Q1: The disinfection efficiency of chlorine in water treatment

ANS:D - is reduced by increased pH value.

A higher pH facilitates the formation of more hypochlorite ions and results in less hypochlorous acid in the water. Lower pH facilitates the formation of fewer hypochlorite ions and more of hypochlorous acid. Hypochlorous acid is the most effective form of free chlorine residual, i.e., chlorine available to kill microorganisms in the water, while hypochlorite ions are much less efficient disinfectants. So disinfection is more efficient at a low pH (with large quantities of hypochlorous acid in the water) than at a high pH (with large quantities of hypochlorite ions in the water). At a high pH, the hypochlorous acid becomes dissociated into the ineffective hypochlorite ion. So, in lower pH water disinfection will be more efficient.