PURPOSE:To enhance a denitrification ratio with the device which admits raw water to a reaction chamber having activated sludge and executes aeration by measuring the rate of utilizing oxygen according to the nitrification reaction of a liquid mixture composed of the activated sludge and the raw water and executing aeration until the value thereof falls down to a threshold value or below. CONSTITUTION:The raw water to be subjected to the activated sludge treatment is continuously supplied through an adjusting vessel 1 to an aeration tank 2 as the reaction chamber where the raw water and the activated sludge are mixed. The liquid mixture is aerated through an air diffusion pipe 5 by a blower 4 to nitrify the oxidation catalyst of org. matter and ammoniac nitrogen. The rate of utilizing the oxygen according to the nitrification reaction of the liquid mixture is determined by an MLSS meter 6 and an rr meter 7 and the aeration is executed until the value falls down to a threshold value or below. The liquid mixture is then stirred in an anaeration state by a submersible stirring pump 3 to denitrify the liquid mixture. Namely, the product by the nitrification is subjected to denitrification by denitrifying bacteria with the org. matter in the flowing water as a hydrogen donor.
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