In the chlorine residual equation, which term describes the chlorine that is consumed by substances in water?

• A. Demand
• B. Dosage
• C. Residual
• D. Contact time

Answer: (A)

The main concept is chlorine demand. This is the portion of added chlorine that reacts with substances in water and is consumed, so it does not appear as residual chlorine. In the chlorine residual equation, the residual is what remains after the demand has been met, meaning Dose minus Demand. Dosage is the amount added initially, and the residual is the remaining chlorine after reactions, while contact time is how long those reactions can occur. So the chlorine consumed by substances in water is the chlorine demand.