Reverse osmosis (1) system having a first membrane unit (2) and at least a second membrane unit (3), the membrane units (2, 3) forming a chain of membrane units, the first membrane unit (2) having a first membrane (4) separating a first feed chamber (5) and a first permeate chamber (6), a first inlet (7) connected to the first feed chamber (5), a first permeate outlet (9) connected to the first permeate chamber (6), and a first concentrate outlet (8) connected to the first feed chamber (5), the second membrane unit (3) having a second membrane (10) separating a second feed chamber (11) and a second permeate chamber (12), a second inlet (13) connected to the second feed chamber (11), a second permeate outlet (15) connected to the second permeate chamber (12), and a second concentrate outlet (14) connected to the second feed chamber (11), wherein the concentrate outlet (8) of a membrane unit (2) in the chain of membrane units is connected to an inlet (13) of a following membrane unit (3) and a concentrate outlet (14) of at least one membrane unit (3) downstream the first membrane unit (2) in the chain of membrane units is connected to a hydraulic motor (18). In such a system the energy consumption should be optimized. To this end the hydraulic motor (18) is operatively connected to a first electric machine (21).