Reverse Osmosis- what it is about

Reverse osmosis is a method of filtration which can remove different types of molecules of different sizes by applying pressure to the solution when it is on one side of a selective membrane. The solute will be retained on the pressurized side of the membrane and the pure solvent will be allowed to pass to the other side. For this process to be carried out, the membrane should not allow large molecules or ions through the pores, but should allow small components of the solution to pass through freely. In other words, the membrane is semi-permeable, thus allowing small substances to pass through while preventing bigger particles from entering.
A common use of reverse osmosis is its use for water purification purposes. Seawater has to be cleared of the salt and other impurities in order to be consumed, so reverse osmosis has to take place to clear out all the impurities and purify the water. The salt particles, along with other dirt particles in the sea water cannot pass through the membrane, however the water particles can, so the salt and dirt particles will gradually be removed from the mixture.
This process is the opposite of osmosis, in which the solvent moves from an area of low solute concentration, through a membrane, to an area of high solute concentration.
The process of reverse osmosis is somewhat similar to membrane filtration. However, membrane filtration includes the use of straining, or size exclusion, so the process can achieve perfect exclusion of particles regardless of the pressure and concentration. On the other hand, reverse osmosis involves a diffusive mechanism so that the separation efficiency depends on the solute concentration, pressure and water flux rate.