UV disinfection has residual?

• A. There is residual with UV
• B. There is no residual with UV
• C. UV residual depends on lamp age
• D. It can cause secondary contaminants

Answer: (B)

UV disinfection inactivates microorganisms by damaging their DNA, but it does not leave a chemical residual in the water. After treatment, there is no ongoing disinfectant to protect against recontamination in the distribution system, which is why many facilities add a secondary disinfectant downstream if a residual is needed. The lamp age affects how much UV energy is delivered (and thus whether the required inactivation is achieved) but it doesn’t create a lasting residual in the water. It also doesn’t introduce secondary contaminants into the treated water.