To provide a sewage treatment system wherein a rotary membrane separation unit is immersed in the endless water passageway in a bioreactor to mix sewage with activated sludge, to ensure solid/liquid separation, to stably performing aerobic and anaerobic operations, and to remove also nitrogen from the sewage.

In a sewage treatment system wherein sewage is mixed with activated sludge in the endless water passageway 10 of a bioreactor 1 equipped with an aeration agitator 12 to oxidatively decompose organic sewage and the mixture is subjected to solid/liquid separation to form the solid component of activated sludge and an aqueous solution, a rotary membrane separator 13 having a structure in which a rotary hollow shaft 13a is provided with a plurality of disk-shaped filter membranes 13b and the sewage is sucked through the hollow shaft 13a is immersed in the endless water passageway 10 to separate the sewage from the sludge.