3 - Reverse osmosis, forward osmosis, and pressure-retarded osmosis

Abstract

When the aqueous solutions of two different salt concentrations are separated by a semipermeable membrane, which allows the transport of water but does not allow the transport of salt, there is a natural tendency for water to flow from the solution of the lower concentration to the solution of the higher concentration. The driving force for the water flow is the difference in osmotic pressure. This phenomenon is called osmosis. Forward osmosis (FO), reverse osmosis (RO), and pressure-retarded osmosis (PRO) processes are all based on this underlying principle. In FO the flow is the natural direction from the lower to the higher salt concentration. In RO the flow direction is reversed by the pressure applied on the solution of the higher concentration. PRO is a process by which the energy of the osmotic flow is taken out by applying pressure also on the high concentration side.