Polymer flocculants bridge coagulated micro-flocs into large, settleable agglomerates. Choosing the right polymer (anionic, cationic, nonionic), preparing it correctly, and dosing at the right G-value determines clarification and dewatering performance. This page covers the engineering.
Long-chain polymers bridge micro-flocs into rapidly-settling agglomerates.
Coagulants neutralise particle charge to allow initial agglomeration. Polymers — long-chain high-molecular-weight molecules — then bridge these primary flocs into large, dense, fast-settling secondary flocs. The mechanism is “inter-particle bridging”: the polymer chain attaches at multiple points to different particles, pulling them together. A second mechanism, “charge patching”, applies when cationic polymer attaches to negative particles and locally reverses their surface charge, allowing nearby unaltered particles to attach. Both mechanisms rely on the polymer being dispersed at the molecular level before contact with the floc — which is why polymer preparation is as important as polymer selection.
Carboxylate functional groups carry a negative charge. Best when used after coagulation — coagulant has already neutralised particle charge; anionic polymer bridges via hydrogen bonding to metal-hydroxide flocs.
Quaternary ammonium groups carry permanent positive charge. Works alone (without coagulant) on negative organic particles — ideal for sludge dewatering.
No formal charge; bridging is purely physical entanglement and hydrogen bonding. Insensitive to pH and ionic strength.
Charge sign matches the particle’s surface charge after coagulation; charge density and MW are tuned to the floc structure needed.
| Application | Polymer type | Charge density | Typical dose |
|---|---|---|---|
| Drinking-water clarification (after alum/ferric) | Anionic PAM | 10–30% | 0.05–0.5 mg/L |
| Industrial DAF (food/brewery) | Cationic PAM | 20–50% | 1–5 mg/L |
| Activated sludge thickening (belt thickener) | Cationic PAM | 30–55% | 3–8 g/kg DS |
| Activated sludge dewatering (centrifuge) | Cationic PAM | 40–70% | 8–15 g/kg DS |
| Anaerobically-digested sludge dewatering | Cationic PAM | 50–80% | 10–20 g/kg DS |
| Mineral tailings thickening | Anionic PAM | 30–60% | 20–100 g/t solids |
| Coal washery clarification | Anionic PAM (high MW) | 15–40% | 0.5–3 mg/L |
| Oily wastewater (DAF after DGF) | Cationic PAM | 30–60% | 1–3 mg/L |
| Phosphate-rich sludge dewatering | Cationic PAM (high MW) | 50–70% | 10–18 g/kg DS |
All values indicative. Polymer selection is best confirmed by laboratory or on-site jar testing using your actual influent or sludge.
A polymer that has not fully hydrated is functionally inert. Get the preparation right.
Polyacrylamide is supplied as fine white powder, dispersion in oil, or pre-dissolved liquid “emulsion” products. All forms must be diluted into water and given time to hydrate before reaching the floc reactor. Inadequate preparation is the single most common reason for polymer performance disappointment.
The preparation cycle has three phases:
| Stock concentration | 0.2–0.5% (powder) / 1–2% (emulsion) |
| Wetting time | 2–5 minutes high-shear |
| Ageing time | 30–60 minutes gentle stir |
| Dilution at injection | 5–10x (final 0.05–0.1%) |
| Storage life (prepared) | 24–72 hours max |
| Make-up water quality | <1 NTU, free of Fe/Mn/Cl₂ |
Polymer mixing intensity defines floc structure and dewaterability.
Polymer is added to the process stream and must disperse uniformly without breaking already-formed flocs. The standard measure of mixing energy is the velocity gradient G (1/s):
G = √(P / μV)
P = mixer power dissipation (W); μ = water viscosity (kg/m·s); V = reactor volume (m³). G has units of 1/s — the rate of velocity change across the fluid.
Too low G: poor dispersion; polymer streams through reactor unmixed; poor floc.
Too high G: polymer bridges break before they can grow; sheared floc cannot be rebuilt.
Right G: rapid dispersion at injection, then gentle aggregation downstream.
Polymer floc bonds, once broken, do not re-form. This is why polymer is always dosed at the FINAL high-shear point before the clarifier/DAF/dewatering machine — not upstream of any pumps, mixers or restrictions.
The largest single polymer cost in most plants. A 20% dose reduction translates directly to Operating expenditure.
Sludge dewatering equipment (belt press, screw press, centrifuge) all need high-cationic-charge polymer to neutralise the strongly-negative organic sludge particles and form a dewaterable floc. The chosen polymer determines:
Trade-offs are non-linear. Optimisation requires polymer screening on actual sludge using small-scale dewatering tests (Buchner funnel, CST, centrifuge bottle test).
View dewatering equipment| Metric | Old polymer | Optimised polymer |
|---|---|---|
| Polymer type | Generic cationic PAM | Higher MW cationic + 60% charge |
| Polymer dose (g/kg DS) | 12 | 8 |
| Cake solids (%) | 22 | 26 |
| Centrate TSS (mg/L) | 1100 | 180 |
| Polymer cost / t DS | ||
| Transport rate / t DS |
Indicative figures — site dependent.
Powder hits water too slowly → outer layer hydrates and gels → gel surrounds dry powder, preventing dispersion. Cure: dispersing eductor at the powder feed, high-shear initial wetting.
30 minutes is the minimum hydration time. Less → partially-coiled polymer → reduced bridging capacity → higher dose needed. Cure: sized two-tank ageing system with sequenced batches.
Prepared polymer hydrolyses over 24–72 hours, losing charge density. After 5 days, solution may be 50% inert. Cure: discard unused stock daily; right-size make-up batches.
Fe, Mn, free chlorine, or pH extremes degrade polymer during preparation. Cure: dedicated polymer-prep water — softened & dechlorinated.
The upstream step before polymer dosing — ferric, alum and PACl selection.
Read MoreMake-up tanks, ageing reactors, eductors and feed pumps.
Read MoreProgressive cavity vs diaphragm pumps for polymer duty.
Read MoreBelt press, screw press and centrifuge — where polymer choice dominates Operating expenditure.
Read MoreSend us your sludge type, dewatering equipment and current polymer / dose / cake quality. We will run a comparative screening, recommend the optimal polymer family and design preparation infrastructure.
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