Phosphorus Precipitation Dose Calculator
Chemical phosphorus removal is a molar-ratio game. Pick alum or ferric, enter the phosphorus to remove, and get the product dose in mg/L, lb/day, and gal/day.
Chemical Dose for P Removal
| Item | Value |
|---|
How the calculation works
Formulas
AlPO4 precipitation is 1:1 Al:P in theory, but competing hydroxide formation pushes practical doses to 1.5–2.5 mol Al per mol P (1.5–2 for Fe).
mg dry alum per mg P = molar ratio × 594 ÷ 62 = ratio × 9.58 (594 = alum MW; 62 = 2×31, two Al per alum, P = 31)
mg FeCl3 per mg P = molar ratio × 162.2 ÷ 31 = ratio × 5.23
Product dose (mg/L) = P to remove (mg/L) × mg product per mg P
Feed (lb/day) = 8.34 × flow (MGD) × dose (mg/L)
Liquid alum (48.5%) carries about 5.33 lb dry alum per gallon; 40% ferric chloride about 4.7 lb FeCl3 per gallon.
AlPO4 precipitation is 1:1 Al:P in theory, but competing hydroxide formation pushes practical doses to 1.5–2.5 mol Al per mol P (1.5–2 for Fe).
mg dry alum per mg P = molar ratio × 594 ÷ 62 = ratio × 9.58 (594 = alum MW; 62 = 2×31, two Al per alum, P = 31)
mg FeCl3 per mg P = molar ratio × 162.2 ÷ 31 = ratio × 5.23
Product dose (mg/L) = P to remove (mg/L) × mg product per mg P
Feed (lb/day) = 8.34 × flow (MGD) × dose (mg/L)
Liquid alum (48.5%) carries about 5.33 lb dry alum per gallon; 40% ferric chloride about 4.7 lb FeCl3 per gallon.
| Chemical | MW | Practical metal:P ratio |
|---|---|---|
| Dry alum Al2(SO4)3·14H2O | 594 | 1.5–2.5 : 1 |
| Ferric chloride FeCl3 | 162.2 | 1.5–2 : 1 |
Notes & limitations
Molar-ratio practice for chemical phosphorus removal follows Metcalf & Eddy, Wastewater Engineering. Ratios above 1:1 reflect hydroxide competition and adsorption, not just AlPO4/FePO4 precipitation — jar testing sets the final dose.
- Feed point matters: dosing ahead of the aeration basin or secondary clarifier gives mixing and flocculation time; verify with jar tests at plant pH and temperature.
- Chemical P removal adds chemical sludge — roughly 2–3 lb of dry solids per lb of alum fed. Account for it in solids handling.
- Alum consumes alkalinity (about 0.5 mg as CaCO3 per mg alum) and can depress pH; ferric is more acidic still. Watch nitrification at low alkalinity.
- Liquid product strengths vary by supplier — confirm the percent active and specific gravity on the SDS or certificate of analysis before setting pump rates.
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