trough 106 so as to form an opening 110 through which floating solids in the holding tank can be moved into the solids receptacle 24 the movable portion 108 can be moved by various mechanisms including but not limited to a suitable actuator not shown such as a pneumatic or hydraulic cylinder or electric motor which actuator can also be controlled by the control system 104 however as further described below in the preferred embodiment the movable portion 108 is simply pushed open by the skimmer of the skimmer apparatus when it nears the trough 106 one or more water level sensors 119 are arranged in the holding tank for sensing the level of the upper surface of the water the sensor 119 can comprise a float type device or other suitable sensor capable of detecting the water level in the tank the sensor 119 is connected to the control system 104 that controls the operation of the pump and valve in the outlet conduit 98 although not illustrated it will be understood that similar pump and valve equipment is included in the inlet conduit system that feeds the waste water into the holding tank and such pump and valve equipment is also controlled by the control system 104 the control system is able to control the water level in the holding tank by suitably controlling the various pumps and valves during the quiescent holding period when solids are allowed to float to the surface the water level is controlled to be well.