Jar Test: Step-by-Step Coagulation & Dose Optimization
The jar test is the bench-scale method for finding the optimum coagulant and flocculant dose: fill identical 1-litre jars with your raw water, dose an increasing series, rapid-mix (~100–150 rpm for 1 min), slow-mix (~30 rpm for 20–30 min), let the floc settle (30–60 min), then read supernatant turbidity and floc quality. The lowest dose that gives clear water with good, fast-settling floc — before overdosing re-stabilizes the particles — is your optimum.
What is a jar test and why it matters
A jar test simulates the full-scale rapid-mix → flocculation → sedimentation train in a row of beakers on a gang stirrer, so you can compare several coagulant doses side by side. It is the standard way to select the coagulant, set the dose, tune pH and pick a flocculant aid before committing chemical at the plant — the difference between clear water at minimum cost and either persistent turbidity (under-dose) or wasted chemical and charge reversal (over-dose).
Equipment and setup
- Gang stirrer (jar tester) with 4–6 paddles at a common, adjustable speed.
- 4–6 identical 1-litre beakers/jars, filled with the same well-mixed raw-water sample.
- Coagulant stock (e.g. 1 % PAC) and, if used, a dilute PAM flocculant (0.1 %).
- Turbidity meter, pH meter, thermometer, syringes/pipettes, stopwatch.
Step-by-step procedure
- Fill and characterize. Put 1 L of raw water in each jar; record turbidity, pH, temperature and alkalinity. Adjust pH if your program calls for it.
- Dose an increasing series. Add a rising coagulant dose to each jar (for example 5, 10, 15, 20, 30, 40 mg/L), keeping one jar as a blank/control.
- Rapid mix. Immediately mix all jars at ~100–150 rpm for about 1 minute to disperse the coagulant and neutralize charge (the flash-mix step).
- Slow mix (flocculation). Reduce to ~30 rpm for 20–30 minutes to let micro-flocs bridge into large flocs. Add flocculant PAM here if testing one.
- Settle. Stop stirring and let the floc settle 30–60 minutes; note floc appearance at 15 and 30 minutes.
- Sample and measure. Draw supernatant from a fixed depth in each jar and measure turbidity (and color/UV₂₅₄, residual metal, pH as needed).
How to read the results and find the optimum dose
Judge each jar on three things: floc size and formation speed, settling rate, and final supernatant turbidity. Plot residual turbidity vs coagulant dose — the curve falls to a minimum then flattens or rises again. The optimum dose is the lowest dose that reaches minimum turbidity with fast-settling floc; going higher wastes chemical and can cause charge reversal that re-stabilizes colloids. Record the pH at the optimum, since coagulation pH and dose interact.
Common mistakes to avoid
- Uneven rapid mix or dosing jars at different times → results aren't comparable.
- Over-shearing during slow mix or adding PAM too early → the floc is torn apart.
- Sampling from different depths → inconsistent turbidity readings.
- Testing only high doses → you miss the true (lower, cheaper) optimum and the charge-reversal point.
- Ignoring temperature — cold water flocculates slower; run the test at plant temperature.
Typical jar-test parameters
| Stage | Speed | Time | Simulates |
|---|---|---|---|
| Rapid (flash) mix | ~100–150 rpm | ~1 min | Coagulant dispersion & charge neutralization |
| Slow mix (flocculation) | ~30 rpm | 20–30 min | Floc growth / bridging |
| Settling | 0 (quiescent) | 30–60 min | Sedimentation basin |
| Measurement | — | at 15 & 30 min | Supernatant turbidity / floc quality |
Watch
Frequently asked questions
What is a jar test in water treatment?
A jar test is a bench-scale procedure that reproduces the rapid-mix, flocculation and sedimentation steps of a treatment plant in a row of 1-litre beakers on a gang stirrer. By dosing an increasing series of coagulant and comparing floc quality and supernatant turbidity, operators find the optimum coagulant, dose, pH and flocculant before applying chemical at full scale.
What are the mixing speeds and times for a jar test?
A common protocol is rapid (flash) mixing at about 100–150 rpm for 1 minute to disperse the coagulant, then slow mixing at about 30 rpm for 20–30 minutes to grow floc, followed by 30–60 minutes of quiescent settling. Speeds and times are adjusted to match the specific plant being simulated.
How do you determine the optimum coagulant dose from a jar test?
Plot residual supernatant turbidity against coagulant dose. The optimum is the lowest dose that reaches minimum turbidity with good, fast-settling floc. Doses above that waste chemical and can cause charge reversal, which re-stabilizes the particles and raises turbidity again. Record the pH at the optimum dose.
Can you jar test coagulant and flocculant together?
Yes. Add the coagulant (e.g. PAC) at the rapid-mix stage for charge neutralization, then add a dilute polyacrylamide (PAM) flocculant at the slow-mix stage to bridge micro-flocs into larger, faster-settling flocs. Vary each independently across jars to find the best combined program.
Where can I get coagulants and flocculants for jar testing?
VCYCLETECH supplies PAC, PolyDADMAC, polyamine and anionic/cationic/nonionic polyacrylamide for bench testing and full-scale use, factory-direct from China with free samples and a COA on every batch. Email sales@vcycletech.com for samples and technical dosing support.
About the manufacturer
VCYCLETECH is a China-based manufacturer of water treatment chemicals — coagulants, flocculants, antiscalants, scale & corrosion inhibitors, biocides, dispersants and paper chemicals — ISO 9001 / 14001 / 45001 certified, with a COA on every batch and OEM/ODM service. See our quality & certifications.
References
- Flocculation & jar test — Wikipedia
- Coagulation (water treatment) — Wikipedia
- The Jar Test — Mountain Empire Community College
Related: PAC · PAM flocculant · PolyDADMAC · All coagulants & flocculants · Coagulants & flocculants guide