What is the primary mechanism by which UV light inactivates microorganisms, and how does this differ from chlorination?

The primary mechanism by which UV light inactivates microorganisms is through damaging their DNA and RNA, preventing them from replicating. UV light at a wavelength of around 254 nm is absorbed by the nucleic acids (DNA and RNA) within the microorganisms. This absorption causes the formation of thymine dimers or other photoproducts, which are essentially kinks or breaks in the DNA/RNA strands. This damage prevents the microorganism from replicating and thus renders it unable to cause infection. Chlorination, on the other hand, inactivates microorganisms through oxidation. Chlorine, in the form of hypochlorous acid (HOCl), penetrates the cell walls of microorganisms and disrupts their cellular processes. This can involve oxidizing essential enzymes, disrupting cell membrane function, or interfering with metabolic pathways. The key difference is that UV light directly damages the genetic material of the microorganism, preventing reproduction, while chlorine disrupts the microorganism's cellular functions through chemical oxidation. UV disinfection does not produce any disinfection byproducts, unlike chlorination which can form trihalomethanes (THMs) and haloacetic acids (HAAs). However, UV disinfection does not provide a disinfectant residual, meaning that it only inactivates microorganisms at the point of application and does not protect against recontamination in the distribution system, which is a key advantage of chlorination.