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Reverse Osmosis

The process of movement of solvent through a semipermeable membrane from the solution to the pure solvent by applying excess pressure on the solution side is called reverse osmosis.

Reverse osmosis is a membrane treatment process primarily used to separate dissolved solutes from water. Reverse osmosis is most commonly known for its use in drinking water purification, particularly with regard to removing salt and other effluent materials from water molecules.

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Introduction to Reverse Osmosis

Reverse osmosis is one of the oldest and most popular separation techniques used mainly for the purification of water. The process was mainly adopted for the desalination of seawater in the year 1950 when the whole process was relatively slow and limited to certain laboratories. However, after a lot of research and advancements in technology, there were significant developments, especially in the field of polymers and the production of efficient membranes.

Today, this technique is extensively used by many around the world to purify water for industrial, residential, commercial and scientific purposes. While reverse osmosis technology is one of humanity’s important scientific innovations we will develop a basic understanding of the whole process here on this page.

What is Reverse Osmosis?

Reverse osmosis which is also commonly referred to as RO is a type of filtration method used for the removal of molecules and ions from a certain solution.

Reverse osmosis involves the application of pressure (usually greater than the osmotic pressure) on one side of the solution where a semipermeable membrane is placed in between the solutions. This membrane is used to filter out contaminants down to the smallest particles. The contaminants are often referred to as RO concentrate.

Reverse Osmosis Principle

To break down the process further, due to the presence of a membrane, large molecules of the solute are not able to cross through it and they remain on the pressurised side. The pure solvent, on the other hand, is allowed to pass through the membrane. When this happens the molecules of the solute start becoming concentrated on one side while the other side of the membrane becomes dilute. Furthermore, the levels of solutions also change to some degree.

In essence, reverse osmosis takes place when the solvent passes through the membrane against the concentration gradient. It basically moves from a higher concentration to a lower concentration.

Reverse Osmosis Process

Osmotic pressure is the minimum pressure required to stop solvent flow through the semipermeable membrane. Therefore, when the solution side (the side where the solute concentration is high) is subjected to a pressure greater than the osmotic pressure, the solvent particles on the solution side move through the semipermeable membrane to the region where the solute concentration is low. Such inverse solvent movement through the semipermeable membrane is called reverse osmosis.
It is important to note that the pressure applied to the solution side must be higher than the osmotic pressure for the reverse osmosis process to proceed. Osmotic pressure is a colligative property, which depends on the concentration of the solution. In water purification, the reverse osmosis process is very important. Many water purifiers used today use reverse osmosis in the purification process as one of the steps.

Experiment of Reverse Osmosis 

The reverse osmosis process is explained below with the help of an experiment.

How does Reverse Osmosis work?

An easy experiment can be conducted by taking some freshwater and a concentrated aqueous solution. The solutions should be kept on opposite sides with a semipermeable membrane placed in between to separate the two solutions. Pressure should be applied on the side with the concentrated solution. Now this will result in water molecules moving through the membrane to the freshwater side. This basically sums up the process of reverse osmosis.

Reverse Osmosis Process

Benefits of Reverse Osmosis

Some of the benefits of reverse osmosis are discussed below –

1. This process can be used to effectively remove many types of dissolved and suspended chemical particles as well as biological entities (like bacteria) from the water.

2. This technique has a wide application in treating liquid wastes or discharges.

3. It is used in purifying water to prevent diseases.

4. It helps in desalinating seawater.

5. It is beneficial in the medical field.

Advantages of Reverse Osmosis

Reverse Osmosis has several advantages, including the following:

        • Bacteria, viruses and pyrogen materials are rejected by the intact membrane. In this respect, RO water approaches distilled water in quality.
        • Available units are relatively compact and require little space. They are well suited to home dialysis.
        • In average use, the membrane has a life of a little more than one to two years before replacement is necessary.
        • Periodic complete sterilization of the RO system with formalin or other sterilant is practical.

Disadvantages of Reverse Osmosis

The disadvantages of RO systems include the following;

        • Cellulose acetate membranes have limited pH tolerance. They degrade at temperatures greater than 35oC. They are vulnerable to bacteria. They eventually hydrolyze.
        • Polyamide membranes are intolerant of temperatures greater than 35oC. They have poor tolerance for free chlorine.
        • Thin-film composites are intolerant of chlorine. High flux polysulfones require softening or deionization of feed water to function properly.


Frequently Asked Questions – FAQs

How does reverse osmosis work?

Reverse Osmosis works by using a high-pressure pump to increase the pressure on the salt side of the RO and force the water across the semipermeable RO membrane, leaving almost all (around 95 to 99 %) dissolved salts in the reject stream behind.

What is an example of the use of reverse osmosis?

Reverse osmosis is a means of pulling clean water out of polluted water or salt water by pushing water through a membrane under pressure. An example of reverse osmosis is the process by which contaminated water is filtered under pressure.

What are the disadvantages of reverse osmosis?

The drawback of RO water is that it can significantly reduce the good minerals which can help in the overall health of the heart and muscles. Those who use reverse osmosis as their primary source of water may suffer from side effects such as tiredness, muscle cramps, general weakness and cardiovascular disorders in severe cases.

What are osmosis and reverse osmosis?

Osmosis is a process in which liquid water flows through a semipermeable membrane from a diluted solution into a more concentrated solution. The process of movement of solvent through a semipermeable membrane from the solution to the pure solvent by applying excess pressure on the solution side is called reverse osmosis.

What is the principle of reverse osmosis?

Reverse osmosis ( RO) is a water purification process that removes ions, unwanted molecules and larger particles from drinking water using a partially permeable membrane. As a result, the solute is kept on the membrane’s pressurised side and the pure solvent is allowed to pass to the other side.

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