PAC vs alum: an honest comparison
Alum is cheaper per tonne. PAC is usually cheaper per cubic metre treated. Here is where each one genuinely wins, including the cases where alum is…
Comparison
PAC vs ferric chloride
Ferric is often cheaper and genuinely effective on some streams — particularly phosphorus removal and sulphide control. It is also aggressive on equipment and stains everything it touches. Here is the honest trade-off.
Side-by-side comparison
| Parameter | Polyaluminium chloride | Ferric chloride |
|---|---|---|
| Effective pH range | Approx. 5.0–9.0 | Approx. 4.0–11.0 — wider |
| Typical dose | Lower on most municipal water | Higher on turbidity, competitive on phosphorus |
| Alkalinity consumed | Low | High |
| Corrosivity to plant | Moderate | Severe — demands rigorous material selection |
| Staining | None | Significant — rust-brown staining of concrete and equipment |
| Sludge volume | Lower, dewaters well | Higher, but often dewaters very well |
| Phosphorus removal | Good | Excellent — usually the better choice |
| Sulphide / odour control | Limited | Excellent — iron precipitates sulphide directly |
| Residual in treated water | Aluminium (regulated in potable) | Iron (can cause colour and taste issues) |
| Handling risk | Moderate | Higher — more aggressive, worse spill consequences |
| Price per tonne | Higher | Usually lower |
Where each one genuinely wins
Ferric wins clearly on two jobs. For phosphorus removal to tight discharge limits it is usually more efficient per kilogram than aluminium. And for sulphide and odour control in septic sewage collection systems, iron chemistry does something aluminium simply cannot — it precipitates sulphide directly.
PAC wins on almost everything else in potable and general municipal treatment. Lower dose, far less alkalinity consumed, no staining, and much gentler on pumps, lines and concrete. Anyone who has run ferric on a plant knows the maintenance story: stained structures, corroded fittings, and a dosing area nobody wants to work in.
The cost comparison is genuinely closer than with alum. Ferric is cheaper per tonne and on some streams the dose is comparable. Where PAC pulls ahead is total operating cost — maintenance, alkalinity supplement, staining remediation and equipment life. Those costs are real but they land on different budget lines, which is why the comparison is often made badly.
One practical note: if you are running ferric today and considering a switch, do not underestimate the pipework question. Lines and seals specified for ferric will handle PAC fine, so switching in that direction is straightforward. Going the other way is not.
FAQ
Common questions
For potable water and general municipal treatment, usually yes — lower dose, less alkalinity consumed, no staining and far less corrosion. Ferric is often better for phosphorus removal and clearly better for sulphide and odour control.
Ferric chloride is generally more efficient per kilogram for phosphorus precipitation. PAC works but often needs a higher dose to hit the same discharge limit.
It is significantly more corrosive than PAC and stains concrete and equipment rust-brown. Material selection has to be rigorous, and maintenance cost over plant life is a real consideration.
Usually yes, and equipment specified for ferric will handle PAC comfortably. Re-run your jar test — the optimum dose will differ — and expect the dosing area to be a much more pleasant place to work.
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