A pump powered partly by a mechanical drive and partly by a pressurised fluid flow, offering energy savings up to 90% in reverse osmosis systems. The pump consists of one or more piston-in-cylinder assemblies. One face of each piston provides a pumping action while a pressurised driving fluid flow is applied to the opposite piston face, thus supplementing the drive force applied to each piston via a piston rod. Input and output of the pumped flow is controlled by pump valves of a non-return poppet type operated by the pumped flow. Input and output of the driving fluid is controlled by poppet-type valves controlled by connection to the pump valves. There are means to control piston movement relative to the cylinder and to the piston rod, and a two-part pump valve to simplify connection to the motor valves.