Which statement describes a UV disadvantage?

• A. No chemicals, no byproducts, minimal contact time
• B. High energy cost, not effective in turbid water, bacteria may repair DNA
• C. Requires chemical pretreatment
• D. Produces chemical residuals

Answer: (B)

Ultraviolet disinfection focuses on inactivating microorganisms by damaging their DNA, so a major limitation is that it requires clear water and steady energy input to work effectively. The high energy cost comes from running the UV lamps and the supporting cooling and control systems continuously, which can add up in large treatment plants. Its effectiveness drops when water is turbid or has color, because suspended particles and dissolved substances absorb or scatter UV light, shielding microbes and making it hard to deliver a sufficient dose. In those cases, pretreatment to reduce turbidity or a higher UV dose is needed, which adds complexity and cost. Another drawback is that some bacteria can repair UV-induced DNA damage if they’re later exposed to light or have strong repair mechanisms, so the disinfection effect isn’t always permanent. This contrasts with statements that UV has no chemicals or byproducts, or that it produces residuals—UV doesn’t add chemical residues and its disadvantages aren’t about chemical byproducts, which is why those options don’t fit as well.