In disinfection practice, which term describes the difference between the amount of chlorine added and the residual chlorine after exposure?

• A. Residual
• B. Breakpoint
• C. Demand
• D. Dosage

Answer: (C)

Chlorine demand is the amount of chlorine that must be added to water to overcome reactions with substances in the water. It represents the difference between the chlorine you dose and the residual chlorine remaining after a contact period. In practice, if you add a certain amount of chlorine and end up with a smaller residual, the missing portion is the chlorine demand—the amount consumed by reactions with organics, ammonia, and other compounds. The residual is just what’s left; breakpoint is the point where adding more chlorine starts to produce free chlorine again after all demand is satisfied; dosage is the total amount added. So the difference between what’s added and what remains as residual is chlorine demand. For example, add 5 mg/L and finish with 2 mg/L residual; the demand was 3 mg/L.