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
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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.
Alum Dosing in Water Treatment: A Practical Guide
Aluminium sulphate — commonly called “alum” in the water industry — is the single most-used coagulant in surface-water treatment worldwide. Its role is to destabilise the colloidal particles (silt, algae, organic colour bodies) that pass through fine pre-screens but are too small to settle by gravity in any reasonable time. When alum is added and rapidly mixed, it hydrolyses to form positively charged Al(OH)₃ floc that neutralises the negative surface charge of the colloids, allowing them to clump together (flocculate) and settle in the clarifier or be removed by the filter.
The Master Equation
Alum dose calculation reduces to the standard chemical-dosing formula: Feed rate (kg/day) = Dose (mg/L) × Flow (ML/day) × (100 / strength %). Liquid alum is typically 48% Al₂(SO₄)₃·14H₂O by mass; dry alum is 17% Al₂O₃ equivalent. So a plant treating 50 ML/day of raw water with a target alum dose of 30 mg/L using 48% liquid alum needs: 30 × 50 × (100/48) = 3,125 kg of liquid alum per day. The dosing pump must deliver this rate at the alum’s specific gravity (~1.33 for 48% liquid). The calculator above performs the conversion and gives both kg/day and L/h pump output.
Setting the Dose: Jar Tests Are Mandatory
The optimum alum dose depends strongly on raw-water characteristics — turbidity, pH, alkalinity, temperature, and natural organic matter (NOM) content. There is no universal alum dose; the only honest way to set it is by jar test. The standard procedure: take six 1-litre samples of the raw water, add increasing alum doses (typically 10, 20, 30, 40, 50, 60 mg/L), rapid-mix at 100 rpm for 1 minute, then slow-flocculate at 30 rpm for 15 minutes, then settle for 30 minutes. Measure residual turbidity and select the dose that gives the lowest turbidity below the regulatory target (typically < 1 NTU after filtration for drinking water). For seasonal raw water, repeat the jar test monthly; for surface water with rapid storm-event changes, daily jar testing during high-turbidity periods is normal.
pH and Alkalinity Considerations
Alum coagulation works best in the pH range 5.5–7.5 with optimum near pH 6.5. Outside this range, the Al(OH)₃ floc is more soluble and residual aluminium in the treated water rises (regulatory limit typically 0.2 mg/L). Alum addition itself is acidifying — each mg/L of alum consumes about 0.5 mg/L of alkalinity (as CaCO₃) and lowers pH. For low-alkalinity raw water (< 30 mg/L as CaCO₃), supplement with lime or soda ash to maintain coagulation pH; the lime dosing calculator sizes this. For high-alkalinity raw water (> 100 mg/L), alum performance is robust and pH compensation is rarely needed.
Worked Example: Sizing a Bulk Tank and Day Tank
A 100 ML/day plant uses 30 mg/L alum dose, 48% liquid: daily consumption = 30 × 100 × (100/48) = 6,250 kg/day = ~4,700 L/day at SG 1.33. For a 30-day bulk-storage tank: capacity ~ 30 × 4,700 × 1.1 (10% reserve) = ~155,000 L → specify a 160 m³ bulk tank. For a day tank (24-hour operating reserve): ~5,200 L → specify a 6 m³ day tank. The dosing pump for 4,700 L/day = 196 L/h, so size to 200–250 L/h with stroke control to handle dose variation. The calculator’s output flows directly into this kind of sizing exercise.
Sludge Production
Alum coagulation generates aluminium-hydroxide sludge: roughly 0.27 kg dry sludge per kg of alum dosed, plus the removed turbidity solids. For our 100 ML/day plant at 30 mg/L alum, that’s about 800–1,000 kg/day of dry sludge requiring dewatering and disposal. This is a significant operating-cost item and a reason why polyaluminium chloride (PACl) is preferred over alum in some plants — PACl gives less sludge per equivalent coagulation performance.
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