For discussion
on 11 June 1999

Legislative Council Panel on Environmental Affairs

208DS - Outlying Islands sewerage, stage 1, phase 1


This paper informs Members of the Administration's intention to seek approval to upgrade part of 208DS to Category A at an estimated cost of $106.7 million in money-of-the-day (MOD) prices for carrying out improvement works at Cheung Chau sewage treatment plant.


2. 208DS was included in Category C and Category B in September and October 1995 respectively for the provision of sewerage improvement works in Cheung Chau, Mui Wo, Siu Ho Wan, Yung Shue Wan and Ngong Ping. In Cheung Chau, improvement works include the upgrading of the sewage sludge dewatering facilities at the sewage treatment plant to enhance its sludge dewatering capability and the construction of a new outfall and its associated pumping station to improve the dispersal capability of the treated effluent.

3. In April 1996, part of 208DS was upgraded to Category A as 209DS for employing consultants to carry out site investigations, environmental impact assessment and preliminary design of the Outlying Islands sewerage stage 1 phase 1 works (completed in January 1998) and detailed design of the proposed works at Ngong Ping and Siu Ho Wan. The detailed design and construction supervision of the proposed works at Cheung Chau, Mui Wo and Yung Shue Wan is undertaken by in-house staff.

4. The detailed design for the sewerage improvement works in Cheung Chau has been completed in March 1999. Construction works are scheduled for commencement in December 1999 for completion in March 2002.


5. The part of the project to be upgraded to Category A comprises the following works at Cheung Chau sewage treatment plant -

  1. upgrading of existing sludge dewatering facilities;

  2. replacement of existing submarine outfall by a new outfall 750 metres long and 600 millimetres in diameter;

  3. construction of an outfall pumping station; and

  4. construction of ancillary civil and drainage works.

A map showing the locations of these facilities is enclosed.

6. The remainder of 208DS for retention in Category B comprises the following -

  1. construction of sewerage system and sewage treatment plant at Ngong Ping on Lantau Island;

  2. upgrading of the preliminary sewage treatment plant constructed under 2429CL "North Lantau Development phase 1 remaining works" at Siu Ho Wan on Lantau Island to chemically enhanced primary level with disinfection; and

  3. construction of a sewage treatment plant and a submarine outfall at Yung Shue Wan on Lamma Island.


7. The Outlying Islands sewerage master plan (SMP) study, under project item 146DS, was completed in December 1994. The study identified the need to dewater the sludge to a dry solids content of 30% by weight before disposal at landfill sites. Dewatering sludge to this standard is necessary because wet sludge which has a higher moisture content would cause operational problems 1 at landfills as well as environmental problems arising from excessive leachate 2 generation.

8. Sewage sludge produced by the Cheung Chau sewage treatment plant is currently dewatered by means of drying beds. The effectiveness of these drying beds is reduced in the wet season when more time is required to dewater sludge to the required standard, leading to accumulation of sludge on drying beds. Excessive accumulation of sludge causes storage and odour problems. To ensure that the sludge dewatering standard is met at all times and overcome the storage and odour problems, it is necessary to replace the sludge drying beds at Cheung Chau sewage treatment plant with membrane filter presses which are capable of dewatering sludge to a dry solids content of not less than 30%.

9. In addition, it is necessary to construct a reinforced concrete sludge dewatering house to accommodate the new presses and ancillary equipment and to store the dewatered sludge before disposal. At present, the dewatered sludge is packed in plastic bags and placed on open ground within the plant before they are collected for disposal. This arrangement is unsatisfactory and causes odour nuisance.

10. The SMP study also revealed that the dispersal of the treated effluent from the existing short submarine outfall at Cheung Chau sewage treatment plant is ineffective. As the existing outfall is close to the Tai Kwai Wan beach, discharge has occasionally affected the water quality of the beach and the Adamasta Channel. To abate the water pollution problem, the SMP study recommended replacing the existing submarine outfall at Cheung Chau with a new and longer submarine outfall with improved dispersal capability. In addition, it is necessary to construct an outfall pumping station to discharge the treated effluent through the long submarine outfall.


11. The capital cost of the proposed works is estimated to be $106.7 million in MOD prices, made up as follows -

$ million
(a) Upgrading of existing sludge dewatering facilities 29.3
(i) civil works15.4
(ii) electrical & mechanical works13.9
(b)Replacement of existing submarine outfall35.8
(c)Construction of an outfall pumping station21.1
(i) civil works16.4
(ii) electrical & mechanical works4.7
(d)Construction of ancillary civil and drainage works3.1
Sub-total 97.0(at December
1998 prices)
(f)Inflation allowance 9.7
Total 106.7(in MOD

12. Subject to approval, the expenditure will be phased as follows -

Year$ million
(Dec 1998)
$ million
1999-20002.01.02625 2.1
2000-200127.71.06217 29.4
2001-200245.01.09934 49.5
2002-200314.91.13782 17.0
2003-20047.41.17765 8.7

13. The MOD estimates are based on Government's forecasts of trend labour and construction prices for the period 1999 to 2004. The civil works of the project will be tendered under a re-measurement contract because of the uncertain quantities of dredged and filled materials for the outfall construction and piling works for the sludge dewatering house. The electrical and mechanical works will be tendered under a fixed price lump sum contract.

14. The additional annual recurrent expenditure for maintenance works is estimated to be $2.1 million.

15. Based on the current level of expenditure on operation and maintenance of sewerage facilities, the proposed works will lead to an increase in the recurrent cost of providing sewage services by about 0.4% which will need to be taken into account in determining sewage charges.


16. The sewerage extension and improvement works proposed under the SMP study was presented to the Islands District Board on 24 April 1995. The Board supported the implementation of the works. Also, after completing the preliminary design for Outlying Islands sewerage stage 1 phase 1 works under 209DS, the Provisional Islands District Board (PIDB) was consulted on 23 February 1998. The PIDB supported the implementation of the stage 1 phase 1 works.


17. The submarine outfall is a designated project under Schedule 2 of the Environmental Impact Assessment (EIA) Ordinance (Cap. 499) and an environmental permit is required for its construction and operation. The EIA study for the Outlying Island sewerage stage 1 phase 1 works was completed in October 1997. The EIA concluded that the environmental impact of the submarine outfall and the odour from the pumping station can be controlled within the established standards and guidelines through the implementation of mitigation measures. The Advisory Council on the Environment endorsed the EIA in October 1997.

18. The pumping station and the sewage sludge dewatering facilities are not designated projects. The Director of Environmental Protection completed an Environmental Review on the dewatering facility in January 1997 which concluded that no EIA was required. To control the odour impact, both the pumping station and the sludge dewatering facilities will be enclosed and provided with deodorisation installations.

19. The cost for implementing the mitigation measures is estimated to be $4.2 million. This cost have been included in the overall project estimate.


20. The project does not require any land acquisition.

Planning, Environment and Lands Bureau
May 1999

1.High moisture sludge is more difficult to compact and forms less stable slopes at landfills.

2.Leachate is liquid draining from a landfill site. As leachate contains pollutants, exceeding the capacity of the existing leachate collection/treatment facilities would result in contamination of the groundwater in the vicinity of the landfill site.