Polymer Dosage: Typical Ranges for Flocculation vs Sludge Dewatering

The first question we ask when someone wants a polymer dosage figure is simple: is the polymer going into water or into sludge? In clarification, polymer dosage is expressed per volume of water treated, in mg/L, and it usually sits somewhere between a fraction of a milligram and a few milligrams per litre. In sludge dewatering, it is expressed per tonne of dry solids (kg/t DS), and a few kilograms per tonne is common. Both are rule-of-thumb ranges only. The real number comes from jar tests and trials on your own equipment.

This article is for operators and process engineers at municipal and industrial treatment plants who need to set up or check a polyacrylamide (PAM) feed.

Two units, two jobs: why polymer dosage figures don't transfer

In clarification, polymer is a flocculant aid. A coagulant such as PAC or ferric has already destabilised the particles, and the polymer bridges those small particles into larger, faster-settling floc. Solids concentration is low, so dose follows flow.

Dewatering is a different job. Thickened sludge carries a lot of solids, and the polymer has to bind them into strong flocs that a belt press, screw press or centrifuge can squeeze. The chemical is effectively fed to the solids, so dose follows dry solids load: sludge flow multiplied by measured percent solids.

Mixing up the two polymer dosage units is a common mistake. A mg/L figure applied to a centrifuge, or a kg/t DS figure applied to a clarifier, can be off by a large margin. Charge matters too. Clarification usually uses anionic or non-ionic grades; organic sludge usually needs cationic grades. With the wrong charge, no amount of polymer will work properly.

Typical polymer dosage for clarification and how to jar test it

For settling and clarification, a typical polymer dosage range is a fraction of a mg/L up to a few mg/L. Clean, low-turbidity water with a well-tuned coagulant sits near the bottom; oily industrial wastewater or water with very fine suspended solids sits near the top. Drinking water plants often face regulatory limits on polyacrylamide, so follow local requirements rather than simply pushing the dose up.

A basic jar test procedure:

1. Fill four to six beakers with the same sample and add your established coagulant dose with rapid mixing.
2. Add a different amount of dilute polymer solution to each beaker, from very little to clearly too much.
3. Switch to slow mixing, note how quickly floc forms and how large it gets, then let it settle and compare supernatant clarity.
4. Take the clearest beaker without loose, fluffy floc as your starting dose.

Use polymer from the same batch, made up and aged the same way as on site, or the jar results won't match the plant. For how coagulant and polymer should be sequenced, see PAC and PAM dosing sequence.

Polymer dosage for sludge dewatering

In dewatering, a common rule-of-thumb range is a few kg per tonne of dry solids. Centrifuges generally use more polymer than belt presses, because the floc must survive much higher shear. Sludge with high organic content, or sludge that feels slimy, also needs more.

Bench tests here are mainly for choosing the product, not fixing the final figure. Compare a few grades with different charge density and molecular weight, pick the one that forms the biggest flocs and releases water fastest, then tune on the machine:

• Hold the sludge feed steady and start the polymer pump mid-range.
• Change dose in small steps and give each step time before judging cake dryness and filtrate clarity.
• Cloudy filtrate and wet cake mean too little polymer. Clear filtrate but sticky cake and blinding cloth usually mean too much.

Watch the feed solids. Polymer dosage per tonne may stay the same, but when the thickener changes, dry solids load changes, and the pump output has to follow. Matching pump flow to sludge load is covered in sizing a polymer dosing pump for sludge dewatering.

Solution strength: make-down concentration vs feed concentration

There are two concentrations to keep apart. Make-down concentration is the strength of the aged stock solution, typically in the range of about one to a few grams per litre; packaged make-down units commonly produce 0.3–5 g/L. Feed concentration is what actually enters the process, and stock is often diluted again inline, especially at clarifier injection points, so it disperses quickly.

Solution strength does not change the polymer dosage itself, but it changes everything downstream. Too strong and the solution is viscous, hard for a metering pump to draw, and mixes poorly. Too weak and you need bigger pumps and a bigger make-down system. For wetting, ageing time and "fish eyes", see our PAM and PAC dissolution tips.

Turning polymer dosage into pump flow

Once trials give you a unit dose, the arithmetic is straightforward:

1. Clarification: active polymer (g/h) = water flow (m³/h) × polymer dosage (mg/L).
2. Dewatering: active polymer (kg/h) = sludge flow (m³/h) × fraction solids × dose (kg/t DS), taking wet sludge as roughly 1 t/m³.
3. Solution flow = active polymer per hour ÷ solution concentration, with consistent units.
4. Size the pump for the maximum case with some margin, so normal duty falls in the middle of the turndown range.

Viscosity drives pump choice. Small clarifier duties are fine with a diaphragm metering pump, mounted below the tank level with a generous suction line. High-volume dewatering duties often use a progressing cavity pump. Packaged make-down and feed units are listed under dosing skids.

Signs your polymer dosage is too low or too high

Under-dosing shows up as small, broken floc and hazy settled water, or cloudy filtrate and wet cake at the press. Over-dosing is just as costly: sticky floc that floats, viscous effluent that blinds downstream filters, cloth blinding and cake sticking to rollers. Beyond the optimum, results get worse, not better, and the extra polymer is wasted.

Conclusion

The right polymer dosage depends on the job. Clarification is dosed in mg/L, typically a fraction of a mg/L to a few mg/L; dewatering is dosed in kg/t DS, typically a few kg per tonne, with centrifuges usually higher than belt presses. Treat these as starting points: confirm clarification doses with jar tests and dewatering doses with product screening plus trials on the machine. Then convert to solution flow and size the pump. Send us your flow or sludge load, solids content and solution strength, and contact our engineers to check pump capacity.

FAQ

Can I estimate polymer dosage from TSS or COD?

Only roughly. Higher suspended solids usually need more polymer, but particle character varies so much that a jar test is still required.

Do anionic and cationic polymers need the same dose?

No, and they are not interchangeable. Clarification usually uses anionic grades, organic sludge usually needs cationic. Dose figures only mean something once the grade is right.

Should I increase polymer dosage in winter?

Often, yes. Cold water slows dissolution and flocculation, so floc is smaller. Allow longer ageing time first, then trim the dose based on results.

How long can made-up polymer solution be stored?

Not long. Performance drops as the solution sits. Making up roughly one day's demand at a time is the safer approach.

The pump setting hasn't changed but results keep getting worse. Why?

Check solution strength first, then calibrate the pump's real output. Caked powder feeding unevenly or valves gummed with polymer both reduce the actual dose.