Abstract: Hydraulic energy is recovered from the waste brine through a control device, having a central rotatable spool piece controlling fluid flow paths therethrough. The control device, in turn, directs the brine stream of reverse osmosis systems by diverting the brine into work exchangers used to pressurize the feed liquor. All components are made of corrosion resistant materials suitable for use in brine environments. Membrane pressure is held stable by equalization lines within the control device. System capacity can be adjusted externally by changing control dwell time.