The chaff produced during the crop season that was attached with calcium was used in an upward type column for the purpose of recovering highly accumulated phosphorus in groundwater. The amount of phosphorus recovery obtained in various hydraulic retention time was 1.5–2.0 mg per 1 g of chaff. Owing to the reaction of calcium on the surface of chaff charcoal with phosphate ion in groundwater, calcium phosphate compound was produced and might be existed as hardly soluble calcium phosphate as it did not elute in citric acid. Thus, phosphorus recovered could not be directly used as fertilizer. It needed support from mycorrhizal fungi in soils, rainwater, etc. to be usable for the farmland. It was also confirmed that pH value was an effective indicator in phosphorus recovery performance because the results show that concentration of phosphate ion in treated water was depend on pH value. Phosphorous recovery using calcium-carrying chaff charcoal is expected to mitigate the severe eutrophication problems at Hachiro Lake.

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