PURPOSE: To permit sufficient execution of a biological treatment of sewage by microorganisms and to reduce the running cost thereof by forming an aeration tank in an oval shape.

CONSTITUTION: An inside pipe 12 having a screw pump 13 in the upper part is provided to the central part of the oval-shaped aeration tank 11. An intake 16 for sewage is provided to the upper part of said tank and an air diffusion pipe 15 and a pipeline 18 to feed treated water to a carrier sepn. tank 17 are respectively provided to the lower part of the tank. The carrier sepn. tank 17 is divided by a partition wall 19 having a passage 20 in the lower part to two chambers A, B. One chamber A is connected to the pipeline 18 and is provided with a pipeline 22 for returning a carrier 10. An overflow port 21 for the treated water is provided to the other chamber B. Since the aeration tank 11 is formed to the oval shape, dead space is decreased and the satisfactory biological treatment of the sewage by the microorganisms is executed. The surface of the sewage is reduced in the area by the formation of the aeration tank 11 to the oval shape and the energy for pushing the carrier is correspondingly decreased. The screw pump of the smaller size suffice, and the running cost is reduced.