Abstract: A scum removal apparatus of a trough type including a weir movable in response to a movable element moving within a treatment pool. This apparatus includes an actuating mechanism operable in response to the movable element moving within the treatment pool and a transmission means for driving the weir, in which the transmission means includes a hydraulic pump and a control valve such that, while the actuating mechanism drives the pump, a hydraulic drive machine is driven selectively in one of two directions opposite to each other with the control valve being switched, thereby to selectively float and sink the weir above and below a water surface.In a different mode of carrying out the invention, the actuating mechanism includes a plurality of cam means each having a series of projections and recesses alternating with each other, said cam means being rotatable about a rotary shaft, and the transmission means is in the form of a mechanical transmission mechanism including the rotary shaft.