Reverse osmosis (
RO) is a water justification technology that uses a
semipermeable membrane to remove larger particles from drinking water. In reverse osmosis, an applied pressure is used to overcome
osmotic pressure, a
colligative property, that is driven by chemical potential, a thermodynamic parameter. Reverse osmosis can remove many types of
molecules and
ions from solutions, including bacteria, and is used in both industrial processes and the production of
potable water. 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 is driven to reduce the free energy of the system by equalizing solute concentrations on each side of a membrane, generating 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, key differences are found 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.
[1] 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.
remember there are no trace minerals in R.O water either.