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In a reverse osmosis system raw water is demineralised without the use (or with a limited use) of chemical reagents.
In case of high salinity water (brackish and sea water), recently even in cases of medium-low salinity water, reverse osmosis demineralisation together with other processes based on ion exchange resins or electro-deionisation allows a significant reduction in costs, if compared with both desalination systems of the evaporator type and systems based on the ion exchange process alone.
The process is based on the property of some membranes (semi-permeable) to be permeated by water but not by the substances dissolved in it.
In other words, if two solutions of different concentration are separated by a semi-permeable membrane, the solvent (water) will flow from the less concentrated to the more concentrated solution (osmosis).
When in the condition of equilibrium the two solutions are at the same concentration and the difference in level between the initially more concentrated and the initially more diluted solution is defined "osmotic pressure".
If the flow is reversed by applying a pressure greater than the osmotic pressure to one solution, it becomes more concentrated; in such case the solvent will flow from the more concentrated to the more diluted solution (reverse osmosis demineralisation process).
The industrial use of the reverse osmosis process is therefore based on special membranes, assembled in pressure vessels and fed by high head pumps (in case of seawater 80 atmospheres can be reached and exceeded).
However, since the membranes are porous and consequently subject to fouling by both colloidal turbidity and saturation of dissolved salts, the feedwater intended for this application must be made to go through a very intensive pre-treatment process (a combination of the precipitation, coagulation, clarification, filtration and ultrafiltration techniques), in order to remove the foreign materials that might alter the state, and consequently the operation, of the membrane surfaces (organic and inorganic fouling, etc.)
The reverse osmosis process, alone or in combination with other technologies (ion exchange, electro-deionisation), has been continuously expanding and has become a widely accepted technology for the production of water for industrial and civil use, e.g. boiler and cooling loop feedwater, ultrapure water and drinking water.
Special processes, such as for instance denitrification of water intended for human consumption, are also available to users.
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