Waste water treatment method using membrane separation-activated sludge

In a waste water treatment method using membrane separation-activated sludge, when, during filtration while supplying air using an immersion-type membrane separation unit loaded with multiple flexible membranes, the filtration differential pressure of the membranes at a set filtration flow rate exceeds a specified value P2 with respect to the filtration differential pressure P1 of the initial period of operation, membrane filtration is continued with air supply stopped until the filtration differential pressure P3 becomes P3≥P1+20 [kPa]. Then with the filtration flux or filtration pressure being smaller or negative compared to when filtering while supplying air, air is supplied and after the filtration differential pressure P4 during filtration reaches P4≤P1+5 [kPa], filtration is returned to filtration while supplying air.