What is the mechanism by which UV disinfection in wastewater treatment inactivates microorganisms?

• A. It Creates Heat That Kills Microbes
• B. It Oxidizes Microorganisms on Contact
• C. It Damages Microbial DNA, Preventing Replication
• D. It Adds Chlorine to the Water

Answer: (C)

Ultraviolet disinfection inactivates microorganisms by damaging their genetic material, so they can’t reproduce. UV-C light is absorbed by DNA (and RNA) and causes covalent linkages between adjacent bases, most notably thymine dimers. These dimers distort the DNA helix and block replication and transcription, so the cell can’t make the proteins it needs and cannot multiply. This is a non-chemical, non-thermal form of disinfection—there’s no heat produced and no oxidants added, and it doesn’t leave residual disinfectant in the water. The effectiveness depends on how clear the water is (UV light must penetrate), the delivered dose of UV (measured in millijoules per square centimeter), and exposure time, along with the organism’s sensitivity. Some microbes with strong repair mechanisms may require higher doses, but the core mechanism is the UV-induced DNA damage that prevents replication.