Reverse Osmosis (R.O.) is a method to reverse the physiochemical process called Osmosis. During this procedure a separation of the components of water takes place, through a semi-permeable cellulose membrane, which practically removes all the soluble components of water  (harmful and useful). Thia means that the osmotic water is almost like distilled water.

R.O. technology is used to remove salts and contaminants, which are difficult to remove via simple filters. This technology manages almost 100% removal of harmful elements in drinking water.

The membrane technology has been applied since 1960. However,  the processes which were using membranes were initially very expensive and as a consequence they were only applied in very specific and sophisticated cases. This changed during the 1990s, due to the tightening of the existing legislation concerning the quality of treated water and because of the lack of water resources. The dramatic reduction in the prices of membranes led to a sudden increase in the use of membranes since 2000 and so the membrane technology is now used in conventional processes and can achieve much better results.

There are two types of membrane processes which are used in the fields of water treatment and waste water treatment. The first category includes Reverse Osmosis (R.O.) and Nanofiltration (NF). This type of membranes consist of a dense, non-porous layer of separable material, mounted on a porous supportive material. R.O. and NF are used for the removal of dilute substances. The second category is filtration membranes in which a micro-porous separable layer provides a barrier for even the smallest particulates, nevertheless, it allows the solvent to pass through. Filtration membranes are often used as an independent treatment method but also as a method for Reverse Osmosis pretreatment.

The primary uses of Reverse Osmosis membranes were mainly for the treatment of industrial waste water and for the production of water for special purposes. All these have been overcome today because of the importance, for the drinking water, of turbidity removal and also of the addition of one more decontamination barrier mainly against parasitic microorganisms. Also one of the most prevalent uses of the Reverse Osmosis membranes is the desalination of sea water.

Concerning the second type of membranes, there are two subtypes of filtration membranes. Ultrafiltration membranes (UF) and Microfiltration membranes (MF). First subtype’s pores' size is about 0.01-0.02 micrometers , and second subtype’s from 0.04 to 0.10 microns . In wastewater treatment applications even more coarse filters are usually used, about 0.2-0.4 microns.