For discussion PWSC(95-96)65
on 7 December 1995


Environmental protection - Sewerage and sewage treatment
200DS - Upgrading of sewage treatment works for sludge disposal

Members are invited to recommend to Finance Committee to upgrade 200DS, entitled ‘Upgrading of sewage treatment works for sludge disposal’ to Category A at an estimated cost of $148 million in money-of-the-day prices.


The existing sludge dewatering equipment of the existing four major secondary sewage treatment works1 (SSTWs) at Tai Po, Yuen Long, Shek Wu Hui and Sai Kung cannot achieve the Director of Environmental Protection’s latest sludge landfill disposal requirement that by mid 1997 all sludge disposed of at the three strategic landfills should have a minimum dry solids content of 30% by weight.


2. The Director of Drainage Services, with the support of the Secretary for Planning, Environment and Lands, proposes to upgrade 200DS to Category A at an estimated cost of $148 million in money-of-the-day (MOD) prices to replace all existing sludge dewatering equipment at four SSTWs by new sludge dewatering systems to produce drier sludge.


3. The whole project comprises -

  1. replacement of all existing sludge dewatering equipment by membrane presses or centrifuges at Tai Po, Yuen Long, Shek Wu Hui and Sai Kung SSTWs;
  2. demolition of existing sludge dewatering facilities in the four SSTWs; and
  3. provision of all civil engineering works including pipeworks, sludge reaction tank, sludge buffer tank and relining of concrete holding tanks.


4. Currently we deposit sludge at the Pillar Point Valley landfill site which we will decommission in the next two years. We will then have to deposit sludge at the three strategic landfills. We completed the works under 119DS “Strategic Sewage Disposal Scheme - Integrated Sludge Disposal Strategy Study” (ISDSS) in June 1993. The purpose of the study was to evaluate the volume of sludge generated by the existing sewage treatment works and the future Strategic Sewage Disposal Scheme (SSDS). The study found that the volume of sludge to be disposed of to the strategic landfills will increase from the current 420 tonnes per day to approximately 2,000 tonnes per day when we commission the SSDS in mid 1997. The vast increase in wet sludge, if maintained at the current moisture content of 14 to 22% dry solids content, will cause extremely difficult operational problems at the landfills, together with the environmental problem of leachate generation and potential surface water contamination.

5. The ISDSS concluded that we will need to dewater the sludge to 30% dry solids content by weight. If we achieve a dry solids content of 30%, there will be a 50% decrease in the sludge volume. Consequently, the strategic landfills will be able to accept more wastes and the effective lives of the landfills will be increased. Moreover, the contracts for the strategic landfills are structured such that the wetter the sludge being deposited, the higher the levy charged. If the sludge is dewatered to a higher dry solids content of 30%, we can avoid paying the landfill contractor at a higher rate.

6. The economic life of typical dewatering equipment is normally considered to be 15 years. The sludge dewatering equipment that we propose to replace in Tai Po, Yuen Long, Shek Wu Hui will be near to the end of their economic life by 1997. In Sai Kung SSTW, although the existing sludge dewatering equipment is of more recent manufacture, it is producing dewatered sludge with a higher moisture content than that produced in other SSTWs. We therefore propose to take this opportunity to improve the dewatering capacity of the Sai Kung SSTW sludge dewatering system.


7. We estimate the capital cost of the proposed works to be $148 million in MOD prices, made up as follows -

$ million

(a) Electrical and mechanical works for


(i) Tai Po SSTW


(ii) Yuen Long SSTW


(iii) Shek Wu Hui SSTW


(iv) Sai Kung SSTW


(b) Demolition of existing sludge dewatering facilities for four SSTWs


(c) Civil engineering works


(d) Contingencies


Sub-total (at December 1994 prices)


(e) Inflation allowance


Total (in MOD prices)


8. Subject to approval, we will phase expenditure as follows -

Year $ million
(Dec 1994)
$ million

1996 - 97




1997 - 98




1998 - 99






9. We derive the MOD estimate on the basis of the Government's forecasts of trend labour and construction prices over the period between 1996 to 1999. We will tender the proposed works under a fixed price, lump sum contract. We have adopted this form of contract as the major part of the works will be the supply and installation of electrical and mechanical equipment for which the scope of works is well defined.

10. We estimate the additional annually recurrent expenditure of the proposed new sludge dewatering systems for the four SSTWs to be $3.3 million. We intend that this expenditure will be a charge to the new Sewage Service Trading Fund. Moreover, subject to the Legislative Council passing a Resolution on appropriation of assets to the Trading Fund, the works under this project will, upon completion, become assets in the Trading Fund valued at cost.

11. There will be about 50% reduction in the volume of dewatered sludge due to the increase of dry solids content to 30% from the current range of 14 to 22%. This volume reduction occupied by the drier sludge will save landfill space and achieve a notional saving of $1.3 million annually in the development costs of the strategic landfill sites. In addition, we estimate that upon completion of the proposed upgrading works, we would have no need to pay the higher charge levied by the contractors for handling sludge of sub-30% dry solids content which we estimate to be about $5.8 million per year.


12. As the works only involve modification and replacement works inside the four existing SSTWs, public consultation is not necessary.


13. The Director of Environmental Protection completed an Environmental Review of the upgrading works in March 1995 and concluded that no Environmental Impact Assessment would be necessary. The new equipment will not only produce drier sludge but also improve noise and odour levels inside the existing buildings. For short-term impact, we shall control noise, dust and site run-off during construction through the implementation of mitigation measures in the works contract.

14. As the Drainage Services Department will transport the dewatered sludge to the three strategic landfills using tipper vehicles and skips, we will take suitable measures during transportation to avoid off-site odour impact.


15. This project does not require land acquisition.


16. The facilities and performance of the existing four SSTWs are -

  1. Tai Po SSTW (commissioned in 1985) - three fixed recess plate filter presses producing dewatered sludge having dry solids content from 18% to 22%;
  2. Yuen Long SSTW (commissioned in 1984) - three fixed recess plate filter presses producing dewatered sludge having dry solids content from 18% to 22%;
  3. Shek Wu Hui SSTW (commissioned in 1984) - six fixed recess plate filter presses producing dewatered sludge having dry solids content from 18% to 22%; and
  4. Sai Kung SSTW (commissioned in 1988) - two belt filter presses producing dewatered sludge having dry solids content from 14% to 18%.

17. We undertook an in-house study of the possible options to achieve the ISDSS objectives for the four SSTWs. The study completed in January 1995 concluded that it is not feasible to modify the existing dewatering equipment at the SSTWs to achieve the required new dry solids content unless we replace all the existing sludge dewatering equipment by new membrane presses or centrifuges.

18. We upgraded the project to Category B in March 1995. We have completed the detailed design and working drawings for the project using in-house staff resources. We have scheduled the proposed replacement works to start in January 1996 for completion in August 1997.

19. Other existing sewage treatment works (STW) which require upgrading of the sludge dewatering facilities are -

  1. Shatin STW - new sludge dewatering facilities will be provided under the 133DS“Sewage Sludge Conditioning Facilities”;
  2. Cheung Chau STW and Mui Wo Oxidation Ditch - new sludge dewatering facilities will be provided under 208DS “Outlying Islands Sewerage Stage I Phase I”; and
  3. Stanley STW - the existing dewatering facilities are able to produce dewatered sludge with dry solids content of 25%. In view of the small sludge quantity of about 1,000 cubic metres per year being produced, it is not cost economical to upgrade the existing dewatering facilities in the coming few years. We estimate that the additional expenditure arising from the higher charges for handling the wetter Stanley STW sludge will be $0.4 million per annum.
1 -- The function of a secondary sewage treatment works is to remove biodegradable organics and suspended solids from wastewater using a biological treatment process after the screening and seimentation processess used in the sewage screening plants and primary sewage treatments works respectively.

Last Updated on 8 December 1998