What is reverse Osmosis

You may not be familiar with the term ‘Reverse Osmosis’. However, that doesn’t mean you are not using it in your daily routine. Something as common as reverse osmosis may pass your notice and fail to gain your attention but nonetheless, it is there may be in your kitchen water filtering system or aquarium water supply. Thus, it is high time you become aware of what this process actually is and how it works.

In this blog post, I bring brief yet comprehensive information about the RO process. All you have to do is read it and know how simple it is and where it is in action.


Reverse osmosis is a process that involves the separation of impurities from water by pushing it through a semi-permeable membrane with full pressure.

RO is a process of liquid filtering; it filters the contaminated liquid and makes it fit for use in daily life.

Reverse osmosis is a process that industry uses to clean water, whether for industrial process applications or to convert brackish water, to clean up wastewater or to recover salts from industrial processes.

It does not remove dissolved impurities like dissolved gases, including

  • Oxygen
  • Carbon dioxide

It is effective against

  • Trihalomethanes(THM’s)
  • Pesticides
  • Organic compounds (VOC’s).

The pressure on the concentrated side of the membrane sends the purified water spiraling down to the RO faucet while discarding the impurities down the drain line.

As an example, consider the marine birds, which drink seawater. Do they drink saline water? They have natural RO system in their throats. Whenever they drink seawater, their throats filter the salt out of the water, thus, swallowing the fresh water and spitting out the waste salt.

RO also has many wide-ranging applications in medicine and industry. It is a process of water-purification or solvent extraction in other cases.

A Scientific Explanation

RO is a relatively complicated filtration process; a separation technique for pure water from a muddy solution. Consider two solutions with different level of concentrations. These solutions have a partition through a membrane. The semipermeable membrane allows some molecules to pass through it while holding back others. The flow of molecules is under pressure, which is greater than the osmotic pressure. This counter pressure in on the solution with a higher concentration of solute; thus, it moves towards the more dilute one.

Water forms links by hydrogen bonding and gets attached to the membrane matrix. These molecules cannot resist pressure and when pressure acts on them, they make way through the membrane and cross it. Most of the compounds weighing above 100g are held back and cannot cross the membrane net so easily. These compounds are oils, other particulates like bacteria, pyrogens etc.

The other ions present in the solution are the salt ions. These ions cannot cross the semipermeable membrane and the flow rejects them. Dielectric interactions repel the ions away from the membrane surface and don’t let them cross the barrier. The percentage of rejection is 85 – 98%.


The central part in the RO process is the membrane. It functions as the heart of the entire system. The materials used in making this semipermeable membrane are:

  • Cellulose acetate
  • Polysulfonate
  • Polyamide

The membrane has a skin and a support layer. It is the skin that lets the water molecules pass through and blocks the passage of salt ions.

Quality of Reverse Osmosis Product Water

When the water filters out by the RO process as permeate, it is not 100% solid free. The quantity of these solids depends on their percentage in the feed water. Let’s say, if the water entering the RO system has 400 ppm dissolved solids, then the permeate has about 10-15 ppm. Thus, the permeate has dissolved solids in a constant percentage of the feed water. This also affects the quality of product water. Its quality is poorer if the recovery percentage is too high or the rejection percentage is too low and vice versa.


One of the naturally occurring processes of utmost importance is Osmosis. It is a special form of diffusion. In this process, a saline solution of lesser concentration moves/migrates towards the one with a higher concentration.

For instance, our body organs absorb water content from out blood. Similarly, roots of different plants take in water from the surrounding moist soil.


Osmosis occurs through a semipermeable membrane, which allows a weaker saline solution to pass towards a concentrated one.

A semi-permeable membrane allows some particles to move through while blocking the others. For instance, a screen door let air to pass through while holding back the pests etc.

If we reverse Osmosis, what we will get is ‘Reverse Osmosis’ Osmosis is a natural process requiring no energy; however, RO requires energy for the concentrated solution to flow towards a dilute solution. This energy is in the form of pressure, applied to the high concentration solution in order to desalinate water. The RO membrane allows the pure water to pass while checking the progress of a majority of impurities.



The performance of the RO system depends on factors, such as

  • Feed pressure
  • Permeate pressure
  • Concentrate pressure
  • Feed conductivity
  • Permeate conductivity
  • Feed flow
  • Permeate flow
  • Temperature

The percentage recovery i.e. the percentage of water entering the unit that is useable also depends on the above factors. RO system needs a good maintenance for better performance, efficiency and maximum water output.

A variety of biocides works with different types of membranes to purify the water, in addition to carbon and sediment filters. A sound knowledge about the fouling is helpful in optimizing the process.


Across the world, reverse osmosis is in use for water treatment and purification. Some of its applications are in

  • Drinking water
  • Wastewater
  • Food industry
  • Car Wash
  • Reef aquariums
  • Desalination
  • Drinking water

The people around the world are using reverse osmosis to make the drinking water free of impurities and contamination. It is prevalent not only on domestic level but also on large-scale industries and factories.

It had found its roots in the U.S. military as R.O.W.P.U.s (Reverse Osmosis Purification Units); now it produces gallons of water in a range of 10000-60000 for a varying number of soldiers with an upper limit of 6000-10000.

This drinking water is also void of NBC i.e. nuclear/biological//chemical particles.

  • Wastewater

In cities where there is a shortage of water for use, it is a practice to collect rainwater in big artificial ponds and then purify it through the process of reverse osmosis. This clean water is then supplied to irrigation channels and industrial cooling.

Even the industrial uses of this process are wide varying. The power plants utilize it to get rid of the minerals in the boiler water. If the water present in the boiler contains minerals, it leaves deposits on the insides and this leads to corrosion, damage etc. of the machinery.

The Singapore authorities had made their intentions publicly known about the treatment of wastewater before spilling it back into the reservoirs using reverse osmosis.

  • Food industry

One of the many benefits of reverse osmosis includes the concentrating or thickening of some food, such as juices of orange and tomato, which, otherwise, can lose their healthful nutrition upon heating.

Furthermore, dairy industry utilizes it in

  • Production of whey protein powder
  • Concentration of milk

It has its uses in wine industry especially in French wineries. There the machines concentrate the grape juices through reverse osmosis, removing acetic acid, alcohol, smoke etc. Maple syrup producers also remove the excess water through this process before the raw maple sap undergoes boiling. It serves three purposes:

  • 80% water elimination
  • Less energy conservation during boiling
  • Less boiling time
  • Car wash

One of the problems associated with water is its hardness. Hardness in water is undesirable. It raises many issues during use; these issues are also present in the industries using hard water. It is responsible for causing water spots on the automobile. This is due to the compounds of magnesium and calcium present in the hard water, which give the water its hardness. That’s the reason, many car wash enterprises employ reverse osmosis machines to carry out the car wash. This process eliminates the water hardness and makes it fit for use.

  • Reef aquariums

In reef aquarium industry, reverse osmosis process makes the water fit for the artificial mix of seawater. The ordinary tap water is full of

  • Amounts of chlorine
  • Chloramines
  • Heavy metals
  • Nitrogen compounds
  • Phosphates
  • Other chemicals

These impurities are harmful for the reef environment. That’s why keepers employ the process to get rid of such contaminants that harm the reef life. It also reduces the running cost.

  • Desalination

Some authorities make the seawater suitable for drinking. This is basically for areas where there are limited amounts of clean water available. Reverse osmosis desalinate the seawater and makes it fit for use. This process is widely in use in the Middle East.


There are two types:

  • 1 stage RO system
  • 2 stage RO system



This percentage tells the effectiveness of the RO membranes in removing contaminants. It is not a measure of the individual performance of the membrane; rather it determines the average performance of the overall system.

This range is 95% to 99% for a properly designed RO system with fully operational RO membranes. It means they will get rid of most of the contaminants. The equation for salt rejection % is:


The greater this percentage, the more efficient the performance of the system is. A lower value of the salt rejection may points towards the cleaning or replacing of membranes.


The inverse of salt rejection % is salt passage %. It is the amount of salt crossing the membrane. For an efficient RO system, we want its value to be a minimum.

Salt Passage % = (1 – Salt Rejection %)


The amount of water that is obtained from the membrane as good permeate water is known as ‘Percent Recovery’

On the other hand, it is the amount of water that doesn’t go down the drain as concentrate; rather it moves to the faucet as a product of RO.

The greater the recovery % , more is the water available for use at the output. However, a higher recovery % can also be a problem as it may cause problems related to scaling, fouling etc. Therefore, the correct recovery is calculated using the formula:



The RO system design also depends on the concentration factor. The recovery water percentage increases, resulting in an increase in the concentration of the salts in the stream. Thus, the scaling potential is higher on the RO membrane surface.

Concentration Factor =    1 /1 – Recovery %

This concept is similar to that operative in a boiler. Whenever the water evaporates, it leaves behind a concentrated salt solution.


This is a comprehensive article on Reverse Osmosis, which is a very essential process in many industries today. With the increase in population, the world is facing water shortage day by day. In such a scenario, availability of fresh water is no less than bliss. You can make use of the RO system or any system employing RO process to obtain fresh water and use it in daily life.

Do not forget to share this useful piece of information with everyone, so that more people know about the benefits of a useful process, RO. Leave your comments in the section below.

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