PURPOSE: To improve the quality of treated water and to reduce the cost of membrane washing by supplying the ozone treated water from the filtration side of a membrane separator and backwashing the membrane in the case the water to be treated is subjected to a biological treatment and the membrane permeated water is subjected to an ozone treatment after the biologically treated water is subjected to a membrane sepn. treatment.

CONSTITUTION: The raw water is introduced from a piping 11 into an aeration tank 1 and is biologically treated. Namely, the permeated water side of the membrane separator 2 disposed in the bottom of the aeration tank 1 is reduced in pressure and sucked by a pump P and, therefore, the raw water is permeated through the membrane separator 2 and is treated. The permeated water is introduced into an ozone treatment tank 3 and is treated by the ozone blown from an air diffusion pipe 3A. Further, the ozone treated water is withdrawn from a piping 13 and is discharged outside the system. The pump P is run backward to allow the water in the ozone treatment tank 3 to flow backward from the piping 12 to the membrane separator 2 at the time of washing the membrane, by which the stains and clogging materials of the membrane are effectively removed by using the residual ozone on the ozone treated water.