A scraping device of various contours used for dislodging sediment or “mud” accumulated at the bottom of clarifying and thickening tanks. It is activated by a rotating arm that moves it circumferentially around the tank riding close to the bottom; it thus transfers the sedimented material from the periphery to the center of the tank where it is discharged into a hopper. Plows are also used in mixing equipment of the Muller type, where they serve to continuously rake the material being mixed into the path of the Muller wheels.