Coagulation is the first and most important chemical step in most water and wastewater treatment plants. For decades the standard coagulant was alum (aluminium sulfate). Today many plants are switching to aluminum chlorohydrate (ACH). This guide explains how the two compare, when switching makes sense and how to test it in your own plant.
What is the difference?
Both products work the same basic way: positively charged aluminium species neutralise the negative charge on suspended particles, colour and organic matter, so they clump together into floc that can be settled or filtered out. The difference is in how the aluminium is delivered.
- Alum (Al₂(SO₄)₃) is an acidic salt. When it dissolves in water it releases acidity, which consumes alkalinity and lowers pH.
- ACH (Al₂(OH)₅Cl) is a highly pre-neutralised (high-basicity) aluminium polymer. Much of the hydroxide the coagulant needs is already built in, so it has very little effect on pH.
Side-by-side comparison
| Alum (aluminium sulfate) | Aluminum chlorohydrate (ACH) | |
|---|---|---|
| Typical aluminium content | Liquid around 8% Al₂O₃; solid around 17% | Liquid around 23% Al₂O₃; powder around 46% |
| Basicity | None | Very high, typically around 83% |
| Effect on pH and alkalinity | Lowers pH, consumes alkalinity | Very small |
| Need for pH correction | Often needs caustic soda or lime | Usually little or none |
| Effective pH range | Narrower | Wider |
| Cold-water performance | Floc forms slowly | Good |
| Sludge volume | Higher | Lower |
| Price per kg | Lower | Higher |
| Dose needed | Higher | Lower |
Exact figures depend on the product grade and supplier, so always check the Certificate of Analysis.
Advantages of switching to ACH
- Less chemical handling: lower doses and often no separate pH-correction chemical.
- Less sludge: less material to dewater and dispose of, which cuts disposal costs.
- Better performance in cold or low-alkalinity water, where alum struggles.
- More stable treated-water pH, which can reduce corrosion in the distribution system.
- No added sulfate in the treated water.
When alum may still be the better choice
- Your raw water has plenty of natural alkalinity and the pH drop is not a problem.
- You need to remove a lot of natural organic matter (colour) and your plant runs best at a lower coagulation pH.
- Budget is tight and the plant already doses lime or caustic for other reasons.
How to compare them: run a jar test
- Collect a fresh sample of your raw water and note its temperature, pH, turbidity and alkalinity.
- Set up a series of jars with increasing doses of alum, and a second series with increasing doses of ACH. ACH doses are usually lower, so start lower.
- Rapid-mix for about one minute, slow-mix for 15–20 minutes, then let the jars settle.
- Compare settled turbidity, colour, final pH, floc size and sludge volume.
- Calculate the total treatment cost for each option: coagulant cost, plus any pH-correction chemical, plus sludge disposal.
Avoid overdosing ACH. Too much can restabilise particles and make results worse, so testing a range of doses matters.
Practical tips when switching
- Check that your tanks, pumps and lines are compatible with ACH (HDPE, FRP and PVC are typical), and flush out the old alum before switching.
- Do not mix ACH and alum in the same tank.
- Reduce or stop pH-correction dosing gradually while monitoring final pH.
- For drinking water, use an ACH grade certified to the standard required in your country.
- Keep disinfection, for example with sodium hypochlorite, separate from coagulant dosing points.
Get a quote for ACH
Saeby Chemical supplies aluminum chlorohydrate in liquid and powder form for water and wastewater treatment, with a Certificate of Analysis and SDS for every batch. Contact us with your plant flow, current coagulant and destination, and we will help you size a trial quantity.
Related guides: How to Store and Handle Sodium Hypochlorite Safely · Caustic Soda Flakes vs Pearls vs Liquid
