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Pounds per day of a chemical equals flow in million gallons per day (MGD) times the dose in milligrams per liter (mg/L) times 8.34, the weight of a gallon of water in pounds. For chlorine, the dose is the demand plus the residual you want to keep; for a product that is not 100 percent chemical, divide the pounds by the product's strength as a decimal, and divide by its weight per gallon to get gallons.
lb/day = Flow (MGD) × Dose (mg/L) × 8.34 lb/gal
| Term | Meaning | Units |
|---|---|---|
| Flow | flow treated | MGD |
| Dose | chemical dose, which for chlorine is demand + residual | mg/L |
| 8.34 | weight of one gallon of water; with MGD and mg/L it folds in the million-gallon and milligram-per-liter conversions | lb/gal |
| Strength | available chemical in the product, as a decimal (12.5 percent = 0.125) | unitless |
The drinking water sheet lists the gas, solid and liquid forms of the formula and dose = demand + residual; the wastewater sheets give the same 8.34 relation for pounds per day of biochemical oxygen demand (BOD) or suspended solids. Official sheets: Exam Conversion Sheet (revised 4/30/2026) (State Water Resources Control Board, Drinking Water Operator Certification Program; Water Treatment and Water Distribution exams); Equivalents and Formulas Sheet, Version 1 (State Water Resources Control Board, Wastewater Operator Certification Program; Wastewater Treatment Plant Operator exams); Equivalents and Formulas Sheet, Version 2 (State Water Resources Control Board, Wastewater Operator Certification Program; Wastewater Treatment Plant Operator exams).
Chemical feed · Original practice item, not a real exam question.
A 6.2 MGD surface water plant measures a chlorine demand of 2.3 mg/L and wants a free residual of 0.7 mg/L leaving the clearwell. It feeds 12.5 percent sodium hypochlorite that weighs 10.0 lb/gal. How many gallons of hypochlorite per day are needed?
Dose = 2.3 mg/L + 0.7 mg/L = 3.0 mg/L6.2 MG/day × 3.0 mg/L × 8.34 lb·L/(MG·mg) = 155.1 lb/day155.1 lb Cl₂/day ÷ 0.125 lb Cl₂/lb product = 1,241 lb product/day1,241 lb/day ÷ 10.0 lb/gal = 124.1 gal/dayAnswer: 124.1 gallons of sodium hypochlorite per day
Yes inside the pounds formula, because there 8.34 converts the flow of water, not the chemical. No when turning pounds of product into gallons: use the product's own weight per gallon, its specific gravity times 8.34. A 12.5 percent solution has a specific gravity of about 1.2, so it weighs about 1.2 × 8.34 = 10.0 lb/gal.
Dose is what you add, demand is what reacts with the water, and residual is what is left. Dose = demand + residual, so a residual reading lower than planned usually means the demand went up.
A milligram per liter is one part per million by weight in water, and a million gallons of water weighs 8.34 million pounds, so one mg/L in one MG is 8.34 lb. The millions cancel.
The free study app has a step-by-step lesson on this calculation, with fraction-tile unit cancellation and graded practice problems.
Open the chemical feed lessonFormulas follow the official sheets linked above, which are the reference at the exam. The worked example and the mistakes are original practice material written for this page; they are not real exam questions, which are confidential. Written by the site's author.
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