Abstract: A method and apparatus for treating a liquid mixture to separate a clarified fraction therefrom leaving a concentrated sludge fraction involve injecting a flocculent material into the liquid mixture; and inletting the liquid material and the flocculent maerial tangentially into a vertical tank having a perforated plate spaced from its upper end dividing the tank into an upper chamber and a lower chamber, a to produce a flow in the lower chamber having a rotational velocity in the horizontal direction adjacent the outer periphery of the lower chamber, and a lower rotational velocity in the horizontal direction adjacent the center of the lower chamber. The holes in the perforated plate define reduced-area passages from the lower chamber to the upper chamber producing a vertical flow of low velocity in the lower chamber and of higher velocity through the passage in the perforated plate.