Aluminium sulphate (alum)
The oldest coagulant still in daily use and the cheapest per tonne. It remains the right answer on a stable, well-buffered water, and the wrong answer almost everywhere else.
Aluminium sulphate, Al₂(SO₄)₃·14H₂O, is supplied as an 8.0–8.3% Al₂O₃ liquid or a 17.0–17.5% Al₂O₃ kibbled or ground solid. It coagulates by hydrolysing in the water to aluminium hydroxide, which means it works only inside a narrow pH window of about 5.8–7.5 and destroys roughly 0.5 mg of alkalinity as CaCO₃ for every mg dosed.
Typical dissolved air flotation doses run 100–400 mg/L as supplied liquid. Where alkalinity is adequate, pH is stable and freight is short, it is the lowest-cost aluminium source available. Where any of those three fail, PAC is normally cheaper in use despite costing more per tonne.
Specification
| Parameter | Liquid | Solid, kibbled |
|---|---|---|
| Al₂O₃ content | 8.0–8.3% | 17.0–17.5% |
| Aluminium as Al | 4.2–4.4% | 9.0–9.3% |
| Free acid as H₂SO₄ | ≤ 0.5% | ≤ 0.3% |
| Iron as Fe₂O₃ | ≤ 0.5% industrial, ≤ 0.05% potable | ≤ 0.5% industrial, ≤ 0.05% potable |
| Water insolubles | ≤ 0.3% | ≤ 0.5% |
| Density at 20 °C | 1.32–1.34 kg/L | Bulk 1.0–1.1 kg/L |
| Crystallisation point | −3 to 0 °C | n/a |
| pH, 1% solution | 2.8–3.6 | 2.8–3.6 |
The alkalinity problem
Alum’s weakness is arithmetic, not chemistry. Every milligram dosed destroys about 0.5 mg of alkalinity as CaCO₃. Run the numbers before selecting it.
| Alum dose | Alkalinity destroyed | Minimum raw alkalinity to stay in window | If short, caustic needed |
|---|---|---|---|
| 50 mg/L | 25 mg/L as CaCO₃ | ≥ 75 mg/L | 20 mg/L NaOH per 25 mg/L shortfall |
| 100 mg/L | 50 mg/L | ≥ 100 mg/L | 40 mg/L |
| 200 mg/L | 100 mg/L | ≥ 150 mg/L | 80 mg/L |
| 300 mg/L | 150 mg/L | ≥ 200 mg/L | 120 mg/L |
| 400 mg/L | 200 mg/L | ≥ 250 mg/L | 160 mg/L |
Dose ranges by application
| Application | Liquid, 8% Al₂O₃ | Solid, 17% Al₂O₃ | Coagulation pH |
|---|---|---|---|
| Potable water, low turbidity | 20–60 mg/L | 10–28 mg/L | 6.5–7.5 |
| Potable water, high turbidity | 60–180 mg/L | 28–85 mg/L | 6.0–7.0 |
| Municipal primary DAF | 80–200 mg/L | 38–94 mg/L | 6.5–7.0 |
| Edible oil refinery | 150–400 mg/L | 70–190 mg/L | 4.5–6.0 |
| Textile and dyehouse | 200–500 mg/L | 94–235 mg/L | 6.0–7.0 |
| Paper mill whitewater | 60–200 mg/L | 28–94 mg/L | 6.0–7.0 |
| Swimming pool clarification | 2–8 mg/L | 1–4 mg/L | 7.2–7.6 |
Handling and materials
- Suitable materials: HDPE, polypropylene, PVC, FRP with vinyl ester, rubber-lined steel, 316L stainless in the liquid at ambient temperature.
- Unsuitable: mild steel, galvanised steel, aluminium, copper, unprotected concrete.
- Solid alum make-up: dissolve at 25–50% w/v in warm water with agitation. Dissolution is endothermic and slow in cold water — allow 30–45 minutes. Kibbled grade dissolves faster than lump.
- Storage: liquid at 5–40 °C, shaded and vented; it crystallises below about −3 °C, which is not an African concern. Solid must be kept dry — alum is hygroscopic and a wet bag sets into a block that has to be broken up mechanically.
- Shelf life: liquid 6–12 months, solid effectively indefinite if kept dry.
When alum is genuinely the right choice
- Raw water alkalinity comfortably above 150 mg/L as CaCO₃, so the alkalinity it destroys costs nothing to replace.
- Stable pH with automatic control, so the narrow window is not a daily operating risk.
- Water temperature consistently above about 18 °C, where the kinetic disadvantage against PAC largely disappears.
- Short freight, or local manufacture. Solid alum is produced in several African countries and the landed cost advantage over imported PAC can be decisive.
- Acid-side duties such as edible oil soapstock splitting, where operating at pH 4.5–6.0 is the intention.
Frequently asked questions
Alum or PAC — which is cheaper?
Alum is cheaper per tonne. PAC is usually cheaper per cubic metre treated. PAC works at roughly a third to a half the equivalent dose on many streams, destroys a fifth of the alkalinity, and produces less sludge. Build the comparison on cost per cubic metre including alkalinity replacement and sludge disposal, and PAC wins on most industrial effluent. On a high-alkalinity, stable, warm water with local alum supply, alum wins.
What pH should alum coagulation run at?
5.8–7.5, with the practical optimum for most waters at 6.2–6.8, where aluminium hydroxide solubility is at its minimum. Outside that band, dissolved aluminium rises sharply, floc quality falls and residual aluminium in the treated water increases. For colour and natural organic matter removal, enhanced coagulation deliberately runs lower, at 5.5–6.0.
How do I convert between liquid and solid alum doses?
Solid at 17% Al₂O₃ is roughly 2.1 times as concentrated as liquid at 8%. A 200 mg/L liquid dose is equivalent to about 94 mg/L of solid. Solid also ships without the water, so on a long sea leg it carries the same aluminium in less than half the tonnage.
Does alum work in cold water?
Less well. Alum relies on hydrolysis happening in the treated water, and hydrolysis slows as temperature falls, so floc formation is slower and the dose must rise. Below about 12 °C the penalty is significant. This is rarely a constraint in African surface water but does matter on highland and deep borehole sources.
Is residual aluminium a problem?
It can be. Well-run alum coagulation leaves 0.1–0.5 mg/L of aluminium in the treated water; poorly controlled pH or overdosing pushes it higher. Where the receiving water or the potable standard sets an aluminium limit — commonly 0.2 mg/L — pH control becomes a compliance issue rather than an efficiency one, and a higher-basicity PAC or ACH is the easier route.