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Legislative Council
Panel on Environmental Affairs
Panel on Planning, Lands & Works

Quality of Dongjiang Water

Background

Currently about 80% of the water demand of Hong Kong is met by Dongjiang water. The remaining 20% is met by rain water collected from local catchments and stored in reservoirs.

2. The water from Dongjiang is delivered to Hong Kong via an 80-odd km long open channel and aqueduct in Southern Guangdong and is, therefore, prone to surface pollution (see the layout plan at Annex I).

3. Since the commissioning of the scheme to supply Dongjiang water to Hong Kong in the early sixties, the quality of the Dongjiang water has been found to be generally satisfactory. With the opening up of China bringing rapid urban development within Southern Guangdong, the run-off surface water has become more polluted than before. Test results at our reception point at the border have indicated an increasing trend of pollution caused mainly by wastewater in the past five years.

Water Quality Monitoring and Water Treatment

4. The quality of the raw water within Guangdong Province is controlled and monitored by the Guangdong authorities. It is closely monitored once entering our reception point in Hong Kong at Muk Wu at the border and again at all treatment works before treatment. Various parameters are analysed generally at frequencies from 4-hourly to 4-monthly, and if necessary as frequently as hourly. Attached at Annex II are water quality monitoring results on Dongjiang water as received at Muk Wu.

5. All raw waters, be they Dongjiang water or rain water collected in our reservoirs, need to be fully treated before supply to the public. The final treated water is also subject to intensive regular monitoring procedure throughout the water supply distribution system, i.e. before leaving the treatment works, at service reservoirs and at consumer taps to further ensure proper quality control and compliance with the World Health Organization (WHO) Guidelines for Drinking-water Quality.

6. The water treatment process comprises coagulation using aluminum sulphate and hydrated lime, rapid gravity filtration preceded by sedimentation at most treatment works, pH value correction with hydrated lime, disinfection by chlorine, and fluoridation.

7. There have been media coverage on test results on quality of Dongjiang water with samples reported to have been taken from Dongjiang. However in any study on water quality, the testing process must follow strictly the internationally recognized procedures, including the way to take the samples and preservation of samples prior to analysis. Otherwise, the analytical results are inaccurate and unreliable. It is also not appropriate to compare the Dongjiang water, an untreated surface water, to the World Health Organization drinking water (fully treated) quality guideline values.

8. In fact, the test procedures by the Water Supplies Department follow strictly the internationally recognized procedures and the treated water in Hong Kong conforms both chemically and bacteriologically to the 1993 Guidelines for Drinking-water Quality recommended by the World Health Organization. The test results are attached at Annex III.

9. As for the trichloromethane (chloroform) produced in the water as a result of using chlorine for disinfection and removal of ammonia, the WHO's 1993 guideline value is 200 ug/l, while the monitoring results of the Water Supplies Department for more than ten years show an average below 50 ug/l. Guideline values from WHO or standards of developed European and American countries all permitted the presence of trace quantities of chloroform as being tolerable to human health.

10. All heavy metals for which WHO has recommended guideline values come under WSD's monitoring scheme. The levels in the treated water are all well below the guideline values or close to the detection limits of sensitive sophisticated instruments used for their measurement.

Enclosed Aqueduct

11. To solve fundamentally the problem of contamination of Dongjiang water along the transfer route, the Guangdong authorities have decided to construct a dedicated closed aqueduct from Dongjiang to Shenzhen to replace the existing open channel supply system. On completion in 2002, this will solve the quality problem in the long term. The scheme will be completed and put into use in phases to achieve early improvement. Preparatory work has already started in December 1998. The progress of the project is monitored jointly by the Guangdong and Hong Kong authorities through regular meetings.

Other Improvement Measures

12. Through various established liaison channels between Hong Kong and the Guangdong authorities, the latter has implemented a number of improvement measures to protect the quality of Dongjiang water before the completion of the enclosed aqueduct:

1. strengthening the control on the development and potential polluting activities along the open aqueduct through which raw water is transferred to Hong Kong;

2. construction of sewage treatment plants along the open aqueduct for the proper treatment and disposal of sewage;

3. relocation of the intake pumping station at Dongjiang to a few hundred metres upstream to avoid contamination from one of its distributory rivers; and

4. construction of a nitrification plant at the inlet of Shenzhen Reservoir to lower the ammonia content and to increase the dissolved oxygen content of the raw water.

Works Bureau
January 1999

Annex II
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Annex II
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Annex III
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Annex III
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