Under favorable conditions, combined chlorine, such as monochloramine and dichloramine, compared to the bactericidal action of free chlorine would require ____ times more combined chlorine residual.

• A. 4
• B. 11
• C. 14
• D. 25

Answer: (D)

Combined chlorine is far less effective as a disinfectant than free chlorine because the active disinfecting species differ. Free chlorine exists mainly as hypochlorous acid, a strong oxidant that kills bacteria quickly. When ammonia is present, chlorine forms chloramines (monochloramine and dichloramine), which are weaker oxidants and react more slowly with microbial cells. To achieve the same level of bacterial inactivation in the same contact time, you must have a much larger residual of combined chlorine. Under favorable conditions, the residual of combined chlorine needed is about 25 times greater than the residual of free chlorine to reach equivalent disinfection.