PURPOSE:To make a reaction tank compact, to prevent the pH rise of treated water and to simultaneously remove nitrogen and phosphorus by adding to raw water a hydrogen donor and an acidic inorg. flocculant whose molar ratio to phosphorus in the raw water is a specific value to form microflocs and subsequently filtering the microflocs by a biological membrane in a denitrifying tank. CONSTITUTION:Raw water is quantitatively supplied to a floc forming tank 5 by a pump 4 but, on the way of supply, a hydrogen donor such as methanol and an acidic inorg. flocculant are added. The acidic inorg. flocculant is added in a molar ratio of 1-3 times phosphorus in raw water because phosphorus can not be sufficiently removed in the case of below one time and the removal ratio of phosphorus is not enhanced corresponding to the increase of the addition amount even in the case of 3 times or more. By this method, a denitrifying tank 7 is made compact and a sedimentation/flocculation tank becomes unnecessary. Since the acidic inorg. flocculant is used, the pH rise of treated water can be prevented and nitrogen and phosphorus can simultaneously be removed.
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