forward and reverse movements of the reciprocating assembly 26 in this way energy is recovered from the pressurized brine to help provide pressurized feed water to the membrane unit 16 fig1 b shows the hydraulic drive unit 18 the hydraulic drive unit 18 has a hydraulic pump 42 two or more hydraulic cylinders 44 a valve set 45 and a controller 90 optionally the valve set 45 may have a control valve 46 for each hydraulic cylinder 44 each hydraulic cylinder 44 has a hydraulic piston 52 connected to a piston rod 40 referring to fig1 a each piston rod 40 is connected to the reciprocating assembly 26 of a water cylinder 14 the hydraulic piston 52 optionally includes an extension 41 that extends from the first side 56 of the hydraulic piston 52 the extension 41 preferably has a different cross sectional area than the piston rod 40 in particular the extension 41 may have a smaller cross sectional area than the piston rod 40 the first side 56 and the second side 60 of the hydraulic piston 52 preferably have different surface areas in particular the first side 56 of the hydraulic piston 52 preferably has a larger surface area than the second side 60 for example the ratio of the surface areas of the first and second side 56 60 may be within about 10 of the ratio of the forces acting on the water pistons 32 as they move in the forward and reverse directions within the water cylinder 14 the ratio.