A self-cleaning assembly comprising an enclosed casing 1 with a movable piston 31 and filters 32,33 therein. The preferred assembly has a casing 1 with a dirty water inlet 5, a first outlet 6 for clean filtered water and a second outlet 7 for the removal of sludge from the casing. The filter assembly is constructed and arranged to provide, in its normal operating mode, a liquid flow path through the filter 32,33 in a first direction between the dirty water inlet 5 and the clean water outlet 6 and, in a cleaning mode, to provide a liquid flow path in a second direction between the dirty water inlet 5 and the sludge outlet 7. In the cleaning mode, the movable member 31 compresses the filters 32,33 during the liquid flow in the second direction and contaminant is released therefrom for removal from the filter assembly via the second outlet 7. In an alternative embodiment, the sludge outlet 7 is omitted and sludge is removed from the casing via the outlet 6.