What is the critical difference in disinfection efficacy between free chlorine and combined chlorine (chloramines) regarding the inactivation of protozoan cysts like *Giardiaand *Cryptosporidium*?

The critical difference in disinfection efficacy between free chlorine and combined chlorine (chloramines) lies in their oxidation strength and, consequently, the contact time required to inactivate protozoan cysts like *Giardiaand *Cryptosporidium*. Free chlorine, which exists primarily as hypochlorous acid (HOCl) at typical drinking water pH levels, is a much stronger oxidant than chloramines. Oxidation is the process of chemically altering or destroying microorganisms. This stronger oxidizing power allows free chlorine to inactivate *Giardiaand *Cryptosporidiumcysts relatively quickly, typically within minutes at commonly used concentrations. Chloramines, on the other hand, are formed when ammonia is added to chlorine. They are weaker oxidants and require significantly longer contact times (often hours) to achieve the same level of inactivation of these protozoan cysts. *Giardiaand *Cryptosporidiumhave a tough outer shell that makes them resistant to many disinfectants. Because chloramines are less reactive, they penetrate this shell more slowly, resulting in a much longer time needed to disrupt their cellular functions and render them non-infectious. While chloramines provide a longer-lasting disinfectant residual in the distribution system, their lower disinfection efficacy against protozoan cysts necessitates higher concentrations or longer contact times compared to free chlorine to achieve adequate protection against waterborne diseases caused by these pathogens. This difference is a key factor in selecting the appropriate disinfection strategy, particularly when dealing with water sources known to be contaminated with *Giardiaor *Cryptosporidium*.