Which description best defines chlorine demand?

• A. The difference between the amount of chlorine added and the residual chlorine remaining after a given contact time
• B. The difference between the total chlorine residual and the free chlorine residual
• C. The difference between the chlorine added and turbidity
• D. The difference between the chlorine added and the pH

Answer: (A)

Chlorine demand is the amount of chlorine that is consumed by reactions with substances in the water during disinfection. When chlorine is added, some of it reacts with organic matter, ammonia, iron, manganese, and other constituents, so less remains as active disinfectant. After a specified contact time, you measure the chlorine that’s left as the residual. The difference between what you added and what’s left as residual is the chlorine demand—the portion of the dose that was used up by those reactions before any disinfection effect can occur.

The other ideas don’t define chlorine demand. The difference between total chlorine residual and free chlorine residual relates to combined chlorine, not to the amount consumed from the dose. Differences involving turbidity or pH don’t describe the amount consumed from the dose; turbidity and pH influence how chlorine behaves, but they aren’t about the amount taken up by reactions during dosing.