LegCo Panel on Planning, Lands and Works
Information Paper on
Provision of Sea Water for Flushing Purposes
The use of fresh water for flushing is a normal practice in all cities in the world. Hong Kong is one of only a few places in the world which have successfully used sea water extensively for flushing purpose, thus achieving a substantial saving in fresh water. In fact, Hong Kong also utilises sea water on a large scale for fire fighting purposes. When Hong Kong introduced sea water for flushing in the late 1950's, it was a pioneer move driven by the then acute shortage of fresh water for potable use, and the use of sea water for flushing was to conserve fresh water. At the initial stage, sea water was pumped to supply government office buildings and government housing estates only. Since then, sea water supply systems were added one after another in the form of sea water pumping stations, sea water service reservoirs and sea water distribution systems, which are totally separated from but coexist with the fresh water supply systems.
2. With the progressive increase in the supply of water from the East River, the further expansion of the sea water flushing system since the mid eighties no longer aims solely at conserving fresh water. The choice of flushing water between fresh water and sea water now depends on several factors viz. the need to conserve fresh water, the economic viability of the new system, the practicability of implementation and phasing with other infra-structure development programme. In forming a proposal for flushing water supply for an area, an economic analysis is normally carried out to determine whether the use of sea water will be cost effective, taking into account both the capital and operating costs. For Sha Tin Flushing Supply System, the unit cost is $3.5 per cubic metre using fresh water against $0.8 per cubic metre using sea water (costs at 1995 price level). Sea water is therefore being provided for flushing to Sha Tin New Town. On the other hand, for Sai Kung Flushing Supply System, the unit cost is $3.6 per cubic metre using fresh water against $5.7 per cubic metre using sea water (costs at 1995 price level). Fresh water is therefore still being used for flushing in Sai Kung.
3. The cost effectiveness for provision of a separate sea water supply system for flushing depends largely on the following factors :-
- The size and density of the population of the area
This affects the per-capita capital cost for the provision of a separate sea water system.
- The distance of the area from a suitable source of sea water
The capital cost and the social disturbance to lay a substantial length of sea water trunk main for the delivery of sea water to the supply area is large if the area is far away from a suitable site for a sea water pumping station. The associated recurrent operation and maintenance costs for the trunk and distribution mains are also large.
- The quality of sea water at the potential intake points
If the quality of sea water at the potential intake points is below the target standard, extra treatment facilities could be required. This increases the necessary capital investment as well as the associated recurrent operation and maintenance costs of the sea water supply system.
- The altitude of the area
Additional costs both capital and recurrent are required for the provision of booster pumping facilities to high level areas.
4. For a large new town development where it is usually justifiable on economic grounds that flushing supply should be of sea water, the provision of a sea water supply system has to be carried out in concert with the phasing of the new town development. Usually, the construction of the sea water pumping station at the seafront can only be completed at a later stage because the reclamation under the new town programme would not have proceeded to a stage which permits the construction of the seafront pumping station and the delivery main from the pumping station to the supply zone. Thus, initially, fresh water is fed into the flushing water distribution system to provide a supply of fresh water specifically for flushing purpose. Flushing supply is later on switched to sea water when the sea water pumping facilities are provided.
5. There may be cases that are not justifiable on economic grounds to provide sea water for flushing to a developing area initially due to the relatively small population involved until a later stage. In such circumstances, separate flushing and potable water distribution networks are still provided at the same time although fresh water is fed initially into the flushing distribution network. In such case the land sale conditions require all internal plumbing for flushing fitments to be constructed of sea water resistant materials even though sea water supplies cannot be provided. This facilitates the conversion of flushing supply to sea water in the future. Generally speaking where sea water supplies are available consumers are obliged to use sea water for flushing and the use of fresh water for flushing is prohibited.
6. At present, there are 19 sea water pumping stations situated at various locations at seafront as shown on Fig. 1. Together with 16 inland booster pumping stations, 42 sea water service reservoirs and about 1,000 km sea water mains, they provide about 440,000 cubic metres of sea water per day for flushing. This represents about 75% of the territorys total current flushing consumption. Projects for extension of sea water flushing systems to Tai Po New Town and Tseung Kwan O New Town are being implemented and are programmed for completion in July 1996 and early 1998 respectively. Another project for providing sea water flushing supply to Pokfulam and Wah Fu Estate is in the design stage and is programmed to be completed in early 1999. A site has been reserved at Tung Chung Planning Area 57 for the proposed Tung Chung Sea Water Pumping Station for supplying Tung Chung and Tai Ho areas at North Lantau. Formation of this site will be completed in early 1999, and it is currently scheduled that the pumping station and the whole sea water supply system will be completed by 2003.
7. It is expected that by the end of the nineties, the percentage of population using sea water for flushing purpose will increase to 90% approximately as compared with 65% in 1991. The remaining 10% of the present population lies outside the already planned sea water flushing supply zones. It is presently uneconomical to provide sea water to these areas for flushing purpose on account of their remoteness from suitable source of sea water or their small population, but a review will be made from time to time to consider any change. These areas include the Peak and South in Hong Kong Island, Yuen Long, Sheung Shui and Fanling, Sai Kung and the Outlying Islands (except North Lantau as mentioned in paragraph 6 above).
8. According to the schedule of charges, consumers are not specifically charged for either sea water supplies or fresh water supplies for flushing, except when the use of fresh water for flushing is in excess of 30 cubic metre per household per 4 months. The cost of providing either fresh or sea water for flushing is met through the general water tariffs. The 30 cubic metres per household per 4 months are normally adequate for domestic flushing purpose. Fresh water provided for flushing is metered for the purpose of charging the quantities above this amount.
9. Whether an area would be supplied with sea water or fresh water for flushing is based on economic analysis and other practical considerations. If the area is not provided with sea water for flushing, fresh water would have to be provided. Therefore the provision of fresh water for flushing in some areas will not result in a higher cost for the supply of flushing water as some people may have thought.
10. The flushing water charging system has no effect on the sewage charging scheme introduced last year. The flushing discharge of all consumers is excluded from the sewage charging scheme, irrespective of whether the flushing medium is fresh water or sea water.
Water Supplies Department
19 June 1996
Last Updated on 21 Aug, 1998