PURPOSE: To enhance the stability and controllability of a control system by a method wherein the quality of the water flowing in an aeration tank is measured to determine necessary aeration quantity on the basis of the measured quality and the inflow amount of sewage and the aeration amount of air is controlled on the basis of the determined aeration quantity and the measured aeration amount of air.

CONSTITUTION: The DO value of the sludge water flowing in an aeration tank 1 is measured by DO meters 31, 32 and the flow rate of the sludge water is,measured by flowmeters 41,42 to be inputted to an aeration air amount regulator 8. The aeration air amount regulator 8 subjects the inputted DO value and flow rate to operational processing at every relay pump through multipliers 91-94 and subtractors 10,11 and delay compensation is performed by delay compensation devices 12,13 to output necessary aeration quantity. The DO value of the aeration tank 1 outputted from a DO meter 5 is compared with a set value by a DO regulator 14 to calculate a DO regulation signal which is, in turn, sent to an adder 16 through a multiplier 15 to be added to the necessary aeration quantity and the obtained value is sent to an aeration air amount regulator 17 as objective aeration quantity. This objective aeration quantity is compared with the air amount outputted from an aeration air amount meter 6 and an aeration air amount control valve 7 is regulated to control the DO value of the aeration tank 1.