Abstract: A method for improving the SVI of the mixed liquor residing in an aeration tank in an activated sludge treatment system which also includes a sedimentation tank is provided. This method comprises the following steps:(A) feeding a concentration system other than said activated sludge treatment system with at least one of the mixed liquor in the aeration tank, the mixed liquor withdrawn from a line connecting the aeration tank and the sedimentation tank, and the settled sludge leaving said sedimentation tank;(B) introducing the concentrated sludge from said concentration system into a first mixing zone where said concentrated sludge is mixed with a resin having high water absorbing capability;(C) introducing a gel-like sludge-resin mixture from said first mixing zone into a second mixing zone where a metal salt is added to said mixture; and(D) returning to said aeration tank a contracted gel-sludge mixture leaving said second mixing zone.