Abstract: A filter apparatus comprising a container, a plurality of sets of rotatable filter elements disposed in the container, each set comprising first and second circular filter plates of different diameters with one or more holes provided in each plate at predetermined distance from the center thereof, and interleaved with third plates of a smaller diameter than the first and second plates, with the sets disposed in a line and sealed in the container with exits provided aligned with the holes. The sets of filter plates are rotated in the same direction. Unfiltered material is supplied to the container at the top and due to the centrifugal force of the rotating plates, the sludge is separated from the liquid and moved toward the outer peripheries of the plates, and then transferred from one plate to the next plate down the line until the sludge is accumulated toward the end of the line.