Alum Coagulant Dose Calculator
Your jar test says 25 mg/L — now what does the feed pump need to deliver? Enter flow and jar-test dose to get dry alum lb/day or liquid alum gal/day.
Alum Feed Rate
| Item | Value |
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How the calculation works
Formulas
Flow (MGD) = gpm × 1,440 ÷ 1,000,000
Dry alum (lb/day) = 8.34 × flow (MGD) × dose (mg/L)
Liquid alum (gal/day) = dry alum (lb/day) ÷ 5.33 (lb dry alum per gallon at 48.5%)
Feed pump rate (mL/min) = gal/day × 3,785.41 ÷ 1,440
Flow (MGD) = gpm × 1,440 ÷ 1,000,000
Dry alum (lb/day) = 8.34 × flow (MGD) × dose (mg/L)
Liquid alum (gal/day) = dry alum (lb/day) ÷ 5.33 (lb dry alum per gallon at 48.5%)
Feed pump rate (mL/min) = gal/day × 3,785.41 ÷ 1,440
Notes & limitations
The 8.34 factor is the weight of one gallon of water in pounds, which makes mg/L × MGD × 8.34 = lb/day work out. Liquid alum at 48.5% carries about 5.33 lb of dry alum per gallon — confirm against your supplier’s spec sheet, since commercial strengths vary.
- Jar-test doses must be on a dry-alum basis for this math to hold. Some suppliers quote doses “as Al2O3” — convert before feeding the pump.
- Alum consumes roughly 0.5 mg/L of alkalinity as CaCO3 per 1 mg/L of alum dosed. Watch finished-water pH and alkalinity when doses climb, especially in low-alkalinity source water.
- Cold water usually needs higher coagulant doses for the same turbidity removal — re-run jar tests seasonally.
- This tool is for operational feed-rate setting. Confirm pump calibration with a drawdown test and follow plant SOPs.
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