LCQ18 : Direct comparison of discharge effluent standards and water quality objectives not appropriate

Following is a question by the Hon Choy So-yuk and a written reply by the Secretary for the Environment, Transport and Works, Dr Sarah Liao, at the Legislative Council meeting today (February 26) :

Question :

The Drainage Services Department ("DSD") plans to construct a sewage treatment plant at Ngong Ping, Lantau Island, and to discharge the treated effluent into the sea via Tung Wan. According to the Environmental Assessment Report for Tung Wan Option submitted by DSD to the Environmental Protection Department, the inorganic nitrogen level at a distance five metres downstream from the discharge point is predicted to exceed, by over 60 times, the relevant water quality objective for the Southern Water Control Zone in which Tung Wan is situated, but the report claims that the discharge will not have grave impact on the quality of the waters concerned. In this connection, will the Government inform this Council:

(a) whether it will adopt remedial measures to minimize the impact of the effluent on the water quality of Tung Wan; if so, of the details; if not, the reasons for that; and

(b) whether, given that the Administration will prosecute those members of the commercial and industrial sector who have caused the discharge of effluent exceeding the prescribed standard, it has adopted equally stringent water quality standard for the effluent of this project of a government department; if not, of the reasons for that?

Reply :

Madam President,

(a) The discharge effluent standards are set according to the "Technical Memorandum on Effluent Standard" under the Water Pollution Control Ordinance (WPCO) to control the quality of effluent discharge. The Water Quality Objectives (WQOs) represent the target of water quality set for water control zones under the WPCO. As the effluent discharged will mix with the receiving waters and be diluted by the natural assimilative capacity of the latter, it is not appropriate to make direct comparison of the discharge effluent standards and the WQOs. In fact, the Technical Memorandum under the Environmental Impact Assessment Ordinance has also pointed out that within the "Mixing Zone", i.e. the area where the initial mixing of the discharge with the receiving waters happens, the WQOs can be exceeded.

In assessing the impact of a particular discharge on the receiving water body, the normal practice is to assess the water quality impacts outside the mixing zone. Ngong Ping Sewage Treatment Works is a tertiary sewage treatment works subject to very stringent effluent standards. The predicted Total Inorganic Nitrogen (TIN) level of the discharge is some 85% lower than the standard specified in the Technical Memorandum under the WPCO. In the worse case scenario, the discharge would only increase the background TIN level at locations beyond a mixing zone of 500 m in radius by less than 0.3%. As this would not cause any adverse impact on the receiving water body, there is no need for any specific remediation measures.

(b) The Environmental Protection Department sets all effluent standards in accordance with the "Technical Memorandum on Effluent Standards" under the WPCO and applies them to monitor and regulate discharges from facilities owned by government departments or commercial and industrial enterprises alike.

End/Wednesday, February 26, 2003

NNNN


Back