UPSC Prelims 2012 · Question 13 of 98

UPSC Prelims 2012 question on UV Water Purification

What is the role of ultraviolet (UV) radiation in the water purification systems?

1. It inactivates/kills harmful microorganisms in water.

2. It removes all undesirable odours from the water.

3. It quickens sedimentation of solid particles, removes turbidity and improves water clarity.

Which of the statements given above is/are correct?

  1. 1 only
  2. 2 and 3 only
  3. 1 and 3 only
  4. 1, 2 and 3
Show answer

Answer: A. 1 only

Verdict

Correct Answer: 1 only

Only Statement 1 is correct because UV radiation disinfects water by inactivating microorganisms, but it does not remove odour, turbidity or suspended particles.

Analysis

Ultraviolet radiation is used in water purification mainly for disinfection. It inactivates harmful microorganisms but does not remove dissolved chemicals, odour or suspended particles.

Statement by statement

1. It inactivates or kills harmful microorganisms in water — Correct

UV radiation, especially UV-C, damages the DNA or RNA of microorganisms such as bacteria, viruses and protozoa.

This prevents them from reproducing, making them inactive and unable to cause infection.

2. It removes all undesirable odours from water — Incorrect

UV radiation is a physical disinfection process, not an adsorption or filtration process.

It does not remove dissolved gases, chlorine, organic compounds or substances causing smell and taste. Activated carbon filters are generally used for odour removal.

3. It quickens sedimentation, removes turbidity and improves clarity — Incorrect

UV light does not remove suspended particles or turbidity.

In fact, turbid water can reduce UV effectiveness because particles may shield microorganisms from UV rays. Sedimentation, filtration or coagulation is needed before UV treatment.

Extra UPSC info

* UV water purification commonly uses UV-C radiation.

* The most effective germicidal wavelength is around 254 nanometres.

* UV treatment does not significantly change the taste, pH or chemical composition of water.

* Unlike chlorination, UV treatment does not leave residual disinfectant in water.

* UV systems require clear water for better effectiveness.

* UV lamps lose germicidal intensity over time and need periodic replacement.

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