What factor primarily affects the residual chlorine levels in treated water?

Residual chlorine levels in treated water are significantly influenced by the pH of the water. When chlorine is added to water, it can exist in several forms, including hypochlorous acid (HOCl) and hypochlorite ions (OCl-). The proportion of these forms is heavily dependent on the pH level of the water.

At lower pH levels (acidic conditions), hypochlorous acid predominates, which is more effective as a disinfectant and is also less stable, leading to higher residual levels as it reacts with contaminants in the water. As the pH increases (more alkaline), the concentration of hypochlorite ions increases, which is less effective for disinfection but is more stable, potentially leading to differences in how long the residual chlorine lasts.

Maintaining appropriate pH levels is essential for optimizing the effectiveness of chlorine as a disinfectant, as well as ensuring that the residual levels are sufficient for safe drinking water.