Reverse Osmosis: Definition, Process, and Uses

Edited By Shivani Poonia | Updated on Jul 02, 2025 08:09 PM IST

The principle of osmosis was first discovered by the French scientist Jean Antoine Nollet. During his study, he observed that water can pass through the membrane namely the semipermeable membrane. The idea of reverse osmosis began to take shape in the 1950s with the development of synthetic membranes and it happened because of researchers aiming to address the water quality and also due to water scarcity.

This Story also Contains

Osmosis
Reverse Osmosis
Some Solved Example
Summary

Reverse Osmosis: Definition, Process, and Uses

Reverse osmosis is basically a process that is used to remove a wide range of contaminants which include, salt, bacteria, and other impurities present in the water or any chemical that is dissolved in the water. It is an important source for the region where the resources of fresh water are limited. Reverse osmosis is also used by the municipalities to improve the quality of water for further supply so that water reaches the people and is pure.

Osmosis

Osmosis: It is the flow of solvent molecules from a solution of low concentration to a solution of higher concentration when they are separated by a semi-permeable membrane(SPM), the concentration obviously being defined with respect to the solute.

Semi-permeable membrane consists of a network of submicroscopic pores or holes. The pore size is such that the smaller solvent molecules can move across the membrane while the movement of larger solute molecules is hindered by the smaller pores of the SPM.

There are many phenomena which include the process of osmosis that we observe in daily lives. For example, raw mangoes shrivel when pickled in brine (saltwater); wilted flowers revive when placed in freshwater, blood cells collapse when suspended in saline water, etc.

Assume that only solvent molecules can pass through these semipermeable membranes. If this membrane is placed between the solvent and solution as shown in the figure given below, the solvent molecules will flow through the membrane from pure solvent to the solution. This process of flow of the solvent is called osmosis.

Reverse Osmosis

The direction of osmosis can be reversed if a pressure larger than the osmotic pressure is applied to the solution side. That is, now the pure solvent flows out of the solution through the semi-permeable membrane. This phenomenon is called reverse osmosis and is of great practical utility. Reverse osmosis is used in the desalination of seawater. A schematic setup for the process is shown in the Figure given below. When pressure more than the osmotic pressure is applied, pure water is squeezed out of the seawater through the membrane. A variety of polymer membranes are available for this purpose.

The pressure required for reverse osmosis is quite high. A workable porous membrane is a film of cellulose acetate placed over suitable support. Cellulose acetate is permeable to water but impermeable to impurities and ions present in seawater. These days many countries use desalination plants to meet their potable water requirements.

Some Solved Example

Example.1

1. RO membranes are made of:

1)Plastic

2)Cotton

3)Silk

4) (correct)Polymer

Solution

The highest recommended applied pressure of commercial membranes presently available is 7.0 Mpa; beyond which compaction will start to occur because RO membranes are made of polymers.
Hence, the answer is the option (4).

Example.2

2. As water viscosity lowers, the water flux of RO membranes?

1)Fluctuates

2)Decreases

3) (correct)Increases

4)None of the above

Solution

When the water viscosity is lowered, the water flux of RO membranes increases and the water viscosity is lowered by an increase in temperature. Thus, the higher the temperature better the flux.

Hence, the answer is the option (3).

Example.3

3. What is the flux considered while designing a reverse osmosis system for treating bore well water?

1)10-15 LMH

2)40-50 LMH

3) (correct)20-30 LMH

4)60-70 LMH

Solution

The flux increases with an increase in pressure. It also increases with increased temperature. In the case of bore well water, the flux considered is 20-30 LMH.

Hence, the answer is the option (3).

Example.4

4. The size of a raw mango shrinks to a much smaller size when kept in a concentrated salt solution which one of the following processes can explain this?

1)Reverse osmosis

2)Diffusion

3) (correct)Osmosis

4)Dialysis

Solution

The size of the mango shrinks when it is kept in a concentrated salt solution. This is due to osmosis in which the water molecules move out from the mango into the salt solution causing the mango to shrink.

Hence, the answer is the option (3).

Example.5

5. What is the maximum acceptable limit of temperature(in ^oC) for RO?

(Response should be an integer value)

1) (correct)40

2)30

3)15

4)50

Solution

To reduce the effects of temperature to a minimum, the acceptable upper limit is 40. RO systems operate at 25^oC usually.

Hence, the answer is the option (1).

Example.6

6.Match List I and List II

Choose the correct answer from the options given below :

1)A-I, B-III, C-IV, D-II

2) (correct)A-III, B-I, C-IV, D-II

3)A-III, B-I, C-II, D-IV

4)A-I, B-III, C-II, D-IV

Solution

(i) Electro osmosis: When the movement of colloidal particles is prevented by some suitable means (porous diaphragm or semi-permeable membranes), it is observed that the D.M. begins to move in an electric field. This phenomenon is termed electrosmosis.

(ii) Solvent molecules pass through a semi-permeable membrane towards the solvent side is termed reverse osmosis.

(iii) When an electric potential is applied across two platinum electrodes dipping in a colloidal solution, the colloidal particles move towards move towards one or the other electrode. The movement of colloidal particles under an applied electric potential is called electrophoresis.

(iv) Solvent molecules pass through a semipermeable membrane towards the solution side is termed as osmosis.

Summary

It has been proved that the phenomenon of reverse osmosis is a highly effective and very versatile technology for the purification of water and has various significant applications in other fields. In this process, water is forced to pass through the semi-permeable membrane that removes all the contaminants of water which results in purified water. The water produced as a result of reverse osmosis is high quality as it has a desirable taste and is free from undesirable taste, odor, and pollutants. As reverse osmosis is very effective, it is also energy intensive and it can be costly in terms of its maintenance and operation. In this process, the semi-permeable membrane needs to be changed frequently because the membrane is the most working part of this process.

Frequently Asked Questions (FAQs)

1. How does reverse osmosis differ from regular osmosis?

In regular osmosis, water naturally moves from an area of low solute concentration to an area of high solute concentration. Reverse osmosis is the opposite - pressure is applied to push water from an area of high solute concentration through a membrane to an area of low solute concentration.

2. What is reverse osmosis?

Reverse osmosis is a water purification process that uses a semipermeable membrane to remove ions, molecules, and larger particles from drinking water. It works by applying pressure to overcome osmotic pressure, forcing water molecules through the membrane while leaving contaminants behind.

3. Why is pressure needed in reverse osmosis?

Pressure is needed to overcome the natural osmotic pressure that would normally cause water to flow from the less concentrated to the more concentrated solution. The applied pressure forces water through the semipermeable membrane against its natural flow direction.

4. What is the role of the semipermeable membrane in reverse osmosis?

The semipermeable membrane is the key component in reverse osmosis. It allows water molecules to pass through while blocking larger molecules, ions, and particles. The membrane acts as a selective barrier, separating purified water from contaminants.

5. What types of contaminants can reverse osmosis remove from water?

Reverse osmosis can remove a wide range of contaminants, including dissolved salts, particles, bacteria, and other microorganisms. It's effective at removing many common pollutants such as lead, chlorine, fluoride, nitrates, sulfates, and many organic compounds.

6. What happens to the contaminants removed by reverse osmosis?

The contaminants removed by reverse osmosis are concentrated in the reject water, also known as brine or wastewater. This concentrated solution is then flushed away or disposed of separately from the purified water.

7. What are the main components of a reverse osmosis system?

A typical reverse osmosis system includes a pre-filter to remove larger particles, the semipermeable membrane, a post-filter for final polishing, a storage tank for purified water, and often an automated shut-off valve to stop production when the tank is full.

8. What is meant by "rejection rate" in reverse osmosis?

Rejection rate refers to the percentage of contaminants that the reverse osmosis membrane prevents from passing through. For example, a 99% rejection rate for a particular contaminant means that 99% of that contaminant is removed from the water.

9. What are the main applications of reverse osmosis?

Reverse osmosis is used in various applications, including desalination of seawater, purification of drinking water, water reclamation in industrial processes, and in the food and beverage industry for concentrating fruit juices and other liquids.

10. What is the concept of "flux" in reverse osmosis?

Flux in reverse osmosis refers to the rate at which water passes through the membrane, typically measured in gallons per square foot of membrane per day (GFD). Higher flux rates generally indicate more efficient systems, but very high flux can lead to faster membrane fouling.

11. How does membrane fouling affect reverse osmosis systems?

Membrane fouling occurs when contaminants build up on the membrane surface, reducing its effectiveness and efficiency. Fouling can decrease water flow, increase energy consumption, and potentially damage the membrane. Regular cleaning and maintenance are necessary to prevent and address fouling.

12. What is meant by "recovery rate" in reverse osmosis?

Recovery rate is the percentage of feed water that is converted into purified water (permeate). For example, a system with a 75% recovery rate produces 75 gallons of purified water for every 100 gallons of feed water, with 25 gallons becoming wastewater.

13. What is the role of pre-treatment in reverse osmosis systems?

Pre-treatment is crucial in reverse osmosis systems to protect the membrane and improve overall efficiency. It typically involves removing larger particles, adjusting pH, and sometimes adding anti-scaling chemicals to prevent membrane fouling and extend the life of the system.

14. What are the environmental concerns associated with reverse osmosis?

The main environmental concerns with reverse osmosis include high energy consumption, especially in large-scale operations like desalination plants, and the disposal of concentrated brine waste, which can impact marine ecosystems if not managed properly.

15. What is meant by "cross-flow filtration" in reverse osmosis?

Cross-flow filtration refers to the way water flows across the surface of the membrane in reverse osmosis. The feed water flows parallel to the membrane surface, with some water passing through (permeate) and the rest continuing along the membrane, carrying away concentrated contaminants.

16. How do multi-stage reverse osmosis systems work?

Multi-stage reverse osmosis systems use multiple membranes in series. The permeate from one stage becomes the feed for the next stage. This arrangement can improve overall efficiency and water quality, allowing for higher recovery rates and more thorough contaminant removal.

17. What are the limitations of reverse osmosis in removing certain contaminants?

While highly effective, reverse osmosis has limitations. It may not effectively remove some dissolved gases like carbon dioxide or chlorine, very small organic molecules, or certain pesticides. Additionally, it cannot remove 100% of any contaminant, though rejection rates are typically very high.

18. What is the role of activated carbon filters in reverse osmosis systems?

Activated carbon filters are often used as pre- or post-treatment in reverse osmosis systems. As pre-treatment, they remove chlorine and organic compounds that could damage the RO membrane. As post-treatment, they can improve taste and remove any remaining odors or organic contaminants.

19. How does reverse osmosis impact the taste of water?

Reverse osmosis typically improves water taste by removing contaminants, dissolved solids, and chlorine. However, some people find RO water "flat" or "bland" due to the removal of minerals. This is why some systems reintroduce minerals or blend RO water with filtered tap water.

20. How does membrane pore size in reverse osmosis compare to other filtration methods?

Reverse osmosis membranes have extremely small pore sizes, typically around 0.0001 micron. This is much smaller than microfiltration (0.1-10 microns) or ultrafiltration (0.01-0.1 microns) membranes, allowing RO to remove even dissolved salts and small molecules that other methods cannot.

21. What is the significance of "total dissolved solids" (TDS) in reverse osmosis?

Total Dissolved Solids (TDS) is a measure of all dissolved substances in water. Reverse osmosis is highly effective at reducing TDS, often by 90% or more. Monitoring TDS levels before and after treatment is a common way to assess the performance of an RO system.

22. What is the concept of "scaling" in reverse osmosis, and how is it managed?

Scaling occurs when minerals in the water precipitate and form solid deposits on the membrane surface. This can reduce efficiency and damage the membrane. It's managed through pre-treatment (like water softening or anti-scalant addition), proper system design, and regular cleaning procedures.

23. How does the presence of chlorine in feed water affect reverse osmosis membranes?

Chlorine can damage many types of RO membranes, particularly thin-film composite membranes. It oxidizes the membrane material, degrading its performance over time. This is why most RO systems include activated carbon pre-filters to remove chlorine before it reaches the membrane.

24. What is meant by "permeate" and "concentrate" in reverse osmosis?

In reverse osmosis, "permeate" refers to the purified water that passes through the membrane. "Concentrate" (also called reject water or brine) is the portion of feed water that does not pass through the membrane and contains the concentrated contaminants removed from the permeate.

25. What is the role of pressure vessels in industrial reverse osmosis systems?

Pressure vessels in industrial RO systems house the membrane elements and withstand the high pressures required for the process. They are designed to efficiently direct the flow of feed water across the membrane surface and separate the permeate from the concentrate.

26. What is meant by "membrane compaction" in reverse osmosis?

Membrane compaction occurs when the physical structure of the membrane is compressed under high pressure, potentially reducing its permeability. This can decrease the system's efficiency over time. Proper system design and operation help minimize compaction and extend membrane life.

27. What is the concept of "differential pressure" in reverse osmosis systems?

Differential pressure in RO systems refers to the difference in pressure between the feed side and the permeate side of the membrane. Monitoring this pressure difference is crucial for assessing membrane condition and system performance, as increases can indicate fouling or other issues.

28. What is the significance of "flux decline" in reverse osmosis systems?

Flux decline refers to the decrease in water flow rate through the membrane over time. It's a common issue in RO systems, often caused by fouling, scaling, or membrane compaction. Monitoring flux helps operators assess system performance and schedule maintenance or membrane replacement.

29. Can reverse osmosis remove all contaminants from water?

While reverse osmosis is highly effective, it cannot remove 100% of all contaminants. It's most effective at removing dissolved salts and larger particles. Some very small molecules, like certain pesticides or volatile organic compounds, may pass through the membrane.

30. How does the pore size of the reverse osmosis membrane affect its performance?

The pore size of the membrane is crucial to its performance. Typically, reverse osmosis membranes have pore sizes around 0.0001 micron. This extremely small size allows water molecules to pass through while blocking most contaminants, including many dissolved salts and larger molecules.

31. How efficient is reverse osmosis in terms of water usage?

Reverse osmosis systems typically use more water than they produce as purified water. The efficiency can vary, but generally, for every gallon of purified water produced, 2-4 gallons of water may be used in the process, with the excess becoming wastewater.

32. How does temperature affect the reverse osmosis process?

Temperature affects the rate of reverse osmosis. Higher temperatures generally increase the rate of water flow through the membrane, improving efficiency. However, extremely high temperatures can damage the membrane, so most systems operate at room temperature or slightly above.

33. Can reverse osmosis remove beneficial minerals from water?

Yes, reverse osmosis can remove beneficial minerals like calcium and magnesium along with harmful contaminants. This is why some systems include a remineralization stage to add back essential minerals to the purified water.

34. How does reverse osmosis compare to other water purification methods?

Reverse osmosis is generally more thorough than many other methods like carbon filtration or UV purification. It can remove a wider range of contaminants, including dissolved salts, which many other methods cannot. However, it typically uses more water and energy than some other methods.

35. How does reverse osmosis work in desalination plants?

In desalination plants, seawater is pressurized and forced through reverse osmosis membranes. The membranes allow fresh water to pass through while blocking salt and other minerals. This process can turn seawater into potable freshwater, though it requires significant energy input.

36. How does pressure affect the performance of a reverse osmosis system?

Increased pressure generally improves the performance of reverse osmosis by overcoming osmotic pressure more effectively. Higher pressure can increase the flow rate of water through the membrane and improve contaminant rejection. However, excessive pressure can damage the membrane.

37. How does reverse osmosis affect the pH of water?

Reverse osmosis tends to lower the pH of water slightly, making it more acidic. This occurs because the process removes alkaline minerals like calcium and magnesium. Some systems include a post-treatment step to adjust the pH back to a neutral level.

38. How does reverse osmosis compare to distillation for water purification?

Both reverse osmosis and distillation are effective at removing a wide range of contaminants. However, reverse osmosis generally uses less energy and can operate at lower temperatures. Distillation can be more effective at removing certain volatile organic compounds that might pass through RO membranes.

39. How does salinity affect the efficiency of reverse osmosis?

Higher salinity in the feed water increases the osmotic pressure that must be overcome, requiring more energy to push water through the membrane. This makes reverse osmosis less efficient and more energy-intensive for highly saline water sources like seawater compared to brackish water.

40. What is the concept of "concentration polarization" in reverse osmosis?

Concentration polarization occurs when dissolved solutes accumulate near the membrane surface, creating a layer of higher concentration. This layer can reduce the efficiency of the system by increasing osmotic pressure and potentially leading to scaling or fouling of the membrane.

41. How does reverse osmosis affect water hardness?

Reverse osmosis is very effective at removing the minerals that cause water hardness, primarily calcium and magnesium ions. As a result, RO-treated water is typically very soft. This can be beneficial for reducing scale buildup in appliances but may require remineralization for drinking purposes.

42. What is meant by "osmotic pressure" in the context of reverse osmosis?

Osmotic pressure is the pressure that would need to be applied to prevent the natural flow of water across a semipermeable membrane from a less concentrated to a more concentrated solution. In reverse osmosis, the applied pressure must exceed this osmotic pressure to force water in the opposite direction.

43. How does reverse osmosis affect water's electrical conductivity?

Reverse osmosis significantly reduces the electrical conductivity of water by removing dissolved ions. Pure water is a poor conductor of electricity, so the conductivity of RO-treated water is much lower than that of the feed water, indicating effective removal of dissolved solids.

44. How does reverse osmosis affect the mineral content of water?

Reverse osmosis significantly reduces the mineral content of water, removing both beneficial and harmful minerals. While this is effective for purification, it can result in demineralized water. Some systems add back beneficial minerals like calcium and magnesium after the RO process.

45. How does reverse osmosis compare to ion exchange for water treatment?

While both methods can remove dissolved ions, they work differently. Reverse osmosis physically separates contaminants using a membrane, while ion exchange replaces unwanted ions with more desirable ones using resin beads. RO is generally more comprehensive but uses more water and energy.

46. How does water temperature affect the performance of reverse osmosis membranes?

Water temperature affects membrane performance in reverse osmosis. Higher temperatures generally increase water flux through the membrane, improving efficiency. However, very high temperatures can damage the membrane or increase the passage of some contaminants, so temperature control is important.

47. How does reverse osmosis affect water's dissolved oxygen content?

Reverse osmosis can reduce the dissolved oxygen content in water as gases, including oxygen, can pass through the membrane. However, the reduction is usually not significant, and dissolved oxygen levels typically re-equilibrate quickly when the water is exposed to air after treatment.

48. What are the energy requirements for reverse osmosis compared to other water treatment methods?

Reverse osmosis generally requires more energy than methods like conventional filtration or UV disinfection due to the high pressures needed. However, it's often more energy-efficient than thermal processes like distillation, especially for treating brackish water or seawater.

49. How does the concept of "osmotic equilibrium" relate to reverse osmosis?

Osmotic equilibrium is the state where the tendency for water to move across a semipermeable membrane due to concentration differences is balanced. In reverse osmosis, the applied pressure must overcome this equilibrium to force water through the membrane against the concentration gradient.

50. How does reverse osmosis technology contribute to water sustainability efforts?

Reverse osmosis plays a crucial role in water sustainability by enabling the use of previously unusable water sources (like seawater or wastewater) for drinking or industrial purposes. It allows for water reclamation and reuse in water-scarce regions, though its energy use and brine production present challenges that ongoing research aims to address.