Florida Wastewater Practice Test

At what pH level is disinfection most effective?

1. 4

2. 5

3. 7

4. 9

The correct answer is: 5

The effectiveness of disinfection processes, particularly when using chlorine or other chlorinated compounds, is significantly influenced by the pH level of the water. Chlorine, a common disinfectant, is most effective at a pH level around 6.5 to 7.5, with peak effectiveness often occurring in the neutral range close to pH 7. At pH levels below 7, chlorine exists primarily as hypochlorous acid (HOCl), which is more effective at killing pathogens compared to its ionized form, hypochlorite ion (OCl-), that dominates at higher pH levels. Therefore, maintaining a pH near neutral enhances the overall efficiency of the disinfection process. Although a pH of 5 is better than more extreme values, it is still not ideal for disinfection because the effectiveness begins to decline as the pH moves significantly away from neutrality. At very high pH levels, such as 9, the effectiveness of chlorine as a disinfectant is reduced even further. Thus, the choice of pH 5 indicates a recognition of the importance of maintaining conditions more favorable to disinfection, though it does not align with the optimal range. Selecting pH 7 would indeed