Jar Test Floc Evaluation: How to Judge PAC and PAM Flocculation by Floc and Effluent

A good jar test floc evaluation answers one question: is the coagulant and polymer program working? Floc that forms quickly, grows large and dense, settles fast and leaves a clear supernatant means the dose is close. Fine pin floc that hangs in the water, or big fluffy floc that carries over into the effluent, means you need to look at dose and dosing order. This guide is for operators at wastewater plants and water treatment works who need a routine way to judge PAC and PAM performance in the jar and in the basins.

What floc is and what each chemical does

Floc is the cluster of suspended particles that the chemicals pull together. PAC, polyaluminium chloride, is the coagulant. It neutralizes the charge on fine particles so they can come together as small pin floc. PAM, polyacrylamide, is the flocculant. Its long molecular chains bridge those small particles into larger, stronger flocs. Because the jobs are different, a jar test floc evaluation should judge them separately: PAC is about whether floc forms at all, PAM is about whether it grows and holds together.

Running a jar test floc evaluation

A jar test means filling several beakers with raw water, dosing each at a different rate, mixing, settling and comparing results. Before changing anything at full scale, run one. During a jar test floc evaluation, watch four things:

1. Time to first floc. After PAC and rapid mixing, how soon do fine particles appear?
2. Size and density. After PAM and slow mixing, are the flocs firm granules or loose cotton-like tufts?
3. Settling speed. After mixing stops, do flocs drop to the bottom quickly or hang mid-jar?
4. Supernatant clarity. After settling, how clear is the top layer? Measure turbidity if you can.

The jar with dense floc, fast settling and the clearest supernatant gives your starting dose for the plant. Water quality varies too much between sites to copy another plant's numbers, so every dose should come from your own jar test floc evaluation.

Making jar test floc evaluation results comparable

One quick look is easy to misread. Sample at the same times and locations each shift: one cup from the end of the flocculation basin, one from the clarifier weir. Use the same clear container, stand it in the same light, and let it settle for a few minutes before judging. Record raw water turbidity, plant flow, both pump settings and the measured pump output on one line. After a week or two the log shows which way the dose needs to move when flow or water quality changes, and shift handovers can rely on data rather than "the water looks bad today".

Three places to check in the plant

Rapid mix and flocculation basin

Downstream of the PAC injection point, the water should quickly turn slightly grainy. By the end of the flocculation basin, flocs should be clearly larger. If they are still fine and scattered at the outlet, PAC is usually short or mixing is poor.

Clarifier

Apply the same eye you use in a jar test floc evaluation here. A sharp sludge blanket interface and clear water above the tube settlers are what you want. Floc drifting above the tubes or spilling over the weir can mean too much PAM making fluffy floc, or a hydraulic overload that has nothing to do with chemistry.

Effluent turbidity

Effluent turbidity is the final measure. With an online turbidimeter, the trend tells you more than any single reading. Without one, take grab samples every shift and log them alongside raw water turbidity, so the plant result can be checked against your last jar test floc evaluation.

Reading jar test floc evaluation symptoms and adjusting

• Fine floc and cloudy effluent: PAC is low or raw turbidity has risen. Recheck the PAC dose first.
• Floc forms but stays small and settles slowly: PAM is low or not fully dissolved.
• Large, loose, stringy floc with carryover: PAM is high. Cut it back.
• Poor results with polymer added before coagulant: the order is reversed. Coagulate first, then flocculate.

Change only one variable at a time, and wait a full settling cycle before judging. Otherwise you cannot tell which change did what.

Rule out these problems before touching the pump setting

Poor floc is often not a dose problem at all. Cold water slows coagulation and gives finer floc in winter. Polymer that has not aged long enough has not uncoiled, so it behaves as if underdosed. PAC crystals can block a metering pump check valve, so the pump runs but delivers nothing. We have seen operators keep raising the setpoint for days before anyone noticed the dosing pump had stopped pumping.

Before you compare plant results with a jar test floc evaluation, verify actual pump output with a calibration column or graduated cylinder. PAC solution is acidic, so wear gloves and eye protection when making up or cleaning tanks.

For more, read about PAC and PAM dosing sequence, the signs of PAC or polymer overdose and finding the right PAC dosage range. Equipment options are in our dosing skid range and diaphragm metering pumps.

Conclusion

Judge PAC and PAM by floc and effluent: fast, large, dense, quick-settling floc with steady effluent turbidity means the program is right. Start from a jar test floc evaluation, check the mixing zone, clarifier and effluent on every round, change one variable at a time, and confirm the pump is really delivering before changing its setting. If you need to size dosing pumps or a complete skid, send plant flow, injection pressure and solution strength and contact our engineers.

FAQ

Is bigger floc always better?

No. Very large, loose flocs break up in the flow or carry into the effluent. Dense, fast-settling floc is the target.

How can I judge results without a turbidimeter?

Hold a clear cylinder of clarifier effluent up to the light and keep retained samples from each shift for comparison. A portable turbidimeter is still worth buying.

How often should I repeat a jar test floc evaluation?

Whenever raw water changes noticeably, such as in the rainy season, after a source change or with load swings. When water is stable, recheck periodically.

Why did the effluent get cloudier after adding PAM?

Usually too much polymer, or poorly dissolved polymer with gel lumps reaching the water. Reduce the dose and check the make-up system.

Results suddenly got worse. Check the chemical or the pump first?

Check the pump first. Confirm actual output and rule out blocked check valves or lost suction before changing the dose.