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?
Contents13
- A1 only
- B2 and 3 only
- C1 and 3 only
- D1, 2 and 3
Show answer
Answer: (A) 1 only
UV radiation in water purifiers works by damaging the DNA of microorganisms, thereby inactivating or killing bacteria, viruses, and protozoa (statement 1 correct).
However, UV treatment does NOT remove odours from water — that requires activated carbon filters (statement 2 wrong).
UV also does NOT help with sedimentation or turbidity removal — in fact, high turbidity reduces UV effectiveness because particles shield microorganisms from UV rays (statement 3 wrong).
Answer: 1 only.
UV radiation damages DNA of bacteria, viruses, and protozoa in water, making it a chemical-free disinfection method used in many modern water purifiers.
The trap here is assuming UV does everything in water treatment - it only disinfects, while odour removal needs activated carbon and turbidity removal needs physical filtration or coagulation.
UPSC is testing whether students can distinguish between different water treatment methods and their specific functions rather than assuming one technology does everything.
UV Radiation in Water Purification
Science And Technology ultraviolet UV radiation water purification
UV Radiation in Water Purification: Mechanism & Limitations
UV radiation kills microorganisms by damaging their DNA structure
UV does NOT remove odours — that requires activated carbon filters
UV does NOT help sedimentation or remove turbidity from water
High turbidity reduces UV effectiveness as particles shield microorganisms
How UV Works
UV radiation disinfects water by penetrating microorganisms and damaging their DNA structure. This prevents bacteria, viruses, and protozoa from reproducing, effectively killing them without adding chemicals to the water.
What UV Can & Cannot Do
Water Treatment Need | UV Effectiveness | Alternative Method Required |
|---|---|---|
Kill bacteria/viruses | Highly effective | None |
Remove odours | Cannot do | Activated carbon filters |
Remove turbidity | Cannot do | Sedimentation, coagulation |
Improve water clarity | Cannot do | Filtration, settling tanks |
Remove dissolved salts | Cannot do | RO, distillation |
UV Disinfection Process
%%{init: {"flowchart": {"wrappingWidth": 460}}}%%
flowchart TD
s1["`**UV Lamp Emission**
UV-C light (254 nm wavelength) penetrates water`"]
s2["`**DNA Absorption**
Microorganisms absorb UV radiation through cell walls`"]
s3["`**DNA Damage**
UV breaks DNA bonds, creating thymine dimers`"]
s4["`**Reproduction Block**
Damaged DNA prevents cell division and reproduction`"]
s5["`**Microorganism Death**
Unable to reproduce, pathogens become inactive/die`"]
s1 --> s2
s2 --> s3
s3 --> s4
s4 --> s5Key Limitations
Requires pre-filtration — turbid water shields microorganisms from UV rays
No residual protection — water can get recontaminated after UV treatment
Power dependent — needs continuous electricity supply to function
Cannot remove chemical contaminants or dissolved impurities
Trap: Statement 2 confuses UV with activated carbon — only carbon removes odours
Trap: Statement 3 confuses UV with coagulation/sedimentation processes
Common error: Thinking UV is a complete water treatment solution — it only disinfects
Reversal trap: High turbidity reduces UV effectiveness, doesn't improve it
Water Treatment & Purification Methods
Science And Technology water purification systems
Water Treatment Methods: Physical, Chemical & Biological
Physical methods: sedimentation, filtration, UV — remove particles and kill pathogens
Chemical methods: chlorination, ozonation — disinfect and remove specific contaminants
Activated carbon: removes odours, colours, and organic compounds
RO (Reverse Osmosis): removes dissolved salts and heavy metals
Water Treatment Methods Comparison
Method | Primary Function | Removes | Cannot Remove |
|---|---|---|---|
Sedimentation | Physical settling | Suspended particles, turbidity | Microorganisms, odours |
Filtration | Physical straining | Particles, some bacteria | Viruses, dissolved salts |
UV Radiation | DNA damage | Bacteria, viruses, protozoa | Particles, odours, chemicals |
Chlorination | Chemical oxidation | Microorganisms, some odours | Heavy metals, fluoride |
Activated Carbon | Adsorption | Odours, colours, organics | Salts, hardness, fluoride |
Reverse Osmosis | Membrane filtration | Salts, heavy metals, most contaminants | Requires high pressure |
Multi-Stage Water Treatment
# Complete Water Treatment Plant
## Pre-Treatment
- Screening
- Coagulation
- Flocculation
- Sedimentation
## Primary Treatment
- Sand Filtration
- Activated Carbon
- Turbidity Removal
## Disinfection
- UV Treatment
- Chlorination
- Ozonation
## Advanced Treatment
- RO
- Ion Exchange
- DistillationWater Treatment Process
Flowchart diagram showing multi-stage water treatment process from raw water intake through sedimentation, filtration, disinfection to final treated water
Complete water treatment requires multiple stages — UV alone cannot purify water
Microorganism Control & Disinfection
Science And Technology harmful microorganisms
Methods of Microorganism Control: Physical vs Chemical
Physical methods: heat, UV, filtration — kill without chemicals
Chemical methods: chlorine, ozone, iodine — oxidize cell components
UV-C wavelength (254 nm) is most effective for DNA damage
Sterilization kills all microorganisms; disinfection reduces to safe levels
Disinfection Methods in Water Treatment
Method | Mechanism | Advantages | Disadvantages |
|---|---|---|---|
UV Radiation | DNA damage | No chemicals, instant kill | No residual protection, power dependent |
Chlorination | Cell wall oxidation | Residual protection, low cost | Forms harmful byproducts (THMs) |
Ozonation | Strong oxidation | No harmful byproducts | Expensive, no residual effect |
Boiling | Protein denaturation | 100% effective, simple | Energy intensive, not practical for large scale |
Target Microorganisms
Bacteria: E. coli, Salmonella, Cholera — killed by UV in seconds
Viruses: Hepatitis, Polio — smaller but equally vulnerable to UV
Protozoa: Giardia, Cryptosporidium — larger parasites, need higher UV doses
Spores: Most resistant form — may require combined treatment methods
Question Context
This 2012 UPSC question tested whether students understood that UV only disinfects — it cannot perform other water treatment functions like odour removal or particle settling that require different mechanisms.