PAC vs Alum: Choosing a Coagulant for Water Treatment

Coagulants make tiny suspended particles in raw water clump together so they can settle or be filtered out. The two most common aluminium-based coagulants are aluminium sulphate (alum) and poly aluminium chloride (PAC).

Alum (aluminium sulphate)

The traditional choice. It is widely available and low in cost per kilogram, and works well in many conventional plants. It consumes alkalinity, so it lowers the pH of the water more, and it performs best in a narrower pH band — roughly 5.5 to 7.5.

PAC (poly aluminium chloride)

A pre-polymerised coagulant. It typically works across a wider pH range (about 5 to 9), forms flocs faster, performs better in cold water and usually needs a lower dose. It consumes less alkalinity and tends to produce less sludge.

Side by side

	Alum	PAC
Aluminium content (as Al₂O₃)	~16–17%	~28–30% (powder)
Effective pH range	~5.5–7.5	~5–9
Effect on pH / alkalinity	Larger drop	Smaller drop
Floc formation	Slower	Faster, larger flocs
Sludge volume	Higher	Lower
Price per kg	Lower	Higher

PAC costs more per kilogram, but because the dose is usually lower and less pH correction is needed, the cost per litre treated is often similar or better. The only reliable way to know is a test on your own water.

Run a jar test

1. Take samples of your raw water in identical beakers.
2. Add different doses of each coagulant.
3. Stir quickly, then slowly, then let it settle.
4. Compare clarity (turbidity), settling time, final pH and sludge.

The best dose and product for your plant is the one that meets your clarity target at the lowest total treatment cost.

Handling

Both products are acidic in solution. Use gloves and eye protection, store them dry and follow the supplier's safety data sheet.

We supply Poly Aluminium Chloride and Aluminium Sulphate — ask us for samples to run your own jar test.