Abstract: An inlet pipe having two branch pipes and an outlet pipe having two branch pipes, between the inlet and outlet pipes of each branch, there being a dump valve and a dump control valve, each outlet branch pipe containing in it a filter, the dump control valve sensing the pressure on each side of the filter and being operable, on pressure differential building up, to open the dump valve; by opening of either one of the dump valves, the water will flow through the inlet tube, the inlet branch tube of the outer dump valve and rearwardly through the filter element. The filter element will thereby be backwashed, and the particulate material which has been built up will be at least partly discharged through the dump valve to drain.