Abstract: In apparatus for the reverse osmosis purification of water or other fluid in a module containing a membrane, the water is forced into the module under pressure using two piston-cylinder assemblies mechanically interconnected with the pressurized fluid from the module applied to the rear face of the piston which, from its front face, is driving water into the module, a low pressure continuously operating pump providing a low pressure on the other piston to supply the necessary extra pressure. Valve means automatically reverse the functions of the two cylinders at each end of each stroke.