AERATOR

PURPOSE:To reduce the amount of consumed electric power for a blower, by providing a sparger pipe for diffusing gas into each group of aeration tanks, a dissolved oxygen meter in optional one of the group of aeration tanks and an airflow-adjusting valve for adjusting the flow amount of gas on the basis of a measured value from the dissolved oxygen meter. CONSTITUTION:An airflow-adjusting valve 7a-7c is provided at each of the inlet sides of a sparger pipe 6a passing crosswise through the vicinity 11 of the inlet of each aeration tank, a sparger pipe 6b passing crosswise through the center 22 of each aeration tank and a sparger pipe 6c passing crosswise through the vicinity 13 of the outlet of each aeration tank. A DO value is detected by a dissolved oxygen (DO) meter 3a provided in the vicinity 11 of the inlet and inputted to a DO-controlling unit 21a. In the DO-controlling unit 21a, the process value measured by the DO meter 3a is compared with an objective DO value preset by a unit 23a for setting a DO value, to perform PI control.