Direct osmosis cleaning

A method of cleaning a fouled Reverse Osmosis (RO) membrane in a module for RO separation, said membrane having a feed side and an opposite permeate side and extending between a front end and a rear end of said module, said module having a front-end feed port and a rear-end brine port in communication with said feed side of the membrane, and at least one permeate port in communication with said permeate side of the membrane, a normal RO separation process in the same module including feeding raw saline solution, which comprises solvent, dissolved salts and a potential foulant, with osmotic pressure PO1, to said front-end feed port at gauge pressure PF1>PO1, collecting weak saline solution called permeate from said permeate port at gauge pressure PP2, and removing residual brine from said rear-end brine port at gauge pressure PB3>PO1, the foulant accumulating at the feed side of the RO membrane, said cleaning method comprising

• a) feeding dilute saline solution to the permeate side of the RO membrane under gauge pressure PD4 and osmotic pressure PO4;
• b) feeding concentrated saline solution to the feed side of the RO membrane under gauge pressure PC5 and osmotic pressure PO5, a net driving differential ΔPN resulting from pressures PO4, PD4, PO5, and PC5 being directed to the feed side, whereby the solvent is being sucked from the permeate side of the RO membrane to the feed side of the RO membrane, penetrates into the interface between the membrane and the accumulated foulant, and separates the foulant from the membrane surface;
• and
• c) withdrawing the concentrated saline solution together with the separated foulant and the penetrated solvent from the feed side of the RO membrane,
• wherein
• said gauge pressure PC5 is between said pressures PF1 and PO1 in the normal RO process, while said ΔPN directed to the feed side is provided by raising said gauge pressure PD4 with respect to said gauge pressure PP2 in the normal RO process, and wherein PD4≦PC5.