Reverse Osmosis (RO) is a membrane separation process in which feed water flows along the membrane surface under pressure. Purified water permeates the membrane and is collected, while the concentrated water, containing dissolved and undissolved material that does not flow through the membrane, is discharged to the drain. The key requirements of Reverse Osmosis (RO) process are a membrane and water under pressure.

Other requirements include prefiltration to remove suspended impurities and carbon to remove chlorine (damages the membrane). Most membranes remove 90-99+ % of the dissolved impurities depending on the impurity and the composition of water. Reverse osmosis systems (RO Systems) remove salts, microorganisms and many high molecular weight organics. System capacity depends on the water temperature, total dissolved solids in feed water, operating pressure and the overall recovery of the system.