PolyDADMAC (polydiallyldimethylammonium chloride) is a synthetic, high-charge-density cationic polymer widely used as a coagulant in water and wastewater treatment. Its performance differs significantly from traditional inorganic coagulants (like alum or ferric salts) and other organic polymers. Below is a detailed comparison across key criteria:
| Coagulant | Type | Charge Mechanism |
|---|---|---|
| PolyDADMAC | Synthetic organic polymer | Permanent quaternary ammonium (+) charge (pH-independent) |
| Alum (Al₂(SO₄)₃) | Inorganic metal salt | Hydrolyzes to form Al(OH)₃ flocs; charge depends on pH |
| Ferric Chloride (FeCl₃) | Inorganic metal salt | Forms Fe(OH)₃ flocs; effective over wider pH than alum |
| PAC (Polyaluminum Chloride) | Pre-hydrolyzed inorganic polymer | Contains polymeric Al species (e.g., Al₁₃⁷⁺); high charge, partial pH independence |
| CPAM (Cationic Polyacrylamide) | Organic polymer | Cationic groups (often quaternary); used mainly as flocculant, not primary coagulant |
✅ Key Advantage of PolyDADMAC: Permanent positive charge—works reliably at any pH.
PolyDADMAC: Charge neutralization (dominant), with minor bridging.
Alum / Ferric / PAC: Sweep flocculation (metal hydroxide precipitates enmesh particles) + some charge neutralization.
CPAM: Polymer bridging (requires pre-destabilized particles).
🎯 PolyDADMAC excels at treating low-turbidity, high-organic waters where sweep floc is inefficient.
| Parameter | PolyDADMAC | Alum | Ferric Chloride | PAC | CPAM |
|---|---|---|---|---|---|
| Effective pH Range | 4–9 (broad) | 6–7.5 (narrow) | 4–9 (broad) | 5.5–8.5 | 4–10 (but needs destabilization first) |
| Sludge Production | Very Low | High | High | Moderate | Very Low* |
| Residual Metal | None | Aluminum | Iron | Aluminum | None |
| Organic Matter Removal | Excellent | Good | Good | Very Good | Poor (alone) |
| Color Removal | Excellent | Moderate | Good | Very Good | Limited |
| Cold Water Performance | Good | Poor | Moderate | Good | Good |
| Corrosivity | Low | High (lowers pH) | High (lowers pH) | Moderate | Low |
*CPAM produces little sludge but is not a standalone coagulant—it’s used after a primary coagulant.
| Factor | PolyDADMAC | Traditional Coagulants (Alum/PAC) |
|---|---|---|
| Dosage Required | Lower (due to high charge density) | Higher |
| pH Adjustment Needed? | Rarely | Often (especially alum) |
| Sludge Handling Cost | Lower (less volume, no metals) | Higher |
| Chemical Storage | Liquid (corrosive but stable) | Alum: solid/crystals; PAC: liquid acid |
| Cost per kg | Higher | Lower |
| Overall Cost Efficiency | Often better due to lower dose, less sludge, no pH correction | Can be higher when factoring in ancillary costs |
PolyDADMAC:
No metal residuals → safer for sensitive ecosystems.
Adds minimal organic carbon (low DBP risk).
Must meet strict limits on residual DADMAC monomer (<0.1%).
Alum/Ferric/PAC:
Leave aluminum or iron residuals (Al concerns in drinking water).
Increase sulfate or chloride levels.
Produce metal-laden sludge (hazardous disposal in some cases).
| Application | Preferred Coagulant(s) |
|---|---|
| Low-turbidity, high-color surface water | ✅ PolyDADMAC or PAC |
| Municipal wastewater (primary clarifier) | Alum, Ferric, or PAC |
| Sludge dewatering | ❌ Not PolyDADMAC → ✅ CPAM |
| Cold climate treatment | ✅ PolyDADMAC or PAC |
| Zero-metal discharge required | ✅ PolyDADMAC |
| High-phosphorus removal | ❌ PolyDADMAC (weak) → ✅ Ferric/Alum |
⚠️ Note: PolyDADMAC is not effective for phosphorus removal, as it doesn’t form insoluble precipitates like metal salts do.
Permanent cationic charge → consistent performance across pH.
Low sludge production and no metal residuals.
Superior for organic matter and color removal.
Simplifies operations (less pH adjustment, compact footprint).
Ideal for drinking water from humic-rich sources (e.g., lakes, rivers).
Not suitable for phosphorus or heavy metal removal.
Higher upfront cost (though often offset by operational savings).
Risk of overdosing → particle restabilization.
PolyDADMAC is not a universal replacement for all coagulants—but it is often the optimal choice for applications prioritizing organic contaminant removal, low sludge, and operational simplicity, especially in drinking water treatment. Many plants use it in combination with PAC or metal salts for synergistic effects.
