PURPOSE: To promote the formation of a biological membrane by providing an agitating mechanism for preventing the drift of a carrier and an agitating mechanism for dividing viable particles in a reaction tank contg. the carrier, and furnishing a pump for supplying a part of the treated water discharged from a settling tank to the bottom of the reaction tank along with water to be treated.
CONSTITUTION: The first agitating mechanism 20 is provided at the location of the carrier having ≥1 sp.gr. in the reaction tank 10, the second agitating mechanism 21 is furnished above the mechanism 20, and both mechanisms are made independently rotatable. The pump 57 for supplying a part of the treated water discharged from the settling tank 32 to the bottom of the reaction tank 10 along with water to be treated is provided. By such a constitution, an upward flow is generated in the reaction tank 10, hence the carriers are fluidized, the drift of the carriers is eliminated when the first agitating mechanism 20 is slowly rotated, and a stabilized carrier fluidized bed is formed. A rolling bed is formed at the upper part of the carrier fluidized bed, and a biological membrane is rapidly formed on the carrier surface and grown into viable particles. At this time, when the second agitating mechanism 21 is appropriately rotated, the enlargement of the viable particle is prevented, and the stabilized suspended bed of the viable particles is formed.