How Does Reverse Osmosis Work

To know what reverse osmosis is, first we need to understand the term 'osmosis'. Osmosis can be defined as a physical process, wherein a solvent diffuses through a semi-permeable membrane. In the process of osmosis, the solvent (liquid) moves from low concentrate solution to its higher concentration of solution. The result of this process is a net reduction in the difference in the concentration of the two solutions. Reverse osmosis, as the name suggests, is exactly opposite to Osmosis. In case of reverse osmosis, the solvent flows from highly concentrated solution to the area of low concentration.

 

When two aqueous solutions of different concentrations are separated by a semi-permeable membrane, the solvent flows through the membrane, towards the more concentrated solution, due to osmotic pressure. When a counter pressure is applied to the concentrated solution, in order to overcome the osmotic pressure, the flow of the solvent changes its direction or is reversed. This physical process is known as reverse osmosis. The process is widely used in water purification systems (seen in industries and households), to purify salted or contaminated water, making it safe to drink. Let us know more

 

In case of reverse osmosis, the solvent (let us take the example of water, for better understanding) is made to flow from the area of high solute (dissolved matter in a solution, salt for example) concentration to the area of low solute concentration. An external pressure is applied for the purpose. This method is widely used to purify water, because as a general fact, the less amount of solute flows freely in comparison to the one that has high amount of solute. The high amount of solute tends to saturate the solution and causes it to become inactive, by reducing its potential.

 

Due to the natural osmotic effect, the solution containing less concentration of solute would flow to the direction of the highly concentrated solution, both being separated by a semi-permeable membrane. The influence of the osmotic pressure is suppressed, when the process of reverse osmosis takes place. As a result, the solution containing high concentration of solute is allowed to pass through the semi-permeable membrane, to flow towards the concentrate with lower solute concentration. This process also demands the application of an external pressure. The pressure applied for reverse osmosis is, thus, higher than the osmotic pressure.

 

Semi-permeable membrane plays a pivotal role in reverse osmosis. Hence, it should be chosen carefully. An ideal semi-permeable membrane used for the purpose of reverse osmosis should be designed in such a way that it allows only the solvent i.e. liquid molecules, to pass through it. Two types of semi-permeable membranes are used for industrial purposes. The first one is Cellulose Triacetate (CTA) membrane, widely used to remove chlorine from tap water. You can see the membrane in commercial water filters.

 

The second membrane is Thin Film Composite (TFC) membrane, used in combination with carbon filters. This membrane is highly resistance to bacteria and used for water purification in commercial desalination systems. Semi-permeable membranes, including Cellulose Ester Membrane (CEM), Charge Mosaic Membrane (CMM), Bipolar Membrane (BPM), Anion Exchange Membrane (AEM), Alkali Anion Exchange Membrane (AAEM) and Proton Exchange Membrane (PEM), are used for purposes other than water purification.