How Does Reverse Osmosis Work?

Reverse osmosis is one of the most important methods used in water purification systems. It is an easy and efficient method for purifying water. RO systems have a lot of commercial benefits, that make them a great success.

Osmosis is the process of the movement of any liquid from an area of low concentration to an area of higher concentration of that solution, through a permeable or semi-permeable membrane. No external force is applied to bring about this process. It is split into two types - forward osmosis and reverse osmosis.

• Solution: The solute and solvent together form the solution.
• Solute: It is the substance that is dissolved in the solution.
• Solvent: It is the liquid in which the solute is dissolved.

• Forward osmosis is the process in which a permeable or semipermeable membrane is used for separating the water from a solution, containing many dissolved solutes as impurities, by the use of osmotic pressure.

• In reverse osmosis, the solvent is separated from the solute by passing it through a semipermeable membrane, while applying hydraulic pressure to the solution.

In this method, the solvent is made to move from an area of high solute concentration to an area of low solute concentration, by the application of external pressure. Generally, any solution that has less amount of solute, flows freely as compared to a solution containing a high amount of solute.

This is because the high amount of solute saturates the solution and causes it to become stagnant by reducing its potential. When separated by a semipermeable membrane, the solution containing lesser solute will flow to the side of the solution containing more solute. This is known as the osmotic effect.

In forward osmosis, the solute and solvent are separated by allowing the solvent to flow under the osmotic pressure influence whereas, this influence is suppressed in the reverse osmosis process. The solvent present on the side with higher solute concentration is forced to pass through the membrane, over to the side with lower solute concentration. The external pressure that has to be applied for the execution of this process is higher than the osmotic pressure.

The permeable or semipermeable membrane used in the reverse osmosis process plays a very important role. It should be designed such that it allows only the solvent i.e. liquid molecules to pass through. Two types of membranes can be used :

• CTA membrane (Cellulose Triacetate): It is specially useful in the removal of chlorine from water. Hence, it is used widely in commercial water filters to purify tap water.
• TFC membrane (Thin Film Composite): It is highly resistant to bacteria and has to be used in combination with carbon filters, if the water to be purified is chlorinated.

• Bacteria, viruses, metals, salts, etc. can be removed efficiently by the RO process.
• It improves the taste of water by removing the excess salts like potassium and sodium from it.
• It eliminates the traces of chlorine from tap water.

• A large amount of water is wasted during purification.
• It is more expensive that the method of UV filtration.

• It is a slow process
• It removes the useful minerals from water.
• It cannot block pesticides, herbicides, chlorine and other harmful elements. Hence, a carbon filter also has to be used in the system.

• RO system requires very less energy for its working.
• It requires very low maintenance.
• The RO system uses an automated technique called cross-flow to clean its clogged membrane.
• RO system, when used in water filters, eliminates 98% of the impurities from tap water.

• The water produced by RO technology, when supplied to a gas turbine, improves operating efficiency, and increases output by more than 10%.
• RO systems are cost-effective, and their price is continually decreasing.
• RO systems do not make use of harmful chemicals, hence are used in many power plants.

Many modern-day water filters use this technique, in combination with other techniques, to dispense pure drinking water. RO systems have numerous benefits and therefore, the disadvantages can be overlooked without evoking any major risks.