Chlorine Dosage Calculator

The single most-tested calculation on water and wastewater operator exams is the pounds formula: pounds per day = dosage (mg/L) × flow (MGD) × 8.34. The 8.34 is the weight of one gallon of water in pounds, and it converts a concentration and a million-gallons-per-day flow into the daily weight of chemical you must feed. A 10 mg/L chlorine dose treating 2 MGD works out to 10 × 2 × 8.34 = 166.8 pounds of chlorine per day.

Getting the dose right starts with chlorine demand. When chlorine enters water it first reacts with organics, ammonia, iron, and other reducing agents — that consumed portion is the demand. Only after demand is satisfied does a measurable free residual remain to keep disinfecting through the distribution system. The relationship is simple: required dose = chlorine demand + desired residual. To hold a 0.5 mg/L residual with a 3.5 mg/L demand, you dose 4.0 mg/L.

Together these two ideas let an operator size chemical feed. Find the dose you need from demand and target residual, then run that dose through the pounds formula at plant flow to get the pounds per day of chlorine to feed. Because both under- and over-chlorination create public-health and compliance problems, operator certification exams test this chain of reasoning closely.