What is Reverse Osmosis and How Does It Work?

What is Reverse Osmosis and How Does It Work?
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Reverse Osmosis, popularly known as RO, is nothing but a technology that is employed in the case of water to get rid of a large number of contaminants from it. This is done by pushing the water under pressure through a membrane that is semi-permeable. In the process of reverse osmosis, water is demineralised or deionised by pushing it through a semi-permeable Reverse Osmosis Membrane.


Osmosis is a process that naturally occurs, in which a weaker saline solution migrates to a stronger saline solution. This is one of the most important processes occurring in nature. It is through this process that the plant roots absorb water from the soil, and the kidneys absorb water from the blood.

It is a general tendency of a lower concentrated solution to migrate to a solution with a higher concentration. For example, if you take a container full of water with a low amount of salt, and another with a higher concentration of salt, and separate them by a semi-permeable membrane, then the water with the lower salt concentration will start to migrate towards the other.

A semi-permeable membrane is a membrane that selectively allows atoms and molecules to pass through it. It only allows some of them to pass. You can take a screen door as an example. It allows only air molecules to pass through it and not pests or dust particles that are larger than the size of the holes in the door.

Reverse Osmosis

Reverse Osmosis can be called as osmosis in reverse. Osmosis is a natural process and does not require any external pressure to occur. But, in the case of reverse osmosis, more pressure needs to be applied to the saline solution with a higher salt concentration. The membrane used in the process of reverse osmosis is a semi-permeable membrane that allows only water molecules to pass through it and not most of the dissolved organics, salts, bacteria, and pyrogens. A high pressure needs to be applied to push the water through this reverse osmosis membrane. This needs to be greater than the naturally occurring osmotic pressure. This desalinates, demineralises, or deionises the water to allow pure water to come out. A majority of the contaminants are held back.

How does Reverse Osmosis Work?

In the process of reverse osmosis, a high pressure pump is used to increase the pressure on the saltier side of the RO and force the water to move across the semi-permeable RO membrane. Around 95% to 99% of the dissolved salts are left behind in the reject stream in this process. The salt concentration of the feed water decides the amount of pressure required for the process. The more the amount of salt in the water is, the more will be the pressure required to be able to overcome the osmotic pressure. ro-diagram

The demineralised or deionised, desalinated water that comes out is known as the permeate, or product water. The water stream that is left behind and carries the concentrated contaminants that were unable to pass through the RO membrane is known as the reject, or concentrate stream.

Once the feed water enters the RO membrane under the pressure applied, the water molecules are able to pass through the semi-permeable membrane. The salts and the other contaminants are not allowed to pass, and they are discharged through the reject stream. This is also known as the concentrate or brine stream. It goes in to the drain or can even be fed back in to the feed water supply to be recycled through the RO stream in order to save water. Permeate, or product water, is the water that comes out of the RO stream. It usually has around 95% to 99% of the dissolved removed from it.

RO system employs the process of cross filtration other than the standard filtration. In standard filtration, the contaminants are collected within the filter media. In cross filtration, the solution passes through or crosses the filter with two different outlets. The filtered one goes via one outlet, and the contaminated one goes through the other outlet. Cross flow filtration allows the water to sweep away the contaminant build up. This avoids contaminant build-up in the filter. It also allows enough turbulence so that the membrane surface is kept clean.

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Contaminants removed by Reverse Osmosis

Reverse Osmosis has the capability to remove almost 99% of the dissolved salts or ions, colloids, particles, organics, bacteria, and pyrogens in water. But, it should not be relied upon to remove 100% of the viruses and bacteria. The RO membrane rejects the contaminants on the basis of their size and charge. If a contaminant has a molecular weight greater than 200, it is more likely to get rejected by the RO filter. In the same way, the greater the charge of the contaminant, the more likely it is to be unable to pass through the membrane. For example, sodium ion has a single charge, which means it is monovalent, and it is not rejected by the membrane. Same is the case of calcium. 

You could use the process of Reverse Osmosis to treat brackish, surface, and ground water for all your applications, be it large or small flows. Boiler feed water, metal finishing, pharmaceutical, food and beverage, and semiconductor industries are some of the common industries that use the process of RO for filtering water.

Reverse Osmosis is known to be a very effective and proven technology for filtering water. The salt produced is suitable for most of the industrial applications that require de-ionised or de-mineralised water. The correct system design, maintenance program, and service support can help your RO system provide many years of efficient supply of high purity water.

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