Reverse Osmosis (RO) | Definition, Principle, Process and Applications

Reverse osmosis refers to the movement of solvent from a region of high concentration of solute to a region of low concentration of solute or pure solvent through a semipermeable membrane. Commonly called RO, it is one of the oldest techniques used in the purification of seawater.

Let's learn about reverse osmosis in detail, including its principle, advantages, and applications.

What is Reverse Osmosis?

When a hydrostatic pressure (greater than the osmotic pressure ) is applied on the higher concentration side, solvent molecules start moving from the higher concentration side to the lower concentration side through the semipermeable membrane (SPM). This process is called Reverse osmosis

• In normal osmosis, solvent molecules automatically move from the low-concentration solution to the high-concentrated side
• It is just the reverse of osmosis i.e. flow of the solvent from high concentrated side to low concentrated side due to external pressure.
• This movement of pure solvent from high concentrated side to the low concentrated side enables the reverse osmosis process to remove many types of suspended and dissolved chemical species and biological species like bacteria from water.
• It is used in the production of pure drinking water and industrial processes as well.

Reverse Osmosis Principle

The principle on which reverse osmosis works is,

When pressure greater than the osmotic pressure is applied on the side of the high-concentration region of the solution, the pure solvent particles start moving from the region of high concentration to low concentration through a semi-permeable membrane leaving behind the suspended particles. 

Reverse Osmosis Process

The working of the reverse osmosis process is explained in the following steps:

Step 1: The two solutions of different concentrations must be placed in contact through a semipermeable membrane which creates a concentration gradient between them or chemical potential between them.

Step 2: This concentration gradient results in the flow of solution from the side of the low-concentration region to the high-concentration region. This process is called osmosis.

Step 3: During osmosis, hydrostatic pressure is exerted by the solution on the higher concentration side. To counter this pressure an external pressure is applied on the region of the high-concentration side. 

Step 4: When this external pressure reaches a certain level then the process of osmosis stops i.e. there is a flow of solution from the low-concentration side to the high-concentration side. At this stage, the applied external pressure is called osmotic pressure.

Step 5: When the applied external pressure exceeds this osmotic pressure level then the solvent particles start moving from the region of high concentration to the low concentration region of solution which is actually the reverse of osmosis. This movement of solvent opposite to the direction of osmosis is called reverse osmosis

To understand the concept of Reverse Osmosis, we need to understand the concept of Osmosis and Osmotic Pressure.

Osmosis Definition

Osmosis is the process in which particles of solvent move from a region of pure solvent or low concentration of solute to a region of high concentration of solute through a semipermeable membrane.

• Suppose two solutions with different concentrations are taken and separated by a semi-permeable membrane (SPM).
• In that case, Gibb's free energy per mole is different for the two regions.
• This difference in the energy of the two regions establishes a chemical potential between the two regions.
• This chemical potential acts as a driving force for the flow of pure solvent to the region of high concentration of solute.
• The molecule of solvent flows through the semipermeable membrane (SPM), until the two sides become the same in concentration i.e. they attain equilibrium.
• This movement of solvent exerts hydrostatic pressure on the other side continuously.
• This movement of solvent can be stopped before attaining equilibrium by applying pressure on the region of high concentration of solute called osmotic pressure.

Osmotic Pressure

Osmotic pressure is the minimum pressure needed to stop solvent molecules from flowing through the semipermeable membrane (SPM), from the region of pure solvent or lower concentration of solute to the higher concentrated side.

• It is denoted by π.
• It is a colligative property that is dependent on the number of particles of solute dissolved in the solution.

Difference Between Osmosis and Reverse Osmosis

These are the difference between osmosis and reverse osmosis:

Osmosis vs. Reverse Osmosis
	Osmosis	Reverse Osmosis
Definition	In osmosis, solvent molecules move from low to high concentrated side, due to Osmotic Pressure.	In reverse osmosis, molecules move from the higher concentrated side to the lower concentration side.
Occurrence	It is a natural process and needs no energy	It is an artificial, commercial process and needs an energy supply.
Direction	It occurs from the low concentration solution to the high concentrated side	It occurs from the higher concentrated side to the lower concentration side.

Reverse Osmosis Experiment

The working of Reverse Osmosis can be best understood by doing a simple experiment. The steps of performing the experiment are mentioned below

Step 1: Take a U-shaped tubular vessel and place a semipermeable membrane in between the vessel. The vessel has now two chambers separated by a semipermeable membrane as shown in the image added below:

👁 Experiment of Reverse Osmosis

Step 2: Fill one side of the vessel with pure solvent and the other side with a concentrated solution i.e. solvent with dissolved solute in it.

Step 3: As soon as the pure solvent and concentrated liquid come in contact with each other through the semipermeable membrane a chemical gradient is developed between them.

Step 4: The chemical gradient so developed results in the flow of liquid from the pure solvent side to the concentrated side. This is called osmosis.

Step 5: Now start applying pressure on the concentrated side with the help of a piston until the movement of liquid from the pure solvent to the concentrated side stops. This pressure is called Osmotic Pressure.

Step 6: Now apply some extra pressure on the concentrated side that exceeds Osmotic Pressure, as soon as the extra applied pressure exceeds the osmotic pressure, the liquid starts moving from the concentrated side to the pure solvent side. This is called Reverse Osmosis.

Reverse Osmosis of Water

We all have been using RO water purifiers in our homes. As the name suggests, the RO water purifier works on the principle of Reverse Osmosis.

Let's understand it's working.

• The filter in the RO purifier which is generally made of graphene act as a semipermeable membrane.
• The RO purifier is then connected to a tap for water supply.
• The water supplied by tap contains dissolved solute thus making it a highly concentrated solution.
• The electric power is used to create pressure which forces the water to move from the highly concentrated region to the pure solvent side removing harmful elements such as arsenic, fluoride, dust, and bacteria.

Thus we get pure water from the Reverse Osmosis purifier.

A similar type of process is followed in purifying seawater by the Reverse Osmosis Process which results in the desalination and de-silting of seawater. This makes it fit for consumption. 

Advantages of Reverse Osmosis

The advantages or Benefits of Reverse Osmosis are :

• Reverse Osmosis helps in effective removal of contaminants from water
• Reverse Osmosis helps in purification of Sea Water
• Reverse Osmosis improves the taste of water
• It is a versatile technique to purify water

Disdvantages of Reverse Osmosis

The disadvantages of Reverse Osmosis are :

• While Purifying, RO also removes important minerals from the water
• Its operation cost is high
• RO process also generates wastewater due to Contaminant concentration
• RO process is slow as it is time consuming process

Applications of Reverse Osmosis

Applications of the reverse osmosis process are :

Freshwater Applications  

The reverse osmosis process improves the quality of water and makes it purer and most suitable for drinking and cooking.

• Systems that use the reverse osmosis process to purify water generally use a number of common steps  
  • Different layers of semi-permeable membranes sediment harmful particles of different sizes and rust from water.
  • Activated charcoal filter to filter contaminants and organic chemicals from water.
  •  A reverse osmosis filter restricts some molecules and makes water purer.

Wastewater Purification 

•  Rainwater is collected from different places (i.e. - storm drains) and reverse osmosis water processors are used to make the water purer. The water becomes reusable for landscape irrigation and industrial cooling processes. This way reuse of wastewater provides a solution to problems like water shortage.
• Boiler water that comes from power plants can also be purified by the reverse osmosis process by separating some minerals.

Food industry 

• The conventional process for concentrating food liquids is the heat-treatment process. But the reverse osmosis process due to low operating cost is a more cost-reducing process. Hence it is used in the food industry for concentrating food liquids.

Related:

• Diffusion
• Difference between Diffusion and Osmosis
• Osmosis and Osmotic Pressure.
• Difference between Osmotic Pressure and Oncotic Pressure