Which statement correctly defines chlorine residual in water treatment?

• A. It equals dosage minus demand
• B. It equals demand minus dosage
• C. It equals dosage plus demand
• D. It equals the product of dosage and demand

Answer: (A)

Chlorine residual is the chlorine that remains in the water after it has reacted with substances that consume chlorine (the chlorine demand). You start with the dosage, the amount of chlorine you add. The water uses some of that chlorine to react with organics, inorganics, nutrients, and microbes—the chlorine demand. The remaining amount is the residual, which provides ongoing disinfection. So the residual equals dosage minus demand. For example, if you dose 2 mg/L and the water’s demand is 1.5 mg/L, the residual is 0.5 mg/L. If the dosage is less than the demand, the residual can drop toward zero, meaning little to no ongoing disinfection.