Abstract: A back-flushing method is practiced with a back-flushing filter construction wherein a perforate filter screen providing filter openings also forms an internal chamber for receiving the effluent from a liquid stream. A filter piston is disposed in the internal chamber and operates to divide the chamber into first and second zones on opposite sides of the piston. By selectively reciprocating the piston, the effluent in the chamber is compressed and moved on both sides of the piston. Thus, the effluent back-flushes the filter openings and creates a rolling action in the liquid surrounding the filter screen on the upstream side to dislodge particulate debris accumulated on the filter screen.