PURPOSE: To perform biological purification with saved energy consumption, by using outdoor air, air in a blower chamber, exhaust gas from an apparatus for heat-treating excess sludge or other waste matter, etc. as the supply source of aerating air for the biological purification of waste water.

CONSTITUTION: Excess sludge 13' formed in a biological purifying step is further thickened by a sludge-thickening cell 13 and changed into dehydrated cake by a dehydrator 14. It is then changed into incineration ash 18 through a drying oven 9 and an incinerator 8. High-temperature incineration exhaust gas 17 is used for heating in the drying oven 9, changed into dry exhaust gas 19 and then discharged through a combustion deodorizing oven 10, an indirect heat-exchanger 11 and a smokestack to the atmosphere. Since the air-sucking openings of a blower 7' are passed to the outdoor by a valve 1', into a blower chamber 7 by a valve 2' and into a drying-incinerating chamber 12 by a valve 3', air 6 having a proper temperature is supplied to the aerating cell 3 by adjusting the opening rates of the valves 1'W3'.