The city could save as many as 170 billion litres of toilet water each year if old cisterns and pipe systems in buildings are altered and if flushing habits were changed. The Toilet Association discovered that although each person released about 550 litres of bodily waste a year in average, every person is flushing about 50,000 litres of waste water into the sea. This means about 340 billion litres of waste water is being released into the sea by 6.8 million people each year. Association secretary Henry Hung Chi-kuen, who is also an engineer, said at least half of the water could be saved if the flushing system was modified so that more water was used for flushing solid waste and less for liquid waste. New toilets usually have two flushing buttons so users can choose the amount of water used. Association vice-president Lo Wing-lok also said if the sewage pipes could be a two or even three-pipe system, then most of the waste could be recycled for other uses. At present, most buildings have a one-pipe system where all solid and liquid waste is flushed into the same pipe, as is other waste water from the shower and wash basin. He said the World Toilet Summit, held in Moscow earlier this month, presented examples of the multiple-pipe system, with the toilet basin having two holes instead of one to collect liquid and solid waste respectively. The waste would be flushed into separate collection tanks for further treatment. Urine is full of phosphorous that can be treated and recycled into fertiliser. Dr Lo, who is also an expert in infectious diseases, said separation of waste could help improve sewage treatment before it was released into the sea, which would decrease the likelihood of spreading viruses. The association admitted that renovating the sewage system citywide would be costly but could be cost-effective in the long term. 'It costs a lot to set it up, but it will cost more to maintain the quality of sea water in Hong Kong,' Dr Lo said. He suggested the concept could first be adopted in the rural area, with village houses adopting the two-pit toilets with one pit for solid waste, which uses ash to cover the waste. The waste would then eventually become fertiliser.