Hong Kong's potable water quality is as good as the best anywhere! This is assured through total compliance with, and in some ways even exceeding, guideline standards for the chemical and bacteriological content of potable water set by the WHO.
Whereas in the past Hong Kong had relied exclusively on its own means of collection and storage of rainwater, it now receives nearly 80 per cent of its fresh water from Dongjiang (East River) in Guangdong province. This has enabled the water authorities to devote more resources to the treatment and, - very importantly too - the delivery of the water to customers' taps.
In the initial stage, raw water from our own storage reservoirs, or from Dongjiang, is carried by gravity or delivered by pumping through pipelines or tunnels to the treatment works. Before the water passes through clarifiers for settlement, alum (sulphate of alumina) is added to assist in the coagulation of the suspended solids into large particles, which settle on the floor of the clarifiers in the form of sludge. The sludge is then collected, thickened, treated and disposed of in the form of cakes in landfill.
Water from the clarifiers meanwhile passes on to the filtration plant, where the more finely divided suspensions are retained on sand filters. The filtered water then passes to clear water tanks.
The filtering process removes colour and turbidity, which are normally caused by solids in suspension in the water.
Hydrated lime is then used to neutralise the acidity of the clear filtered water caused by the addition of alum. This gives the water a slightly alkaline characteristic to reduce corrosion of water pipes and fittings.
The water is dosed with chlorine in solution for disinfection and fluoride compound is added for dental care.
With the completion of the entire process - and water samples taken and tested in the laboratory - the potable water is ready for distribution to customers.
Throughout the whole production process, and in the water distribution network, the quality of the water is being continuously monitored by professionally qualified chemists under the Water Science Division of the Water Supplies Department.
Samples - amounting in all to some 150 000 a year - are regularly taken at intakes, storage reservoirs, treatment works, service reservoirs, trunk mains and customers' taps for chemical, bacteriological, biological and radiological tests.
In the overall endeavour to provide the best in drinking water, the Water Supplies Department has sought also to enhance its quality control and monitoring systems.
Towards this end, its Mainland East Laboratory, having complied with the relevant technical criteria, was accredited in 1996 by the Hong Kong Laboratory Accreditation Scheme (HOKLAS) for a number of tests under the category of environmental testing.
The HOKLAS technical criteria are based on the requirements of the ISO/IEC Guide 25, as well as the relevant requirements of the ISO 9000 series of standards, including those described in ISO 9002.
The quality of the raw water supply from Guangdong has been regularly monitored ever since the scheme began in 1960.
Examinations are carried out on more than 100 physical, chemical and bacteriological parameters. These include turbidity, conductivity, manganese, dissolved oxygen, ammoniacal nitrogen, nitrate nitrogen, total phosphate and the bacteriological examination of coliform organisms.
Though there have been some signs of deterioration in raw water quality in recent years, the tests carried out show the quality of Dongjiang water is within the treatment capability of the Hong Kong treatment works.
At the same time, in response to Hong Kong's request, the Guangdong authorities have taken steps to improve raw water quality.
One of the major measures will be the construction of a closed aqueduct system conveying the water to Hong Kong from its source some 83 kilometres away.
Other measures that have also been taken, or will be taken, include the relocation of the intake source to the upper flow of the river to obtain better quality water, construction of a biological nitrification plant in Shenzhen, construction of more sewage treatment plants, removal of nearby factories, and removal of sediment from the Shenzhen Reservoir.