PRESSURE RETARDED OSMOSIS SYSTEM

A pressure retarded osmosis system (1) includes a membrane unit (2) having a high-pressure inlet connection (3) and a high-pressure outlet connection (4) on one side of a membrane arrangement (5) and a low-pressure inlet connection (6) and a low-pressure outlet connection (7) on the other side of the membrane arrangement (5). Such a pressure retarded osmosis system should be operated at a pressure higher than 140 bar. To this end the high-pressure inlet connection (3) is connected to a pressure exchanger (9), and the high-pressure outlet connection (4) is connected to a motor (20) and to a high-pressure inlet port (12) of the pressure exchanger (9), characterized in that the pressure exchanger (9) includes at least a first stage (10) and a second stage (11), the first stage (10) being arranged nearest to the high-pressure inlet connection (3) of the membrane unit (2), wherein the high-pressure inlet port (12) of the first stage (10) is connected to the high-pressure outlet connection (4) of the membrane unit (2) and a high-pressure inlet port (16) of the second stage (11) is provided with fluid having a lower pressure and a high-pressure outlet port (17) of the second stage (11) is connected to a low-pressure inlet port (14) of the first stage (10).