Polymer Selection for Dewatering
The right polymer at the right dose can lift cake dryness five percentage points on the same centrifuge, or halve chemical spend on the same filter press. The wrong polymer wastes both product and machine capacity. This guide sets out how to select and apply dewatering polymers in practice.
1. Cationic, anionic or non-ionic
Cationic polymers dominate biological sludge dewatering (municipal WAS, food-industry biomass) where the sludge carries a net negative charge.
Anionic polymers are the default for mineral and inorganic sludges (mining tailings, coagulant residuals, drilling waste).
Non-ionic polymers occupy a smaller niche — flocculation of neutral or lightly charged suspensions.
2. Molecular weight and charge density
Higher molecular weight generally gives larger, faster-settling flocs but shears more easily. Lower MW is more robust but produces smaller flocs.
Charge density should be matched to the surface charge of the sludge — high-charge polymers overshoot on lightly charged sludges and cause floc breakdown.
3. Dry vs emulsion
Dry polymer: lowest cost per kg active, longest shelf life, requires a properly aged make-up system.
Emulsion polymer: faster activation, simpler make-up, higher cost per kg active. Ideal for smaller installations and mobile units.
4. Make-up matters as much as chemistry
Under-aged polymer solution never reaches its full activity. A properly sized make-up skid dissolves, wets and ages polymer at controlled concentration (typically 0.25–0.5% active for dry polymers).
Solution water quality — hardness, chlorine residual, temperature — directly affects polymer performance. Never mix polymer with raw chlorinated water.
5. Dose control
Optimum dose is a narrow band. Under-dose and flocs are weak; over-dose and polymer costs run away with no improvement in cake dryness.
Automated dose control tied to sludge flow (feed-forward) or centrate turbidity (feedback) delivers consistently better cake than manual dosing.
6. Bench-test before you commit
A properly conducted jar test with the actual sludge and shortlisted polymers takes half a day and de-risks the entire supply decision. It routinely reveals cheaper polymers that outperform the incumbent.
If a dewatering unit under-performs on a stable sludge, the polymer is the first thing to check — chemistry, age, make-up concentration and dose. Machine problems almost always come second.
- Cationic for biological, anionic for inorganic — start there.
- Match molecular weight and charge density to the sludge, not the catalogue.
- A properly aged make-up solution is half of polymer performance.
- Automated dose control pays back in a matter of weeks.
- Jar testing beats vendor promises every time.