PURPOSE: To execute continuously an initial adsorption treatment by a low sludge load operation by adopting an aerobic system over the entire part of a treatment tank including a solid-liquid sepn. part, i.e., all aerobic system by making use of upward current circulating aeration.

CONSTITUTION: The conventional sepn. part 3 is connected integrally to the down stream side of a divided sewage inflow part 2 where thorough mixing is executed. Said part is operated under the BOD-MLSS load having inclination or having small inclination if such inclination exists within the range where the propagation rate of the sludge is approximately equal to or smaller than the attenuation rate of the sludge. The initial adsorption effect of the activated sludge is thus continued. More specifically, the aerobic system over the entire part of the treatment tank 1 including the part 3, i.e., all aerobic system is adopted by making use of the upward current circulating aeration by a circulating aeration tank 4 to eliminate an oxygen deficient part. The MLSS concn. is thereby considerably increased and the inclined load over the entire area of the tank inside is eliminated. The initial adsorption treatment by the low sludge load operation is thus continuously executed.