Reverse osmosis (RO) is a process that overcomes many of the shortcomings of distillation and ion exchange. To explain reverse osmosis let us first look at osmosis. This is a natural process, which occurs whenever a dilute solution is separated from a concentrated solution by a semi-permeable membrane. Water, driven by a force caused by the concentration difference - the osmotic pressure - passes through the membrane into the concentrated solution. The flow of water continues until the concentrated solution is diluted and back pressure prevents any further flow through the membrane (osmotic equilibrium).
If a pressure greater than the osmotic pressure is applied to the higher concentration side of the membrane, the normal direction of osmotic flow is reversed, pure water passes through the membrane from the concentrated solution and is thus separated from its contaminants. This is the basic principle of reverse osmosis (sometimes call hyper-filtration).
In practice, feed water is pumped into a pressure vessel containing a spiral or set of hollow fibers of semi-permeable membranes. The purified water passes through the membrane to form the ´permeate´. The contaminants accumulate in the residual water, called the ´concentrate´, which is bled continuously to drain. The latest generation of polyamide thin film composite reverse osmosis membranes which have replaced early cellulosic membranes, remove 95-98% of inorganic ions, together with virtually all the large non-ionic contaminants and organic molecules with a molecular weight greater than 100.
What are the benefits of Reverse Osmosis (RO)?
Reverse Osmosis (RO) is the most efficient and effective method of water purification known to man. It uses a special, semi-permeable membrane which removes impurities as small as 0.0001 micron (i.e. 0.00000004 inches) in size, cleansing water of all biological impurities, suspended particles, dissolved solids (TDS), salts, metals and chemicals.
What are the limitations of Reverse Osmosis (RO)?
Reverse Osmosis is limited by the relatively slow rate of production and is, therefore, normally used to fill a reservoir prior to use or further purification. Reverse osmosis tends to protect the system from bacteria and pyrogens. It is often combined with ion exchange to considerably improve product water quality.