Reverse Osmosis (RO) is a water purification technology that uses a semipermeable membrane to remove ions, molecules, and 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 differences in the solvent, a thermodynamic parameter. Reverse Osmosis can remove many types of dissolved and suspended species from water, 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, but should allow smaller components of the solution to pass freely.
The normal osmosis process, the solvent naturally moves from an area of low solute concentration, through a membrane, to an area of high solute concentration. The driving force for the movement of the solvent is the reduction in the free energy of the system when the difference in solvent concentration on either side of the membrane is reduced, generating osmotic pressure due to the solvent moving into the more concentrated solution. Applying an external pressure to reverse the natural flow of pure solvent, thus, in 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 efficiency regardless of parameters such as the solutions pressure and concentration. RO also involves diffusion, making the process dependent on pressure, flow rate, and other conditions. RO is also known for its use in drinking water purification from sea water, removing the salt and other effluent materials from the water molecules.