Water treatment — effect of pH on chlorine disinfection efficiency How does increasing pH affect the disinfection efficiency of free chlorine in conventional water treatment?

Introduction / Context:
Chlorination remains one of the most common disinfection methods in drinking water treatment. The germicidal potency of free chlorine depends strongly on its chemical speciation, which is controlled by the water’s pH.

Given Data / Assumptions:

• Disinfectant considered: free chlorine (hypochlorous acid/hypochlorite system).
• Typical treatment pH range 6–9.
• Contact time and temperature assumed unchanged.

Concept / Approach:
Free chlorine exists in equilibrium: HOCl ⇌ H^+ + OCl^−. At lower pH, the more powerful disinfectant hypochlorous acid (HOCl) predominates; at higher pH, the less potent hypochlorite ion (OCl^−) dominates. Therefore, disinfection efficiency decreases as pH increases, holding dose and contact time constant.

Step-by-Step Solution:
Recognize HOCl as the stronger species; OCl^− is weaker.Increasing pH shifts equilibrium to OCl^−, reducing germicidal activity.Hence, for the same CT (concentration × time), higher pH leads to lower inactivation.

Verification / Alternative check:
CT tables used in regulatory guidance explicitly show larger required CT values at higher pH for the same log inactivation, confirming reduced efficiency.

Why Other Options Are Wrong:
“Not dependent” and “constant” contradict established speciation chemistry. “Increased with pH” reverses the true trend. A simple monotonic decrease is observed in the usual pH range; the suggested non-monotonic option is not standard.

Common Pitfalls:

• Confusing free chlorine with chloramines, which have different pH sensitivity.
• Ignoring temperature effects that also alter CT requirements.

Final Answer:
It is reduced by increased pH value