Natural osmosis occurs when solutions with two different concentrations are separated by a semi-permeable membrane.
Osmotic pressure drives water through the membrane, the water dilutes the more concentrated solution, and the end result is equilibrium.
In reverse osmosis (RO), carefully applied hydraulic pressure counteracts the natural osmotic pressure and forces the solution back through the membrane to remove unwanted solutes. In plainer terms, pure water is forced through a membrane with pores so small that impurities such as sodium, chlorine, arsenic, and over 99.99% of total dissolved solids are unable to pass through.
A typical RO system combines a reverse osmosis membrane with a series of additional filters designed to remove various tastes, chemicals and sediments. These pre-filters help to extend the life of the reverse osmosis membrane. Because the membranes are very restrictive, they yield very slow flow rates. Storage tanks are required to produce an adequate volume in a reasonable amount of time.
RO systems are frequently used in residential homes in combination with a water softener, especially for individuals who are sensitive to minor amounts of sodium in their drinking water.