It is a solute-solvent separation process that uses a dense asymmetric semi-permeable membrane that is highly permeable to water and highly impermeable to solutes. A pressurized feed solution is passed over one surface of the membrane. As long as the applied pressure is greater than the osmotic pressure of the feed solution, “pure” water will flow from the more concentrated solution to the more dilute through membrane. RO membrane is widely used to remove salts from seawater or brackish water. RO membranes can remove solutes in the 0.0001 to 0.1 micron size range.