Quick answer: Polymer dose for a filter press is set by jar test, not by rule of thumb, and then trimmed on the press. Start from the typical range for the sludge (biological sludge 3–8 kg active polymer per dry tonne, mixed municipal 2–5, chemical and mineral sludge 0.5–2 or none), find the dose that gives the fastest free drainage and clearest filtrate in the jar, then run the press at that dose and step it down until cycle time or filtrate clarity starts to suffer. Over-dosing is the more common mistake: it costs money, blinds cloth with unreacted polymer and produces a slimy cake. Inorganic coagulants (ferric chloride, alum, lime) are dosed first where the sludge is oily, fine or has poor floc strength, and can halve the polymer requirement.
What conditioning does in a filter press
A filter press separates water from solids by forcing the water through a cake of the solids themselves. That only works if the solids form a porous cake. Fine, gelatinous or colloidal solids, which describes most biological and many chemical sludges, pack into an impermeable layer and stop the flow within minutes. Conditioning chemicals bind those fine particles into larger flocs that leave channels for water to escape, and they strip some of the water that is bound to the particle surfaces.
Two families are used. Inorganic coagulants (ferric chloride, ferric sulphate, alum, polyaluminium chloride, lime) neutralize surface charge and add a mineral skeleton to the cake. Organic polymers (polyacrylamides, mostly cationic for sludge) bridge particles into large, strong flocs at doses a hundred times smaller by weight. Filter presses historically used lime and ferric; today most sludge presses use polymer alone or a low coagulant dose followed by polymer. The choice matters for the cake: lime and ferric add 15–30% to the dry solids sent to disposal, polymer adds almost nothing. Our article on filter cloth for polymer-conditioned biosolids covers the cloth side.
Typical dose ranges
| Sludge | Cationic polymer, kg active per dry tonne | Coagulant (if used) | Notes |
|---|---|---|---|
| Waste activated sludge (WAS) only | 5–10 | Ferric 3–6% of DS or PAC where floc is weak | Hardest case; high charge demand; membrane press helps |
| Mixed primary + WAS, undigested | 3–6 | Usually none | Primary fibre helps |
| Anaerobically digested biosolids | 6–12 | Ferric or lime for struvite and odour control | Higher charge demand after digestion |
| Aerobically digested | 5–9 | Rarely | Fine floc, slow drainage |
| Industrial biological (food, brewery, paper) | 3–8 | PAC 100–300 mg/L where oil or fines present | Wide range; jar test essential |
| Physico-chemical sludge (coagulation/precipitation) | 0.5–2 | Already coagulated upstream | Often needs only a small anionic or non-ionic polymer |
| Metal hydroxide sludge | 0.5–1.5 | None | Anionic polymer common |
| Mineral, tailings, sand washing, ceramics | 0–1 | None or lime for pH | Often filtered with no chemicals; anionic if used |
| Oily sludge (refinery, tank bottoms, DAF float) | 2–5 | Ferric or PAC first, 5–15% of DS | Coagulant breaks emulsion; polymer alone does little |
These are starting points for the jar test, not settings. Two sludges with the same name can differ by a factor of three in polymer demand depending on sludge age, temperature, the presence of filamentous bacteria, and what the plant discharges upstream.
The jar test: how to run it for a filter press
A jar test for a press is different from one for a clarifier because the question is drainage, not settling. The method most plants use:
- Make up the polymer at 0.1–0.2% active in clean water, let it age 30–60 minutes, and confirm the concentration with the supplier’s data.
- Take 500 mL of sludge at the concentration that will feed the press. Test the actual feed, not a diluted or thickened sample.
- Add polymer at five doses spanning the expected range (for example 3, 4, 5, 6, 8 kg/t), mixing each for 10–20 seconds at a steady moderate speed. Over-mixing shears the floc; under-mixing leaves polymer unreacted.
- Pour each sample onto a piece of the press’s own filter cloth stretched over a funnel and time how long it takes to collect 200 mL of filtrate (the free drainage or “time to filter” test). Note the filtrate clarity and whether the solids left on the cloth are firm and release cleanly.
- The best dose is the lowest that gives near-maximum drainage rate and clear filtrate. Above it the drainage stops improving and the floc becomes slimy and sticks to the cloth.
Capillary suction time (CST) and specific resistance to filtration (SRF) are laboratory versions of the same test with numbers attached; a CST below about 20 seconds usually indicates sludge that will press well. A Buchner funnel test under vacuum gives the SRF and, with some arithmetic, an estimate of cycle time. Senjie runs these tests on customer samples as part of press sizing; see our laboratory filter press and scale-up article.
Trimming the dose on the press
The jar test lands within about 20% of the right dose. The press finds the rest. Run three or four cycles at the jar-test dose and record cycle time to reach the target pressure, filtrate clarity in the first ten minutes, cake solids and how cleanly the cake drops. Then reduce the dose in 10% steps. The signs that you have gone too low are, in order: filtrate turning cloudy at the start of the cycle, cycle time lengthening, cake going soft and wet in the centre. The signs of too much are slimy cake, cloth that stays slick after washing, and a filtrate that foams. The correct dose is one step above the point where the first sign of under-dosing appears.
Repeat this exercise whenever the sludge changes: season, a change in the biological process, a new upstream discharge, or a new polymer batch. Plants that set the dose once and leave it typically over-dose by 30–50% for years.
Polymer selection: charge, molecular weight and form
Cationic polyacrylamides are sold by charge density (low, medium, high) and molecular weight (medium to very high), in dry powder, emulsion or bead form. For filter presses the general pattern is:
- Charge: biological sludge needs medium to high cationic charge; digested sludge often very high; chemical and mineral sludges are usually anionic or non-ionic. Charge is the first variable to screen.
- Molecular weight: presses prefer medium to high, not the very highest. Very high molecular weight makes a large, soft floc that is ideal for a belt press or centrifuge but compresses into a slimy layer under filter press pressure.
- Form: powder is cheapest per kilogram of active and stores well but needs a proper make-up unit with ageing; emulsion is easier to dose at small plants but carries 50–60% oil and water you pay to ship.
Screen three or four products from the supplier’s range by jar test before settling; the difference between the best and worst product on a given sludge is commonly two to one in dose.
Make-up and dosing equipment
Most polymer problems on presses are make-up problems. Powder that is not fully dissolved forms “fish eyes” that blind cloth and waste product; solution that is dosed too concentrated does not mix into the sludge. The rules: make up at 0.1–0.5% and dilute further to 0.05–0.1% at the injection point; age powder solutions 30–60 minutes; inject through a mixer or static mixer into the sludge line with 10–30 seconds of contact before the feed pump; and never dose polymer into the suction of a centrifugal feed pump, which shears the floc. A progressive cavity or diaphragm feed pump is gentler. Our page on air-operated diaphragm pumps and our wastewater treatment equipment range include polymer preparation units and dosing pumps sized to the press.
When to use a coagulant as well
Add ferric chloride, PAC or lime ahead of the polymer when the jar test shows weak floc that breaks under pressure, when the sludge is oily, when the filtrate carries fine colloids that polymer alone cannot capture, or when the polymer dose is climbing above 10 kg/t. Ferric at 3–8% of dry solids typically cuts the polymer dose by a third to a half and firms the cake, at the price of more dry solids to dispose of and corrosion protection for the dosing equipment. Lime at 10–20% of dry solids was the classic filter press conditioner and is still used where the cake goes to land and the pH rise is useful for pathogen control. Our heavy metal precipitation article covers coagulant chemistry in that context.
Frequently asked questions
How much polymer does a filter press need?
Typically 3–8 kg of active cationic polymer per dry tonne for biological sludge, 6–12 for digested biosolids, 0.5–2 for chemical and metal hydroxide sludge, and often none for mineral slurries. Set the actual dose by jar test and trim it on the press.
How do I know if I am over-dosing polymer?
Slimy, sticky cake; cloth that stays slick after washing; foaming filtrate; and no improvement in cycle time when the dose is raised. Reduce in 10% steps until the filtrate starts to cloud, then go back up one step.
Which polymer is best for a filter press?
Usually a medium-to-high charge cationic polyacrylamide of medium-to-high (not very high) molecular weight for biological sludge, and an anionic or non-ionic product for chemical and mineral sludge. Screen three or four products by jar test; the difference in dose between products can be two to one.
Should I use ferric chloride or lime instead of polymer?
Use them in addition to polymer when floc strength is poor, the sludge is oily, or the polymer dose is climbing above about 10 kg/t. They firm the cake and cut polymer demand, but add dry solids to disposal.
Where should polymer be injected?
Into the sludge line through a static mixer or inline mixer 10–30 seconds ahead of the feed pump, at 0.05–0.1% solution strength. Never into the suction of a centrifugal pump, which shears the floc.
Why does the right dose keep changing?
Sludge changes with season, sludge age, upstream discharges and the biological process. Re-run the jar test and trim on the press whenever the plant changes or a new polymer batch arrives; plants that set the dose once typically over-dose by 30–50%.
Send us a sludge sample and we will run the drainage and CST tests, recommend polymer type and starting dose, and size the press: request a conditioning and sizing test.
References: US EPA — Biosolids Technology Fact Sheet: Recessed-Plate Filter Press (conditioning and performance data), US EPA — Biosolids Technology Fact Sheet: Belt Filter Press (polymer dose ranges by sludge type).