Short answer
For most drinking-water and effluent plants, PAC clarifies faster, works across a wider pH range (roughly 5 to 9 against about 6 to 7.5 for alum), needs a lower dose, consumes less alkalinity and produces less sludge. Alum is cheaper per kilogram and still suits plants with plenty of natural alkalinity and stable raw water. The fair comparison is cost per cubic metre treated, settled by a jar test on your own water.
Key points
- 01PAC is pre-hydrolysed, so it forms floc almost instantly and holds up in cold, low-turbidity water.
- 02Alum lowers pH and alkalinity more, which often means dosing lime or soda ash alongside it.
- 03PAC powder carries around 30% Al₂O₃ against roughly 16–17% for solid alum, so less product moves per tonne of aluminium delivered.
- 04Compare on treated-water cost, sludge volume and residual aluminium, not on price per kilogram.
What each coagulant is
Aluminium sulphate, usually just called alum, has been the standard coagulant for more than a century. Dosed into raw water, it hydrolyses to aluminium hydroxide, which neutralises the charge on suspended particles and sweeps them into floc that settles or filters out.
Poly Aluminium Chloride (PAC) does the same job, but part of that hydrolysis has already been done during manufacture. The aluminium arrives as charged polymeric species, measured as basicity, so it acts faster and depends less on the water's own chemistry to work.
Side by side
Typical figures. Your raw water decides the real numbers, which is why jar testing comes first.
| PAC | Alum | |
|---|---|---|
| Aluminium content | ~30% Al₂O₃ (powder); 10–18% (liquid) | ~16–17% Al₂O₃ (solid); ~8% (liquid) |
| Effective pH range | About 5 – 9 | About 6 – 7.5 |
| Speed of floc formation | Fast, dense floc | Slower, lighter floc |
| Alkalinity consumed | Low | High: often needs lime or soda ash |
| Cold / low-turbidity water | Performs well | Performance drops |
| Sludge volume | Lower | Higher |
| Price per kg | Higher | Lower |
Where PAC usually wins
PAC earns its higher price per kilogram where its dose is much lower, where the plant would otherwise have to correct pH, and where sludge disposal is expensive. It is the common choice for municipal drinking water, for raw water that swings in turbidity with the monsoon, and for industrial effluent from textiles, dyeing and paper mills.
- Low-alkalinity or soft raw water, where alum would push pH out of range
- Cold water and winter operation
- Plants with tight residual-aluminium or sludge limits
- Effluent with colour, oil or fine colloids
Where alum still makes sense
Alum is not obsolete. With hard, well-buffered raw water and a stable process, it gives good results at the lowest chemical cost, and many plants have dosing equipment and operators built around it. It is also used outside water treatment, for example in paper sizing.
How to decide: run a jar test
Take samples of your raw water and run a jar test with both products across a range of doses. Record settled turbidity, final pH, sludge volume and, if you measure it, residual aluminium. Then multiply each optimum dose by the delivered price, add any pH-correction chemical, and compare the cost per cubic metre. That number, not the price per kilogram, is the one that matters.
Polyelectrolytes such as polyacrylamide are often dosed after either coagulant to build larger floc, so include them in the trial if your plant uses one.
Questions
Also asked.
Usually yes. Liquid PAC doses through the same kind of metering pumps, and powder PAC is dissolved like solid alum. The dose will be lower, so pump settings need to be recalibrated after a jar test.
Generally less, when dosed correctly. PAC works across a wider pH range, so less aluminium stays dissolved. Overdosing either product raises residual aluminium.
Powder at around 30% Al₂O₃ suits long export routes because you ship less water. Liquid grades at 10% or 17% suit plants that dose directly from a storage tank.
