<<返回上一页

Oysters do their bit for cleaner beaches

发布时间:2019-03-08 08:04:02来源:未知点击:

By Peter Hadfield in Tokyo DISCARDED oyster shells are to be used to clean household waste water in a prototype plant being built in Japan. The plant will purify water that would otherwise pollute local beaches. “Oyster shells harbour large numbers of anaerobic and aerobic microbes on their surfaces,” says Toyokuni Asahina, one of the plant’s designers at the Kesennuma City Council. “Dirty water is food for these microbes.” The £3.5 million prototype is being built on Oshima, an island 200 kilometres north of Tokyo where oysters are farmed, and is due to start working in March 2000. About 250 tonnes of shells will be required annually to maintain layers 4 metres deep in the plant’s six filtration tanks. Filthy kitchen, bath and laundry water from 265 households will be filtered through the tanks, passing through each in turn. The more organic waste there is in water, the more oxygen microorganisms need to break it down. Untreated waste water on Oshima has a biochemical oxygen demand (BOD) of about 200 parts per million, but this should be reduced to less than 20 ppm after the water has been filtered through the oyster shells, Asahina says. Suspended solids will be reduced from 200 ppm to fewer than 50 ppm. Microorganisms are already widely used to treat water, but the new plant should be more effective than gravel beds and will provide a use for hundreds of tonnes of waste shells. Another advantage is that oyster shells make acidic waste water alkaline,