PAC vs Aluminium Sulfate vs Ferric Sulphate: Choosing a Coagulant
Coagulant choice drives the whole treatment train: dose, pH correction, sludge volume and the clarity you can achieve. The three workhorses are polyaluminium chloride (PAC), aluminium sulfate (alum) and ferric sulphate. This article compares them on the things that matter in practice – pH window, dose, alkalinity, sludge and application – so you can shortlist before you jar-test.
1. What each coagulant is
| Coagulant | CAS | What it is |
|---|---|---|
| Polyaluminium chloride (PAC) | 1327-41-9 | Pre-hydrolysed aluminium polymer, [Al2(OH)nCl6-n]m; supplied as solid or liquid |
| Aluminium sulfate (alum) | 10043-01-3 | Classical aluminium salt; solid or liquid |
| Ferric sulphate | 10028-22-5 | Iron(III) coagulant; strong, acidic |
2. Side-by-side comparison
| Factor | PAC | Aluminium sulfate | Ferric sulphate |
|---|---|---|---|
| Effective pH window | Wide (roughly 5-9) | Narrow (roughly 5.5-7.5) | Wide (roughly 4-9) |
| Alkalinity consumed | Low (pre-hydrolysed) | High | High |
| Speed of floc formation | Fast, dense flocs | Slower | Fast |
| Works in cold water | Good | Poorer | Good |
| Sludge volume | Lower | Higher | Higher |
| Colour / iron carry-over | Low | Low | Can add colour; residual iron |
| Cost per tonne | Higher | Lowest | Moderate |
| Best for | Drinking water, variable water, cold water, tight sludge budgets | Simple, high-alkalinity water; low cost | Phosphorus removal, high-colour or high-turbidity water |
3. How to read the comparison
- If your water is variable, cold, or low in alkalinity, PAC usually wins: it works across a wider pH, needs less pH correction and forms flocs faster.
- If your water is stable and high in alkalinity, aluminium sulfate is often the most economical choice.
- If you must remove phosphorus (for example municipal wastewater with a strict P limit), a ferric coagulant is usually the first choice.
4. Cost is not price per tonne
PAC costs more per tonne than alum, but it is often dosed lower and needs less pH correction, so the cost per treated cubic metre can be lower. Compare coagulants on:
- Dose needed to reach your target turbidity or phosphate (jar test).
- pH correction chemicals required (lime, caustic).
- Sludge volume and the cost of handling it.
- Chemical storage and dosing equipment.
5. You still have to jar-test
No table can replace a jar test on your water. Coagulant performance depends on turbidity, alkalinity, pH, temperature, NOM and the mixing regime. Test at least two doses of each shortlisted coagulant, compare settled turbidity, residual aluminium or iron, and sludge volume, then pilot the winner.
6. Supply
All three are available from stock grades: PAC (solid or liquid, by Al2O3 content and basicity), aluminium sulfate (solid or liquid) and ferric sulphate. Tell us the application (drinking water, wastewater, industrial), the water character and the delivery port for a quotation.
Frequently Asked Questions
Is PAC better than aluminium sulfate?
Not always – it depends on the water. PAC works across a wider pH range, consumes less alkalinity and forms flocs faster, which helps for variable or cold water; alum is cheaper and fine for simple, high-alkalinity water.
When should I use ferric sulphate instead?
Mainly when phosphorus removal or high-colour/high-turbidity treatment is required; ferric coagulants are strong but can add colour and residual iron.
Which coagulant makes less sludge?
PAC typically produces less sludge than alum or ferric salts, because less is dosed and less alkalinity is consumed.
Related reading
- Coagulant selection chart: PAC vs aluminium sulfate vs ferric sulphate vs ferrous sulphate
- Water treatment chemicals overview: coagulants, flocculants, biocides and dechlorination
- Polyaluminium chloride (PAC): properties, production and applications
- Polyaluminium chloride (PAC) in water treatment in Africa
- How to buy polyaluminium chloride (PAC): specs, Al2O3 and basicity
- Aluminium sulfate in water treatment
PolyBlueChem supplies polyaluminium chloride (PAC), aluminium sulfate and ferric sulphate for water and wastewater treatment, in solid and liquid form, with per-batch COA and SDS. Contact us with your water character and volume for a quotation.