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?

Only statement 1 is correct. UV disinfection works by damaging the DNA and RNA of bacteria, viruses and protozoan cysts, so they cannot reproduce. That is inactivation, not removal. Statement 1 is therefore right.

• Statement 2 is wrong. UV has no effect on dissolved chemicals that cause odour or taste. Odour is handled by activated carbon adsorption, or by oxidation through aeration, chlorine or ozone. The word "all" makes it wrong too.
• Statement 3 is wrong. UV does not settle particles or remove turbidity. Those are physical stages: coagulation and flocculation (alum), sedimentation, and rapid sand filtration. UV only works on water that is already clear, because turbid water shields microbes from the light.

The core concept is the multi-barrier principle: physical clarification, then chemical treatment, then disinfection. Mains questions on safe drinking water, water-borne diseases and decentralised treatment under schemes like Jal Jeevan Mission depend on this sequence.

Trap: assuming that a disinfection method also clarifies water, or that it handles every taste and odour problem. Read each statement for what the process physically does.