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Reverse Osmosis – Definition, Principle, Process, Applications

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  • Last Updated : 22 Aug, 2022
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Osmosis is the process in which molecules move from high to low concentration. Suppose two solutions with different concentrations are taken and separated by a semi-permeable membrane (SPM). In that case, solvent molecules gradually move from the low concentration solution to the high concentrated side through the semipermeable membrane (SPM), until the two solutions contain the same concentration.

Osmotic Pressure 

The minimum pressure is needed to stop solvent molecules from flowing through the semipermeable membrane (SPM), from the lower to higher concentrated side.

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).  The process is known as the Reverse osmosis process.

In normal osmosis, solvent molecules automatically move from the low concentration solution to the high concentrated side, whereas this process is just the reverse flow of the solvent due to external pressure. The reverse osmosis process removes many types of suspended and dissolved chemical species and biological species like bacteria from water and is used in both productions of pure drinking water and in industrial processes as well. The is an artificial, commercial process that has various applications in the industry, including freshwater applications, wastewater applications, power stations, removing harmful elements from water sources, the food industry, etc.

Osmosis and Reverse Osmosis


How does the Reverse osmosis process work?

In osmosis, solvent molecules move from low to high concentrated side, due to Osmotic Pressure. In reverse osmosis, a hydrostatic pressure, greater than Osmotic Pressure is applied on the higher concentration side, which makes solvent molecules move to the opposite side i.e. from the higher concentration side to the lower concentration side through the semipermeable membrane (SPM). 

Why do we use reverse osmosis?

The reverse osmosis principle is used in 

  • Softening hard waters – softens hard water by limiting different types of elements in a semi-permeable membrane making the water pure. 
  • Removing Harmful elements –  removing different types of harmful elements from the water (Ex – iron, potassium, zinc ) and making the water worthy to be used for daily needs.
  • Power stations – Reverse osmosis process is also used in power stations to get rid of the minerals from boiler water.

Applications of reverse osmosis process 

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.

 Frequently Asked Questions

Question 1: Define a semi-permeable membrane?


 A semi-permeable membrane is a  synthetic or biological material that allows selective particles to move through the membrane and is used in the process called osmosis. The semi-permeable-membranes consist  of  polyamide, cellulose acetate, or polysulphone 

Question 2: What are the advantages of reverse osmosis?


  • Water Purification – Through the process of reverse osmosis ionic, colloidal, and nonionic harmful particles are removed to purify the water.
  • Removing salt from ocean water –  Reverse osmosis Desalinating facilities removes the salt from the ocean water. It is a straining procedure that helps in removing the salts from the ocean water and getting fresh water. 

Question 3: What are the disadvantages of reverse osmosis?


 While filtrating impurities from water sometimes minerals that are needed in our body also get filtrated. That reduces benefits in the water. That can lead to often weakness, muscle-related problems, etc.

Question 4: What are the conditions for performing the reverse osmosis process?


  • Two different concentration solutions –  There must be 2 different solutions with different concentrations.
  • Semi-permeable-membrane – a semi-permeable-membrane would be separating those two different concentration solutions
  • Pressure should be kept on high concentration solution.
  • External pressure must be greater than the osmotic pressure

Question 5: Difference between osmosis and reverse osmosis. 




Reverse Osmosis


 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 occurs from the low concentration solution to the high concentrated side occurs from the higher concentrated side to the lower concentration side.

Question 6: Similarity between osmosis and reverse osmosis?


  • Both processes needs a semi-permeable membrane as a separator between two solutions.
  • Both processes don’t allow solute particles to go through a semi-permeable membrane, mainly water molecules pass through semi-permeable membranes.
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