To provide a treatment system using a membrane separation activated sludge method which performs solid-liquid separation of water in which activated sludge is mixed, in a treatment tank by a membrane separator installed in the treatment tank so as to be immersed, while treating sewage by the activated sludge after introducing the sewage into the treatment tank, can reduce the biological load of the activated treatment in the treatment tank and the solid-liquid separation load of the membrane separator, and prevents the membrane surface of the membrane separator from being clogged by a residual coagulant.

A feed passage 20 that feeds the sewage into the treatment tank 11 is equipped with a mixing part 21 that adds a polymer coagulant to the sewage and mixes them, and a solid-liquid separator 22 that is located downstream of the mixing part 21 to separate pollutants from the sewage. An auxiliary feed passage 44 is formed for making a part of the sewage flowing through the feed passage 20 branch out of the upstream of the mixing part 21 and joining it to the downstream of the solid-liquid separator 22.

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