1.1 Background of Project
The Yuen Long and Kam Tin Sewerage Master Plan (YLKTSMP) was completed in early 1992. Subsequently, the Review of Yuen Long and Kam Tin Sewerage and Sewage Treatment Requirements (YLKTSMP Review) completed in early 1999 indicated that the original YLKTSMP needed to be revised to cater for the redistribution and increase of the future population in accordance with the latest Territorial Development Strategy Review. The YLKTSMP Review recommended the Yuen Long and Kam Tin Sewerage and Sewage Disposal (YLKTSSD) scheme (Stage 1 and Stage 2) and the upgrading of the sewage treatment facilities at San Wai Sewage Treatment Works (STW) from preliminary treatment to chemically enhanced primary treatment (CEPT) with disinfection.
Figure 1 shows the key elements recommended in the YLKTSMP Review and Figure 2 shows the locations of the existing NWNT effluent tunnel and Urmston Road outfall.
1.2 Description of Project
The Project is to expand the sewage facilities at San Wai STW and Ha Tsuen Pumping Station (PS) to cater for the projected increase of population arising from Hung Shui Kiu New Development Area (HSKNDA) and other planned developments at Yuen Long and Kam Tin areas and upgrade the sewage treatment facilities at San Wai STW from preliminary treatment to chemically enhanced primary treatment (CEPT) with disinfection for the attainment of water quality objectives on the aquatic environment in North West New Territories (NWNT).
The proposed key items of works of this Project are shown in Figure 3. The current schedule for these key items is to commence the construction in mid 2004 for commissioning in late 2007. The key items of this project are identified as Designated Projects under Part I F.1 and Part I F.3(b) of Schedule 2 of the Environmental Impact Assessment Ordinance (EIAO).
Upgrading and Expansion of San Wai STW
Ø Expand the preliminary treatment works at San Wai STW from 164,000 m3 per day to 246,000 m3 per day;
Ø Upgrade the preliminary treatment level at San Wai STW to CEPT;
Ø Add centralised disinfection at San Wai STW for the effluent after CEPT from San Wai STW and effluent after secondary treatment from Yuen Long STW;
Ø 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 the NWNT effluent tunnel is out of operation; and
Ø Rearrange the existing preliminary treatment facilities of San Wai STW to conform to the upgraded treatment works layout.
The layout plan of the upgraded and expanded San Wai STW is shown in Figure
Expansion of Ha Tsuen PS
Ø The expansion of Ha Tsuen PS involves mainly expanding pumping capacity of Ha Tsuen PS from 164,000 m3 per day to 246,000 m3 per day.
The layout plan of the expanded Ha Tsuen PS is shown in Figure 5.
1.3 Enhancement Measures to Control Emergency Discharge
Figure 6 shows schematically the emergency discharge scenarios for the existing San Wai STW and Ha Tsuen PS. Under the emergency event when the NWNT effluent tunnel or the Urmston Road outfall is out of operation, raw sewage will be discharged directly from the Ha Tsuen PS into the Tin Shui Wai Drainage Channel to avoid uncontrolled overflow of raw sewage in the catchment area. Under both the controlled and the uncontrolled emergency discharge scenarios, the raw sewage will eventually go to Deep Bay.
In order to reduce the chance of discharging raw sewage into Tin Shui Wai Drainage Channel and hence Deep Bay, 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. Figure 7 shows schematically the emergency discharge scenarios for the future San Wai STW and Ha Tsuen PS. Under the emergency event when the NWNT effluent tunnel or the Urmston Road outfall is out of operation, raw sewage from the Ha Tsuen PS will be treated at the San Wai STW and then discharged via the proposed emergency bypass culvert to nearby drainage channel and then to Tin Shui Wai Drainage Channel and Deep Bay. With this enhancement measure, the effluent discharged to 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.
Comparing Figure 6 and 7, other than the scenario with the break down of the NWNT tunnel or the Urmston Road outfall, the other emergency discharge scenarios for the existing and the future arrangements are the same. Having said that, the chance of discharging raw sewage into the Tin Shui Wai Drainage Channel is very remote and had not happened since the commissioning of the existing San Wai STW, the Ha Tsuen PS, and the NWNT effluent tunnel and Urmston Road outfall for more than 10 years. Besides routine monitoring, inspection, and maintenance to ensure satisfactory working condition of the system, the following have been and will be applied to the Ha Tsuen PS to further reduce the chance of system failure:
Ø Standby pumps and screens to facilitate maintenance and repair of equipment;
Ø Back-up power in the form of dual power supply;
Ø 24-hour manned pumping station; and
Ø Hand-cleaned bar screen at overflow bypass to prevent discharge of floating solids.