PURPOSE: To enhance purification efficiency by eliminating the break-up or expansion of an aerobic microorganism, by providing a flow amount detecting apparatus for detecting the flow amount of inflow sludge and operating a blower alone or in a combination of two or more by the output of said detecting apparatus.

CONSTITUTION: When the sewage flow amount of an inflow pipe 1 gradually increases and a water level reaches a lowermost level flow amount detecting sensor 73a, a detection signal is outputted from the sensor 73a. The input terminal of a switch circuit 8 is turned ON by this signal and a driving current is flowed to a blower 4a to operate only the same having an 1/4 air emitting amount. When the sewage flow amount is further increased and a water level reaches a next low level flow amount detecting sensor 73b, the detection signal thereof is outputted and a driving current is flowed to a blower 4C to operate only the same having an 1/2 air emitting amount.