Reverse osmosis (RO) is a membrane-technology filtration method that removes many types of large molecules and ions from solutions by
applying pressure to the solution when it is on one side of a selective membrane. The result is that the solute is retained on the
pressurized side of the membrane and the pure solvent is allowed to pass to the other side. To be "selective," this membrane should
not allow large molecules or ions through the pores (holes), but should allow smaller components of the solution (such as the
solvent) to pass freely.
In the normal osmosis process, the solvent naturally moves from an area of low solute concentration (High Water Potential), through a
membrane, to an area of high solute concentration (Low Water Potential). The movement of a pure solvent to equalize solute
concentrations on each side of a membrane generates osmotic pressure. Applying an external pressure to reverse the natural flow of
pure solvent, thus, is reverse osmosis. The process is similar to other membrane technology applications. However, there are key
differences between reverse osmosis and filtration. The predominant removal mechanism in membrane filtration is straining, or size
exclusion, so the process can theoretically achieve perfect exclusion of particles regardless of operational parameters such as
influent pressure and concentration. Moreover, reverse osmosis involves a diffusive mechanism so that separation efficiency is
dependent on solute concentration, pressure, and water flux rate. Reverse osmosis is most commonly known for its use in drinking
water purification from seawater, removing the salt and other effluent materials from the water molecules.