PAC and PAM Ratio: Find It With a Jar Test, Then Tune It on Site

The PAC and PAM ratio is not a number you can copy from another plant. It is simply what you get after finding the best dose of each chemical separately. When the water changes, the PAC dose changes, the polymer dose changes, and the ratio moves with them. The reliable method is a two-stage jar test: fix the PAC (polyaluminium chloride) dose first, then find the PAM (polyacrylamide) dose on top of it, then fine-tune on site against effluent quality. This article is for wastewater and water treatment operators and commissioning engineers who need to turn a jar test into two pump settings.

Why a fixed PAC and PAM ratio does not work

PAC is a coagulant. It neutralizes particle charge so fine solids destabilize. PAM is a flocculant. It bridges those small flocs into large, fast-settling ones. The two do different jobs, so their doses depend on different things.

PAC demand follows turbidity, suspended solids, alkalinity and temperature. PAM demand follows the floc size left after coagulation, the settling target, and the polymer's charge type and molecular weight. Ratios quoted online rarely say what water, which products or what basis they used, which is why they so often fail in practice.

Stage 1 of setting the PAC and PAM ratio: PAC alone

Leave the polymer out. Fill several beakers with the same sample, add PAC in steps, mix rapidly to disperse it, then slowly to grow floc, and let it settle.

Watch three things: how early floc appears, how clear the supernatant is, and how fast solids settle. The range from the first clear improvement to the point where extra PAC adds nothing is your working range. Our guide on finding the right PAC dosage range covers step sizes and common test errors.

Do not add polymer to rescue a poor result at this stage. If PAC is short, extra PAM only wraps part of the fines and the water stays cloudy.

Stage 2: hold PAC, step the polymer to set the PAC and PAM ratio

Run a fresh set of beakers, each with the chosen PAC dose. After rapid mixing, switch to slow mixing and add PAM in steps. Polymer must go in after the coagulant; reversing the order performs much worse, as explained in PAC and PAM dosing sequence.

At the right dose, floc grows large and dense, settles quickly and leaves clear water. Overdose shows as sticky, loose floc, stringy water and a haze of fine particles in the supernatant. Pick the lowest dose that gives the good result.

Stage 3: a small cross-check

The two chemicals interact. A little more polymer sometimes lets you cut PAC. Once you have a pair of doses, run a small matrix: PAC at the chosen value and one step either side, PAM the same, and compare the combinations.

This step is often skipped, yet it answers a practical question: which PAC and PAM ratio gives equal performance at the lowest total chemical use. Record water temperature, sample time and chemical batch so the next test is comparable.

Calculate the PAC and PAM ratio on product mass

This is where most sites go wrong. Express the PAC and PAM ratio as grams of each product per cubic meter of water, then compare the two.

Many crews compare the flow rates of the two pumps instead. That is misleading. PAC solution is usually batched fairly strong, while polymer is batched very dilute so it dissolves fully. The solution flow ratio can differ from the product mass ratio by an order of magnitude, and it shifts every time someone changes a batch strength even though the real ratio has not changed.

Polymer dose also depends on duty. Clarification is dosed per cubic meter of water; sludge dewatering is usually dosed per unit of dry solids. Do not compare the two, as covered in polymer dosage ranges.

From jar to plant: turning the PAC and PAM ratio into pump settings

Convert each dose to chemical feed per hour, divide by its batch strength to get solution flow, and set each metering pump accordingly.

Plant results rarely match the beaker exactly, because mixing energy and reaction time differ. When adjusting:

1. Change one chemical at a time and hold the other constant, or you will not know which change worked.
2. Wait at least one full retention time before judging the effluent.
3. Log pump settings, plant flow and effluent turbidity for every change.

Where flow varies a lot, pace both pumps from a 4-20mA flow signal so the PAC and PAM ratio holds as flow swings.

When to retest

Retest when influent quality shifts, when you change chemical supplier or batch, at seasonal temperature changes, or when effluent deteriorates and the pumps check out fine. Turning both pumps up by feel is not a substitute.

Safety note: spilled polymer makes floors extremely slippery, so hose down the batching area. PAC solution is acidic; wear gloves and eye protection.

Conclusion

The PAC and PAM ratio is found, not assumed. Test PAC alone, hold it and step the polymer, then cross-check a small matrix, and always express the ratio on product mass. On site, adjust one chemical at a time, wait a full retention time, and retest when the water changes. Each chemical normally gets its own chemical metering pump, or you can choose a packaged dosing skid with tanks included. Send flow, jar test results and batch strengths and contact our engineers to size both pumps.

FAQ

Is there a typical PAC and PAM ratio to start from?

Only as a jar test starting point. Water and product differences make the final ratio site specific.

How often should I run a jar test?

Spot-check regularly when water is stable, and retest whenever quality, season or chemical batch changes.

Can I just compare the two pump flow rates?

No. Convert both to product mass first, because batch strength changes distort the flow ratio.

Will a higher polymer share in the PAC and PAM ratio cut PAC use?

Within limits, sometimes. Too much polymer makes floc sticky and effluent worse, so confirm with a cross-check.

Plant results differ from the jar test. What now?

Verify actual pump output and batch strength, then check mixing and reaction time, and only then change doses.