The process of osmosis through semipermeable membranes was first observed in 1748 by Jean Antoine Nollet. For the following 200 years, osmosis was only a phenomenon observed in the laboratory. In 1949 the University of California at Los Angeles (UCLA) first investigated desalination of seawater using semipermeable membranes in 1949. Researches from both UCLA and the University of Florida successfully produced freshwater from seawater in the mid-1950s, but the flux was too low to be commercially viable. The future of RO is promising. By the end of 2001, about 15,200 desalination plants were in operation or in the planning stages worldwide. 
A semipermeable membrane coil used in desalinization.
Formally, reverse osmosis is the process of forcing a solvent from a region of high solute concentration through a semipermeable membrane to a region of low solute concentration by applying a pressure in excess of the osmotic pressure.
The membranes used for reverse osmosis have a dense barrier layer in the polymer matrix where most separation occurs. In most cases the membrane is designed to allow only water to pass through this dense layer while preventing the passage of solutes (such as salt ions). This process requires that a high pressure be exerted on the high concentration side of the membrane, usually 2–17 bar (30–250 psi) for fresh and brackish water, and 40–70 bar (600–1000 psi) for seawater, which has around 24 bar (350 psi) natural osmotic pressure which must be overcome.
This process is best known for its use in desalination (removing the salt from sea water to get fresh water), but it has also been used to purify fresh water for medical, industrial and domestic applications since the early 1970s.
Osmosis describes how solvent moves between two solutions separated by a semipermeable membrane to reduce concentration differences between the solutions. When two solutions with different concentrations of a solute are mixed, the total amount of solutes in the two solutions will be equally distributed in the total amount of solvent from the two solutions. 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. Since equilibrium cannot be achieved by the movement of solutes from the compartment with high solute concentration to the one with low solute concentration, it 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 influencing the movement of water: the pressure caused by the difference in solute concentration between the two compartments (the osmotic pressure) and the externally applied pressure.
Drinking water purification
Around the world, household drinking water purification systems, including a reverse osmosis step, are commonly used for improving water for drinking and cooking.
Such systems typically include a number of steps:
• a sediment filter to trap particles including rust and calcium carbonate
• optionally a second sediment filter with smaller pores
• an activated carbon filter to trap organic chemicals and chlorine, which will attack and degrade TFC reverse osmosis membranes
• a reverse osmosis (RO) filter which is a thin film composite membrane (TFM or TFC)
• optionally a second carbon filter to capture those chemicals not removed by the RO membrane
• optionally an ultra-violet lamp for disinfecting any microbes that may escape filtering by the reverse osmosis membrane
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 from 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 remove chlorine from the final product water.
Portable reverse osmosis (RO) water processors are sold for personal water purification in various locations. To work effectively, the water feeding to these units should best be under some pressure (40 psi or greater is the norm). Portable RO water processors can be used by people who live in rural areas without clean water, far away from the city's water pipes. Rural people filter river or ocean water themselves, as the device is easy to use (Saline water may need special membranes). Some travelers on long boating trips, fishing, island camping, or in countries where the local water supply is polluted or substandard, use RO water processors coupled with one or more UV sterilizers. RO systems are also now extensively used by marine aquarium enthusiasts. In the production of bottled mineral water, the water passes through an RO water processor to remove pollutants and microorganisms. In European countries, though, such processing of Natural Mineral Water (as defined by a European Directive) is not allowed under European law. (In practice, a fraction of the living bacteria can and do pass through RO membranes through minor imperfections, or bypass the membrane entirely through tiny leaks in surrounding seals. Thus, complete RO systems may include additional water treatment stages that use ultraviolet light or ozone to prevent microbiological contamination.)
Membrane pore sizes can vary from .1 to 5,000 nanometers (nm) depending on filter type. "Particle filtration" removes particles of 1,000 nm or larger. Microfiltration removes particles of 50 nm or larger. "Ultrafiltration" removes particles of roughly 3 nm or larger. "Nanofiltration" removes particles of 1 nm or larger. Reverse osmosis is in the final category of membrane filtration, "Hyperfiltration", and removes particles larger than .1 nm.
In the United States military, R.O.W.P.U.'s (Reverse Osmosis Water Purification Unit, pronounced "roh-poo") are used on the battlefield and in training. They come ranging from 1500 GPD (gallons per day) to 150,000 GPD and bigger depending on the need. The most common of these are the 600 GPH (gallons per hour) and the 3,000 GPH. Both are able to purify salt water and water contaminated with N.B.C. (Nuclear/Biological/Chemical) agents from the water. During a normal 24 hour period, one unit can produce anywhere from 12,000 to 60,000 gallons of water, with a required 4 hour maintenance window to check systems, pumps, R.O. elements and the engine generator. A single ROWPU can sustain a force of a battalion size element or roughly 1,000 to 6,000 soldiers.