Difference Between Osmosis and Reverse Osmosis with Examples
Reverse osmosis is a modern biological and technological process widely used to purify water by removing salts, microbes, and many dissolved impurities. At its core, reverse osmosis (commonly abbreviated as RO) works by using pressure to force water through a very thin, semi-permeable membrane. This process is found in many household water filters and is also used for large-scale water treatment, such as turning seawater into fresh drinking water and producing ultra-pure water for laboratories and industries.
Understanding Osmosis and Reverse Osmosis
Osmosis is a natural process seen in biology, where water moves from an area of low solute (such as salt or sugar) concentration to an area of high solute concentration through a semi-permeable membrane. For example, plant roots absorb water from the soil by osmosis, and kidneys use a similar process to filter blood.
Reverse osmosis, as the name suggests, is the opposite. Here, by applying external pressure to the concentrated (salty) side, water is forced to move in the reverse direction—from the side with higher solute concentration to the side with lower solute concentration. The semi-permeable membrane allows only water molecules to pass, blocking most contaminants.
How Does Reverse Osmosis Work?
In an RO system, water containing dissolved impurities is pressurized using a pump. This pressurized water is then pushed through an RO membrane, which permits water molecules to pass while blocking many dissolved salts, harmful bacteria, and organic contaminants.
As a result, two streams are created:
- The "permeate" or product water (clean water that passes through the membrane).
- The "concentrate" or reject/brine (water that carries away the removed contaminants).
Key Biological and Practical Examples
- Plants: Roots absorb water from the soil naturally by osmosis, while reverse osmosis is not natural but engineered.
- Kidneys: Human nephrons filter blood plasma, using semi-permeable membranes and pressure, which is analogous to RO filtration.
- Water Purifiers: Home RO filters create clean drinking water from tap or groundwater by removing most salts, bacteria, and organic compounds.
Important Definitions and Principles
| Term | Definition | Significance |
|---|---|---|
| Osmosis | Natural movement of water from low to high solute concentration through a semi-permeable membrane | Biological cell hydration; root and kidney functions |
| Reverse Osmosis | Applied pressure moves water from high to low solute concentration via a semi-permeable membrane | Used for water purification, desalination, and industrial processes |
| Semi-permeable Membrane | Barrier that lets only select molecules, usually water, pass through | Crucial for osmosis, reverse osmosis, and cell function |
| Permeate | Clean water that passes through the RO membrane | Collected for practical use in homes and industries |
| Concentrate/Brine | The leftover water containing concentrated impurities | Usually discarded or recycled |
Step-by-Step Breakdown of the RO Process
- A high-pressure pump pushes source water into the RO system.
- The semi-permeable membrane allows mainly water molecules to pass through, holding back most salts, organic matter, and bacteria.
- Clean water (permeate) is collected, while the concentrated impurities (concentrate or brine) are removed from the system.
Which Contaminants Does Reverse Osmosis Remove?
RO membranes can remove 95–99% of dissolved salts (ions), particles, bacteria, and most organic substances from water. Larger molecules and those with higher ionic charge (like calcium) are blocked more effectively than small, uncharged particles or dissolved gases like carbon dioxide.
Reverse Osmosis vs. Osmosis: Key Differences
| Feature | Osmosis | Reverse Osmosis |
|---|---|---|
| Direction of Water Flow | Low to high solute concentration | High to low solute concentration (with pressure) |
| Energy Requirement | Passive, no external energy | Active, requires pressure |
| Example in Nature/Technology | Plant roots, cell hydration | Water purifier, desalination plant |
| Applications | Maintaining biological balance in cells | Producing pure water for drinking or industry |
Advantages and Limitations of Reverse Osmosis
| Advantages | Limitations |
|---|---|
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Helpful Tips for Students
- To remember: Osmosis is natural and passive; reverse osmosis is engineered and requires pressure.
- In daily life, RO is mainly seen in water purifiers and treatment plants.
- In biology, the semi-permeable membrane concept applies to both cell membranes and RO filters.
Practice Questions
- What is reverse osmosis and how does it differ from osmosis in plants?
- Describe the stepwise process by which an RO filter purifies water.
- List two advantages and two limitations of the reverse osmosis method.
- Why can't RO remove all dissolved gases from water?
- Give one example of the use of RO in industry.
Explore More on Vedantu
- Water Pollution and Its Control
- Water Cycle
- Water: Importance and Functions
- How Can We Conserve Water?
Reverse osmosis is an essential process in biology and technology for providing clean and safe water. Knowing its mechanism, significance, and differences from natural osmosis helps students excel in biology and environmental science topics, and make better decisions about water use and management.
FAQs on What is Reverse Osmosis? Principle, Process & Uses
1. What is reverse osmosis?
Reverse osmosis (RO) is a process where water molecules are forced through a semi-permeable membrane from a region of higher solute concentration to a region of lower solute concentration by applying external pressure. This purifies water by removing dissolved salts, bacteria, and impurities. It is the reverse of natural osmosis and is widely used in water purification systems.
2. What is the main difference between osmosis and reverse osmosis?
Osmosis is the passive movement of water from low to high solute concentration across a semi-permeable membrane, requiring no external energy. Reverse osmosis (RO) uses applied pressure to force water from high to low solute concentration, opposite to natural flow, and needs external energy. Osmosis occurs in cells (e.g., roots absorbing water); reverse osmosis is used in water purifiers and desalination plants.
3. What contaminants are removed by reverse osmosis?
Reverse osmosis removes up to 95–99% of dissolved salts, heavy metals, bacteria, viruses, and organic impurities.
- Dissolved ions like calcium, magnesium, and sodium
- Heavy metals such as lead and mercury
- Bacteria and some viruses
- Organic chemicals and particles
4. Is reverse osmosis water safe to drink?
Reverse osmosis water is generally safe for drinking as it removes harmful contaminants, bacteria, and dissolved salts. However, it may also remove essential minerals (e.g., calcium, magnesium), so remineralisation is sometimes recommended for improved taste and health benefits. Always ensure proper system maintenance for safe use.
5. List two advantages and two disadvantages of reverse osmosis.
Advantages:
- Removes up to 99% of bacteria, viruses, and dissolved impurities
- Improves water taste and clarity
- Removes beneficial minerals along with contaminants
- Wastes a part of input water during purification
6. How does a reverse osmosis filter work (briefly explain with process steps)?
A reverse osmosis filter works as follows:
- Saline or impure water is pressurized and sent through a semi-permeable membrane.
- Water molecules pass through the membrane, leaving most dissolved salts and impurities behind.
- The filtered water (permeate) is collected; the concentrated waste (brine) is discarded or treated.
7. Why is reverse osmosis used for seawater desalination?
Reverse osmosis is preferred for seawater desalination because it effectively removes high concentrations of dissolved salts and minerals, producing potable (drinkable) water. It is more energy-efficient than distillation for large volumes and requires less infrastructure. RO membranes selectively allow water to pass, blocking most solutes present in seawater.
8. Can reverse osmosis remove all bacteria and viruses?
Reverse osmosis membranes can remove most bacteria and many viruses owing to their small pore size (typically 0.0001 microns). However, absolute sterilisation is not guaranteed; additional disinfection (like UV treatment) may be necessary in critical applications to ensure complete microbial safety.
9. What are common applications of reverse osmosis?
Common applications of reverse osmosis include:
- Domestic RO water filters and purifiers
- Seawater desalination plants
- Pharmaceutical and laboratory pure water production
- Industrial wastewater treatment
- Food and beverage industries
10. How does external pressure influence the reverse osmosis process?
In reverse osmosis, external pressure must be applied to the side with higher solute concentration to overcome natural osmotic pressure. This forces water molecules to move in the opposite direction of osmosis—across the semi-permeable membrane—while leaving most solutes and impurities behind. Sufficient pressure is critical for efficient water purification.
11. What are the key components of a reverse osmosis system?
Key components of an RO system include:
- Pre-filters (sediment/carbon filters)
- High-pressure pump
- Semi-permeable RO membrane
- Post-filters (optional)
- Storage tank
- Wastewater (brine) drain
12. Why is remineralisation sometimes needed after reverse osmosis?
After reverse osmosis, water may lack essential minerals like calcium and magnesium, which are removed during filtration. Remineralisation restores these beneficial minerals, improving taste and supporting health—especially for regular drinking purposes. Many modern RO systems include remineralisation cartridges for optimum water quality.
