PAC Dosage Calculation: From Jar Test Dose to Metering Pump Flow

PAC dosage calculation comes down to three lines of arithmetic. Chemical feed (kg/h) equals flow (m³/h) times dose (mg/L) divided by 1000. Solution feed (L/h) equals chemical feed divided by solution strength and density. Pump capacity equals solution feed times a safety margin. The only number you cannot calculate is the dose itself, which has to come from a jar test. This article is for operators and engineers sizing polyaluminium chloride (PAC) feed systems in water and wastewater plants, and it focuses on where each number comes from and where people usually go wrong.

Step 1 of the PAC dosage calculation: chemical mass per hour

Chemical feed (kg/h) = flow (m³/h) × dose (mg/L) ÷ 1000.

One mg/L is one gram per cubic meter, so dividing by 1000 converts grams to kilograms. In water, ppm and mg/L are effectively the same thing.

Base the PAC dosage calculation on design peak flow, not the daily average. Municipal sewage has morning and evening peaks, and a pump sized on average flow runs flat out at peak and still falls short.

Set the dose by jar test. The same plant can need very different doses in wet and dry seasons or when influent solids swing. Our guide on finding the right PAC dosage range covers how to run the test.

Product basis or active basis?

This is the most common error in any PAC dosage calculation. PAC strength is usually expressed as aluminium oxide content, and it varies between solid and liquid grades and between suppliers. If the jar test dose was recorded as commercial product, use it directly. If the lab reports the dose as active content, divide by the product's active fraction to get the amount of product to weigh out.

Always confirm whether the mg/L figure is product or active basis. Mixing the two can throw the result off several times over, and the operator sees a plant that is "dosing by the formula" and still not clarifying.

Liquid PAC adds one more step. It is already a solution, the label strength is a mass fraction, and converting to volume needs the density. If the data sheet does not give it, ask the supplier.

Step 2 of the PAC dosage calculation: solution volume

The pump moves solution, not powder:

Solution feed (L/h) = chemical feed (kg/h) ÷ solution strength (as a fraction) ÷ solution density (kg/L)

Dilute solutions are close to 1 kg/L, which is fine for a first pass. Stronger batches are denser, so use a measured value when accuracy matters.

Solution strength is a trade-off. Too dilute and the tank, the pump and the batching workload all grow. Too strong and the solution gets viscous, crystallizes in check valves, and the pump runs near the bottom of its stroke range where accuracy suffers. PAC dosing pump selection discusses dilution in more detail.

Step 3: turn the PAC dosage calculation into pump size

Pump capacity = solution feed × margin. A common rule is about 1.3 times the maximum solution feed.

More margin is not always better. On a mechanical diaphragm pump with manual stroke adjustment, the usable range is often around 20-100% of rated flow, so check the specific manual. An oversized pump spends its life near the low end. Aim for normal flow in the middle of the range with headroom for peaks.

Check pressure too. If the injection point is on a pressurized main, the pump rating must exceed line pressure, and a back pressure valve may be needed.

Worked PAC dosage calculation (hypothetical numbers)

The figures below are assumptions for illustration only. They do not describe a real project, and your dose must come from your own jar test.

1. Assume peak flow of 100 m³/h and a jar test dose of 50 mg/L as product.
2. Chemical feed = 100 × 50 ÷ 1000 = 5 kg/h.
3. Assume a 10% solution at roughly 1 kg/L. Solution feed = 5 ÷ 0.10 ÷ 1 = 50 L/h.
4. Pump capacity = 50 × 1.3 = 65 L/h, then pick the next model up.
5. Daily use = 5 × 24 = 120 kg of product, about 1200 L of solution per day, which sets tank size and batch frequency.

Once the PAC dosage calculation is done, check two things: where the pump stroke sits at normal flow, and how long one batch lasts. A crew batching three times a day will eventually miss one.

What the formula does not cover

The formula gives a design value. When influent load changes, the dose changes with it. For plants with large flow swings, a metering pump paced by a 4-20mA flow signal turns the flow term into a live value.

Winter deserves attention. Cold water coagulates less readily and often needs a higher dose, and cold solution is more viscous and harder to draw up. For conversions that apply to other chemicals, see the general chemical dosing calculation for wastewater. What makes PAC dosage calculation different is mainly the active content question and the choice of solution strength.

Safety note: PAC solution is acidic. Wear gloves and eye protection when batching and rinse any splashes with clean water.

Conclusion

The PAC dosage calculation itself is simple: flow times dose gives mass, divide by solution strength for volume, multiply by a margin for pump size. The result depends on three inputs: peak flow, a jar test dose on a clear product or active basis, and a solution strength that balances tank size against pump accuracy. PAC is usually fed with a chemical metering pump, and multi-pump systems with tanks are available as a packaged dosing skid. Send us your flow, jar test dose and injection pressure, and contact our engineers to check the pump size.

FAQ

Can I use another plant's dose in my PAC dosage calculation?

Only as a starting point for your jar test. Water quality differs, and so does the dose.

Does the PAC dosage calculation work for both solid and liquid PAC?

Yes, but the inputs differ. Solid PAC uses the batch strength you mix; liquid PAC needs its active content and density.

What solution strength should I mix?

There is no single answer. Balance tank size, batch frequency and the pump's low end, and avoid batches strong enough to crystallize.

The calculated pump flow is tiny and no model fits. What now?

Dilute the batch so the pump runs mid-range. That is steadier than forcing a very small pump.

Pump setting and calculation do not match. Where do I start?

Measure actual output with a calibration column first, then confirm the batch strength, and only then question the dose.