Water & Wastewater
Chemical Dosing Calculator
Chemical feed rate from dose and flow, in pounds per day and in gallons per day of solution, with the metering pump setting.
Calculated reference results
- Chemical required
- 31.28lb per day
- Solution feed rate
- 25.64gal per day
- Solution feed rate
- 1.068gal per hour
- Metering pump setting
- 67.4mL per minute
- Solution consumed
- 250.20lb per day
- Process flow
- 1,042gpm
- Ratio of solution to process flow
- 1 : 58,500
Useful for checking that the injection point gives adequate mixing.
The arithmetic
- Chemical = 2.50 mg/L x 1.5000 MGD x 8.34 = 31.28 lb per day
- Solution required = 31.28 / 0.125 strength = 250.20 lb per day
- Solution volume = 250.20 / (8.34 x 1.17) = 25.64 gal per day
What this does not account for
- Dose is what you apply. Residual is what remains after demand. Sizing a feed system on residual rather than dose will undersize it.
- Sodium hypochlorite loses strength in storage, faster when warm. A tank that started at 12.5% may be considerably weaker weeks later, and the feed rate has to compensate.
- Chemical feed systems require their own safety review: containment, compatible materials, ventilation, eyewash, and personal protective equipment. Nothing here addresses that.
- Verify the actual feed rate by drawdown against a calibration cylinder. Metering pump dial settings are not accurate enough to trust without checking.
How this is calculated
The pounds formula gives chemical feed as dose in milligrams per litre multiplied by flow in million gallons per day, multiplied by 8.34. To convert that to a solution feed rate, divide by 8.34, by the solution specific gravity, and by the strength as a decimal fraction.
Formulas
lb per day = dose (mg/L) x flow (MGD) x 8.34
- 8.34 — pounds per gallon of water
solution gpd = (lb per day / strength) / (8.34 x specific gravity)
Assumptions and limitations built into this calculator
- The 8.34 constant assumes water at standard conditions.
- Assumes the solution strength you enter is the actual current strength, not the as-delivered strength.
- Does not account for chemical demand, decay, or reaction kinetics.
Frequently asked questions
- Where does 8.34 come from?
- It is the weight of one US gallon of water in pounds. The pounds formula relies on it, which is why it is only strictly correct for water at standard conditions.
- Why is my residual lower than the dose?
- Because the water exerts a demand that consumes chemical before any residual remains. Dose minus demand equals residual, and demand varies with the water quality.
Direct contact
Have a controls question?
Reach Eric Sullivan directly about anything on this site, a controls or automation topic, or one of his personal projects.