and thus to increase the cleaning of the screen 5 if now a mixture of sewage water and storm water flows in via the water inlet 1 in an excessive volume the excessive amount of water will flow over the spillway 4 and via the discharge channel 6 directly into the main discharge channel of the water purification plant such as indicated by the arrows in fig1 this water which flows over spillway 4 into the discharge channel 6 is now screened by the screen 5 to remove at least coarse particles or contaminants accordingly deposits 11 will form at the bottom side of screen 5 after a certain duration and adhere mainly at the upstream side of screen 5 such that the throughflow resistance thereof will be substantially increased or such deposits may eventually clog the screen 5 completely accordingly this screen must be cleaned periodically to this end the container i e the dump flushing member 8 will be filled with screen cleaning water flowing out of the supply or feed tube 21 as soon as member 8 is filled it will rotate around its pivot axis 10 as mentioned above whereby this rotational movement is an abrupt movement such that a solid body of water forming the surge of water will be generated the dump flushing member 8 is dimensioned and shaped such that a surge is shaped which acts upon the complete surface of the screen 5 the framing of this screen 5 prevents an escape of the cleaning water surge and forces the entire body of water forming the surge to flow through screen 5 and thus to enhance.