A system for both producing high quality water from saline water and producing power from two aqueous streams of different salt concentrations, which system comprises: a semipermeable membrane element capable of operating in either an RO or a PRO mode, which element has a first subchamber and a second subchamber separated by semipermeable membrane material said element also having an inlet and an outlet connected to said first subchamber and first and second ports (21,25) connected to said second subchamber, a hydroturbine power generator (31,33), first conduit means connecting said outlet from said first subchamber of said element to said hydroturbine, means for delivering a liquid feedstream of high salt concentration water from a source of such saline water under high pressure which includes a second conduit leading to said first subchamber inlet, third conduit means having first and second branches connected to said first port of said second subchamber, means for supplying low salt concentration water to said second subchamber through said first branch (41 a), while said second branch serves as a discharge tube for aqueous permeate, means for withdrawing some low salt concentration water from said second port of said second subchamber to achieve sweeping flow along said semipermeable membrane material, and control means for alternately operating said system either in a power-producing PRO mode wherein said first branch operatively connects said low salt concentration water supply means to said first port of said second chamber, or in a high quality water-producing RO mode wherein said second discharge branch (41 b) is operatively connected to said second subchamber for flow of permeate therethrough. a semipermeable membrane element capable of operating in either an RO or a PRO mode, which element has a first subchamber and a second subchamber separated by semipermeable membrane material said element also having an inlet and an outlet connected to said first subchamber and first and second ports (21,25) connected to said second subchamber, a hydroturbine power generator (31,33), first conduit means connecting said outlet from said first subchamber of said element to said hydroturbine, means for delivering a liquid feedstream of high salt concentration water from a source of such saline water under high pressure which includes a second conduit leading to said first subchamber inlet, third conduit means having first and second branches connected to said first port of said second subchamber, means for supplying low salt concentration water to said second subchamber through said first branch (41 a), while said second branch serves as a discharge tube for aqueous permeate, means for withdrawing some low salt concentration water from said second port of said second subchamber to achieve sweeping flow along said semipermeable membrane material, and control means for alternately operating said system either in a power-producing PRO mode wherein said first branch operatively connects said low salt concentration water supply means to said first port of said second chamber, or in a high quality water-producing RO mode wherein said second discharge branch (41 b) is operatively connected to said second subchamber for flow of permeate therethrough.