PolyDADMAC (Polydiallyldimethylammonium chloride) is a high molecular weight, cationic (positively charged) polymer widely used as a coagulant and flocculant in water and wastewater treatment processes.
Here's a breakdown of its key characteristics and uses:
Chemical Nature: It is a synthetic polymer composed of repeating units of diallyldimethylammonium chloride. Its strong positive charge is central to its function.
Primary Function (Coagulation): In water treatment, PolyDADMAC acts primarily as a primary coagulant. Its mechanism involves:
Charge Neutralization: Most particles in water (like clay, silt, organic matter, and microorganisms) carry a negative surface charge, which causes them to repel each other and remain dispersed. The highly positive charge of PolyDADMAC neutralizes these negative charges on the particles.
Destabilization: Once the repulsive charges are neutralized, the particles can come closer together.
Agglomeration: The destabilized particles collide and begin to clump together, forming small aggregates (microflocs).
Applications:
Drinking Water Treatment: Used to remove turbidity, color (especially from natural organic matter like humic acids), algae, and some pathogens.
Wastewater Treatment: Effective in removing suspended solids, colloidal matter, and certain dissolved organic compounds from industrial and municipal wastewater. It's particularly useful for treating wastewater with high organic loads.
Paper Manufacturing: Used as a retention and drainage aid to help keep fine particles and fillers on the paper sheet.
Swimming Pools & Spas: Sometimes used to clarify cloudy water.
Advantages:
High Charge Density: Very effective at charge neutralization, often requiring lower dosages than traditional metal coagulants (like alum or ferric chloride).
Wide pH Range: Generally effective over a broader pH range compared to metal coagulants.
No pH Adjustment: Often doesn't require significant pH adjustment of the water.
Less Sludge Production: Typically generates less chemical sludge than metal coagulants since it doesn't hydrolyze to form metal hydroxides.
Stable: Relatively stable solution.
Disadvantages/Considerations:
Cost: Can be more expensive per kilogram than traditional coagulants (though dosage might be lower).
Organic Loading: Adds organic carbon to the water, which can be a concern for downstream processes or if disinfection by-products (DBPs) are a worry.
Toxicity/Biodegradability: While effective, it is a synthetic polymer. Its environmental impact and biodegradability are considerations, and residual levels in treated water are regulated.
Not a Flocculant Alone: While it initiates floc formation, larger, settleable flocs often require the addition of a separate, high molecular weight flocculant (like an anionic polyacrylamide) after coagulation with PolyDADMAC.
Potential for Overdosing: Overdosing can restabilize particles (due to excess positive charge) or increase dissolved organic carbon (DOC) in the treated water.
In summary, PolyDADMAC is a powerful, positively charged synthetic polymer used to destabilize and clump together negatively charged particles in water, making them easier to remove through sedimentation or filtration. It's a key chemical in modern water clarification processes.
