Polymer Dosing Calculation: From Jar Test Dose to Pump Flow

A polymer dosing calculation comes down to three lines: polymer mass per hour equals the flow you treat times the dose; solution flow equals that mass divided by the make-down concentration; pump capacity equals solution flow times a safety margin. The math is easy. What goes wrong on site is units, and confusion between dry product, emulsion product and active polymer. This guide is for plant operators, process engineers and buyers sizing a polymer feed pump. The dose itself must come from a jar test or a trial on the dewatering machine; every number below is an assumed example, not a recommendation.

The three numbers in any polymer dosing calculation

Dose. For clarification and flocculation it is usually expressed in mg/L, which is the same as ppm by weight in water. For dewatering it is usually kg of polymer per tonne of dry solids (kg/t DS), or lb per dry ton in US plants. The two bases are not interchangeable; our article on typical polymer dosage for flocculation vs dewatering explains why.

Solution concentration. The strength of the mother solution after make-down, normally a fraction of a percent. A 0.1% solution is 1 g/L, which is also 1 kg/m³. Keep that conversion in your head and most of the arithmetic disappears.

Solution flow. The volume of polymer solution the pump has to deliver, in L/h. This is what you size the pump on, not the polymer mass.

Polymer dosing calculation for clarifiers

For a clarifier the polymer dosing calculation starts with mass: polymer mass (g/h) = water flow (m³/h) × dose (mg/L). Because 1 mg/L equals 1 g/m³, you simply multiply.

Assume a clarifier with a peak flow of 200 m³/h and a jar test dose of 2 mg/L:

1. Polymer mass = 200 × 2 = 400 g/h, or 0.4 kg/h.
2. Assume a 0.2% solution (2 g/L). Solution flow = 400 ÷ 2 = 200 L/h.
3. Add roughly 30% margin: size the pump for about 260 L/h and pick the next model up.

Change the solution strength and the pump changes with it. The same 0.4 kg/h made down at 0.1% needs 400 L/h. Fix the concentration before you buy the pump.

Polymer dosing calculation for sludge dewatering

Dewatering adds one step: convert wet sludge to dry solids. Dry solids (t/h) = sludge flow (m³/h) × solids fraction, taking wet sludge density as roughly 1 t/m³. Then polymer mass (kg/h) = dry solids × dose (kg/t DS).

Assume 20 m³/h of sludge at 2% solids and a trial dose of 4 kg/t DS. Dry solids = 0.4 t/h, polymer = 1.6 kg/h. At an assumed 0.1% solution, that is 1.6 m³/h of solution. At this scale many plants use progressive cavity pumps; sizing a polymer pump for sludge dewatering covers that choice.

Use measured solids, not the design figure. Feed solids drift with season and wasting practice, and a polymer dosing calculation built on the wrong solids value can be off by a factor of two. Size the pump on the high readings and budget chemical on the average.

Emulsion polymer and active content in the polymer dosing calculation

If you buy liquid emulsion polymer instead of powder, check what the dose refers to. Some jar tests report neat product, others report active polymer. If the dose is in active terms, divide by the active fraction from the supplier's data sheet to get neat product. Get this wrong and the polymer dosing calculation is off by the same ratio.

Dilution water: before or after the pump?

Many skids add post-dilution water downstream of the pump to thin the solution before injection. Because that water joins after the pump, size the metering pump on mother solution only and set the dilution water on its own rotameter. If the design dilutes before the pump, the pump must handle the total diluted flow and gets much bigger. Check the P&ID before you calculate.

Batch size and run time

The same polymer dosing calculation sizes your make-down batch. Polymer per batch (kg) = usable tank volume (m³) × concentration (kg/m³). An assumed 2 m³ tank at 0.2% takes 4 kg per batch, which lasts about 10 hours at 0.4 kg/h. Remember to include aging time: freshly mixed solution is not ready to pump. If operators would be mixing several batches a day, a continuous three-chamber unit from our dosing skid range usually pays for itself in avoided mistakes.

Check the polymer dosing calculation after start-up

Back-calculate once the pump runs: actual polymer mass = calibrated pump flow × solution concentration; divide by the treated flow to get the real dose and compare it with the jar test. If they disagree, look at three things: how the powder was weighed, whether the solution fully dissolved, and the pump's true output on viscous solution. Nameplate capacity is rated on water, so draw down a calibration column. The same back-calculation approach appears in our PAC dosage calculation guide. For pump hardware, browse mechanical diaphragm metering pumps and keep the suction line generous and flooded.

Conclusion

A sound polymer dosing calculation is dose times flow, divided by solution concentration, times a margin. Use mg/L for clarifiers and kg/t DS for dewatering, convert wet sludge to dry solids, settle where dilution happens, and confirm everything with a calibration test after start-up. Never copy another plant's dose. Send us your flow, trial dose, solution strength and injection pressure and contact our engineers to check pump size and turndown.

FAQ

Is 0.1% the same as 1,000 ppm?

Yes. A 0.1% solution is 1 g/L, or roughly 1,000 mg/L. Writing doses in g/L on drawings avoids factor-of-ten mistakes.

Should the polymer dosing calculation use active or neat product?

Use whatever basis the trial used. If the jar test weighed neat product, calculate in neat product; if it reported active polymer, convert with the active fraction.

Can I use a dose from a similar plant?

Only as a starting point. Water chemistry, sludge type and polymer grade all shift the dose, so run your own trial.

The pump runs at a very low stroke setting. What now?

The pump is oversized. Make down a weaker solution to raise the solution flow, or drop one pump size so normal duty sits mid-range.

Do I need to recalculate in winter?

It is worth checking. Cold water changes floc formation, the dose may rise, and the solution gets thicker, so recalibrate the pump.