A jar test is the cheapest way to stop guessing at chemical doses. In about an hour, with six beakers and a stirrer, your plant team can compare coagulants, find the dose that gives clear water and see how much sludge each option makes. Most plants that overspend on coagulant have simply never run one, or ran it once years ago when the influent was different.
What you need
- Six 1-litre beakers (or a gang stirrer with six paddles)
- A fresh, representative sample of the water or effluent you treat, collected the same day
- Stock solutions of each coagulant you want to compare, for example 1% PAC, 1% alum and 1% ferric chloride, made up in distilled water
- A 0.1% polyelectrolyte solution if you use a flocculant aid
- Syringes or pipettes, a pH meter and, ideally, a turbidity meter
Make the polymer solution at least 30 to 60 minutes before the test so it can fully dissolve. Half-dissolved polymer gives misleading results.
The procedure
- Fill and check: fill each beaker with the same volume of sample and note its pH and turbidity.
- Dose: add increasing doses of one coagulant across the six beakers, for example 10, 20, 30, 40, 50 and 60 mg/L.
- Rapid mix: stir fast (around 100 to 120 rpm) for about one minute so the coagulant spreads instantly.
- Slow mix: drop to a gentle 20 to 40 rpm for 15 to 20 minutes. If you use polymer, add it at the start of this stage.
- Settle: stop the stirrer and let the beakers stand for 20 to 30 minutes.
- Measure: take a sample from just below the surface of each beaker and record turbidity, pH, and colour or COD if those matter for your discharge limits.
Repeat the series with each coagulant you are considering. Plants that run industrial effluent often also repeat it at two or three pH values, because coagulants work best in different pH windows.
Reading the results
Pick the lowest dose that meets your target, not the beaker that looks best. Beyond a certain point extra coagulant does nothing except add sludge and cost, and overdosing can even restabilise the particles and make the water cloudy again.
Also look at the floc. Large, fast-settling floc means your clarifier will cope better with flow surges. Fine, slow floc usually means you need a polymer, a different coagulant or a pH correction.
Note the sludge volume in each beaker after settling. Lower sludge is a real saving, because sludge handling and disposal often cost more than the coagulant.
Turning beaker results into a plant dose
Jar-test doses are a starting point. Set the plant dose close to the jar-test optimum, then fine-tune over a few days while watching outlet turbidity or COD. Re-run the test whenever the influent changes noticeably: a new product line in the factory, the monsoon, or a change of coagulant supplier.
If you are comparing suppliers, test their samples side by side on the same day with the same water, so the comparison is fair.
Chemicals to compare
Our PAC, alum and ferric chloride pages list the grades we stock, and the polyelectrolyte page covers flocculant aids. For background on which coagulant suits which water, see How to Choose a Coagulant for Effluent Treatment.