UPSC Prelims 2012 Q13: Science & Technology – Energy & Environment
What is the role of ultraviolet (UV) radiation in the water purification systems?
1. It inactivates/kills the harmful microorganisms in water.
2. It removes all the undesirable odours from the water.
3. It quickens the sedimentation of solid particles, removes turbidity and improves the clarity of
water.
Which of the statements given above is/are correct?
- (a) 1 only
- (b) 2 and 3 only
- (c) 1 and 3 only
- (d) 1, 2 and 3
Answer: (a) 1 only
Explanation
Ultraviolet radiation in water purification
UV light damages the DNA of microbes, so they cannot multiply.
| Statement | Verdict | Fact |
|---|---|---|
| 1. It inactivates/kills the harmful microorganisms in water | Correct | Works against bacteria, viruses and protozoa |
| 2. It removes all the undesirable odours from the water | Incorrect | Odour is removed by activated carbon |
| 3. It quickens sedimentation, removes turbidity and improves clarity | Incorrect | Done by filtration and settling; UV even works poorly in cloudy water |
Stages of a home water purifier:
| Stage | Removes |
|---|---|
| Sediment filter | Mud and particles |
| Activated carbon | Odour, taste, chlorine |
| Reverse osmosis (RO) | Dissolved salts and heavy metals |
| UV lamp | Germs |
Limits of UV: it adds nothing to the water, leaves no residual protection and does not remove chemicals.
Answer: (a) 1 only.
Why each option is right or wrong
- (a) CORRECT. UV radiation only inactivates or kills microorganisms.
- (b) Incorrect. UV does not remove odours or turbidity.
- (c) Incorrect. Statement 3 is false; UV does not help sedimentation.
- (d) Incorrect. Statements 2 and 3 are false.
Key point
UV purification kills microbes by damaging DNA; it does not remove odour, turbidity or dissolved salts (carbon filter, sediment filter and RO do).
Remember
UV = germ killer only, nothing else.
Common mistakes
- Thinking UV cleans the water of dirt and smell.
- Confusing UV (germs) with RO (dissolved salts).
- Forgetting that UV needs clear water to work.
Exam tip
UV kills germs; it does not filter, deodorise or soften water.
Sources
- Bureau of Indian Standards — IS 14724: Water purifiers with ultraviolet disinfection
- World Health Organization — Guidelines for drinking-water quality (UV disinfection)