Question

The disinfection efficiency of chlorine in water treatment

• A. remains constant at all pH value.
• B. is reduced by increased pH value.
• C. is not dependent on pH value.
• D. is increased by increased pH value.

Answer 1

The correct option is B is reduced by increased pH value.

Chlorination is most effective at a certain pH range.

Increasing or decreasing pH reduces efficiency of chlorination.

At pH < 5, chlorine doesn't react with water and remains as free chlorine.

Water should be maintained at a pH slightly below 7 since HOCl is predominant at this pH and is 80% more effective than $OC{l}^{-}$.

$HOCl\underset{pH<7}{\overset{pH>8}{\leftrightarrows }}{H}^{+}+OC{l}^{-}$

HOCl is unstable at higher pH and breaks down as the reaction shown above, thus reducing efficiency.