Reverse osmosis, particularly for water desalination, uses semipermeable membranes which selectively permeate purified water from a feed solution pressurized by a reciprocating feed pump driven by rotating crankshaft. Pump action is assisted by depressurizing concentrate fluid within the pump cylinder, which has an internal displacer separating feed and concentrate fluids. Directional valves controlling concentrate fluid flow to and from pump cylinder are actuated mechanically by crankshaft. A dwell interval is provided in displacer motion to permit completion of directional valve actuation while displacer is stationary and displacer rod is providing compression or decompression action to approximately equalize pressure differences across valve ports prior to valve actuation. Detrimental concentration polarization is reduced by reducing pressure and flow fluctuations across membranes by using a differential surge absorber, a multi-feed cylinder pump in which the feed cylinders are phased apart, or a single feed cylinder pump with one or more recirculating cylinders phased equally from the feed cylinder to supply pulses of recycled concentrate fluid. In the multi-cylinder pumps, actuation of the piston rod of one cylinder can be used to actuate a directional valve of another cylinder, which can be a spool valve integral with displacer rod.