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. In simpler terms, reverse osmosis is pushing a solution through a filter that traps the solute from one side and allows the obtainment of the pure solvent from the other side.
This process is used in treating sea water to get fresh water.
When two solutions with different concentrations of a solute are mixed together, the total amount of solutes in the two solutions will be equally distributed in the total amount of solvent from the two solutions. This is achieved by diffusion, in which solutes will move from areas of higher concentration to areas of lower concentrations until the concentration in all the different areas of the resulting mixture are the same, a state called equilibrium.
Instead of mixing the two solutions together, they can be put in two compartments where they are separated from each other by a semipermeable membrane. The semipermeable membrane does not allow the solutes to move from one compartment to the other, but allows the solvent to move. Equilibrium cannot be achieved by the movement of solutes from the compartment with high solute concentration to the one with low solute concentration. Equilibrium is, instead, achieved by the movement of the solvent from areas of low solute concentration to areas of high solute concentration. When the solvent moves away from low concentration areas, it causes these areas to become more concentrated. On the other side, when the solvent moves into areas of high concentration, solute concentration will decrease. This process is termed osmosis. The tendency for solvent to flow through the membrane can be expressed as "osmotic pressure", since it is analogous to flow caused by a pressure differential.
In reverse osmosis, in a similar setup as that in osmosis, pressure is applied to the compartment with high concentration. In this case, there are two forces to consider regarding the movement of water: the force of solute concentration difference between the two compartments (the osmotic pressure) and the force caused by the externally applied pressure. In the same manner, the solute cannot move from areas of high pressure to areas of low pressure, because the membrane is not permeable to it. Only the solvent can move in this way. When the effect of the externally applied pressure is greater than that of the concentration difference, net solvent movement will be from areas of high solute concentration to low solute concentration, and reverse osmosis occurs.
Reverse Osmosis in Use
Drinking water purification
In July 2002, Singapore announced that a process named NEWater would be a significant part of its future water plans. It involves using reverse osmosis to treat domestic wastewater before discharging the NEWater back into the reservoirs. 
In the United States, household drinking water filtration systems, including a reverse osmosis step, are commonly used for improving water for drinking and cooking. Such systems typically include four or five stages:
- a sediment filter to trap particles including rust and calcium carbonate
- optionally a second sediment filter with smaller pores
- a carbon filter to trap organic chemicals and chlorination
- a reverse osmosis filter with a thin film composite membrane (TFM or TFC)
- optionally a second carbon filter
In some systems, the carbon pre-filter is omitted and cellulose triacetate membrane (CTA) is used. The CTA membrane is prone to rotting unless protected by the chlorinated water, while the TFC membrane is prone to breaking down under the influence of chlorine. In CTA systems, a carbon post-filter is needed to eliminate the chlorine.
Maple Syrup Production
Starting in the 1970s, some maple syrup producers started using reverse osmosis to remove water from sap before being further boiled down to syrup. The use of reverse osmosis allows approximately 75 to 80 percent of the water to be removed from the sap.