A jar test establishes coagulant and flocculant dose on the wastewater that actually reaches the treatment plant, not on a sample that was made clean. The usable result is the dose that gives the lowest turbidity at working pH, with floc large enough to settle or float, and a sludge volume the plant can still handle.
Why the dose cannot come from a catalogue
Coagulants work by neutralising the charge on colloidal particles so they can stick together. Underdose and the charge is not neutralised, so turbidity stays high. Overdose and the charge reverses, restabilising the particles — the symptom looks identical, so operators add more and make it worse. Every dose curve has a minimum, and that minimum can only be found by testing the real water.
Flocculants play a different role. These polymers bridge small floc into larger aggregates, and their dose is usually far smaller than the coagulant — often a fraction of a milligram per litre up to a few mg/L. Because the dose is small, failure is more often caused by how the stock solution is prepared and added than by the choice of polymer.
A repeatable procedure
- Take a representative sample, hold it near plant temperature, and record pH, turbidity, TSS and colour. A sample left standing for hours has already changed and no longer represents the stream.
- Prepare coagulant and flocculant stock solutions at known concentrations, and record volumes so each dose can be converted to mg/L.
- Fill the jars to equal volume, mix rapidly to disperse the coagulant, then mix slowly so floc grows without being sheared.
- When mixing stops, watch floc formation, size and settling time, then measure supernatant turbidity at the same depth in every jar.
- Repeat across a few pH values where the wastewater is poorly buffered, because each coagulant has its own effective window.
- Once the best coagulant dose is found, test flocculant against that fixed dose and compare with no flocculant at all.
- Measure the sludge volume produced and estimate dewatering load; the clearest water is not automatically the cheapest to treat.
Rapid mixing that runs too long, or slow mixing that is too vigorous, breaks floc. If results across jars look random, check mixer speed and addition technique before concluding the dose is wrong.
Decision table
| Decision | What to check | Risk if skipped |
|---|---|---|
| Coagulant type | Water pH, alkalinity, colour, oil content, sludge handling capability | Coagulant operates outside the water’s pH window, so dose climbs |
| Coagulant dose | Turbidity-versus-dose curve at working pH | Overdose reverses charge and obscures the result |
| Flocculant type | Particle charge, floc size required, separation method | Wrong charge means floc never forms |
| Preparation method | Stock concentration, solution age, addition rate | Polymer gels before reaching the tank and dose is not what was intended |
| Injection point and method | Mixing location, mixing energy, retention time | Floc breaks up before reaching the separation zone |
Technical checklist
- Record the wastewater source and its daily variation; one sample does not represent the whole day.
- Measure pH, alkalinity, turbidity, TSS, colour and oil before testing, not after.
- Use the same mixer and the same speed across all jars so the comparison is valid.
- Log every dose in actual mg/L rather than millilitres of stock, so it can be transferred to the plant.
- Test two or three pH values where the wastewater is unstable.
- Measure turbidity at the same depth and settling time for every jar.
- Keep dose, result and water condition records as the basis for the next adjustment.
- Verify lab results against plant performance after the dose is applied; large gaps usually come from injection point or mixing, not from the dose.
Moving the result into the plant
A dose that wins the jar test does not automatically win in the plant. Three things usually explain the gap: coagulant added where mixing is too weak, flocculant injected too close to a pump so it is over-sheared, and retention time in the separation zone too short for the floc that formed. Any dose change should therefore come with a check of injection point and mixing energy.
For fluctuating load, a fixed dose is almost always a poor compromise. What holds up better is a dose band with measurement triggers — influent turbidity or pH, for example — so adjustment follows actual conditions.
Next steps
- Coagulants for wastewater treatment for PAC, ACH and POLYDADMAC options.
- Flocculants for clarification for anionic, cationic and nonionic polymers.
- Dosing pumps for feed that can be calibrated at small doses.
- DAF for light floc separation where flotation suits separation better than settling.
- Wastewater physical separation for the upstream stage before coagulation.
To build a program, bring the jar test results, several days of wastewater analysis, the plant flow scheme and the effluent target. Those decide the coagulant and flocculant selection, not the product name.