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Anionic or Cationic Polyacrylamide How to Choose and Dose
Anionic Polyacrylamide and Cationic Polyacrylamide have different application area and cases. Normally, use anionic polyacrylamide when your suspended solids behave positively charged, and use cationic polyacrylamide when treating negatively charged sludge/organics. Confirm the choice with a quick jar test and then optimize the dose for fast settling (clarification) or low filtrate turbidity and strong cakes (dewatering). Below are the steps:
▶ Fast selection: anionic polyacrylamide or cationic polyacrylamide
The practical rule is to match polymer charge to particle behavior: opposite charges promote attachment, then high molecular weight drives bridging into larger flocs.
| If your stream looks like this | Start with | Typical target outcome |
|---|---|---|
| Inorganic-rich solids (e.g., clays, sand fines, metal hydroxide flocs) | Anionic polyacrylamide | Rapid settling, clearer supernatant |
| Organic/biological sludge (activated sludge, digested sludge, oily emulsions after coagulant) | Cationic polyacrylamide | Low filtrate turbidity, strong flocs for dewatering |
▶ Practical dosing benchmarks (so you do not start blind)
Start with a conservative range, then tighten to the best-performing window using jar tests (clarification) or belt press/centrifuge trials (dewatering).
Clarification and solids settling (mg/L in water)
- Common jar-test ranges for wastewater clarification are often 0.5–15 mg/L depending on solids loading and upstream chemistry.
- If you already run an inorganic coagulant (alum/ferric), you can often start in the lower half of that range because particles are pre-aggregated.
Sludge dewatering (kg active polymer per ton dry solids)
- A practical starting band for cationic polyacrylamide on digested wastewater sludge is 5–15 kg/t DS.
- Published case data commonly lands near the middle of that band; for example, one study reported an optimum near 7.6 kg/t DS (dry solids) for best dewatering metrics in its test conditions.
▶ Jar test method to confirm the best polymer and dose
Use this approach to compare anionic polyacrylamide versus cationic polyacrylamide quickly, and to avoid overdosing (which can re-stabilize solids and increase turbidity).
- Prepare fresh polymer solutions at the same active concentration (example: 0.1% w/w) and label clearly.
- In identical beakers, dose a ladder (for example: 0.5, 1, 2, 5, 10 mg/L for clarification) for both anionic and cationic products.
- Mix rapidly for short contact, then slow mix to build flocs; stop and observe settling rate, floc strength, and supernatant clarity.
- Pick the best performer by a measurable endpoint (turbidity/TSS, settling time, filtrate clarity), then run a narrower dose ladder around it.
Stop increasing dose once performance plateaus. The best dose is usually near the inflection point where clarity improves sharply and then levels off.
▶ Field examples that make selection easier
When anionic polyacrylamide is typically favored
- Sediment control and construction dewatering where fines dominate; documented applications have shown large TSS reductions (for example, 42 mg/L down to 13 mg/L after filtration in one field guide context).
- Mining and mineral processing tailings thickening where bridging high-solids slurries is the primary goal.
When cationic polyacrylamide is typically favored
- Wastewater sludge dewatering (belt press, centrifuge) where biosolids are strongly negative; cationic charge neutralization plus bridging improves cake solids and filtrate clarity.
- Systems rich in soluble organics or emulsified oils after primary coagulant, where a cationic polymer can improve capture and drainage.
▶ Troubleshooting (common failures and quick fixes)
- Flocs are tiny and fragile: increase molecular weight (or reduce shear), and verify you are using the correct charge type (anionic vs cationic).
- Supernatant turns cloudy after higher doses: you are likely overdosing; step back to the last dose before turbidity worsened.
- Dewatering cake is slimy or drains poorly: reduce dose slightly and/or move to a lower charge density cationic polyacrylamide to avoid over-neutralization.
- Performance changes day to day: track pH, solids concentration, and coagulant dose; re-run a short jar test whenever upstream chemistry changes.
Bottom line: the “right” polymer is the one that meets your measurable endpoint at the lowest stable dose—anionic polyacrylamide for many inorganic suspensions, cationic polyacrylamide for most sludge/organics—validated by a quick test.
