PURPOSE:To improve denitrification efficiency by installing a movable gate in a water discharging part of an aeration tank, making anaerobic and aerobic conditions periodically in every prescribed period following rise of water level in the aeration tank, lowering the movable gate after treatment completion in the aeration tank, and discharging water batch-wise. CONSTITUTION:Raw water is continuously supplied to an aeration tank 1, and air supply to a gas dispersing nozzle apparatus 12 is stopped until the water level reaches the low level 14 so as to keep the aeration tank 1 anaerobic condition. A under-water propeller 11 rotates all the time independently of anaerobic and aerobic conditions, and the raw water turns round an endless water ditch 2. After a prescribed time passes, air is supplied to the gas dispersing nozzle apparatus 12 so that the tank becomes aerobic condition when the water level reaches the high level 15. As a result, anaerobic and aerobic zones are completely separated. Moreover, since untreated raw water does not overflow a movable gate 13, abrupt flow of the raw water is prevented and consequently denitrification efficiency in the aeration tank 1 is improved. After the treatment completion in the aeration tank 1, the movable gate 13 is lowered and primarily treated water above the low level 14 is discharged in batch to precipitation pond.
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