PURPOSE: To prevent nitrification reaction from lowering by setting the aerobic and anaerobic time ranges in a periodic aeration cycle in prescribed cases and increasing the amount of air supply and the rotary frequency of a stirrer until the measured DO value in a tank in the aerobic time range becomes a prescribed value.
CONSTITUTION: An aerator 3 under water is set in the inside of a periodic aeration tank 1 as a reaction tank. Air is supplied periodically to the aerator 3 under water from a blower 4 and a stirrer of the aerator 3 under water is driven continuously. Meanwhile, the treated wastewater discharged out of the periodic aeration tank 1 is led to flow in a precipitation tank 2 to be separated into solids and a liquid and a part of sludge is returned to the periodic aeration tank 1 by a pump P. In this case, the operation is carried out while the aerobic time range during which air is supplied and the anaerobic time range during which air is not supplied are set to give a prescribed time ratio (within 0.4-1.2) in the periodic aeration cycle. In the aerobic time range, during the time from the air supply start to the time measured value of a DO meter 5 becomes a prescribed value (0.5-1.5mg/range), the amount of the air supply and the rotary frequency of the stirrer are increased.
IWABE HIDEKI
KOBAYASHI YASUO
