To provide a method of preventing the flow out of activated sludge which hardly has an effect upon activated sludge microorganisms and surely accelerates the sedimentation of the activated sludge in a waste water treatment method by introducing a mixed liquid into a sedimentation tank to sediment and separate and returning a part of the seprated sludge to an aeration tank in an activated sludge method for introducing waste water to the aeration tank holding the activated sludge and biologically treating the waste water.

In the waste water treatment method for introducing the mixed liquid to the sedimentation tank to sediment and separate and returning a part of the separated sludge to the aeration tank in the activated sludge method for introducing the waste water to the aeration tank holding the activated sludge to biologically treat the waste water, the flow out of the activated sludge is prevented by adding a water soluble polymer dispersed liquid as a precipitant in which a cationic or an amphoteric water soluble polymer fine particle having ≤100 μm particle diameter and a polyalkyl imine neutralized product are coexisted into a path from the aeration tank to the sedimentation tank.

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