Polydadmac (poly diallyldimethylammonium chloride) is a highly effective cationic organic polymer widely used in drinking water purification systems. It is one of the most important modern coagulants due to its strong positive charge density, rapid reaction speed, and ability to remove turbidity, natural organic matter, and fine suspended solids. In drinking water treatment plants, Polydadmac is mainly used in coagulation and clarification processes to improve water quality, reduce chemical consumption, and enhance overall treatment efficiency.

1. Characteristics of Polydadmac in drinking water treatment

Polydadmac is a water-soluble quaternary ammonium polymer synthesized from diallyldimethylammonium chloride (DADMAC). It is typically supplied as a clear or slightly yellow viscous liquid.

Key characteristics include:

• High cationic charge density: Strong ability to neutralize negatively charged particles
• Liquid form: Easy to dose directly into water treatment systems
• Stable chemical structure: Resistant to hydrolysis and stable across wide pH ranges
• Low to medium molecular weight (or tailored grades): Suitable for fast coagulation and floc formation
• Excellent solubility in water: Ensures rapid and uniform dispersion

These properties make Polydadmac highly suitable for potable water treatment applications where stability, efficiency, and safety are critical.

2. Role in drinking water purification

In drinking water treatment plants, Polydadmac is primarily used in the coagulation stage. Its main roles include:

• Removing turbidity from surface water
• Reducing natural organic matter (NOM)
• Eliminating fine suspended solids and colloids
• Improving sedimentation and filtration efficiency
• Reducing color and odor-causing substances
• Lowering disinfection by-product (DBP) precursors

It is often used alone or in combination with inorganic coagulants such as PAC (polyaluminum chloride).

3. Mechanism of action

Polydadmac improves drinking water quality through several key mechanisms:

(1) Charge neutralization
Most particles in raw water (clay, silt, organic matter) carry negative charges. Polydadmac neutralizes these charges, eliminating electrostatic repulsion and allowing particles to aggregate.

(2) Adsorption onto colloids
Polydadmac adsorbs strongly onto particle surfaces, changing their surface properties and promoting aggregation.

(3) Patch flocculation mechanism
Positively charged regions on polymer chains attract negatively charged particles, forming microflocs.

(4) Aggregation and floc growth
Once destabilized, particles collide and form larger flocs that settle more easily in sedimentation tanks.

4. Application process in drinking water plants

Polydadmac is used in the conventional water treatment process, which includes several stages:

(1) Raw water intake

Surface water from rivers, lakes, or reservoirs is collected and screened to remove large debris.

(2) Rapid mixing (coagulation stage)

Polydadmac is added into the rapid mixing chamber. It immediately reacts with suspended particles and dissolved organic matter, destabilizing colloids.

(3) Flocculation stage

Gentle mixing allows microflocs to grow into larger, denser flocs.

(4) Sedimentation

Flocs settle under gravity in clarifiers, producing clearer water.

(5) Filtration

Remaining fine particles are removed using sand filters, activated carbon filters, or membrane systems.

(6) Disinfection

Final treatment step using chlorine, ozone, or UV ensures microbiological safety.

5. Advantages of Polydadmac in drinking water purification

(1) High turbidity removal efficiency
Effectively removes suspended solids and colloidal particles even at low dosages.

(2) Reduction of natural organic matter (NOM)
Helps remove humic and fulvic acids, reducing color and improving taste.

(3) Lower formation of disinfection by-products (DBPs)
By reducing organic precursors, it minimizes formation of harmful compounds such as trihalomethanes (THMs).

(4) Fast reaction speed
Rapid coagulation reduces hydraulic retention time in treatment systems.

(5) Lower sludge production
Compared with inorganic coagulants, it generates less sludge volume.

(6) Stable performance across variable water quality
Effective under changing seasonal conditions and raw water fluctuations.

6. Dosage and influencing factors

Typical dosage ranges in drinking water treatment:

• 0.5–10 mg/L for low turbidity water
• 1–20 mg/L for moderate turbidity surface water
• 5–30 mg/L for high turbidity conditions

Key influencing factors include:

• Raw water turbidity and particle size
• Natural organic matter concentration
• Water temperature
• pH level (usually effective between pH 5–9)
• Mixing intensity and contact time
• Seasonal variation (rainy vs dry season)

Jar testing is essential to determine optimal dosage for each water source.

7. Combination with other treatment chemicals

Polydadmac is frequently used in combination with other coagulants and treatment chemicals:

(1) Polyaluminum chloride (PAC):
Enhances coagulation strength and improves removal efficiency of turbidity and organic matter.

(2) Aluminum sulfate (alum):
Used in traditional systems to improve floc formation and reduce cost.

(3) Polyacrylamide (PAM):
Improves floc size and settling speed during flocculation.

(4) Activated carbon:
Used for additional removal of organic compounds, odor, and taste issues.

This combined approach ensures higher efficiency and more stable water quality.

8. Safety and regulatory considerations

Because Polydadmac is used in drinking water production, it must comply with strict safety standards:

• Must meet potable water treatment regulations (NSF/ANSI or equivalent standards)
• Residual polymer levels must be controlled
• Proper dosing ensures no harmful by-products are formed
• It is considered safe when used within recommended limits

Water treatment plants must ensure proper monitoring and control during application.

9. Limitations and considerations

(1) Overdosing risk
Excess Polydadmac can cause charge reversal, leading to poor floc formation and reduced water clarity.

(2) Sensitivity to water quality changes
Performance may vary with seasonal or source water changes.

(3) Cost compared to inorganic coagulants
More expensive than alum or PAC, but often more efficient at lower dosages.

(4) Need for precise control
Requires proper dosing systems and monitoring.

10. Operational best practices

To ensure optimal performance in drinking water treatment:

• Conduct routine jar testing
• Adjust dosage based on raw water changes
• Maintain proper rapid and slow mixing conditions
• Avoid overdosing
• Combine with PAC or PAM when necessary
• Continuously monitor turbidity and organic content

11. Future trends

The use of Polydadmac in drinking water purification is expected to increase due to:

• Stricter drinking water quality regulations
• Rising concern over disinfection by-products
• Increased demand for water reuse and reuse systems
• Development of hybrid polymer coagulants
• Advancements in membrane-based water treatment

Future developments will focus on improved efficiency, lower dosage requirements, and enhanced environmental safety.