Coagulation is the step that turns cloudy raw water into water that can be filtered. A coagulant neutralises the electrical charge on tiny suspended particles so they stick together into flocs that settle out. The two most common choices are aluminium sulphate, known as alum, and polyaluminium chloride, known as PAC.
Aluminium sulphate
Alum has been used for over a century. It is economical, widely available and well understood by operators.
- Works best in a pH range of roughly 6 to 7.5.
- Consumes alkalinity, so soft water may need lime or soda ash to hold the pH.
- Slows down in cold water.
Polyaluminium chloride
PAC is a pre-polymerised coagulant. Part of the chemistry that alum performs in the tank has already been done during manufacture.
- Works across a wider pH range and in colder water.
- Uses less alkalinity and usually produces less sludge.
- Costs more per kilogram, though a lower dose can close the gap.
How to decide
The only reliable answer comes from a jar test on your own raw water. Run both coagulants at several doses, then compare settled turbidity, floc size, final pH and sludge volume. Factor in the cost per cubic metre treated, not just the price per bag.
Our technical team can help you set up the test and supply alum for your trials.
Questions about this topic?
Our chemists can advise on your specific process.
