Reverse osmosis 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. This is the reverse of the normal osmosis process, which is the natural movement of solvent from an area of low solute concentration, through a membrane, to an area of high solute concentration when no external pressure is applied. The membrane here is semipermeable, meaning it allows the passage of solvent but not of solute.
To illustrate, imagine a semi permeable membrane with fresh water on one side and a concentrated aqueous solution on the other side. If normal osmosis takes place, the fresh water will cross the membrane to dilute the concentrated solution. In reverse osmosis, pressure is exerted on the side with the concentrated solution to force the water molecules across the membrane to the fresh water side.
The membranes used for reverse osmosis systems have a dense polymer barrier layer in which separation takes place. Since Reverse Osmosis does not occur naturally, it must be created by applying pressure to the high solids water in order to force it through the membrane, with pressures from 8 - 14 bar for fresh and brackish water, and 40 - 70 bar for seawater, which has around 24 bar (350 psi) natural osmotic pressure which must be overcome.
The feed water, depending on its source, may contain various concentrations of suspended solids and dissolved matter. Suspended solids may consist of inorganic particles, colloidal particles and biological impurities such as microorganisms and algae. Dissolved matter may consist of highly soluble salts, such as chlorides, and sparingly soluble salts, such as carbonates, sulfates, and silica.
Demineralization ( DM ) Water Treatment Plants
Demineralization is the process of removing mineral salts from Water by using the ion exchange process.
Demineralised Water is Water completely free ( or almost ) of dissolved minerals as a result of one of the following processes :
Membrane filtration (reverse osmosis or nanofiltration)
Or other technologies.
Demineralized Water also known as Deionized Water, Water that has had its mineral ions removed. Mineral ions such as cations of sodium, calcium, iron, copper, etc and anions such as chloride, sulphate, nitrate, etc are common ions present in Water. Deionization is a physical process which uses specially-manufactured ion exchange resins which provides ion exchange site for the replacement of the mineral salts in Water with Water forming H+ and OH- ions.