LEGCO QUESTIONS

Impact of sewage treatment on quality of water bodies in Hong Kong

25 Apr 2018
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LCQ22: Impact of sewage treatment on quality of water bodies in Hong Kong
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Following is a question by the Hon Kenneth Leung and a written reply by the Secretary for the Environment, Mr Wong Kam-sing, in the Legislative Council today (April 25):

Question:

The Drainage Services Department announced in February this year that replacement and maintenance works would be carried out from early this year to early 2020 for the electrical and mechanical facilities at the Stonecutters Island Sewage Treatment Works (SCISTW). The works include the replacement of two large inlet penstocks at the Main Pumping Station No. 1 installed deep underground that have been operating for over 16 years. It has been reported that the inlet penstocks need to be replaced due to corrosion, and during the works period when the pumping station concerned needs to be closed, around 930 000 cubic metres (equivalent to 370 standard swimming pools) of preliminarily treated/screened effluent will be discharged into the Victoria Harbour each day. Regarding the impact of sewage treatment on the quality of water bodies in Hong Kong, will the Government inform this Council:

(1) of the estimated expenditure for the aforesaid replacement of inlet penstocks; the other sewage treatment works (STWs) currently using inlet penstocks of the same model; and whether it has regularly monitored the corrosion situation of those inlet penstocks; if so, of the details; if not, the reasons for that;

(2) of the following information in respect of each STW: (i) the year of commissioning, (ii) the average volume of sewage treated per day, (iii) the average volume of treated effluent discharged per day, (iv) the water control zone (WCZ) into which the treated effluent is discharged, and (v) the name(s) of the beach(es) in the vicinity, with the locations of the various STWs, their outfalls and nearby beaches marked on a map;

(3) of the respective Water Quality Objectives (WQO) compliance rates of the water bodies in each WCZ in terms of total inorganic nitrogen, unionised ammonia nitrogen, E. coli bacteria, etc., in each of the past five years (set out in a table);

(4) of the respective WQO compliance rates of the water bodies in the various gazetted beaches in the vicinity of each STW in terms of dissolved oxygen, E. coli bacteria, turbidity, etc., in each of the past five years (set out in a table);

(5) in respect of the incidents in the past five years in which STWs discharged effluent not having gone through the established treatment procedure, of the following details of each incident: (i) the name of the STW concerned, (ii) the date on which the incident happened and (iii) the number of days the incident lasted, (iv) the cause(s) of the incident, and, during the incident, (v) the level(s) of treatment which the discharged effluent had undergone, (vi) the average volume of sewage treated per day and (vii) the volume of effluent discharged per day (set out in a table); and

(6) whether it has assessed the impacts of the incidents mentioned in (5) on the quality of water bodies in the vicinity, and whether it has monitored the relevant situation and adopted mitigation measures; if so, of the details; if not, the reasons for that?

Reply:

President,

(1) The maintenance works carried out by the Drainage Services Department (DSD) from February 20 to March 4, 2018 mainly involved the first stage of replacement of two large penstocks, which were located at 34 metres below ground level inside the Main Pumping Station No.1 (MPS1) of the Stonecutters Island Sewage Treatment Works (SCISTW). According to the current estimate, the works would cost about $70 million. As these two penstocks were situated at a unique location at the outermost part of the MPS1 where the deep sewage tunnels of the Harbour Area Treatment Scheme (HATS) were connected to the SCISTW and taking into account safety consideration, it was necessary to carry out the temporary bypass of sewage at some of the upstream Preliminary Treatment Works and empty MPS1 of sewage before technicians may enter to carry out the works.

The DSD has been providing timely inspection and maintenance to its facilities, including the deeply located penstocks. The penstocks that require replacement has been in operation since 2001. After operating continuously for over 16 years, the current conditions of the main bodies of the penstock gates are generally satisfactory, but their function is affected by the rusting and corrosion occurred around the edges that are in contact with the penstock frames and supporting facilities. The overall conditions of these penstocks tally with the duration of their operation and they are now due for replacement.

The situation of the penstocks at SCISTW that require replacement is rather unique. The other DSD’s sewage treatment works (STWs) do not have similar deeply located penstocks.

(2) The details of the major STWs in Hong Kong are shown in Annex 1.

(3) The compliance rates of the key marine Water Quality Objectives (WQOs) parameters, including dissolved oxygen (DO), total inorganic nitrogen (TIN), unionised ammonia (UIA) and E. coli, for the 10 Water Control Zones in the past five years (2013 to 2017) are listed in Annex 2.

(4) All gazetted beaches in Hong Kong have fully complied with the WQO parameter of E. coli for the past five years. There is no relevant WQO on DO and turbidity for bathing beaches.

(5) The details of the incidents involving the discharge of sewage that was not fully treated due to operational or engineering problems of STWs in the last five years are shown in Annex 3.

(6) The incident occurring on August 25, 2014 at Tuen Mun Pillar Point Sewage Treatment Works involved the mechanical failure of the plant equipment. During the incident, about 95 000 cubic metres of untreated sewage was discharged through the emergency submarine outfall. As a precautionary measure, the Leisure and Cultural Services Department (LCSD) temporarily closed 14 beaches in Tsuen Wan and Tuen Mun districts in the evening on that day. The Environmental Protection Department also collected water samples at these beaches in the morning and afternoon of the following day, and confirmed that the water quality at all the 14 beaches was suitable for swimming. The LCSD subsequently re-opened all the beaches concerned.

Besides, three incidents occurred in the North West Kowloon Preliminary Treatment Works, Tai Po Sewage Treatment Works and Ma Wan Sewage Treatment Works in 2017. They were mainly due to minor malfunction of the plant facilities and electricity supply problem of the power company. Due to the short duration of these incidents and small volume of sewage discharged, the impact caused to the water quality nearby was small and the problems were swiftly dealt with.

The replacement of penstocks for SCISTW from February 20 to March 4, 2018 had necessitated the temporary bypass of sewage as explained in (1) above. Overall speaking, 55 per cent of the total volume of HATS flow was bypassed, while the remaining 45 per cent was conveyed by Main Pumping Station No.2 and other system as usual to SCISTW for proper treatment and disposal.

When planning for the maintenance works, the DSD had employed consultants to conduct detailed water quality impact assessment. The assessment indicated that the potential impact on water quality of Victoria Harbour would be slight and transient. And water quality would resume to normal quickly in a few days after the cessation of bypass.

Notwithstanding that, in order to minimise the potential impact of the bypass to the environment and the public, the Government has carried out detailed assessment and preparatory work at the outset, formulated and implemented contingency plans as well as various mitigation measures (including means to enhance the dilution and dispersion of the sewage discharged, addition of odour suppressants and installation of deodorisation units). Furthermore, the maintenance works were conducted during the non-bathing season to minimise potential impact to the public.

During the bypass, all relevant government departments collaborated closely to monitor the situation. The DSD had conducted comprehensive water quality monitoring at 47 monitoring sites, including beaches, fish culture zones, coral sites, promenades and typhoon shelters. The monitoring results indicated that the water quality was generally in line with the consultant’s predictions. Overall speaking, the actual levels of water quality impacts of the bypass were slightly lower than the predictions and, as expected, the water quality of different parts of Victoria Harbour resumed to normal within one to several days after the cessation of bypass.