Major potential air quality impacts during the construction phase of the project would result from dust arising from site clearance and preparation, excavation and filling, open site erosion, construction of foundation and superstructure, and handling and transportation of construction and demolition material. There may also be cumulative dust impacts due to the concurrent construction activities of the proposed Deep Bay Link and HSKNDA in the area.
Practicable and effective dust suppression measures should be implemented to minimise the dust nuisance arising from the construction activities. Specific measures recommended in the EIA include twice daily watering with complete coverage of active construction sites and limit the vehicle travel speed on unpaved site areas to no more than 10 km per hour. Besides, relevant dust control requirements set out in Parts I, III and IV of Schedule 1 of the Air Pollution Control (Construction Dust) Regulation should be adopted by the site agent while carrying out construction works.
With the implementation of effective dust control measures, adverse dust impacts from the construction works of the project is not expected. The highest predicted cumulative dust levels at the air sensitive receivers near the San Wai STW and Ha Tsuen PS are all below 87% of the Air Quality Objectives for total suspended particulates under the mitigated scenario.
During the operational phase, major air quality impact would be related to the potential odour emissions from the expanded and upgraded San Wai STW and the expanded Ha Tsuen PS. Odour mitigation measures have been recommended in the EIA. For the San Wai STW, the odour sources namely the fine screen, the detritors, the sludge holding tanks, the solid handling house, the sludge dewatering house, the rapid mix and flocculation tank, and the primary sedimentation tanks should all be contained within either building structures or enclosure to minimise direct emission of odour to the atmosphere. All odour emissions from the odour sources should be ventilated to a deodorization unit with a hydrogen sulphide removal efficiency of 96% or better before discharging to the atmosphere.
For the Ha Tsuen PS, it is recommended that the air ventilated from the existing and the new wet wells should be treated by deodorisers with a hydrogen sulphide removal efficiency of 97% or better before discharge to the atmosphere.
The computer dispersion modelling results showed that the odour levels at the air sensitive receivers around the San Wai STW and Ha Tsuen PS could be reduced to below the EPD odour criteria of 5 odour unit with the implementation of the recommended odour control measures.
The construction noise impacts of this project will be associated with the expansion and upgrading of the existing San Wai STW, the expansion of the existing Ha Tsuen PS, and the construction of the emergency bypass culvert. There may also be cumulative construction noise impacts due to the concurrent construction activities of the proposed Deep Bay Link and HSKNDA in the area.
The identified existing and future noise sensitive receivers
are located at considerable distance from the construction sites of the San Wai
STW and Ha Tsuen PS. The predicted construction noise levels at the noise
sensitive receivers are all within the construction noise criteria with the use
of quiet plant and/or temporary noise barriers. Adverse residual noise impacts
are not expected.
During the operational phase of the expanded and upgraded San Wai STW and the
expanded Ha Tsuen PS, it is recommended to house all noisy equipment underground
or within building structure made of material with a sound transmission loss of
30 dB(A) or better (e.g. concrete with thickness of 300mm) to enclose all noise
emissions. Adverse residual noise impacts at nearby noise sensitive receivers
are not expected.
Construction activities related to this project namely excavation, filling, and construction of superstructure would generate site runoff. The nature of site runoff is usually characterised by high concentration of suspended solids. Suitable mitigation measures as recommended in ProPECC PN 1/94 Construction Site Drainage need to be undertaken. Site drainage systems and treatment facilities would be required to minimise water pollution. A discharge licence issued by the Environmental Protection Department (EPD) would be required for discharging effluent from the construction sites.
Through effective implementation of the practicable mitigation measures, the water quality impacts associated with the construction activities of this project would be reduced to acceptable levels.
The upgrading and expansion of San Wai STW would enhance the treatment efficiency and increase the capacity of the existing facilities. Irrespective of the final selection of the treatment option, the effluent quality from the upgraded treatment works would be improved as compared with the current preliminary treatment process. The residual impacts are expected to the low.
As discussed in Section 2.1 above, CEPT with disinfection is
identified as the preferred treatment option for the expanded and upgraded San
Wai STW. Due to the dispersive characteristics of tidal flows in the vicinity of
the NWNT Outfall, the water quality modelling results showed that discharge of
effluent after CEPT treatment is not likely to cause adverse water quality
impacts in the North Western and Deep Bay Water Control Zones. In addition, the
selection of CEPT with disinfection rather than biological treatment options
would not jeopardise the compliance of the relevant WQOs for DO, UIA and TIN.
The disinfection process would also minimise the impact to the aquatic
environment in terms of spreading of faecal coliform and satisfy the
precautionary requirement for protection of dolphins.
The EIA has also assessed the water quality impacts for the following emergency
discharge scenarios:
Ø Discharge of treated effluent via Tin Shui Wai Drainage
Channel into Deep Bay if NWNT Outfall/Tunnel is not in operation;
Ø Discharge of raw sewage into Deep Bay via Tin Shui Wai Drainage Channel when
both San Wai STW and NWNT Outfall/Tunnel are not in operation or during the
breakdown of Ha Tsuen PS; and
Ø Discharge of raw sewage via the NWNT Outfall in case the San Wai STW is not
functioning.
For the first and the second emergency discharge scenarios, the modelling results showed that the increases in E.coli, total inorganic nitrogen (TIN) and unionised ammonia (UIA) would cause a short-term deterioration of the water quality conditions in the inner part of Deep Bay during the emergency discharge period. The impacts are apparently related to the duration of emergency discharge period. The water quality conditions would quickly recover after the termination of emergency discharge. The potential impacts to the water quality sensitive receivers located further away from the discharge location would be less significant.
The emergency discharge at NWNT Outfall under the third emergency discharge scenario would cause lower water quality impacts when compared to the discharge through Tin Shui Wai Drainage Channel into Deep Bay.
3.4 Waste Management Implications
The construction activities of this project including the site formation and foundation works at the proposed expanded and upgraded San Wai STW and the expanded Ha Tsuen PS will generate construction and demolition (C&D) material. The construction of the proposed emergency bypass culvert will be carried out by traditional open trench method and will also results in a certain amount of C&D material. It is estimated that there will be a total of about 57,000m3 of excavated spoil and most of them are soft clayey materials which are not suitable for the foundation of the expanded and upgraded STW. About 10,600m3 of the excavated spoil will be reused onsite and 46,400m3 will be disposed off site. About 123,400m3 of suitable filling material will be imported for the construction of the expanded and upgraded STW foundation.
If appropriate mitigation measures are implemented during the handling, collection, and disposal of construction waste material, the residual environmental impacts would be reduced to acceptable levels. These mitigation measures can be enforced by incorporating them into a waste management plan as part of the contract document. Environmental monitoring and audit will be necessary to ensure the implementation of correct disposal requirements for the various wastes generated from construction works.
The sludge generated during the operation of the expanded and upgraded San Wai STW should be dewatered to more than 30% dry solids and transported to the landfills. The operators of future UV disinfection facility should work with the supplier/manufacturer on recycling and reuse of the defective UV lamps as far as possible. In handling the UV lamps, cautions should be exerted to avoid breakage of lamps and release of contaminants.
The handling, storage and disposal of spent UV lamps and spent odour scrubbing fluids should follow the requirements under the Waste Disposal (Chemical Waste) (General) Regulation. Unless the spent fluids are treated to meet discharge standards as stipulated in the Technical Memorandum under Water Pollution Control Ordinance, direct discharge into local sewerage systems should not be allowed.
In the current design, the expanded Ha Tsuen PS will be
entirely located within the currently site boundary of the Ha Tsuen PS and no
additional land will be required. The potential of land contamination impacts is
considered minimal.
For the expanded site of San Wai STW, most part of site, other than the existing
San Wai STW, is occupied by the two fishponds to the north and the container
yard to the east. Based on the review of the past and present land uses, land
contamination issue related to the area of the two fishponds is not anticipated.
The area of the container yard was previously agricultural
land and fishpond and was then converted to container yard since early 1990s.
Localised land contamination issues within this area would be related to vehicle
and plant maintenance and repairing activities. Similar land contamination
concern also applies to the open storage area to the west outside but adjacent
to the proposed expansion site.
In order to verify the low potential of land contamination impact prior to the
commencement of the construction of the San Wai STW expansion, a land
contamination assessment includes intrusive sampling and testing will be carried
out during the detailed design stage of the project when access to the concerned
areas is gained. A Contamination Assessment Report (CAR) will be prepared and
submitted to the Environmental Protection Department (EPD) for approved. If the
findings confirmed the potential of land contamination, a Remediation Action
Plan (RAP) will be formulated and implemented prior to the construction stage of
the project.
Deep Bay is facing long-term pollution problems which threaten the sensitive ecosystem in Deep Bay. HKSAR Government is planning to substantially reduce pollution loadings in Deep Bay. Under the Yuen Long/Kam Tin Sewerage Master Plan, sewage networks will be provided to unsewered villages and new developments, and sewage effluents from Yuen Long and Kam Tin will be transported out of Deep Bay to the better flushed water in Urmston Road for disposal (EPD 2001). The proposed project is an important component of the Plan.
Flora and fauna recorded in the Study Area during field surveys covering wet and dry seasons are generally of low conservation importance and disturbance tolerant. Habitats within the Study Area are mostly man-made, highly fragmented and disturbed. The project will mainly cause permanent loss of habitats of low ecological importance, hence ecological impact is considered minor. The project will greatly improve water quality in Deep Bay, a site of recognised international importance. Tiny increases of unionised ammonia (1%) would be expected near the outfall location and in the Sha Chau and Lung Kwu Chau Marine Park. The changes, however, would be very small and would be outweighed by the significant drop of E.coli count there. Therefore, the overall impact of the project will be positive.
An emergency bypass culvert, in the form of box culvert, will be constructed from San Wai STW to nearby drainage channel as part of this project. This culvert will largely reduce the chance of discharging raw sewage into Deep Bay. Under the current emergency situation, raw sewage will be discharged directly from the Ha Tsuen Pumping Station into the Tin Shui Wai Drainage Channel and then to Deep Bay when the NWNT effluent tunnel or the Urmston Road outfall is out of operation. The proposed project will provide an alternative pathway for San Wai STW to dispose treated effluent. With this enhancement measure, the effluent discharged to Tin Shui Wai Drainage Channel and Deep Bay under the emergency event when the NWNT effluent tunnel or the Urmston Road outfall is out of operation will be treated effluent instead of raw sewage. The associated environmental impacts would certainly be lower when compared with the current arrangement.
A review of existing information supplemented by the results
of recent field surveys on commercial fisheries resources confirmed that the
area of concern supports fisheries resources that are neither abundant nor of
high value in comparison to resources in other fishing areas of Hong Kong.
Potential impacts to fisheries resources and operations may arise from permanent
loss of fishponds, and changes in water quality. Due to the small percentage
compared to the overall resources in Hong Kong, permanent loss of fisheries
resources due to the project are predicted to cause minor impacts to Hong Kong
fisheries. As changes in water quality would be mostly positive, adverse impacts
to fisheries resources are not predicted. In the North Western Water Control
Zone, the much higher tidal flushing rate would effectively dilute and disperse
the pollutant concentrations that would occur at higher levels in the effluent
discharged from the upgraded San Wai STW.
The cultural heritage impact assessment has identified significant remains of cultural heritage in the San Wai and Ha Tsuen areas, including historic buildings at San Sang Tsuen and an archaeological site with Neolithic deposit and late historical ruins at Tseung Kong Wai. Besides, two graves with a possible pre-war dating have been identified at So Kwun Tsai and Hau Tsz Shan.
The Tseung Kong Wai potential site of cultural heritage with archaeological significance will be encroached upon by the Alternative 3 alignment of the proposed emergency bypass culvert. The Alternative 3 alignment is thus not preferable from a cultural heritage preservation point of view.
The historic buildings and pre-war graves identified in the San Wai and Ha Tsuen areas will not be affected directly by this project. It is recommended that appropriate mitigation measures should be taken by the project proponent to protect the potential sites of cultural heritage from any indirect impact potentially caused by the proposed construction.
3.9 Landscape and Visual Impact
In both the expanded sites of San Wai STW and Ha Tsuen pumping station, this project is anticipated to result in a temporary loss of vegetation during construction. In the Ha Tsuen pumping station compound approximately 30 trees comprising a mix of Acacia and Eucalyptus species planted as amenity planting will need to be removed to accommodate the new extension works. Subject to a detailed tree survey it may be possible to transplant some of the better specimens to the perimeter to reinforce the existing screen planting. In addition, upon completion of the construction works, it is proposed to provide compensatory tree planting to create denser and more effective boundary screen planting. The net landscape impact will be a loss of 0.19 ha. of mixed woodland and 0.33 ha. of lawn and shrubs.
At San Wai STW, an area of fishpond will need to be backfilled and approximately 50 trees comprising a mix of Acacia, Celtis, Mallotus and Macaranga species established on the pond bunds will need to be removed to accommodate the new extension works. Subject to a detailed tree survey it may be possible to transplant some of the better specimens to the proposed 5m wide planting reserve around the perimeter of the existing STW works to establish an effective visual screen to the site. The peripheral planting will also create a beneficial impact by replacing a portion of the unsightly container yards adjacent to the site. The net landscape impact will be a gain of 0.55 ha. of mixed woodland and a loss of 0.01 ha. of grassland and 1.75 ha. of fishpond.
In addition to the proposed compensatory planting the EIA also recommended the following mitigation measures to ameliorate the landscape and visual impacts.
Ø Align new utilities clear of existing trees and boundary
line of project sites
Ø Erect protective hoarding around existing trees to be retained to prevent
damage during construction
Ø Store any good excavated topsoil on site for reuse in screen planting and
hydroseeding works
Ø Select recessive colour scheme for the new buildings to help blend them into
the surrounding landscape.
With the implementation of the recommended mitigation measures the residual landscape and visual impacts are considered acceptable during the construction and operational phases of this project.