Osmosis is a natural phenomenon where by a solvent (typically water) moves from a zone of low solute (dissolved ions) concentration to a zone of high solute concentration by passing through a semi-permeable membrane. Whenever there is a difference in solute concentration across a semi-permeable membrane there will be a resultant osmotic pressure created that is proportional to the difference in concentration.  A simple example of this would be if two halves of a U-tube were separated by a semi-permeable membrane and one half was filled with fresh water and the second half was filled with sea water.  


  • Reduced chemical use and waste compared to ion exchange demineralisation 

See the Xion ‘Knowledge Hub’ to find a wealth of material covering the theory of reverse osmosis and helpful hints on typical RO trouble shooting and plant operation.


  • Sea water desalination 

  • Boiler feed water/steam generation 

Osmosis will attempt to equalise the concentration of the two solutions, requiring movement of the water from low solute concentration side, through the membrane, to the high solute concentration side. However, most uses of water require desalinated or purified water. This can be formed by a process which forces the water to act against the natural osmotic behaviour, hence the term reverse osmosis. This produces pure water from salt or brackish sources. To overcome the natural osmotic force, pressure is applied to the water via a high pressure pump, typically 8-17 bar for brackish water and 50-80 bar for seawater sources. When the applied pressure exceeds the osmotic pressure, pure water will permeate through the membranes leaving behind the dissolved salts, suspended particles and organic compounds. Reverse osmosis separates the feed water into a permeate stream and a concentrate stream. The permeate is considered as pure water, free of dissolved salts, suspended solids and organic compounds. The concentrate stream contains a fraction of the feed water and the bulk of the dissolved solids and suspended particles, typically the concentrate goes to drain as the waste stream.