The previous PAC vs CPAM piece covered the two flocculants we get asked about most. The full family is wider than that — and the chemistry-selection logic is more interesting once you see all four chemistries side by side.
The four flocculant chemistries
| Chemistry | Type | Charge | MW (Da) | Mechanism |
|---|---|---|---|---|
| PAC (Poly Aluminium Chloride) | Inorganic coagulant | Positive (Al³⁺ family) | ~1,000–10,000 | Charge neutralisation + sweep coagulation |
| CPAM (Cationic Polyacrylamide) | Organic flocculant | Positive (quaternary ammonium side groups) | 5–15 million | Charge neutralisation + bridging |
| APAM (Anionic Polyacrylamide) | Organic flocculant | Negative (carboxylate) | 10–25 million | Bridging (after upstream cationic coagulation) |
| NPAM (Non-ionic Polyacrylamide) | Organic flocculant | Neutral | 3–10 million | Bridging only; works across wide pH and ionic strength |
The two-stage logic
Coagulation and flocculation are two distinct chemical operations, not synonyms. Coagulation neutralises particle surface charge so colloids can come into contact. Flocculation bridges neutralised particles into large, dense flocs that settle or dewater. Most wastewater plants need both. The question is what to use for each stage.
- Stage 1 (rapid mix, 30–60 sec): coagulant. Almost always PAC in industrial applications. Dose: 20–200 ppm depending on raw water turbidity and COD load.
- Stage 2 (slow mix, 5–15 min before settling or dewatering): flocculant. Type and dose depend on what the upstream water now looks like — that is, what charge state remains, what ionic strength, what pH.
When to use which polyacrylamide
CPAM — when colloids are still net-negative after coagulation
Default choice for sludge dewatering, especially organic-rich sludges (municipal sewage, food-industry waste, paper-mill primary sludge). Also the right choice when PAC dose has been minimised and residual negative charge remains on suspended solids. Typical dose: 1–10 kg per dry tonne sludge (for dewatering), 0.5–5 ppm (for clarification).
APAM — when colloids are net-positive or charge-neutral, and you need maximum bridging
Right choice after lime-based coagulation (water is alkaline, charges are positive), or for inorganic sludges where bridging matters more than charge neutralisation (mineral processing tailings, ash sluice water, sand-wash water). Typical dose: 0.2–2 ppm.
NPAM — when ionic strength is high and charged polymers lose efficacy
The undervalued chemistry. In high-salinity wastewater (textile dye-bath effluent, oil-and-gas produced water, brine streams), cationic and anionic polymers lose their charge-driven activity. NPAM bridges purely by molecular weight and works where the charged variants fail. Also useful for paper-mill white-water where calcium hardness interferes with anionic polymer charge. Typical dose: 0.5–3 ppm.
PAC alone — when sludge production is not a concern
Drinking water clarification on low-turbidity surface water, or municipal sewage primary clarification where downstream sludge treatment is sized for the volume. PAC is cheap on a per-kg basis but generates substantial chemical sludge — every kilogram of Al₂O₃ added creates ~3 kg of Al(OH)₃ floc on hydrolysis. CPAM finishing reduces that sludge volume by 40–60%.
Decision matrix by application
| Application | Stage 1 coagulant | Stage 2 flocculant | Notes |
|---|---|---|---|
| Municipal drinking water (surface) | PAC, 5–25 ppm | Optional NPAM, 0.1–0.5 ppm | Polish only if clarifier throughput-limited |
| Municipal sewage primary | PAC, 30–80 ppm | None typically | Add CPAM only for tertiary polish or sludge thickening |
| Textile dye-house effluent | PAC, 80–200 ppm | NPAM or APAM, 1–3 ppm | Adjust pH 5.5–7.0 for colour removal |
| Paper-mill primary clarifier | PAC, 20–60 ppm | CPAM, 0.5–2 ppm | Cationic critical for negatively-charged fines |
| Sewage sludge dewatering (centrifuge) | none (already coagulated) | CPAM (high cat), 4–8 kg/tDS | Match polymer cationicity to sludge VS:TS ratio |
| Mineral tailings thickening | none | APAM, 30–80 g/t solids | Bridging dominates; cationic ineffective on negative mineral surfaces |
| Oil-and-gas produced water | FeCl₃ or PAC | NPAM, 1–3 ppm | High salinity disables charged polymers |
Jar testing is non-negotiable
No selection table replaces an actual jar test on the actual feed. Run six beakers with stepped doses across the candidate chemistry, observe floc formation rate, settling rate, and supernatant turbidity, then optimise from the best two or three. A two-hour jar test correctly performed saves three weeks of dosing-room troubleshooting.
If you have an effluent stream that isn't responding to your current chemistry, send us the analysis and we'll run jar tests in our Karachi lab against multiple candidate products. Request a jar-test review. See also PAC specifications, CPAM specifications, and CPAM application at a Lahore paper mill.