Reverse osmosis is the process in which water, at a certain pressure, passes through a semipermeable membrane from a more concentrated to a less concentrated solution, i.e., in the reverse direction for osmosis. In this case, the membrane passes water, but does not allow the soluble substances in it.

Reverse osmosis has been used since the 1970s to treat water, extract drinking water from seawater, and obtain clean water for medical, industrial and other purposes.

Reverse osmosis is one of the most promising and widely used water treatment and treatment methods. The reverse osmosis plant is capable of removing particles from water with dimensions of 0.001-0.0001 mkm. This range includes hardness salts, sulphates, nitrates, sodium ions, small molecules, dyes. The use of pre-cleaning steps (mechanical cleaning and micro-, ultra- or nano-filtration) that remove larger particles is recommended for more efficient operation.

The production of fresh water from sea requires pressure greater than osmotic sea water pressure. This value is quite high - existing installations develop a pressure of 10-12 atm.

Water purification systems typically use synthetic semi-permeable membranes. The membrane retains high molecular contaminants, but passes low molecular substances such as oxygen, chlorine, carbon dioxide, etc. Some gases may determine the taste of water. Purified water may have a mildly acidic reaction (pH < 7) due to the presence of dissolved carbon dioxide.

The property of almost completely purifying water from all impurities deprives it of important trace elements (if they were in it before desalination). Therefore, adding the necessary salts to desalinated water is the next step in the production of quality drinking water.

Unlike distillation (another method of desalination), in the process of reverse osmosis water is practically not heated, energy is spent only on the pump, which not only pumps water (a small share of energy), but also overcomes high osmotic pressure (major energy inputs).