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Advisory Council on the Environment

Environmental Impact Assessment Ordinance (Cap. 499)


(ACE-EIA Paper 01/2003)
For Advice

 

 

Environmental Impact Assessment
Upgrading and Expansion of San Wai Sewage Treatment Works and Expansion of Ha Tsuen Pumping Station

Purpose

This paper presents the key findings and recommendations of the Environmental Impact Assessment (EIA) Report for the Upgrading and Expansion of San Wai Sewage Treatment Works and Expansion of Ha Tsuen Pumping Station submitted under Section 6(2) of the Environmental Impact Assessment Ordinance (EIAO) with application no. EIA-086/2002. Comments from the public and ACE will be taken into account by the Director of Environmental Protection when he makes his decision on the approval of EIA Report under the EIAO.

Advice Sought

2. Members' views are sought on the findings and recommendations of the EIA Report.

Need for the Project

3. The Project and two other sewerage projects (i.e. the Yuen Long and Kam Tin Sewerage and Sewage Disposal Stage 1 and Stage 2) were recommended under the "Review of Yuen Long and Kam Tin Sewerage and Sewage Treatment Requirements" (YLKTSMP Review) completed in early 1999. ACE was consulted on the EIA Report "Yuen Long and Kam Tin Sewerage and Sewage Disposal Stage 1, Packages 1A-1T and 1B-1T - Kam Tin Trunk Sewerage Phases I and II" and the report was endorsed with conditions by ACE on 29.7.02. The EIA Report was approved with conditions under the EIA Ordinance on 26.8.02. The subject Project is to expand the sewage facilities at San Wai Sewage Treatment Works (San Wai STW) and Ha Tsuen Pumping Station (Ha Tsuen PS) to cater for the projected increase of population in NWNT and other planned developments in Yuen Long and Kam Tin areas and potential development for Hung Shui Kiu, and upgrade the sewage treatment facilities at San Wai STW from preliminary treatment to Chemically Enhanced Primary Treatment (CEPT) for the attainment of WQO on the aquatic environment in NWNT.

Description of the Project

4. The Project consists of the following elements:

  • Expand the preliminary treatment works at San Wai STW from 164,000m3/d to 246,000m3/d for discharge into the existing NWNT Effluent Tunnel and Urmston Road Outfall;
  • Upgrade the preliminary treatment level at San Wai STW to CEPT;
  • Add centralized disinfection at San Wai STW for the effluent after CEPT from San Wai STW and effluent after secondary treatment from Yuen Long STW;
  • Construct and operate an emergency bypass culvert, from San Wai STW to nearby drainage channel, in order to provide an alternative discharge route for San Wai STW in an emergency event when NWNT effluent tunnel is out of operation;
  • Rearrange the existing preliminary treatment facilities of San Wai STW to conform to the upgraded treatment works layout; and
  • Expand the pumping capacity of Ha Tsuen PS from 164,000m3/d to 264,000m3/d.
5. The Project is a Designated Project under Item F.1 and F.3(b), Part I, Schedule 2 of the Environmental Impact Assessment Ordinance (EIAO). The planned construction programme for the Project is from 2004 to 2007. The locations of the expanded San Wai STW, expanded Ha Tsuen Pumping Station and the 4 alternatives of the emergency bypass culvert are shown in Fig. 1 MSWORD. Fig. 2 MSWORD is a location plan of the existing NWNT Effluent Tunnel and Urmston Road Outfall.

Specific Environmental Aspects to Highlight

6. The key environmental issues identified are operational water quality, ecology, noise and odour

Operational Water Quality

7. The potential water quality impacts during the operational phase will be mainly related to the effluent discharge from the San Wai STW. Key concerns are the discharge of treated effluent through the outfall at Urmston Road for different treatment options.

8. The EIA Report concludes with the model predictions that there would be no adverse water quality impacts for the adoption of Option 2 (i.e. CEPT with disinfection) as a preferred treatment option at San Wai STW. Disinfection would satisfy the precautionary requirement for protection of dolphins and the water quality standards at Butterfly Beach and other recreational areas would be met during the operation of San Wai STW.

9. Depending on the emergency event, treated or untreated sewage would be discharged via Tin Shui Wai Drainage Channel to the Deep Bay waters or directly to North Western waters. The three emergency discharge scenarios are: (i) discharge of raw sewage via the NWNT Outfall in case the San Wai STW is not functioning; (ii) discharge of treated effluent via Tin Shui Wai Drainage Channel into Deep Bay if NWNT Outfall/Tunnel is not in operation; and (iii) 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.

10. For the occurrence of emergency discharge, the EIA Report considers the chance is very remote and has not happened since the commissioning of the existing San Wai STW, the Ha Tsuen Pumping Station, and the NWNT Effluent Tunnel and Urmston Road Outfall in the last 10 years. Although the EIA Report considers that the probability of occurrence of the emergency discharge is low, the following precaution measures were exhausted in the EIA Report to further minimize the occurrence of the emergency discharges:

  1. Regular maintenance and checking of plant equipment to prevent equipment failure;
  2. Use standby pump in case of pump failure or maintenance required and;
  3. Use dual power supply to keep the pump in operation in case of main power failure. The EIA Report also recommends that a contingency plan to be developed at the detailed design stage.
11. The EIA Report predicts with the model results that the increase in E.coli, TIN and UIA would cause a short-term deterioration of the water quality conditions in the inner part of Deep Bay in case of the emergency discharge period. The EIA Report, however, considers that the water quality conditions would quickly recover after the termination of emergency discharge for both wet and dry seasons.

Ecology

12. The proposed expanded sites of San Wai STW will cause the loss of 1.75 ha of fishponds. The two fishponds supports dense vegetation on the pond bunds and the bund slopes are steep (>1:1). The EIA Report considers these habitats offer few perches for wading birds to deed on or from the pond shores. Water depths in the two ponds are probably deep (>2m). The two ponds are isolated from other continuous clumps of fishponds in northwest and north New Territories. In addition, disturbances from surrounding industrial developments and human residences is high. Uses of the two fishponds by both wetland and non-wetland birds are limited by these factors. The overall ecological value of the two ponds as bird habitat is ranked as low in the EIA Report. Mitigation for this loss of fishponds due to the Project is not considered necessary in the EIA Report.

13. The EIA Report considers that the frequency of occurrence of emergency discharge is very low. There have not been any emergency discharges from Ha Tsuen Pumping Station and San Wai STW since their commissioning over a decade ago. All emergency discharge scenarios would be episodic and short-term. Furthermore, the Project could help to reduce the pollution loadings in Deep Bay. The sewage processed by the expanded and upgraded San Wai STW is predominantly domestic sewage. Existence of heavy metals concentrations in domestic sewage above the background level in the marine environment is not likely. The emergency discharge would be diluted and naturally decomposed. The long-term water quality improvement in the entire activity range of the dolphins would be a more significant factor for dolphin health than the episodic accidents. Though water quality might decline during emergency discharge period, based upon the low potential of the emergency discharges and the rapid restoration after the termination of discharges, the ecological impact is ranked minor to moderate in the EIA Report.

Noise

14. The EIA Report anticipates that there will be no construction work carried out during restricted hours. The identified NSRs are all located at considerable distance from the construction site of the San Wai STW and Ha Tsuen Pumping Station. The predicted construction noise levels at the NSRs are all within the construction noise criteria with the use of quiet plant and temporary barrier. No residual noise impacts are expected during the construction phase of San Wai STW and Ha Tsuen Pumping Station.

15. With mitigation measures in the form of quiet plant and temporary noise barrier in place, the predicted cumulative noise impacts at the identified NSRs during the construction phase near San Wai STW under the worst-case scenario are 68 to 75 dB(A), which are all within the construction noise criteria. Whereas the predicted cumulative noise impacts at the identified NSRs during the construction phase near Ha Tsuen Pumping Station under the worst-case scenario are 60 to 74 dB(A), which are all within the construction noise criteria except at NSR N1 during the examination periods of school with 3dB(A) exceedance.

16. The EIA Report recommends all the noisy equipment of the expanded and upgraded San Wai STW and Ha Tsuen PS to be housed underground or within building structures made of suitable materials (e.g. concrete) and should be a complete enclosure with minimal openings for which these openings should not be facing any NSRs. A silenced ventilation system incorporating silencers at the air intakes and discharge openings should be employed. The EIA Report concludes that the residual noise impacts at the nearest NSRs are not expected provided that all the indoor noisy equipment will be housed underground or within building structure made of suitable material.

Odour

17. Computer dispersion modelling has been undertaken in the EIA Report to assess the potential operational phase air quality impacts due to odour emissions. Odour mitigation measures have been recommended. These include enclosing the odour sources and ventilating all the odour emissions through deodorisers before discharging to the atmosphere. The EIA Report predicts with the modelling results that no exceedance of odour criteria at all the identified existing and future air sensitive receivers in the vicinity of the proposed works under the mitigated scenario.

Environmental Monitoring and Audit (EM&A)

18. An EM&A programme has been proposed in the EM&A Manual submitted together with the EIA Report and included a detailed Implementation Schedule. The EM&A requirements will be enforced as Environmental Permit conditions.

Public Consultation

19. DSD has made the EIA Report, EM&A Manual and Executive Summary available for the public to comment under the EIAO on 25 February 2003. Members will be briefed about any comments received from the public at the meeting.

Environmental Protection Department
March 2003


 

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Friday, 28 April, 2006