Abstract: A cylindrical insert for a pre-existing pressure vessel has an integral accumulator chamber defining a head for the vessel. Inner end surfaces of the accumulator chamber, along with the walls of the pressure vessel, define a filter element chamber, the inner end of the accumulator chamber also supporting an array of permanent filter elements. The accumulator chamber is of annular form, around a central pipe which serves, alternately, as the outlet for filtered fluid passing from the filter chamber or as the inlet for filtered clean fluid to be forcefully pulsed back through the filter elements by the release of compressed air from the accumulator to flush them of an accumulation of filtered waste material. The insert system includes piping associated with the accumulator chamber for charging the chamber with highly compressed air and for selectively releasing the charge for backflushing, and control valves for placing the insert system in filtering and backflushing modes.