Choosing a Coagulant for Industrial Water Treatment

Coagulation is usually the first chemical step in a water treatment train, and it is also the step where the wrong choice costs the most — in chemical consumption, sludge volume and operator time. This note compares the three coagulant families most commonly used in Thailand and explains what to compare before you switch or specify.

The three families you will meet

Polyaluminium chloride (PAC)

PAC is a pre-hydrolysed aluminium salt. Because it is already partly neutralised, it works across a wider pH band than alum and typically produces less sludge for the same turbidity removal. It is the default choice for many industrial and municipal plants, and is supplied as a powder (typically around 30% Al₂O₃) or as a liquid.

Aluminium sulphate (alum)

The traditional coagulant. It is inexpensive per tonne, well understood by operators, and effective on a wide range of raw waters. The trade-offs are a narrower effective pH window (roughly 5.5–7 for many waters), higher sludge generation than PAC, and a greater impact on alkalinity — which may mean adding lime or soda ash to compensate.

Ferric salts (ferric sulphate and ferric chloride)

Iron-based coagulants form dense, fast-settling flocs and work well at low temperature and on waters containing sulphide or colour. They are often used where phosphorus removal matters. The disadvantages are corrosiveness, the need for acid-resistant dosing equipment, and residual iron colour if overdosed.

What to compare before you choose

• Active content. PAC is sold on Al₂O₃ content and basicity; alum on Al₂O₃ equivalent. Compare cost per kilogram of active, not per tonne of product.
• Working pH range. Ask for the supplier’s recommended range and compare it with your actual raw-water pH through the year.
• Sludge volume and dewatering. A lower dose does not always mean less sludge; floc density matters.
• Alkalinity consumption. If your water is poorly buffered, factor in the cost of the alkali you will need to add back.
• Dosing equipment. Powder PAC needs make-down tanks; liquid PAC and ferric salts need corrosion-resistant pumps and lines.
• Residual limits. Check the aluminium or iron residual your discharge consent or process allows.

Running a jar test properly

Jar tests remain the fastest way to compare coagulants on your own water. Use the same raw water sample, the same rapid-mix and slow-mix speeds and times, and the same settling period for every candidate. Increase dose in fixed steps and record turbidity, pH and settled sludge volume at each step — a single-point test tells you very little.

Practical selection checklist

1. Define the target: turbidity, colour, phosphorus or organic load.
2. Establish the raw-water variability across a full season, not a single sample.
3. Shortlist two or three coagulants and compare on a cost-per-cubic-metre basis, including any pH correction chemical.
4. Confirm the specification you need and ask for the COA and SDS for the exact grade offered.
5. Pilot at full scale for at least a week before committing to contract volumes.

Where we can help

King Chemical supplies PAC, aluminium sulphate, ferric sulphate, polyacrylamide flocculants and disinfection chemicals to water treatment operators in Thailand. If you send us your raw-water analysis and target, we can confirm the grades we hold and the documentation available for your evaluation.