Control Kiosk

2.6

1.5

1.5

5.85m2

P2

 

310 m2

Pumping Station

11.85

4.0

300mm

14.22m2

Deodourizer

3.5

1.5

3.2

9m2

Standby Generator

5

6

3.5

105m2

Control Kiosk

2.6

1.5

1.5

5.85m2

STW

Refer to Figure 10.13 in the EIA Report for layout

Refer to Figure 10.13 in the EIA Report for layout

7.0m max.

1529 m2

Note: *         Height above ground level

 

3.31.   Vegetation loss would be small - 1% (2283 m2) of the study area.  Mitigation measures at construction phase include minimise damage and protect existing vegetation, conservation of topsoil for reuse, careful transplant of the identified tree, short excavation sections and immediate back filling upon completion, and compensatory planting would alleviate the adverse impact to negligible to slight adverse.  However, at the Exposed Coastline area, the presence of the sea-borne construction traffic at the outfall site would disturb the natural context of the coastline, the landscape impact would be relatively high - slight/moderate adverse.  Other than the new ancillary buildings (STW, 3 nos. pumping stations), the impacts at construction phase would be reversible.  Mitigation measures in the operation phase include architectural and soft landscape finishes to the ancillary buildings and planting to reformed slope to establish a coherent context within the overall landscape.

 

Visual Impact Assessment

 

3.32.   In general, the major visual impact is during construction where the sewer is at parts adjacent to residential units and to the waterfront restaurants, which leads to slight/moderate to moderate adverse visual impact.  Mo Tat Wan would have a moderate adverse impact during construction since it is close to the location of the outfall affecting its tranquil quality.  If night lighting is required for the marine fleets near Mo Tat Wan, the light source should be directed away from the residential units.  Mitigation measures in construction phase include short excavation sites and immediate backfilling upon completion of sections, minimise damage to vegetation along footpath, and use of site hoarding appropriate to the natural site context to screen trenches.  Mitigation measures in operation phase include architectural and soft finishes to the pumping stations and STW appropriate to its site context, planting to reformed slope behind the STW, and appropriate paint finishes to the exposed section of pipeline.  Upon implementation of the mitigation measures, the visual impact in operation phase would be reduced to negligible to slight.

 

Evaluation of the Project

 

3.33.   The major adverse landscape and visual impacts occur in the construction phase, and these impacts are reversible with the completion of construction.  Since the major length of the pipeline alignment would be underground and the scale of the ancillary buildings would be relatively small, the landscape and visual impact in operation would be limited. 

 

3.34.   Overall in the context of Annex 10 of the EIAO-TM, the landscape and visual impacts would be acceptable with mitigation measures.