Reverse osmosis is the process of forcing a solvent from a region of high solute concentration through a membrane to a region of low solute concentration by applying a pressure in excess of the osmotic pressure. This is the reverse of the normal osmosis process, which is the natural movement of solvent from an area of low solute concentration, through a membrane, to an area of high solute concentration when no external pressure is applied. The membrane here is semipermeable, meaning it allows the passage of solvent but not of solute.
RO systems use Cross Filtration and not the standard filtration where the contaminants get accumulated in the filter media. In this case, the solution passes through the filter in two different ways. Contaminated water goes in one direction and the filtered water in other. Cross flow filtration cleans the contaminant build up so that the membrane surface remains clean. When the water molecules pass through the membrane under very high pressure, they pass through it, and salts or contaminants hold back. They get discharged through the system by the reject stream. Rejected water may be drained out or sent to the feed water supply, depending on the design.