Coagulant Selection Chart: PAC vs Aluminium Sulfate vs Ferric Sulphate vs Ferrous Sulphate
Coagulant choice sets the dose, the pH correction, the sludge volume and the clarity you can achieve. This page pulls the four common water-treatment coagulants into one selection chart so you can shortlist before you jar-test. The four are polyaluminium chloride (PAC), aluminium sulfate (alum), ferric sulphate and ferrous sulphate.
1. The two families: aluminium vs iron
- Aluminium coagulants (PAC, alum) – the workhorses for drinking water and general clarification; grab the turbidity and colour without adding colour themselves.
- Iron coagulants (ferric, ferrous) – strong coagulants, first choice where phosphorus removal or heavy colour/turbidity must be handled; they can add residual iron/colour if overdosed.
Within a family, the second question is whether the coagulant is pre-hydrolysed (PAC) or a simple salt (alum, ferric, ferrous). Pre-hydrolysed PAC works over a wider pH and consumes less alkalinity.
2. Side-by-side comparison
| Factor | PAC | Aluminium sulfate | Ferric sulphate | Ferrous sulphate |
|---|---|---|---|---|
| CAS | 1327-41-9 | 10043-01-3 | 10028-22-5 | 7720-78-7 |
| Effective pH window | Wide (about 5-9) | Narrow (about 5.5-7.5) | Wide (about 4-9) | Wide but needs oxidation |
| Alkalinity consumed | Low | High | High | High |
| Floc speed / density | Fast, dense | Slower | Fast, dense | Slower (needs oxidant) |
| Cold water performance | Good | Poorer | Good | Moderate |
| Colour / residual metal | Low | Low | Can add colour / iron | Can add colour / iron |
| Phosphorus removal | Some | Some | Strong | Strong (after oxidation to Fe(III)) |
| Sludge volume | Lower | Higher | Higher | Higher |
| Typical cost | Higher per tonne, often lower dose | Lowest | Moderate | Low |
3. Selection chart by treatment goal
| Your goal | First choice | Notes |
|---|---|---|
| Clarify a variable or cold water | PAC | Wide pH window, good in cold water, less pH correction |
| Lowest chemical cost on stable, high-alkalinity water | Aluminium sulfate | Cheap and well understood; watch the alkalinity drop |
| Remove phosphorus (wastewater) | Ferric sulphate | Strong Fe(III) coagulant; see phosphorus removal |
| High colour / high turbidity | Ferric sulphate | Strong coagulant for difficult water |
| Minimise sludge volume | PAC | Lower dose and less alkalinity consumption mean less sludge |
| Drinking water, simple water | PAC or alum | Use the drinking-water grade and check residual aluminium |
| Metal-finishing / wastewater where ferrous is on hand | Ferrous sulphate | Must be oxidised to Fe(III) to coagulate well |
4. Selection chart by water character
| Water character | Reach for |
|---|---|
| Low turbidity, low temperature | PAC (works when alum struggles) |
| High turbidity | Ferric sulphate or PAC |
| High natural organic matter (NOM) / colour | Ferric or PAC at the right pH |
| High alkalinity, stable quality | Aluminium sulfate (economical) |
| Soft, low-alkalinity water | PAC (less alkalinity consumption) |
| Wastewater with a strict P limit | Ferric sulphate |
5. Quick decision rules
- Phosphorus or heavy colour? Go iron (ferric).
- Cold or variable water? Go pre-hydrolysed aluminium (PAC).
- Stable, high-alkalinity water and cost matters? Alum is usually the cheapest.
- Want less sludge? PAC – less dose, less alkalinity consumed.
- Low alkalinity? Avoid alum; PAC needs less pH correction.
- Compare on cost per treated m3, not cost per tonne: dose, pH correction and sludge handling all count.
- Always jar-test the shortlist on your water before committing.
6. Typical dosage
| Coagulant | Typical dose (starting point) |
|---|---|
| PAC | A few to a few tens of mg/L (as product or Al2O3, per quote) |
| Aluminium sulfate | About 5 – 50 mg/L |
| Ferric sulphate | Set by phosphorus or turbidity target |
| Ferrous sulphate | Higher, because it must be oxidised to Fe(III) |
All doses are set by jar test on the actual water – the figures above are starting points for a trial.
7. What changes the choice
| Factor | Effect |
|---|---|
| pH | Sets how well each coagulant hydrolyses and works |
| Alkalinity | Alum and iron salts consume it; low-alkalinity water needs PAC or pH correction |
| Temperature | Cold water slows alum; PAC copes better |
| Residual metal limits | Aluminium or iron residuals may limit the dose (drinking water) |
| Sludge handling cost | Can flip the economics toward PAC |
Frequently Asked Questions
Is PAC better than aluminium sulfate?
Depends on the water. PAC works across a wider pH, consumes less alkalinity and forms flocs faster (good for cold/variable water); alum is cheaper for simple, high-alkalinity water.
When should I use a ferric coagulant?
Mainly for phosphorus removal and for high-colour or high-turbidity water. Ferric sulphate is strong but can add colour and residual iron if overdosed.
What is the difference between ferric and ferrous sulphate?
Ferric is Fe(III) and coagulates directly; ferrous is Fe(II) and must be oxidised to Fe(III) before it works well. See ferric vs ferrous sulphate in water treatment.
Which coagulant makes the least sludge?
Usually PAC, because it is dosed lower and consumes less alkalinity, so less sludge is produced.
Which coagulant for drinking water?
PAC or aluminium sulfate with the drinking-water grade and appropriate residual-metal limits; confirm the approvals for your market.
Related reading
- Water treatment chemicals overview: coagulants, flocculants, biocides and dechlorination
- PAC vs aluminium sulfate vs ferric sulphate: choosing a coagulant
- Ferric vs ferrous sulphate in water treatment
- Ferric sulphate for phosphorus removal in wastewater
- How to buy polyaluminium chloride (PAC): specs, Al2O3 and basicity
- PolyDADMAC (CAS 26062-79-3): the organic coagulant alternative to inorganic salts
PolyBlueChem supplies PAC, aluminium sulfate, ferric sulphate and ferrous sulphate for water and wastewater treatment, in solid and liquid form, with per-batch COA and SDS and FOB/CIF delivery. Contact us with your water character and volume for a quotation.