Aluminum sulfate (alum, CAS 10043-01-3) is one of the oldest and most widely used coagulants in water treatment. From drinking water clarification to industrial wastewater and papermaking, alum remains a cost-effective workhorse — despite being centuries old. This guide covers how aluminum sulfate works in water treatment, the dosing practice, and its role alongside modern coagulants.
1. The Role of Alum in Water Treatment
Aluminum sulfate is a coagulant: when added to water, it hydrolyzes to form aluminum hydroxide flocs that neutralize the negative charge of suspended particles and sweep them out of suspension. The result:
- Clarification: removes turbidity, color and suspended solids.
- Pathogen removal support: removes particles that shelter microorganisms before disinfection.
- Phosphorus removal: precipitates phosphate in wastewater treatment.
2. How It Works: Coagulation and Flocculation
- Coagulation: alum is dosed with rapid mixing; it hydrolyzes and neutralizes particle charge (charge neutralization), forming micro-flocs.
- Flocculation: gentle mixing lets micro-flocs collide and grow into larger settleable flocs.
- Settling: the flocs settle in the clarifier, taking impurities with them.
Alum works best in a specific pH window (roughly pH 5.5-8, depending on the water); pH adjustment is often required.
3. Typical Dosing
| Application | Typical dose |
|---|---|
| Drinking water clarification | 5-50 mg/L (as alum) |
| Industrial wastewater | 50-300 mg/L (system-dependent) |
| Phosphorus removal | Al:P ratio typically 1.5-3:1 |
Doses are confirmed by jar testing on the actual water; overdosing wastes chemical and can re-stabilize particles.
4. Alum vs Modern Coagulants
| Aspect | Alum | PAC (polyaluminum chloride) | Ferric salts |
|---|---|---|---|
| Cost | Lowest | Higher | Medium |
| pH sensitivity | Narrow | Broad | Broad |
| Sludge volume | Higher | Lower | Higher |
| Cold-water performance | Weaker | Better | Better |
Alum remains the economical default; PAC and ferric salts are chosen for cold water, wide pH or lower sludge needs. See our PAC guide for the modern alternative.
5. FAQ
Is aluminum sulfate safe in drinking water?
Yes, when dosed correctly; residual aluminum is controlled within regulatory limits, and the flocs are removed before disinfection.
Why does alum need pH control?
Alum’s coagulation efficiency peaks in a narrow pH window; outside it, performance drops and residual aluminum rises.
Does alum remove phosphorus?
Yes — it precipitates phosphate, making it a standard chemical for phosphorus removal in wastewater.
How do I optimize the alum dose?
By jar testing: vary dose and pH, measure turbidity/color removal, and pick the lowest effective dose.
6. References and Further Reading
- Water treatment literature on coagulation with aluminum sulfate
- Standard jar-test protocols for coagulant optimization
This article is published for industrial and technical reference by PolyblueChem. Doses are starting points — optimize by jar testing.
PolyblueChem supplies aluminum sulfate (CAS 10043-01-3) with COA and MSDS. Contact us for specifications and quotations.