LegCo Paper No. CB(1) 1973/95-96
(These minutes have been seen by the Administration)
Ref : CB1/PL/EA
LegCo Panel on Environmental Affairs
Minutes of Special Meeting
held on Wednesday, 12 June 1996 at 10:15 a.m.
in Conference Room A of the Legislative Council Building
Members Present :
Dr Hon John TSE Wing-ling (Deputy Chairman)
Hon Edward HO Sing-tin, OBE, JP
Dr Hon Samuel WONG Ping-wai, MBE, FEng, JP
Hon IP Kwok-him
Hon MOK Ying-fan
Hon NGAN Kam-chuen
Member Attending :
Hon CHAN Wing-chan
Members Absent :
Hon Christine LOH Kung-wai (Chairman)
Dr Hon LEONG Che-hung, OBE, JP
Hon Emily LAU Wai-hing
Dr Hon LAW Cheung-kwok
Public Officers Attending :
- Mr Danny TSUI
- Principal Assistant Secretary for Planning, Environment & Lands (Environment)
- Mr Edward S CHU
- Assistant Secretary for Planning, Environment & Lands (Environment)
- Dr Michael CHIU
- Assistant Director of Environmental Protection (Waste & Water)
- Mr David Hall
- Acting Principal Environmental Protection Officer
Attendance by Invitation :
- International Review Panel
- Professor Donald Harleman
- Professor Poul Harremoes
- Professor QIAN Yi
- Environmental Management Ltd
- Dr Martin White
- Project Manager
Staff in Attendance :
- Miss Odelia LEUNG
- Ms Sarah YUEN
In the absence of the Chairman, who was engaged in another meeting, Dr John TSE, Deputy Chairman, took the chair. The Deputy Chairman welcomed the three International Review Panel (IRP) members, the representative of the Environmental Management Ltd and the government officials to the meeting.
2. At the invitation of the Deputy Chairman, Professor Poul Harremoes briefed members on the paper provided by the Administration (LegCo Paper No. CB(1)1591/95-96), highlighting the background to and the progress of the Pilot Plant Study on Chemical Dosing and Disinfection. Professor Donald Harleman and Professor QIAN Yi supplemented the following points:
- Chemical dosing was a cost-effective waste water treatment technology. A very low dosage of chemical additives would result in a significant increase in efficiency, thus enabling Hong Kong to handle a substantial volume of sewage flows with a relatively small plant. Through effecting a more rapid settling rate, chemical addition could, in a cost-effective manner, remove more than 80% of suspended solids and about 60% of organic materials measured in terms of Biochemical Oxygen Demand (BOD).
- Past experience in San Diego showed that Chemically Enhanced Primary Treatment (CEPT) greatly enhanced efficiency. Hong Kong was at present in the forefront of this technology. The Stonecutters Island Sewage Treatment Works (SISTW), upon completion, would become the worlds largest treatment plant of its kind and the dosage could be as low as 10 parts per million, i.e., for every million litre of waste water, only 10 litres of ferric chloride would be added. Hong Kong Government should be given the credit for having the foresight to adopt this innovative technology.
- The IRP considered the progress of the Study satisfactory and believed that the results of the pilot plant trials and the environmental impact assessment (EIA) studies would facilitate the implementation of the Strategic Sewage Disposal Scheme (SSDS).
3. On whether the results of the EIA studies might negate the CEPT method recommended by the IRP, Professor Poul Harremoes opined that such possibility did not exist because in choosing this particular technology, the IRP had taken care to ensure that any need for further improvement to water quality identified by the EIA studies could be accommodated.
4. Concerning the quality of treated water and its impact on marine life, Professor Donald Harleman reported that the results of the pilot plant trials, which were in a sufficient scale to be meaningful, had confirmed and even exceeded the expectation as to the treatability of Hong Kongs waste water reported in SSDS Stage II Options Review. The impact of the addition of ferric chloride was negligible on the marine environment because the chemical was most commonly used in treating waste water and the small dosage should leave no residual.
5. On bacteria contamination, Professor Donald Harleman and Professor Poul Harremoes pointed out that one advantage of CEPT was its high capacity of removing suspended solids, which was comparable to that of biological treatment. The effectiveness of disinfection by chlorine or by ultra-violet (UV) irradiation would be known within the coming months. It was believed that UV irradiation which had a more benign residual effect on the ocean water would be feasible. If the EIA studies showed that there was a need for bacteria disinfection, UV irradiation would provide an option. The two professors however pointed out that while the reduction of bacteria concentration to a very low level was technically feasible, there were always other sources of contamination in the Harbour, due to stormwater discharge and other factors, which could not be centrally collected and taken to the SISTW. Moreover, although investigations showed that disinfection intended to bring the bacteria content in line with the water quality standards was achievable, there were other uncertainties which could only be uncovered by the EIA studies.
6. Addressing members concern about the relation between the efficiency of the preliminary SISTW and the outfall location in achieving the objectives of desirable water quality, Professor Donald Harleman and Professor Poul Harremoes said that the effectiveness of the additional removal of organic materials related to the question of the impact on dissolved oxygen in the receiving water. It was a choice between either employing CEPT with a longer outfall or undergoing secondary treatment to achieve a lower BOD with a shorter outfall. It was possible to achieve essentially the same impact by having lesser removal of organic matters, as measured by BOD, but an outfall which placed the effluent into deeper water with stronger tidal trends. In either case protection against low dissolved oxygen could be achieved . There was no particular advantage of one over the other. The interaction between the level of treatment and the outfall location was an economic question.
7. Professor Donald Harleman and Professor Poul Harremoes further stressed that the setting of objectives in respect of water quality was a political rather than a technical decision and the IRP was only concerned how best to achieve the objectives cost-effectively. In this regard, the preliminary SISTW and the outfall were very cost-effective in bringing about a significant improvement to water quality in the Harbour while allowing the maximum flexibility for future development. At its last visit, the IRP recommended that further water quality investigations should be conducted in respect of the impact of the discharge of organic matters on the oxygen content in the receiving water. With the benefit of information obtained from such investigations and the EIA studies, the IRP would be in a position to recommend whether any improvement was necessary to achieve the required water quality standards. If deemed necessary, biological treatment could be included. In fact, CEPT would maximise the efficiency in subsequent biological treatment process, so that the secondary treatment plant could be much smaller and less costly. The process was thus either a terminal treatment should the EIA studies reveal the oxygen problem not being an issue, or the first cost-effective stage of any subsequent treatment.
8. On whether the design of the SISTW could accommodate the switch from the use of lime to the use of chemical additives, Professor Donald Harleman explained that lime was originally chosen to achieve a certain level of disinfection by bringing about a higher pH value. However, even at the planned dosage of 125 milligrammes per litre, lime already produced an enormous amount of sludge. To achieve any effective disinfection, there was a need to double the quantity of lime used. Hence the use of lime was out of the question. It was a sensible decision to switch from lime to ferric chloride which was compatible with the design of the SISTW. Professor Poul Harremoes supplemented that the design of the STW would enable it to operate for a long time provided that maintenance was adequate.
9. The meeting ended at 11:00 a.m.
19 August 1996
Last Updated on 18 Aug, 1998