Which factor has the greatest influence on chlorine effectiveness?

The effectiveness of chlorine as a disinfectant in water treatment is significantly influenced by pH. At different pH levels, the forms of chlorine present in the water vary, particularly between hypochlorous acid (HOCl) and hypochlorite ion (OCl−). Hypochlorous acid is generally more effective as a disinfectant than hypochlorite ion.

At lower pH levels (typically below 7.5), more hypochlorous acid is present, which enhances chlorination effectiveness against pathogens. As the pH increases, hypochlorous acid levels decrease while hypochlorite ion levels increase, reducing chlorine's disinfection efficiency. Therefore, maintaining an optimal pH level is crucial for maximizing chlorine's efficacy in water treatment processes.

While temperature, contact time, and concentration also play important roles in chlorine disinfection, pH directly impacts the chemical form of chlorine that actively participates in disinfection. This is why pH is considered the factor with the greatest influence on chlorine effectiveness.