Alkalinity Remaining After Alum
Worked example: 60 mg/L alkalinity, 50 mg/L alum → 37.5 mg/L left. Press Try an example to run it live, then adjust anything.
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Where Alkalinity Remaining After Alum fits
- A_0Total Alkalinity as CaCO₃ from Species
- DProduct Dose from Active Strength · Jar Test Dose Scale-Up
See also
Jar Test Dose Scale-Up: Both use alum dose.Chemical Feed Rate (lb/day = mg/L × MGD × 8.34): Both use alum dose.Chemical Feed Rate from Dose: Both use alum dose.Closed Loop Slug Dose Volume: Both use alum dose.Dose Achieved from Chemical Added: Both use alum dose.Product Dose from Active Strength: Both use alum dose.9 more related
Cooling Tower LSI at Cycles of Concentration: Both use alkalinity remaining.Langelier Saturation Index from a Full Water Analysis: Both use alkalinity remaining.Larson–Skold Index: Both use alkalinity remaining.Lime Dose for Softening (as CaCO₃): Both use alkalinity remaining.Predicted Cooling Tower LSI and Maximum Cycles, from the Makeup Analysis: Both use alkalinity remaining.Puckorius (Practical) Scaling Index: Both use alkalinity remaining.Saturation pH (pHs) for Langelier's Index: Both use alkalinity remaining.Soda Ash Dose for Softening (as CaCO₃): Both use alkalinity remaining.Total Alkalinity as CaCO₃ from Species: Both use alkalinity remaining.The jar test →
UniversityProcess & Water Chemistry
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Alkalinity Remaining After Alum explained
Filter alum, Al₂(SO₄)₃·14H₂O, does not simply dissolve: it hydrolyses, and in doing so it releases acid that consumes the water's natural bicarbonate. The stoichiometry works out at 0.45 mg/L of alkalinity as CaCO₃ destroyed, and about 0.44 mg/L of carbon dioxide produced, for every mg/L of alum fed. A raw water at 60 mg/L alkalinity dosed with 50 mg/L alum finishes at 60 − 22.5 = 37.5 mg/L, which is comfortable. Dose the same alum into a soft 25 mg/L water and you land at 2.5 mg/L: no buffer left, the pH crashes below the range where aluminium hydroxide is insoluble, and the coagulation stops working just when you needed it most.
That is the whole reason lime, soda ash or caustic feed exists in soft-water plants: you are not raising pH for its own sake, you are restoring the alkalinity the coagulant consumed. Keep 20–30 mg/L of alkalinity as CaCO₃ in the settled water as a working floor, and remember that ferric chloride is hungrier still at roughly 0.55 mg/L of alkalinity per mg/L of ferric, while polyaluminium chloride consumes far less, often a third, which is exactly why PACl has taken over so many soft-water plants. Downstream, the released CO₂ turns finished water aggressive, so a plant that coagulates hard and never adjusts afterwards will find its own distribution mains and household copper paying the bill.
Alkalinity Remaining After Alum formula
- = Alkalinity remaining (mg/L)
- = Raw water alkalinity (mg/L)
- = Alum dose (mg/L)
Cite this page
formula.expert. (2026, October 31). Alkalinity Remaining After Alum Formula & Calculator. https://formula.expert/solvers/alum-dose-alkalinity-balance
https://formula.expert/solvers/alum-dose-alkalinity-balance