Shells for safer seas
discarded oyster shells are to be used to clean household wastewater 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,' Asahina was quoted recently by the British media.
The prototype is being built in Oshima, an island 200km north of Tokyo where oysters are farmed, and will start functioning in March 2000. About 250 tonnes of shell will be required annually to maintain layers 4m deep in the plant's six filtration tanks. Waste from some 250 households will be filtered through these tanks, passing through each in turn. Untreated wastewater in Oshima has a biochemical oxygen demand ( bod ) of some 200 parts per million (ppm). bod is the important parameter in monitoring wastewaters from industrial and domestic sources. It plays an important role in quantification of organic pollution in water and wastewater.
The Oyster shell treatment will bring bod levels down to a safe 20ppm. Similarly, suspended solids will be reduced from 200ppm to 50ppm, Asahina said.