PolyDADMAC (Polydiallyldimethylammonium chloride) differs significantly from other common coagulants in its chemistry, mechanism, and performance. Here's a comparison with the main categories:
| Feature | PolyDADMAC | Alum (Al₂(SO₄)₃) / Ferric (FeCl₃, Fe₂(SO₄)₃) |
|---|---|---|
| Chemistry | Organic, cationic polymer | Inorganic metal salts |
| Primary Mechanism | Charge neutralization (dominant) | Charge neutralization + Sweep flocculation |
| pH Sensitivity | Low - effective over a wide pH range (5-9+) | High - optimal pH narrow (Alum: ~6-7, Ferric: ~5-8) |
| Sludge Production | Lower - no metal hydroxide precipitate | Higher - forms voluminous metal hydroxide sludge |
| pH Adjustment | Usually not required | Often required (e.g., add lime/soda ash) |
| Organic Addition | Yes - adds DOC (potential DBP precursor) | No - inorganic |
| Dosage | Often lower (mg/L range) | Typically higher (tens to hundreds mg/L) |
| Floc Characteristics | Smaller, denser microflocs initially | Larger, more voluminous flocs (sweep floc) |
| Residuals | Residual polymer (regulated) | Residual aluminum/iron, sulfate/chloride |
| Cost | Higher chemical cost, lower sludge handling | Lower chemical cost, higher sludge handling cost |
Key Difference: Metal coagulants rely heavily on sweep flocculation where the metal hydroxide precipitate enmeshes particles. PolyDADMAC relies almost purely on electrostatic charge neutralization.
| Feature | PolyDADMAC | Cationic Polyacrylamide (CPAM) |
|---|---|---|
| Molecular Weight | Lower to Medium (typically 200k-1M Da) | Very High (often > 1M Da, up to 20M Da) |
| Charge Density | Very High | Medium to High |
| Primary Role | Primary Coagulant (destabilizes) | Flocculant (bridges, builds large flocs) |
| Mechanism | Charge neutralization | Charge neutralization + Polymer bridging |
| Application Order | Added first | Added after a primary coagulant (like PolyDADMAC or metal salt) |
| Effect on Floc | Forms small microflocs | Binds microflocs into large, fast-settling flocs |
| Viscosity | Lower viscosity solutions | Can form viscous solutions at lower concentrations |
Key Difference: PolyDADMAC is primarily a coagulant for destabilization. High MW CPAM is primarily a flocculant for agglomeration. They are often used together (PolyDADMAC first, then CPAM).
| Feature | PolyDADMAC | Natural Coagulants (e.g., Chitosan) |
|---|---|---|
| Origin | Synthetic | Natural / Biopolymer |
| Consistency | Highly consistent, pure product | Variable (depends on source, extraction) |
| Effectiveness | High and predictable | Often lower, less predictable |
| Biodegradability | Low to Moderate (persistent) | High (readily biodegradable) |
| Toxicity Concerns | Potential aquatic toxicity, regulated | Generally considered low toxicity, eco-friendly |
| Cost | Moderate to High | Can be very low (if locally sourced) |
| Application | Industrial scale, high-performance needs | Small-scale, decentralized, sustainable systems |
Key Difference: PolyDADMAC offers reliability and high performance but is synthetic and less eco-friendly. Natural coagulants are sustainable and biodegradable but often less effective and consistent.
Choose PolyDADMAC over Metal Salts when you want lower sludge production, wider pH tolerance, and simpler pH control, and can manage the added organic carbon.
Use PolyDADMAC with CPAM for optimal performance: PolyDADMAC destabilizes, CPAM builds large, settleable flocs.
Choose Natural Coagulants for sustainability and biodegradability, accepting potential trade-offs in performance and consistency.
PolyDADMAC's Niche: It excels as a high-efficiency, low-sludge primary coagulant, particularly effective for removing color and organic matter through charge neutralization, making it a popular choice in modern water treatment, especially where sludge disposal is costly or pH adjustment is undesirable.
