Water tools
Coagulant dose: alkalinity consumed, precipitate formed, base to add
Adding a metal salt is adding a strong acid. Every mole of trivalent metal hydrolyses to its hydroxide and releases three protons that the water's alkalinity must absorb, or a base must.
How it works
The commercial salt hydrolyses to the amorphous hydroxide and releases protons, for example for alum:
Each proton consumes one equivalent of alkalinity (50 mg/L as CaCO₃ per meq/L). With nH protons and nM metal ions per mole of salt of formula weight FW:
- coagulant dose, mg/L of the commercial salt
- formula weight of the salt, g/mol
- protons released per mole of salt
- metal ions per mole of salt
- alkalinity consumed, mg/L as CaCO₃
- metal hydroxide precipitated, mg/L
- formula weight of the hydroxide, 78 for aluminium, 107 for iron
- caustic to replace the alkalinity, mg/L
Alum and ferric sulfate give six protons and two hydroxides per mole; ferric chloride three and one. Lime and soda ash supply two equivalents per mole. The hydroxide here is the coagulant's own sludge; add the removed turbidity (about 1.4 mg TSS per NTU) and any polymer for the total. Residual soluble metal depends on pH: aluminium is least soluble near 6.2, iron near 8. The dose itself cannot be predicted from chemistry; jar tests set it.
Related reading
These calculators use standard published formulas and are provided for preliminary engineering guidance. Confirm against measured data and vendor projections before design. Model your full water matrix in Nepti or post your project to compare provider proposals.