PROBLEM TO BE SOLVED: To enable high load treatment without increasing plottage. SOLUTION: A reaction tank 10 is divided into reaction chambers 10a, 10b, 10c and raw water is allowed to flow in the reaction chamber 10a and treated water is discharged from the immersion type membrane separator 20 disposed in the reaction chamber 10c. Surface aerators 14 are arranged in the reaction chambers 10a, 10b and an air distributing member 16 distributing air from a circulating blower 18 is disposed in the reaction chamber 10c. Especially, since the immersion type membrane separator 20 is disposed above the air distributing member 16, the membrane washing of the immersion type membrane separator 20 can be performed by diffused air. Since oxygen enriched air from an oxygen generator 12 is introduced into the reaction chamber 10a to successively flow through the reaction chamber 10b, 10c, the concn. of oxygen in the high load reaction chambers 10a can be enhanced and the discharged oxygen enriched air becomes near to exhaust of aeration by air and oxygen is effectively utilized.
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