PURPOSE: To prevent an excessive operation and operate a membrane separating device with a degree of efficiency commensurate with the anticipated amt. of sewage to be treated in order to reduce the cake layer adhering to the membrane surface by varying the intermittent operation time of the membrane separating device in accordance with changes in the water level in a flow-rate. adjusting tank.

CONSTITUTION: After being stored in a flow-rate. adjusting tank 1, the liq. 2 to be treated is supplied into an aeration tank 5 in prescribed portions, i.e., a control device 15 causes a liq. supply pump 6 to be driven at the lower limiting value of a second level meter 7 and to be stopped at the upper limiting value thereof. Only when the first level meter 3 indicates the lower limiting or higher value, the liq. supply pump 6 is made operative. In the aeration tank 5, a jet of air is applied from a blower device 9 through an air diffusion pipe 8 to the liq. 2 so as to aerate it and, at the same time, the surface of the separation membrane of a membrane separating device 10 is cleaned with an upward stirring stream produced by the lifting action of air. The membrane separating device 10 is intermittently operated under the negative pressure caused by a suction pump 12 and the liq. separated from the solid is discharged through a treated water discharge pipe 11 to the outside.