The term reverse osmosis comes from the process of osmosis, the natural movement of solvent from an area of low solute concentration, through a membrane, to an area of high solute concentration if no external pressure is applied.

In simple terms, reverse osmosis is the process of pushing a solution through a filter that traps the solute on one side and allows the pure solvent to be obtained from the other side. More formally, it 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. The membrane here is semipermeable, meaning it allows the passage of solvent but not of solute.

The membranes used for reverse osmosis have no pores, the separation takes place in a dense polymer layer of only microscopic thickness. In most cases the membrane is designed to only allow water to pass through. The water goes into solution in the polymer of which the membrane is manufactured, and crosses it by diffusion. This process requires that a high pressure be exerted on the high concentration side of the membrane, usually 2 - 14 bar for fresh and brackish water, and 40 - 70 bar  for seawater, which has around 24 Bar  natural osmotic pressure which must be overcome.

Reverse osmosis-manufactured plants are used to produce high-quality demineralised water, especially where the client looks to minimise the use of chemicals which are the regenerants associated with the more conventional demineralised ion exchange process.
R O Plants are used extensively in the food and beverage industry (especially the brewing industry), pharmaceutical and the electronics industries.

 
Reverse osmosis has emerged as a serious alternative to chemical treatment systems due to a number of reasons. The minimal use of chemicals makes it environmentally desirable. Reverse osmosis is a simple process and the operational and maintenance costs are lower. It has higher production capacity to space ratio via-a-via other technologies. Ion exchange processes, on other hand, incur high resin replacement costs, substantially higher regeneration costs and inordinate downtime costs due the need for replacing ion exchange columns. While the biggest operating costs for ion exchange systems are regeneration chemicals, the major cost component in RO is energy i.e. electrical power to operate the RO feed pump.