Jar testing is the standard bench-scale method for selecting coagulants and optimizing their dose before any change is made at the treatment plant. A gang stirrer mixes a series of raw-water samples dosed with different chemicals or dose levels under controlled rapid-mix, flocculation, and settling conditions; the jar with the best settled-water quality at the lowest workable dose points to the plant setting. Done well, a jar test answers in an afternoon what plant-scale trial and error would take weeks to resolve.
Equipment and Sample Handling
The core equipment is a multi-position gang stirrer with matched paddles and identical jars — square two-litre jars reduce vortexing and give more reproducible mixing than round beakers. Collect raw water as close to the plant intake as possible and test it fresh: particle and organic character changes on standing, so run tests within 24 hours of collection, and always at the actual raw-water temperature. A refrigerated sample warmed back to room temperature will mislead you about winter performance. For wastewater or highly variable sources, collect a composite sample over the event you want to represent rather than a single grab.
Prepare fresh working dilutions of each coagulant on the day of the test — typically around 0.1–1% strength, so that dose volumes are large enough to pipette accurately. Dilute metal-salt coagulants such as ferric chloride and PAC age quickly at high dilution as the hydrolysis chemistry proceeds, so discard working solutions after the session. Record the product concentration used, so that bench doses convert correctly to plant feed rates later. Label every jar and log its dose before mixing starts — once flocs form, identically hazy jars are easy to confuse.
Step-by-Step Procedure
- Measure and record baseline raw-water quality: turbidity, pH, alkalinity, temperature, and color or UV254 where relevant.
- Fill all jars with the same measured sample volume and set them on the stirrer.
- Dose each jar with its planned coagulant amount — bracket the expected optimum, for example five doses stepped across the likely range plus one undosed control.
- Rapid mix at high speed for about one minute to disperse the coagulant.
- Reduce to slow mixing for 15–20 minutes to flocculate; note the time to first visible floc in each jar.
- Stop mixing and allow 15–30 minutes of undisturbed settling.
- Draw supernatant from the same depth in every jar and measure settled turbidity and pH.
- Repeat the best-performing jar in a second run to confirm the result before recommending a plant change.
What to Measure and Record
Settled turbidity ranks the jars, but the supporting data explains them. Record final pH — a jar whose pH crashed below the coagulant effective range failed for a fixable reason. Note floc character: large, dense, fast-settling floc suggests a workable dose; fine pinpoint floc that will not settle suggests underdosing or poor pH; a haze that reappears at higher doses signals restabilization. Where the goal is organics or color removal, measure UV254 or color on the supernatant as well, since the best turbidity jar is not always the best organics jar. Track temperature too — results shift with it, and a log of test temperature explains seasonal disagreements between sessions.
Plot settled turbidity against dose. The optimum is usually the knee of the curve — the dose beyond which extra chemical buys little further clarity. Choosing the knee rather than the absolute minimum saves chemical, limits sludge production, and reduces residual metal carryover. If two adjacent doses perform equally, prefer the lower one and re-test around it at finer dose steps.
Common Mistakes That Invalidate Results
- Testing stale or re-warmed samples instead of fresh water at plant temperature.
- Mixing at the wrong intensity — too fast shears floc, too slow prevents collisions; keep speeds consistent between sessions.
- Using aged, diluted coagulant working solutions instead of freshly prepared ones.
- Sampling supernatant from different depths or at different times from jar to jar.
- Testing a single dose per chemical and calling the comparison decided.
- Ignoring the pH shift each coagulant causes instead of recording it.
From Bench to Plant
A jar test identifies the promising dose window; the plant confirms it. Full-scale mixing energy, retention time, and hydraulic short-circuiting all differ from the bench, so implement jar results as a starting setpoint, then trim against settled and filtered water turbidity. Repeat the exercise whenever the raw water changes character — seasonally at minimum, and after any storm event or source change that shifts turbidity or organics significantly. Keep a simple log linking each jar test to the raw-water conditions of the day; over a year it becomes a dosing map of your source. Where the plant layout allows it, trial a new dose on one parallel treatment train first before rolling it out plant-wide.
Support for Your Jar Testing Program
CHIMI ART supplies the coagulants and flocculants most jar test programs compare — ferric chloride, liquid and powder PAC grades, and polyacrylamide flocculants — and our technical team can help design the dose matrix, interpret the results, and plan the plant-scale trial. Contact us to arrange samples and dosing support for your water.