appropriate agitation and oxidizing an organic substance by the effect of the microorganism the precipitation step is a step of cutting off the inflow of oxygen into the reaction tank and precipitating the activated sludge with denitrification in an anaerobic state the effluence step is a step of collecting the supernatant of the reaction tank and discharging it as a treated water the control of the amount and quality of activated sludge and the aeration amount with respect to the inflow load and the control of the oxidation of bod and cod the oxidation nitrification of ammonia and the reduction denitrification of the resulting nitric acid and nitrous acid are very important from the standpoint of maintaining the purification function in the reaction tank if these controls are improper the purification function for bod cod ss and tn is impaired and a predetermined quality of treated water cannot be maintained the control of the activated sludge is performed by a treatment of drawing excess sludge out of the system but a constant concentration cannot be maintained at all times the control of the inflow load is performed by disposing a raw water storage tank having a required capacity before the reaction tank and controlling the inflow into the reaction tank but accurate equalization of the amount and quality cannot be attained accordingly the primary means of the operational management for a purification tank is in the inflow aeration.