COMPARISON
PAC vs Alum vs Ferric Sulfate: Which Coagulant Fits Your Water?
All three chemicals do the same basic job - destabilise colloids so they can be removed by settling or flotation. They differ in pH window, alkalinity demand, floc behaviour and sludge volume. Here is how to choose without a chemistry degree.
The short comparison
Prices shift with freight and local supply, so read the last row as a pattern, not a quote. The real comparison is always run on your own water in a jar test.
| Property | PAC | Alum (Al2(SO4)3) | Ferric sulfate |
|---|---|---|---|
| Effective pH window | 5.5 – 8.5 | 5.5 – 7.5 | 4 – 11 |
| Alkalinity consumption | Low | High | Moderate – high |
| Floc formation speed | Fast | Moderate | Fast, dense floc |
| Temperature sensitivity | Low – moderate | High (weak below 10 °C) | Low |
| Sludge volume | Compact | Bulky | Dense but heavy |
| Residual colour / turbidity | Good | Moderate | Good (may tint water) |
| Relative cost per t | Higher | Low | Moderate |
Where PAC wins
PAC is pre-hydrolysed, so it spends less of your water's alkalinity and works in a wider pH band. That makes it the practical choice in four situations: cold water (below 10 °C, where alum goes sluggish), low-alkalinity soft water, high-colour surface water, and plants where sludge volume is a disposal cost. Drinking water plants also like PAC's lower residual aluminium when dosing is controlled properly - our drinking water grade carries Al2O3 29% to GB 15892-2020.
Where alum and ferric sulfate win
Alum is cheap, globally available and decades of operating data exist for it. If your water is straightforward and alkalinity is plentiful, alum remains a rational choice. Ferric sulfate earns its keep in specific jobs: it precipitates phosphate effectively (useful for effluent phosphorus limits), binds sulfides, and tolerates a wide pH range including alkaline conditions where aluminium chemicals dissolve. Its flocs are dense and settle fast, which suits high-load industrial streams - the trade-off is a yellowish tint in overdose and more corrosive handling.
Many plants run combinations: ferric for phosphorus and sulfide plus PAC for polish, or PAC as the main coagulant with a small PAM dose as flocculant aid.
A five-question decision checklist
- Is your water cold for part of the year? If yes, PAC or ferric beat alum.
- Is alkalinity below about 50 mg/L as CaCO3? If yes, PAC consumes the least.
- Is phosphorus removal the main goal? Ferric deserves a trial.
- Is sludge disposal priced per tonne? Compact PAC flocs cut sludge volume.
- Is budget the only criterion and your water easy? Alum still works.
Run the comparison on your water
We run side-by-side jar tests on customer samples: three beakers, one per coagulant, identical mixing, settling curves recorded and photographed. Send us pH, turbidity and a litre of raw water and the comparison comes back with your quotation. Ask for it through the quotation page.
Frequently asked questions
Is PAC always better than alum?
No. PAC costs more per tonne and, on easy water with good alkalinity, alum can be the more economical choice. PAC's advantages appear in cold water, soft water, high-colour water and where sludge volume or residual aluminium matters. Compare on your own water, not on a datasheet.
Can I switch from alum to PAC without changing my dosing system?
Usually yes for solid products - both are prepared as solutions in the same kind of make-down tank. Expect to dose less PAC than alum by weight (typically 30-50% less for the same effect) and re-run your jar test before switching the plant over.
Does ferric sulfate stain equipment?
Ferric solutions are corrosive and any drips leave rust-coloured marks. Use 316L or lined tanks and pumps, and keep wash-down water available around the dosing skid. PAC is milder in this respect.