Chlorine residual equals which of the following?

• A. Demand minus dosage
• B. Dosage minus demand
• C. Volume minus dosage
• D. Temperature minus demand

Answer: (B)

Chlorine residual is the amount of chlorine left after the water’s chlorine demand has been satisfied. In practice, it’s the difference between what you dose and what the water consumes. So the residual = dosage minus demand. For example, if you dose 2 mg/L and the water’s demand is 0.8 mg/L, you’d have a residual of 1.2 mg/L. If the demand exceeds the dose, the residual is effectively zero, since all added chlorine is used up.

The other options don’t describe this balance: volume minus dosage isn’t how chlorine is consumed, and temperature minus demand isn’t the definition of residual (though temperature can influence reaction rates).