Abstract: The new floating sludge evacuating unit for circular tanks having a drain, and a scraper blade movable in relation thereto is characterized in that the scraper blade is lifted repeatedly in order to evacuate the tank surface by sections, advanced to a position remote from the drain where it is stopped and lowered, while the drain follows the scraper blade and pushes the floating sludge together, and that prior to the lowering motion causing the floating sludge to be evacuated a push-down unit causes the intake edge of the drain to be pre-lowered to a position which takes the level in the tank into account. Then, the lowering path for floating sludge evacuation will always be correct, and evacuation of water will be avoided. A scraper blade interacting with several stationary drains is operating according to the same principle. It is lowered behind every drain and pushes the floating sludge toward the next drain.