Abstract: A method and apparatus for filtering a fluid flowing from a fluid supply duct in a first direction through a filtering medium in which separate solid particles or concentrate are released from the filtering medium by a back-flush. The method and apparatus has a moving means, such as a diaphragm, which receives the pressure of the fluid at both the inlet and outlet of the filter. In response to a changing pressure, such as by opening of an outlet valve, the moving means operates, e.g., the diaphragm changes its position, and operates to send a supply of the fluid through the filter medium in a second direction which is opposite to the normal first direction of fluid flow.