Abstract: Apparatus for reverse osmosis purification of water or other fluid by passing the fluid under pressure across a membrane, through which membrane a part of the fluid passes in purified form, makes use of a piston or diaphragm pump in which the front face of the piston or diaphragm is used to force the fluid from one end of a cylinder into the module and in which the return fluid is applied to the rear face of the piston or diaphragm so that the pressures on the two faces are substantially balanced. The piston or diaphragm is operated by a piston rod attached to the rear face of the piston or diaphragm; the system is self-regulating to give an extraction ratio equal to the ratio of piston rod area to piston area.