PURPOSE: To make uniform the feed of COD an to improve an aerating effect, by attaching a bearing to the top end of a horizotal shaft to which an inclination angle-controllable buffle plate is fixed to exert horizontal rotation power inside the lower portion of a draft pipe at the water-suction side.

CONSTITUTION: An inclination angle-controllable buffle plate 16 to apply horizontal rotation power to a horizontal shaft 12 with a sucked water curent is fixed to the lower inside of a draft pipe 5, and a bearing 13 is attached to the top end of the horizontal shaft 12. The bearing 13 is supported at its upper and lower parts by track frames 14a, 14b. Organic waste water and returned aqueous sludge are poured into an aeration vessel 2 through a pipe 1, and a rotary shaft 10 is rotated to perform aeration by the operation of a motor 9. When the water comes into collision with the buffle plate 16a, the buffle plate 16a rotates on a horizontal plane between the track frames 14a and 14b. When the purification of water is completed, a pump 6 is driven to feed the water through a pipe 7 into the next treating vessel. At this time, the water surface inside the vessel 2 is gradually lowered. When the water surface is lowered to a predetermined water level, the drainage by the operation of the pump 6 is stopped.