4.1
Water Quality Mitigation Measures
Since water generated from the subject site will be
discharged into foul sewers leading into Government sewage treatment
plants, standards for effluents discharged as stipulated in the Technical
Memorandum for Effluents Discharged into Drainage and Sewerage Systems, Inland
and Coastal Waters shall be followed.
The EIA report has not recommended
any water quality monitoring, however, site specific controls and mitigation
measures to manage surface runoff from construction sites were recommended. The
Contractor shall be responsible for the design and implementation of these
measures. The Contractor shall implement water quality mitigation measures to
include, but not be limited to, the following:
· If possible, works shall be planned to avoid the rainy season so as to minimise the runoff and reduce the amount of soil that could be carried off site.
· The site shall be kept clean and tidy to avoid construction material and waste being washed off site.
· Runoff shall be directed to silt traps or sedimentation basin before reuse or discharge with the help of channels, earth bunds or sand bag barriers for suspended solids removal prior to its being discharged to storm water drain. The hoarding gaps shall be tightly sealed to avoid the seepage of wastewater to the nullah and outside the site.
· Perimeter channels shall be provided at site boundaries (where necessary) to intercept storm-water runoff from outside the site so that it does not wash across the site.
· Silt traps, sedimentation basins, channels and manholes shall be regularly cleaned to remove the deposited silt and grit.
· Silt trap design shall conform to the guidelines laid down in Appendix A1 of ProPECC PN 1/94[1].
· Temporarily exposed slope surfaces and construction material stockpiles shall be covered with tarpaulin or similar fabric to prevent erosion.
· Wastewater likely to be contaminated with oil or grease shall be passed through an oil separator or grease trap before entering the site drainage system.
· Water used in ground boring shall as far as practicable be re-circulated after sedimentation. Thus, the consumption of water and the amount of effluent discharge from piling works can be reduced. The final discharge of the water shall be via silt removal facilities.
· Vehicles and plants shall be cleaned before they leave the site to prevent deposition of mud, debris, etc. on roads by them. Water collection tank(s) shall be provided near the wheel washing facilities at the site entrance. Thus, the wastewater can be reused. The wash water shall have silt and sand removed prior to discharge into storm drains.
· Wastewater from toilets, kitchen, etc. shall be discharged into a foul sewer or a sewage treatment facility either directly or indirectly by means of pumping or other means considered appropriate by the Engineer. Chemical toilets may be used temporarily during the construction phase or on the day of Dragon Boat Festival to cater for additional demand. For long-term purpose, any sewage/wastewater generated from the proposed site should be diverted and discharged to the existing foul sewers in order to avoid further water quality impacts.
·
Bentonite slurries will be reused where possible. The residues will be
disposed of in accordance with the ProPECC PN 1/94. Disposal of the residues as
effluent discharge is possible if adequate treatment is provided in order to
comply with the effluent discharge standards as stipulated in the TMES.
Slurries may be disposed of at the marine disposal ground subject to the
provision of licence by the EPD.
· Construction site discharges are controlled under the Water Pollution Control Ordinance (WPCO) and as such a discharge license will be needed for discharging the wastewater generated on site.
· ArchSD site supervisory staff will monitor the quality of water discharge to ensure compliance.
If the above measures
are not sufficient to reduce the water quality to the discharge limit upon the
advice of ET Leader, the Contractor shall liaise with the ET Leader on some
other mitigation measures, propose to ER for approval, and implement the
mitigation measures.
[1] Practice Note for Professional Persons, ProPECC PN 1/94, Professional Persons Environmental Consultative Committee, 1994