Guide
PAC, PFS or alum? How to choose a coagulant
Polyaluminium chloride (PAC), polyferric sulfate (PFS) and aluminium sulfate (alum) are the three inorganic coagulants most plants start with. All three neutralise the charge on fine particles so they clump into floc that can settle or be filtered. None of them is "the best" in general: the right one depends on your raw water, your treatment targets and what happens to the sludge afterwards.
This guide explains where each one tends to work well, what to watch for, and how to confirm the choice before you buy in bulk.
Quick comparison
| PAC | PFS | Alum | |
|---|---|---|---|
| What it is | Pre-hydrolysed aluminium polymer | Pre-polymerised iron (ferric) sulfate | Simple aluminium salt |
| Typical form | Powder or liquid | Powder or liquid | Crystals, powder or liquid |
| Strong points | Works across a wide range of waters, often at a lower dose than alum; less pH drop | Dense, fast-settling floc; also reduces colour, oil, COD and heavy metals | Well understood, easy to dissolve, low cost per kg |
| Watch for | Grade matters (Al₂O₃ content and basicity) | Can tint water if overdosed; acidic solution needs suitable dosing materials | Uses more alkalinity, so pH can fall; may need lime or caustic |
| Common uses | Drinking and process water, municipal and industrial wastewater | Textile, paper, tannery, plating and mining effluents | Drinking water, wastewater, paper sizing |
When PAC is a good fit
PAC is "pre-hydrolysed": part of the reaction that alum goes through in your tank has already happened during manufacture. The share of that pre-neutralisation is called basicity. In practice this usually means:
- Less alkalinity is consumed, so the pH drops less after dosing. This helps with soft, low-alkalinity water.
- Lower doses are common. In one published comparison on surface water, the optimum dose was about 8 mg/L for PAC against 16 mg/L for alum and 40 mg/L for ferric chloride. Your own water may give different numbers, which is why a jar test matters.
- It copes with changing conditions, such as turbidity spikes after rain.
What to check: PAC grades differ in Al₂O₃ content and basicity. Drinking-water use also needs a grade that meets your local standard. Ask for the TDS of the exact grade you are quoted.
When PFS is a good fit
PFS is the iron-based counterpart. Iron floc is heavier, so it tends to settle quickly and form a compact sludge. PFS is often chosen for industrial effluents where you need more than turbidity removal:
- Colour and COD in textile and dyeing wastewater
- Oil and fine suspended solids
- Heavy metals in plating and mining water, and fluoride-bearing effluent
What to check: an overdose of iron can leave a yellow-brown tint. PFS solutions are acidic, so tanks, pumps and pipework must be suitable for them. Ask for the Fe₂O₃ content of the grade you are offered.
When alum is a good fit
Alum (aluminium sulfate) has been used for over a century and many operators know exactly how it behaves. It dissolves easily and is cheap per kilogram. It is also used in papermaking for sizing.
What to check: alum consumes more alkalinity than PAC, so the pH can drop noticeably. Low-alkalinity water may need lime or caustic soda to keep coagulation on target. In some waters the alum dose, and so the sludge volume, is higher than with PAC. Compare cost per cubic metre treated, not price per tonne.
What about PAFC?
Polyaluminium ferric chloride (PAFC) combines aluminium and iron in one product. It hydrolyses quickly and tolerates swings in turbidity, alkalinity and organics. It is worth including in a jar test when neither PAC nor PFS alone gives the result you want.
How to decide: test, don't guess
The fastest way to choose is a jar test with two or three candidates on your own water:
- Take a fresh sample of the water you actually treat.
- Test each coagulant at a range of doses.
- Compare the clarity of the treated water, the pH after dosing, floc size, settling speed and sludge volume.
- Work out the cost per cubic metre: dose (mg/L is the same as g/m³) × price per gram.
Our step-by-step guide, How to run a jar test before you order a coagulant, walks through the procedure.
Get samples to test
We supply PAC, PFS, PAFC and aluminium sulfate. Samples of about 1 kg are free; you pay only the courier. Every quotation comes with the TDS and SDS for the grade we offer.
Request samples and a quote · PAC · PFS · PAFC · Aluminium sulfate