Guide

How to run a jar test before you order a coagulant

27 October 2026 · 4 min read

The right coagulant and dose depend on your raw water, so the cheapest way to avoid a wrong order is a jar test. In about an hour it shows how fast floc forms, how clear the water gets and how much sludge you will handle. You can test samples side by side before you commit to a container.

The procedure below follows the common approach of the ASTM D2035 standard practice for jar tests. Adapt the speeds and times to match your plant's mixing and settling.

What you need

• A jar tester with four to six paddles, or several magnetic stirrers
• 1-litre beakers, one per dose
• Syringes or pipettes for dosing
• A turbidity meter and a pH meter (a colour meter if colour matters)
• A timer
• About 10 litres of fresh raw water, taken from the point where the coagulant is dosed in your plant
• The coagulants you want to compare, for example PAC, PFS and alum

Step 1: Make a stock solution

Dose from a dilute stock solution so small doses are easy to measure.

For powders, a 1% solution is convenient: dissolve 10 g of product in water and make it up to 1 litre. Then:

1 mL of a 1% solution in a 1-litre jar = 10 mg/L

So 2 mL gives 20 mg/L, 3 mL gives 30 mg/L, and so on. For liquid products, check the active content on the TDS and adjust. Make the stock solution on the day of the test, because some coagulant solutions change over time.

Step 2: Choose a dose range

Fill each beaker with 1 litre of raw water. Start with a wide series, for example 10, 20, 30, 40, 50 and 60 mg/L. If you already know your current dose, centre the series around it. After the first run you can test a narrower range around the best result.

Step 3: Rapid mix (about 1 minute)

Set the paddles to a fast speed, typically around 100–120 rpm. Add the coagulant doses to all jars at the same time if you can, and mix for about one minute. This spreads the coagulant evenly before floc starts to form.

If you plan to use a polymer such as PAM as a flocculant aid, add it at the end of the rapid mix or at the start of the slow mix, at a small dose.

Step 4: Slow mix (about 20 minutes)

Reduce the speed to a gentle stir, typically 20–40 rpm, for about 20 minutes. This lets small particles collide and grow into floc. Write down when floc first appears in each jar and how large it gets.

Step 5: Settle (15–30 minutes)

Stop the paddles and let the jars stand. Watch how quickly the floc settles. After the settling time, draw a sample from the same depth in every jar, just below the surface, without disturbing the sludge.

Step 6: Measure and compare

For each jar, record:

How to read the results

• Pick the lowest dose that reaches your target, not the dose with the very lowest turbidity.
• If turbidity goes up again at higher doses, you have overdosed. Too much coagulant can re-stabilise the particles.
• Check that the pH after dosing is still in your required range.

Step 7: Compare cost per cubic metre

Price per tonne is only half the story. A dose in mg/L is the same as grams per cubic metre, so:

Daily use (kg) = dose (mg/L) × flow (m³/day) ÷ 1,000

Example: at 30 mg/L and 1,000 m³/day you use 30 kg of product a day. Multiply by the price per kg for each candidate and compare.

Tips for reliable results

• Test the water within a few hours of sampling and at a temperature close to the plant's.
• Use the same mixing speeds and times for every product, or the comparison is not fair.
• Repeat the best two or three results with narrower dose steps.
• Treat the jar test as a guide: confirm the result with a short trial in the plant before switching fully.

Get samples for your jar test

We send samples of about 1 kg free of charge; you pay only the courier. Tell us your water type, current chemical and target, and we will suggest two or three grades to compare, with the TDS and SDS for each grade.

Request samples · Not sure which coagulants to test? Read PAC, PFS or alum? How to choose a coagulant.