the drive shaft 1 10 and its protective housing 1 14 are mounted in the center of the receiving tank 1 6 in other embodiments the drive shaft 1 10 could also be mounted off center in the receiving tank 1 6 the safety valve 1 15 is concentric with respect to the drive shaft 1 10 also concentric with respect to the drive shaft 1 10 and its protective housing 1 14 is a prefilter 1 16 the prefilter 1 16 is installed near the inside wall of the receiving tank 1 6 and serves to keep coarse material such as leaves small twigs gravel etc away from the suction device 1 8 there is a certain gap between the inside circumference of the prefilter 1 16 and the protective housing 1 14 of the drive shaft 1 10 the upper suction connection 1 2 is located at the level of the maximum water or fill level the flow route is limited on the inside by a wall 1 17 of the centrally mounted safety valve 1 15 so that the incoming sludge containing liquid is diverted downward into the prefilter 1 16 adjacent to the safety valve and then emerges from this filter on all sides or at least radially toward the inside the safety valve 1 15 is limited laterally on the outside by the wall 1 17 and on the inside by the protective housing 1 14 in the enclosed space of the safety valve 1 15 a ring shaped float 1 18 is provided which at low water or fill levels closes off an opening 1 19 at the bottom the.