The addition of disinfectant producing a cancer-causing substance is necessary to ensure drinking water is germ-free, the Water Supplies Department insisted yesterday. Department officials admitted that chlorine disinfectant, once added to water, would produce among its by-products tricholromethene, which can cause cancer. Chief chemist Cheung Tze-leung said if disinfectants were not used to clean the water of germs, an outbreak of diarrhoea and gastro-enteritis could occur quickly. 'On the other hand, the chances of getting cancer by disinfectants is rated by the World Health Organisation as 100,000 to one after 70 years.' The organisation has recommended in its guidelines that in each litre of water, the substance's presence should not exceed more than 200 micrograms. In Hong Kong, it has never exceeded 50 micrograms. The department expected less chlorine and other disinfectants to be used once water quality has been improved with the completion of a $4.1 billion project in Guangdong by 2002. Guangdong is moving a pumping station up the East River - a major source of Hong Kong's drinking water - after complaints about pollution downstream. There are also plans to build a treatment plant at the Shenzhen Reservoir to lower the ammonia content of the water. In a paper to be discussed at Friday's joint meeting of the Legco environmental affairs panel and the planning, lands and works panel, the Works Bureau admits 'test results at the border have indicated an increasing trend of pollution caused mainly by waste water'.