PURPOSE:To automatically properly control the purification of sewage, by measuring the amount of dissolved oxygen in treated water at the outlet of an aeration tank in a sewage disposal plant, and adjusting the amount of air to be sent into the tank on the basis of the measured signal. CONSTITUTION:In purifying waste water containing organic contaminants, e.g. sewage, with microbes, air is sent through a pipe 21 into the waste water inside an aeration tank 1 to accelerate the purification with microbes. Hereon, if dissolved oxygen inside the aeration tank 1 is insufficient, the activity of activated sludge in the waste water would be lowered so that the purification would be unsatisfactory. If dissolved oxygen is excessive, sludge would be easily dispersed without precipitation so that the quality of treated water would be lowered. To inhibit these defects, dissolved oxygen in the waste water is measured by two dissolved oxygen densitometers A, B provided in the vicinity of the tank 1 and compared with a proper value preset in a judging part 3. On the basis of the resulting output signal, the flow amount of air from a blower 2 is adjusted to automatically control the amount of dissolved oxygen at the proper value. Hence, the purification is held under the optimum condition.
