Abstract: The invention relates to a sludge extraction, in particular for the extraction of scum in a sedimentation basin, and a process for selecting the sludge partitions. The sludge extraction of the invention comprises a float; a mixing unit to generate a fluid flow away from the sludge extraction; a catch tank with an overflow rim, which runs partially or completely around the sludge extraction; and a conveyor, which conveys the sludge, flowing into the catch tank, through an outlet line. The flow generated by the mixing unit moves the entire scum of the surface layer to the sludge extraction, where the scum can be collected. With the aid of this sludge extraction the scum and the light-weight sludge partitions can be extracted selectively and from over a large area.