LCQ6: Replacement and maintenance works for the electrical and mechanical facilities at the Stonecutters Island Sewage Treatment Works
The Drainage Services Department (DSD) announced on February 12 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 (SISTW) under the Harbour Area Treatment Scheme (HATS) Stage 1. The works include the replacement of two large inlet penstocks installed deep underground at the Main Pumping Station No.1 of SISTW. During the works period, the DSD needs to discharge effluent, which has been initially treated at some of the upstream preliminary treatment works, into the Victoria Harbour by five bypasses, with each bypass not expected to exceed two weeks. The DSD has indicated that according to the outcome of a consultancy study, the potential impact of the bypasses on the water quality of the Victoria Harbour is expected to be mild and transient, and the DSD has devised appropriate contingency and mitigation measures. However, the water quality monitoring results published earlier by the DSD showed that on some days during the first bypass (from February 20 to March 5 this year), the total Escherichia Coli (E. coli) levels in three beaches near Tusen Wan ranged from 180 to 610 counts/100 millilitres (mL) (a poor ranking for beach water quality), and the total E. coli levels in the two typhoon shelters in To Kwa Wan and Gin Drinkers Bay ranged from 10 000 to 50 000 counts/100 mL (a very poor ranking for beach water quality). In this connection, will the Government inform this Council:
(1) as it has been reported that the aforesaid inlet penstocks needing replacement were made of cast iron and wrapped with phosphor bronze on all sides, and they have suffered serious corrosion due to prolonged exposure to effluent, of the details of the new inlet penstocks (including their design, materials used, how they compare with the original ones in terms of anti-corrosion capacity, anticipated service periods and replacement procedure in future);
(2) whether the materials used for and the design of the inlet penstocks of the Main Pumping Station No. 2 under HATS Stage 2A are the same as those of the aforesaid inlet penstocks needing replacement; whether the DSD will examine the corrosion condition of the former during the said replacement and maintenance works to assess when they need to be replaced;
(3) whether the DSD activated the aforesaid contingency and mitigation measures during the first bypass; if so, of the details; if not, the reasons for that;
(4) of the DSD's new measures to further alleviate the impacts on the water quality brought about by the bypasses yet to be carried out;
(5) as the DSD has indicated that it will, after conducting the first bypass, review (i) the arrangements for the replacement and maintenance works and (ii) the effectiveness of the contingency and mitigation measures, whether the review has been completed; if so, of the outcome; and
(6) whether the DSD will take measures to (i) reduce the number of bypasses that need to be carried out and (ii) shorten the duration of each bypass?
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). 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 temporary bypass of sewage at some of the upstream Preliminary Treatment Works and to empty MPS1 of sewage before technicians can enter to carry out the works. Overall speaking, 55% of the total volume of HATS flow was bypassed, while the remaining 45% was conveyed by Main Pumping Station No.2 (MPS2) and another system to SCISTW as usual for proper treatment and disposal.
When planning the maintenance works, the DSD had employed consultants to conduct detailed water quality impact assessment. The assessment results indicated that the potential impact on water quality of Victoria Harbour would be slight and transient. In addition, the Government had, during the bypass, implemented a series of mitigation measures, closely monitored the water quality, formulated contingency plans and informed the public through different channels to minimise the impact to the environment and the public.
Our reply to the question raised by the Hon Martin Liao is as follows:
(1) Since the commissioning of MPS1 of the SCISTW in 2001, the DSD has been providing timely inspection and maintenance to its facilities, including the deeply located penstocks. The penstocks in MPS1 were designed and made with materials in accordance with the prevailing design standards, with a normal service life of 15 to 20 years. 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. In designing the new penstocks, the DSD would make reference to the current technologies and use materials with enhanced corrosion and abrasion resistance properties. The service life of the new penstocks is estimated to be up to 30 years.
(2) The design and facilities of MPS2 of SCISTW are different from those of MPS1. The pipes and gate valves (instead of penstocks) adopted in MPS2 have an expected service life of up to 30 years.
(3) 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. Besides, throughout the bypass period, the DSD had conducted comprehensive and detailed water quality monitoring, uploaded the latest monitoring results on the Internet for public inspection and set up a telephone hotline and email address for public enquiry. All relevant Government departments had also collaborated closely to monitor the bypass situation. With the above measures in place, the water quality conditions observed during the bypass were in line with the predictions.
(4) to (6) During the bypass, the DSD 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 impact 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 the bypass. The monitoring results also indicated that baseline water quality at the beaches in Tsuen Wan, the fish culture zones and coral sites close to Victoria Harbour were of similar level before and after the bypass, indicating that they were not affected by the bypass. As regards the conditions at the typhoon shelters at To Kwa Wan and Gin Drinkers Bay, it is not appropriate to apply beach water quality grading standard to evaluate the impact there as they are not bathing beaches, notwithstanding that the elevation of E. coli level had only lasted for a short period of time and the water quality had quickly resumed to normal in two to four days after the cessation of the bypass.
According to the original plan, the maintenance works have to be implemented in stages and require a total of five rounds of bypass, including the first one conducted recently, with due consideration of safety and sequencing of works. Upon completion of the first stage of the scheduled works (i.e. detailed survey and partial dismantling of the existing penstocks), we are consolidating the experience of this round of bypass and reviewing the environmental monitoring results with a view to ensuring safety of works, minimising the duration of bypass and reducing potential environmental impact of the future works.
Ends/Wednesday, April 25, 2018
Issued at HKT 15:45
Issued at HKT 15:45