Reverse Osmosis

We all know that presently the need for clean water is paramount. This water is unusable due to the presence of salt in the water. This is precisely where reverse osmosis comes into effect. Reverse osmosis helps to separate the salt particles from the contaminated water to give out pure and clean water. Reverse osmosis is a type of a critical desalination process. It is used for water treatment, recycling, and it can also help in producing energy. Before trying to understand the reverse osmosis principle of operation, we need to understand the principle of osmosis and reverse osmosis together. We will also go through the reverse osmosis definition in this article.


To explain reverse osmosis, you need to have a proper understanding of the osmosis principle. In osmosis, the pure water flows from a dilute solution to a concentrated solution when it passes through a semipermeable membrane. A semipermeable membrane will only allow small particles to move through it, and it will block all large particles. 

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The image shows a container with different concentrations of solution separated by a membrane.

Consider a container, as shown in the figure above. It is divided into two parts with the help of a semipermeable membrane. Saltwater is kept in one compartment, and pure water in the other. The membrane will only allow pure water to pass through it, and not the saltwater. 

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The image shows the process of osmosis taking place.

Now what happens is, the system will want to stay in equilibrium. Therefore, it tries to attain equal concentration on both sides. The only way it can be achieved is when pure water passes through the membrane and mixes with the saltwater. Hence the water level in the saltwater compartment will rise and create a positive pressure called osmotic pressure.

Reverse Osmosis Principle

Now that you know about osmosis, we will look at the reverse osmosis definition. Reverse osmosis (RO) is a type of process which is opposite of the osmosis principle. Reverse osmosis is a process wherein a concentrated solution passes through a membrane in a direction opposite to the natural osmosis process, due to external pressure, which is higher than the osmotic pressure. This is the reverse osmosis definition.

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This image depicts the reverse osmosis simple explanation.

From the above image, we can see that when higher pressure is applied on the concentrated side, the solution passes through the semipermeable membrane into the other side of the container. With the help of reverse osmosis principle, we can convert any contaminated water to pure and clean water very quickly.

Advantages of Reverse Osmosis

Some of the advantages of the reverse osmosis process are as follows.

  • It is the best method for water softening.

  • The semipermeable membrane will block all ion particles.

  • Maintenance of the system is very simple.

  • It gives us clean and pure water by blocking all contaminants.

  • The available RO systems are very compact, and it requires little space.

  • The useful life of the full system, including the membrane, is over two years.

  • This system does not require any use of chemicals to purify water.

  • The energy requirement for the RO system is very low.

  • RO systems are totally automated and are designed to start and stop on their own.

Disadvantages of Reverse Osmosis

Some of the disadvantages of the reverse osmosis process are as follows.

  • Sometimes reverse osmosis leads to clogging of the whole system.

  • It requires routine filter changes and maintenance.

  • The installation cost of a reverse osmosis system is high.

  • The whole process is very slow when it comes to household application, as the pressure used is very low.

  • The process does not help in disinfecting the water. You will require a separate process to disinfect the water.

  • Hard water can damage the system.

  • The damaged membrane will allow any small microorganism to pass through it.

  • The applied pressure has to be more than the osmotic pressure, or the system won’t work.

  • The RO system is not self-sustaining.

Applications of Reverse Osmosis

  • Reverse osmosis is widely used in the residential and commercial water filtration system. Other than that, it has plenty of applications in various industries. Some of them are mentioned below.

  • Reverse osmosis is a type of a process which is used to remove dissolved chemical particles from water.

  • Reverse osmosis is a type of a process which is used to remove dissolved biological entities from water.

  • It is highly used in desalinating seawater.

  • It has crucial applications in the medical field.

  • It is used to purify water to prevent any diseases.

  • It has a wide application in water treatment and water purification.

  • It is used in food industries, and it is applied for the concentration of juices, milk, and other beverages.

  • It is used to provide clean water for the community water supply.

FAQ (Frequently Asked Questions)

1. How is osmosis different from reverse osmosis?

Osmosis is the natural tendency of a solvent to flow across a semipermeable membrane in the direction of higher solute concentration. Think of it as the solvent (e.g. water) on one side, and a solution containing more solute (e.g. salt) at the other side of the membrane flow in that direction with the intent to equalize solute concentration at both sides. This process induces pressure on the semipermeable membrane or osmotic membrane. Such pressure is defined as the osmotic pressure. The method of applying pressure in the opposite direction to reverse the flow direction is described as reverse osmosis. The reverse thus indicates the inverted direction of flow.

2. What is not removed by reverse osmosis?

RO systems do not remove dissolved molecules which are about as small as water molecules. That includes most dissolved substances which do not ionize when dissolved. Dissolved gases like oxygen and carbon dioxide pass through the membrane. Most dissolved organic molecules also can pass through the membrane.