Aluminium Sulphate (Alum) Coagulant Dosing Equation
Aluminium sulphate (alum) is the most widely used coagulant in conventional water treatment. Its dose is set by jar test — typically 5–80 mg/L as Al₂(SO₄)₃·14H₂O — and then scaled to plant flow. Dry feed uses kg/hr of granular alum; liquid feed uses 48–50% liquid alum at SG 1.32–1.34.
Where:
- Flow = Main flow rate in m³/hr
- Dose_PPM = Target concentration in mg/L or ppm
- Strength_% = Percentage active ingredient of the stock chemical
- SG = Specific Gravity (density relative to water) of the stock
Related dosing calculators
Other chemical-specific dosing calculators in the same series — same formula, different defaults:
- Chemical Dosing Calculator (generic) — the universal seed page
- Coagulant Dosing Calculator — Coagulation · PAC · FeCl₃
- Polymer Dosing Calculator — Wastewater · Flocculant · Make-down
- Lime Dosing Calculator — pH Correction · Alkalinity Boost
- All Chemical Engineering Calculators →
Frequently Asked Questions
Multiply the optimum jar-test dose (mg/L) by the plant flow rate (m³/hr). That gives kg/hr of dry alum required. For liquid alum, divide by the active strength in g/L (e.g. 50% × 1.33 SG × 1000 = 665 g/L). Always validate at the plant — full-scale dose is sometimes 10–20% higher than jar-test optimum.
10–80 mg/L for most lakes and rivers; 80–150 mg/L for high-turbidity tropical rivers; 5–20 mg/L for low-turbidity pre-filtered water. Cold water (<10°C) often needs 1.5× the dose of warm water for the same coagulation result.
1 mg/L of alum (as Al₂(SO₄)₃·14H₂O) consumes ~0.5 mg/L as CaCO₃ alkalinity. Soft waters with <30 mg/L alkalinity often need lime or soda-ash co-dosing to keep coagulation pH in the 6.0–7.5 floc-friendly range.
Liquid alum (50%) is the standard for plants >500 m³/day: no dust, easy metering, accurate dosing. Dry alum (granular, ~17% Al₂O₃) is cheaper per kg of active and has indefinite shelf life — preferred for small plants or remote sites without reliable power for liquid metering.
No. PAC (polyaluminium chloride) is 2–3× more potent per mg of Al than alum and consumes about half the alkalinity. A PAC dose of 10–15 mg/L often replaces a 30–50 mg/L alum dose. Always re-jar-test when switching coagulants.