At a WWTP, the chlorine feed is 120 lb/day expressed as Cl2 equivalent. The plant effluent flow is 2.0 MGD (about 1,389 gpm). Using the pounds formula, the operator calculates the applied chlorine dosage is 7.2 mg/L. After the contact basin, the measured chlorine residual is 1.5 mg/L. What is the chlorine demand (mg/L) across the basin?

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A) 5.7 mg/L — Correct. Chlorine demand is the amount of chlorine consumed by reactions, so Demand = Dosage − Residual = 7.2 mg/L − 1.5 mg/L = 5.7 mg/L. Sanity check: demand should be less than the applied dosage and positive; 5.7 mg/L fits.

Answer

B) 7.2 mg/L — Incorrect. 7.2 mg/L is the applied dosage, not the demand. Choosing this confuses “how much you fed” with “how much was used up.”

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C) 1.5 mg/L — Incorrect. 1.5 mg/L is the residual, meaning what remains after the contact time. Demand is what was consumed, not what is left.

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D) 8.7 mg/L — Incorrect. 8.7 mg/L comes from adding dosage and residual (7.2 + 1.5), which reverses the relationship. Residual is not added to dosage; it is what remains after demand has consumed part of the dosage.

Chlorine Demand vs Dosage vs Residual

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