Forward osmotic and water hammer method of membrane cleaning

A method for cleaning a semi-permeable membrane in a pressure retarded osmosis module, the method comprising:

• providing a pressure retarded osmosis module comprising:
• an enclosure, the enclosure comprising a semi-permeable membrane with a first side and a second side opposite the first side, with foulant located on at least the second side,
• a raw saline solution having an osmotic pressure POr at a gauge pressure PGr for entering the first side of the semi-permeable membrane,
• a fluid stream having a gauge pressure PGp and an osmotic pressure POp for entering the second side of the semi-permeable membrane,
• wherein at least part of a fluid from the fluid stream penetrates from the second side of the semi-permeable membrane to the first side according to a net driving pressure (NDP) defined by a balance of pressures PGr, POr, POp, and PGp, as pressure retarded osmosis (PRO),
• wherein a remainder of the fluid exits at least periodically as a residual fluid stream from the second side of the semi-permeable membrane via an outlet,
• wherein the raw saline solution and the penetrated fluid exits as a residual brine stream from the first side of the semi-permeable membrane via a residual brine outlet;
• and
• periodically changing the balance of pressures from functioning as the PRO to functioning as reverse osmosis (RO) by altering the NDP from negative to positive, thereby inducing the RO process to create backward flow towards the second side of the semi-permeable membrane, thereby lifting the foulant and enhancing evacuation of the foulant.