Direct-osmosis (DO) method for cleaning a semi-permeable membrane in a RO separation module, the membrane having a feed side with foulant located thereon, and an opposite permeate side. A normal RO separation process in the same module includes: feeding, under gauge pressure PGr, raw saline solution having osmotic pressure POr to the membrane feed side; collecting permeate (solvent) having osmotic pressure POp from the permeate side, under gauge pressure PGp; and removing residual brine from the membrane feed side. The method comprises feeding to the feed side of the membrane, for a predetermined injection time, super saline solution having osmotic pressure POs>POr, such that backward flow of permeate towards the feed side of the membrane is provided so as to lift the foulant from the feed side.