The center clarifier of an endless channel type activated sludge sewage treatment plant is provided with a steep conical floor, a conical baffle and a circular effluent launder. The upper periphery of the baffle includes an overflow trough to receive the scum which is allowed to collect between the trough and the tank wall. The feed to the clarifier is directed through a circular series of spaced submerged ports in the tank wall and toward the conical baffle below the scum collection zone to promote and allow separation of the scum which is removed by overflowing into the scum trough.