PURPOSE: To miniaturize the title apparatus separating bacteria and water, to easily ensure the establishment area of the apparatus and to enable inexpensive treatment by arranging a drum screen used as a separation means in the aeration tank of the apparatus.

CONSTITUTION: In an aeration tank 5, a sufficient amount of air is sent into the org. waste water within the tank 5 from the air emitting piping 6 provided to the bottom part of the tank 5 to decompose decomposable org. matter in the org. waste water 1 by bacteria. The drum screen 14 in the aeration tank 5 separates bacteria and water while it is rotated by a motor. The water separated to enter the drum screen 14 is sucked by a pipe 22 through a suction pipe 12 and discharged through a discharge passage 10. Since the waste water is held to a stationary state on the side of the bottom part of the aeration tank 5, sludge is settled and the sludge precipitate 11 is sent to a sludge treatment process 13 to be treated.