If disinfection is used, which parameter is commonly monitored in the effluent?

• A. Chlorine residual.
• B. Color
• C. Turbidity
• D. pH only

Answer: (A)

Disinfection leaves a measurable residue in the water, so the most direct way to know if the step is still effective is to monitor the chlorine residual in the effluent. In chlorine-based systems, maintaining an adequate residual free chlorine ensures that pathogens remain inactivated as water leaves the plant and complies with downstream requirements. Color and turbidity reflect appearance and particle content, not the disinfection's ability to inactivate microorganisms. While pH influences how well chlorine works, it does not by itself confirm ongoing disinfection, so the practical, commonly monitored indicator is the chlorine residual.