apparatus for dehydrating sludge originating from water purification stations , comprising a draining grid part positioned above a pressing - belt filter part , and a device for washing the grid wherein the device is positioned above the grid , so as to limit the drop height of the flocculated sludge between the grid and the belt , thus avoiding deflocculation of the sludge between the thickening stage and the dehydration stage , the washing of the grid being performed concurrently with the movement of the sludge on the grid .