Question

a water treatment plant operates at ph 9.5 while maintaining the same chlorine dose and contact time as before. which statement best explains why microbial inactivation decreases?

• chlorine demand increases because more trihalomethanes are formed
• more hypochlorous acid (hocl) is formed, increasing disinfection efficiency
• a larger proportion of chlorine exists as hypochlorite ion (ocl\\(^{-}\\)), which is significantly less effective as a disinfectant.
• chloramines decompose rapidly at high ph, reducing total chlorine

Explanation:

Analyze chlorine speciation in water as a function of pH

$$ \text{HOCl} ightleftharpoons \text{H}^+ + \text{OCl}^- \quad (\text{p}K_a \approx 7.5) $$

Determine dominant species at pH 9.5

$$ \text{pH} = 9.5 \gg \text{p}K_a \implies [\text{OCl}^-] \gg [\text{HOCl}] $$

Evaluate disinfection efficacy of the dominant species

$$ \text{Disinfection Efficiency: } \text{HOCl} \gg \text{OCl}^- $$

Answer:

• (A) Chlorine demand increases because more trihalomethanes are formed
• (B) More hypochlorous acid (HOCl) is formed, increasing disinfection efficiency
• (C) A larger proportion of chlorine exists as hypochlorite ion (OCl⁻), which is significantly less effective as a disinfectant. (Correct answer)
• (D) Chloramines decompose rapidly at high pH, reducing total chlorine