Alkalinity Remaining After Alum

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.