Reverse Osmosis is a process that is used to remove a wide range of salts and contaminants to give water of a very high purity.
Osmosis is a natural process involving a fluid flow across a semi-permeable membrane. It is known as the process by which most nutrients feed the cells in our bodies and how water gets to the leaves and branches at the top of trees.
If you separate any solution of salts from pure water using a basic thin semi-permeable membrane not unlike a sausage skin, the pure water passes through the membrane and tries to dilute the salt solution. If the salt solution is connected to a vertical pipe then the progressively diluted solution will fill the pipe until the 'osmotic pressure' drawing the pure water though the membrane is the same as the head pressure of the diluted solution. This process can be reversed - hence 'Reverse Osmosis'
By applying a higher pressure to the salt solution. Pure water will then pass the other way through the reverse osmosis membrane in a process that is easy to visualize as 'filtration' where the filter will only let through the small water molecules and retain almost all of the other molecules and other contaminants.
This means that water containing a high level of natural salts can be purified without the need for chemical regenerates such as the acid and caustic used in demineralization plants. Reverse Osmosis is therefore considered a much safer route of producing pure water for many residential commercial and industrial applications, and additionally the plant does not need to be taken out of service for regeneration as a Demineralisation plant does.
Rejection rates of salts from water is generally in the region of 95-99.5% dependent upon the membrane type used and the raw water feed quality. With improvements in membrane technology reverse osmosis effiency is constantly improving.
Reverse osmosis systems can be designed to utilise the wide range of membranes available, which will give different permeate water qualities. Standard designed reverse osmosis units are manufactured utilising the Low Energy Membranes which will give a permeate water quality of approx. 10 microsiemens from an input water of between 500-700 microsiemens.