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Reverse Osmosis is a membrane treatment procedure that is used to separate the dissolved solutes from water. It is simply a process used for water purification.
- Reverse Osmosis involves the use of a semi-permeable membrane to remove and filter all the unwanted particles from the water, thus giving us clean water free from bacteria or viruses.
- This process helps to remove the dissolved ions present in the water.
- The technology of reverse osmosis is widely used these days to clean water by removing all contaminants.
The process of Reverse Osmosis (RO) was adopted mainly for the desalination of seawater in 1950 when the entire method was comparatively slow and limited to certain laboratories.
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Key Terms: Osmosis, Water, Purification, Semi-permeable membrane, Filter, Bacteria, Virus, Ions, Water plants, Water industries, Desalination
What is Reverse Osmosis?
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Reverse osmosis can be defined as:
| “The process of solvent movement via a semipermeable membrane from the solution to the pure solvent by means of applying excess pressure on the solution side.” |
- A process that is used to separate the dissolved particles that are present in the water from the water itself.
- The salt particles, along with other effluent materials, are removed from the contaminated water to obtain clean and clear water.
- It is a type of desalination process that is used in treating water.
- This method is one of the oldest methods that is used for the purification of water.
- It is widely used to produce potable water in industries and water plants.
Experiment of Reverse Osmosis
The reverse osmosis process can be explained with an experiment below:
How does Reverse Osmosis work?
The experiment shows how reverse osmosis work:
- Take some fresh water, along with a concentrated aqueous solution.
- Keep the solutions on opposite sides with a semipermeable membrane between them to separate the two solutions.
- Apply the pressure on the side with the concentrated solution.
- It will result in water molecules moving via the membrane to the freshwater side.
This procedure will exhibit reverse osmosis.
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Reverse Osmosis Principle
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The principle of reverse osmosis is based on the use of applying pressure on one side of a particular solution.
- The semipermeable membrane used in this process is kept in between the solutions.
- Due to the presence of a membrane, large solute molecules are unable to pass via it, thus remaining on the pressurised side.
- However, the pure solvent passes through the membrane.
- During this, the solute molecules start concentrating on one side, while the other side becomes diluted.
- The pressure applied is greater than the osmotic pressure, helping the molecules to move from a highly concentrated to a less concentrated solution.
- Reverse osmosis occurs when the solvent passes via the membrane against the concentration gradient, thus moving from a higher concentration to a lower one.
Working of Reverse Osmosis
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Reverse Osmosis is a method of filtration used for removing ions and molecules from a given solution. In the process of reverse osmosis, a semipermeable membrane is placed between the concentrated solution and the freshwater.
- High pressure is applied on the side of the concentrated solution.
- Due to this, the water starts to move through the membrane to the freshwater side, leaving all the contaminants on the other side.
- Thus, the water molecules pass towards the freshwater.
- A concentrated solution passes through a membrane that is in a direction opposite to the natural osmosis process.
- When it passes through the semipermeable solution, the solute concentration is relatively low.
- The membrane helps to filter down the contaminants that are present in the water to the smallest fine particles.
- These particles are also known as concentrates.
Through this process, the contaminants are left behind and we get clean and pure water. This process has become common these days. Major water purifiers use this process in the purification of the water.
Contaminants removed by Reverse Osmosis from Water
Reverse Osmosis helps to remove most of the dissolved contaminants from the water such as bacteria, viruses, salt, and other unwanted substances. The contaminants present are separated through the use of the present RO membrane.
- The size and the charge of the contaminant decide what contaminants will be left behind and what will pass through it.
- The smaller the size of the charge of the contaminant, the higher the chances for it to pass through the RO membrane.
- There are many solvents in the water that the RO membrane cannot remove like the carbon dioxide present in the water is completely dissolved in it and is highly ionized, thus it can't be removed.
Applications of Reverse Osmosis
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The process of reverse osmosis has many applications. Some of them are:
- The process of reverse osmosis is heavily used in the commercial industry for purifying municipal water to have a supply of good quality water. This process helps to reduce the dissolved impurities, bacteria and virus that is present in the water. This helps to supply clean and purified water to the community.
- Many industries like pharmaceutical, chemical, mechanical, and lump industries use this system of reverse osmosis for pre-boiling water treatment.
- The combination of freshwater and saltwater is considered to be brackish water. To desalinate this brackish water, the process of reverse osmosis is used.
- Reverse osmosis is also used to treat the wastewater that is produced by the industries before its disposal. This process is an important application in the industrial field for treating water.
- In the beverage industry, this process is also used for the concentration of milk, juice, and other drinks.
Advantages of Reverse Osmosis
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Reverse Osmosis is a useful process that has many advantages such as:
- This process helps to remove all kinds of bacteria from the water.
- The semipermeable membrane that is present in the process helps to remove all kinds of ion particles, viruses, and bacteria and thus gives us pure and clean water that is fit for drinking.
- The available current systems of RO are very small and compact, acquiring very little space. It is used in homes for water purification.
- Reverse osmosis helps in desalinating the seawater and it is also helpful in treating any kind of liquid discharge.
- The reverse osmosis systems have a very low maintenance cost and the energy that is required for the running of the machine is also very low.
- This process is also used in the beverage industry for applying concentration on beverages like juice, milk, etc.
Disadvantages of Reverse Osmosis
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If there are benefits of reverse osmosis then there are some disadvantages too stated as below:
- The pH tolerance of the cellulose acetate membrane is very low. They start to degrade at a temperature above 35 degrees
- The polyamide membrane present has a poor tolerance for free chlorine.
- If there is hard water then it will completely damage the system and due to the damaged membrane, any small microorganism or bacteria will pass through it very easily.
- The speed of the process is slow and if it has to be used on the household premises then the pressure that will be provided will be very low.
- This process helps to remove all the dirty particles or unwanted substances from the water but it doesn't help in the disinfection of the water. If one needs to disinfect the water they have to go through a different process to disinfect the water.
- The thin-film composites that are present are intolerant to chlorine.
Difference between Reverse Osmosis and Osmosis
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Below mentioned are some of the differences between reverse osmosis and osmosis.
| Reverse Osmosis | Osmosis |
|---|---|
| Reverse osmosis is an artificial process. | Osmosis is a natural process. |
| The process of reverse osmosis involves the use of energy. | The process of osmosis does not involve the use of energy. |
| The movement of reverse osmosis is against the potential gradient. | The movement of the osmosis occurs along the potential gradient. |
| Reverse osmosis is a process where the solution passes through the membrane to remove the unwanted substances from it and give out freshwater. The pressure applied is greater than the osmotic pressure. | Osmosis is a process wherein the solvent molecules pass through the semipermeable membrane from a high concentration to a low concentration of water. |
Things to Remember
- Reverse osmosis involves the use of a semi-permeable membrane to remove and filter all the unwanted particles from the water, as a result giving us clean water free from any kind of bacteria or virus.
- Reverse osmosis and osmosis are two different processes.
- Reverse osmosis helps to remove particles like sodium and lead from the water.
- The combination of freshwater and saltwater is considered to be brackish water. To desalinate this brackish water, the process of reverse osmosis is used.
- The principle of reverse osmosis is based on the use of applying pressure on one side of a particular solution.
- Reverse osmosis is also used to retreat the water and this water is also used in producing energy and power generation.
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Previous Year Questions
- The mass of one mole of a substance in grams is called its…
- The mass of CaCO3 is required to react with 25 mL of…
- The number of moles of water present in 180gm of water… (AIIMS 1996)
- 4 g of a hydrated crystal of formula AxH2O has 0.8 g of water… (KCET 2013)
- The number of moles of KMnO4 that will be needed to react… (NEET 2007)
- The number of gram molecules of oxygen in 6.02 x 1023… (NEET 1990)
- The mole fraction of benzene in a solution containing… (COMEDK UGET 2012)
- One mole of NaCl contains 6.023 x 10 molecules of sodium…
- Equal moles of different substances contain the same number of…
- The density of 2 M aqueous solution of NaOH is 1.28 g… (NEET 2019)
- When 25 g of a non-volatile solute is dissolved in 100 g of water… (BITSAT 2008)
- 0 .15 g of a substance dissolved in 15 g of a solvent higher by… (NEET 1999)
- 200 mL of an aqueous solution of a protein contains its… (NEET 2011)
- At 25∘C, the highest osmotic pressure is exhibited by… (NEET 1994)
- A 5% solution of cane sugar (MW = 342) is isotonic with… (NEET 1998)
Sample Questions
Ques. What is RO membrane? (1 mark)
Ans. Reverse osmosis (RO) membranes are used in wastewater treatment units since they can be used to remove salts and other pollutants very effectively. The performance of RO membrane is affected by several factors like Operational parameters and feed characteristics, during operation.
Ques. How does the process of Reverse Osmosis work? (1 mark)
Ans. Reverse Osmosis works by means of a high-pressure pump helping to increase the pressure on the RO’s side of the salt, thus forcing the water across the semipermeable RO membrane. It helps leave almost all (roughly 95 to 99%) dissolved salts in the reject stream behind.
Ques. What is osmosis? (1 mark)
Ans. Osmosis can be defined as the process by means which the molecules of a solvent are passed from a solution of low concentration to a solution of high concentration via a semi-permeable membrane. It is simply the movement of water molecules. The three types of Osmotic Solutions are isotonic, hypertonic and hypotonic solutions.
Ques. What is the formula of osmotic pressure? (1 mark)
Ans. The formula of Osmotic Pressure is Π = MRT.
Ques. What are the common issues with the reverse osmosis process? (3 marks)
Ans. This process helps to take out all the bacteria and viruses that are present in the water but this process is so powerful that it can also take out or remove the natural minerals that are present in the water for human consumption and are healthy for us like magnesium and calcium, due to this we can lose out on important minerals.
- One of the common issues with the reverse osmosis process is that it can clog the membranes very easily.
- Fouling and scaling of the membranes is a very common issue. This happens when the material starts to pile up on the membrane and this causes the slow down of the water. This also reduces the lifespan of the membrane and because of this, it has to be made sure that the membrane is changed fast so that proper cleaning of the water is done. If this is ignored then this may increase the operating cost and will also damage the equipment.
- Wasting water is one of the other big issues with the reverse osmosis process. They use a lot of water to run their operation and in the filtering process. Water is one of the most important resources that need to be conserved and utilized but the reverse osmosis system does not help in saving water.
Ques. What is the practical use of reverse osmosis? (5 marks)
Ans. The process of reverse osmosis is used in the cleaning and purification of water.
- Salt and other effluent particles, bacteria, and viruses are removed from the water molecules to make them clean and fit for drinking.
- The process is chemical-free. This is a natural process for cleaning the water and is used in many different areas for the purification of the water.
- This process is used by local municipal corporations to supply clean water to people in their homes.
- Reverse osmosis is also used in many industries like chemicals, pharmaceuticals, lumps, and mechanical industries for pre-boiling water treatment.
- This helps to remove all kinds of bad substances from the water.
- This process is also used for wastewater treatment.
- Industries use this process to filter out all the waste before disposing of the water into the rivers.
- It also helps in the desalination of the brackish water, meaning removing salts from the freshwater and making it fit for drinking.
Ques. What is the difference between osmosis and reverse osmosis? (5 marks)
Ans. In the process of osmosis, water flows through the semipermeable membrane from a dilute solution to a concentrated solution until equilibrium is reached. Whereas in reverse osmosis, the concentrated solution passes through the membrane that is in the direction opposite to the natural osmosis process.
- In the process of reverse osmosis, water molecules move against the concentration of the gradient across the semipermeable membrane. Whereas in the osmosis process, water molecules pass along with the concentration gradient.
- Osmosis occurs from a high concentration to a low concentration direction but in the case of reverse osmosis, it is the opposite.
- Osmosis is a naturally occurring process whereas the process of reverse osmosis is used in the treatment of water.
- To conduct the reverse osmosis process there is a need for energy but energy is not required in the process of osmosis.
Ques. Does reverse osmosis help in the complete purification of the water? (5 marks)
Ans. The reverse osmosis process may not completely help in the purification of the water but it helps to remove all the unwanted and dirty particles from the water. However, if one has to purify the water they have to go through the other process for the purification.
- Reverse osmosis helps to remove all the dirt and bacteria that are present in the water. This process is so powerful that sometimes it may remove important minerals from the water like calcium and magnesium which are important for the human body.
- The RO membrane that is present helps to keep away all the contaminants on the other side only allowing the clean water to move to the other side. This process is used to treat salty and brackish water that may have several impurities and an excess amount of salt content.
- Reverse osmosis is a widely used technology in industries and homes used for the cleaning and purification of water. This process is commonly used these days.
- To make sure that the water is cleaned properly, it is important that the RO membrane should be clean and changed from time to time as required.
Ques. What are the various disadvantages of reverse osmosis? (5 marks)
Ans. Reverse osmosis can have various disadvantages and some of which are:
- It doesn't help in the disinfection of the water. It may remove all the dirty and unwanted particles but it may not purify the water.
- The speed of process of reverse osmosis is very slow and if it has to be used for household use then it may not give the required pressure that is needed in the process.
- This process also includes a large amount of wastage of water due to which there is a lot of loss of water which is not good because water is a scarce resource and we should try to save it as much as we can.
- The operating cost of the RO machinery may increase if proper care is not taken of it.
- The RO membrane should be changed timely because if it is not changed the unwanted substances can damage the membrane and the machinery also.
- Clogging is also one of the main and common issues that arise in the reverse osmosis process. This may damage the reverse osmosis membrane and will create an issue in the cleaning of the water.
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