PolyDADMAC (Polydiallyldimethylammonium chloride) is used in water treatment as a primary coagulant or a coagulant aid, primarily valued for its extremely high, permanent positive charge density. Here's how it functions within the treatment process:
The Problem: Raw water contains suspended particles like clay, silt, organic matter (humic/fulvic acids causing color), algae, and microorganisms. These particles typically carry a negative surface charge, which causes them to repel each other and remain dispersed (colloidally stable), making them hard to remove by settling or filtration.
The Solution - PolyDADMAC: As a highly cationic (positively charged) polymer, PolyDADMAC is dosed into the raw water.
Mechanism: The long chains of positively charged quaternary ammonium groups along the PolyDADMAC molecule are attracted to the negatively charged particles.
They adsorb onto the particle surfaces.
This neutralizes the negative charges on the particles.
Once the repulsive forces are overcome (charge neutralization), the particles can come close together.
PolyDADMAC is typically introduced early in the treatment train:
Rapid Mixing (Flash Mixing):
Location: Immediately after the chemical feed point, often in a rapid mix basin or using inline static mixers.
Purpose: To ensure the PolyDADMAC solution is quickly and thoroughly dispersed throughout the entire water stream. This maximizes contact between the polymer and the suspended particles for effective charge neutralization. Mixing is intense but brief (seconds).
Flocculation:
Location: Follows rapid mixing, in a flocculation basin with slow, gentle agitation.
Purpose: After charge neutralization, the destabilized particles begin to collide and stick together due to van der Waals forces. The long PolyDADMAC molecules can also act as "bridges," linking multiple particles together (bridging flocculation). This process forms larger, visible aggregates called flocs.
Sedimentation (or Flotation):
Location: Flocs flow into a sedimentation basin (clarifier).
Purpose: The large, heavy flocs settle out of the water due to gravity, forming sludge at the bottom, which is then removed.
Filtration:
Location: Water from the clarifier passes through filters (e.g., sand, anthracite, membrane).
Purpose: Removes any remaining fine flocs or particles that didn't settle.
Disinfection:
Final Step: The clarified water is disinfected (e.g., with chlorine, UV) to kill pathogens before distribution.
High Efficiency: Its very high charge density makes it extremely effective at neutralizing negative charges, often requiring lower dosages than traditional metal coagulants (like alum or ferric salts) or weaker cationic polymers.
Broad Applicability: Effective over a wide pH range (typically pH 4-9), offering operational flexibility.
Reduced Sludge Production: Produces significantly less chemical sludge compared to metal coagulants because it doesn't hydrolyze to form bulky aluminum or iron hydroxide precipitates. The sludge is primarily organic/inorganic flocs bound by the polymer.
Less pH Impact: Does not significantly lower the pH of the water (unlike alum), reducing or eliminating the need for alkalinity addition (e.g., lime).
Excellent Color and Organic Matter Removal: Particularly effective at removing dissolved organic carbon (DOC) and natural organic matter (NOM) that cause color and can form disinfection by-products (DBPs) when chlorinated.
Tolerance to Cold Water: Performs well even in low-temperature conditions where some other coagulants struggle.
Drinking Water Treatment: Primary coagulant for surface water sources high in turbidity, color, and organic matter.
Wastewater Treatment: Used in municipal and industrial plants for primary clarification, phosphorus removal (as a tertiary treatment), and improving sludge dewatering characteristics.
Recycling Water: Clarification in processes like cooling tower blowdown or process water reuse.
Swimming Pools & Spas: As a clarifier to combat cloudy water.
Overdosing Risk: Adding too much PolyDADMAC can reverse the charge of the particles (making them positive), causing them to repel each other again and preventing floc formation ("restabilization"). Careful dosage control via jar testing is essential.
Organic Carbon Addition: As an organic polymer, it adds a small amount of organic carbon to the water. While usually negligible, this is considered in overall DBP formation potential assessments.
Cost: Can be more expensive per kg than traditional coagulants, but its higher efficiency often makes it cost-effective overall.
In summary, PolyDADMAC is a powerful synthetic cationic polymer used early in water treatment to destabilize negatively charged contaminants through charge neutralization. This enables the formation of large, settleable flocs, leading to efficient removal of turbidity, color, and organic matter, with benefits including reduced sludge and minimal pH impact.
