Aluminium sulphate (alum) and polyaluminium chloride (PAC) are both aluminium-based coagulants. They work on the same principle, but they behave differently in the plant. Here is a practical comparison.
The basic difference
Alum releases simple aluminium ions that hydrolyse in the water after dosing. PAC is pre-hydrolysed: part of that reaction already happened during manufacture, so PAC contains polymeric aluminium species from the start. The degree of pre-hydrolysis is expressed as basicity.
How they compare
| Aspect | PAC | Alum |
|---|---|---|
| Effect on pH and alkalinity | Smaller drop, because it is pre-neutralised | Larger drop; extra alkali (lime or soda) is often needed |
| Effective pH range | Generally wider | Narrower |
| Cold, low-turbidity water | Often performs better | Floc forms more slowly |
| Floc formation | Usually faster, denser floc | Slower, lighter floc |
| Dose | Often lower for the same result | Often higher |
| Sludge | Often less sludge | Often more sludge |
| Price per kg | Higher | Lower |
These are general tendencies reported across the industry. Actual results depend on your water and the grades compared.
Compare cost per cubic metre, not cost per kg
Alum usually costs less per kilogram. But if PAC works at a lower dose, needs less pH correction and produces less sludge, the total treatment cost per cubic metre of water can be similar or lower. To compare fairly, add up:
- Coagulant dose × price
- Alkali needed for pH correction
- Polymer dose, if used
- Sludge handling and disposal
When alum may still be the better choice
- Your water has plenty of natural alkalinity and stays at a comfortable temperature.
- Your plant is built and tuned around alum and works well.
- Sludge disposal is cheap and the lowest price per kg matters most.
How to decide
Run a jar test with your current alum dose as the reference and PAC at several doses. Measure settled turbidity, pH and sludge volume, then calculate the cost per cubic metre. That gives you a fair answer for your own water.
Want to try this with our PAC? Request a free 1 kg sample with COA. The sample is free; you cover the courier.