At what pH would disinfection be at its greatest?

Disinfection in wastewater treatment is significantly influenced by the pH level, as it affects the efficacy of various disinfectants, notably chlorine. Lower pH levels enhance disinfection efficiency because chlorine exists predominantly in its more effective hypochlorous acid form (HOCl) in that range.

At a pH of 5, the concentration of hypochlorous acid is maximized, allowing for better disinfection capabilities against pathogens. This is because hypochlorous acid penetrates microbial cell walls more easily and is much more lethal to various microorganisms. Additionally, acidic conditions help to reduce the formation of chloramines, which are less effective as disinfectants compared to hypochlorous acid.

In contrast, higher pH levels, such as those indicated in the other choices, lead to an increase in hypochlorite ion (OCl⁻) concentration, which is less effective than hypochlorous acid. This is why pH is a critical parameter in wastewater disinfection processes, emphasizing the importance of maintaining slightly acidic conditions for optimal effectiveness.