Abstract: A sedimentation basin for purifying waste water includes first and second basins separated by a baffle and a tube settler mounted substantially horizontally in the baffle. The tube settler comprises a plurality of individual tubes which communicate liquid between the two basins. A mechanism is provided to flow unclarified liquid into the first basin and slowly through the tube settler, whereby particles suspended in the unclarified liquid settle onto an inner surface of the tubes, and the resultant clarified liquid flows into the second basin. A tube cleaner is mounted on the device for intermittent registry with an upstream end of the tubes, and creates a suction backwash therein during the registry which reverses the direction of fluid flow with sufficient velocity to dislodge and draw the settled particles from the inner surface of the tubes, and flush the same into a sludge disposal duct.