How Are Poly(dimethyldiallylammonium chloride)–ferric Sulfate Used for Composite Flocculation?

As global water treatment standards continue to rise, industries such as industrial wastewater, municipal sewage, drinking water purification, and sludge treatment demand increasingly efficient, stable, and cost-effective water treatment chemicals. Single-component flocculants often struggle to simultaneously achieve multiple objectives, including advanced phosphorus removal, efficient solid-liquid separation, low-temperature adaptability, reduced sludge production, and operational cost control.

The inorganic-organic composite flocculation system formed by poly(diallyldimethylammonium chloride) (PDADMAC) and ferric sulfate has emerged in recent years as a widely adopted and consistently effective solution in water treatment engineering. This system leverages the synergistic effects of electrostatic neutralization, adsorption bridging, entrapment and sweeping, and complexation precipitation.

1. Material Properties of Poly(diallyldimethyl)ammonium Chloride with Ferric Sulfate

1.1 Structure and Functional Characteristics of Ferrous Sulfate

Ferrous sulfate is a classic inorganic polymeric coagulant that rapidly hydrolyzes in water to form polyhydric hydroxy iron complexes and Fe(OH)₃ colloids. Its primary characteristics include:

High positive charge density provides strong electrostatic neutralization of negatively charged colloids, suspended solids, and microorganisms.
Exhibits specific precipitation with phosphate ions, delivering significant deep phosphorus removal.
Capable of co-precipitation removal of color, humic acids, oils, and certain heavy metals.
Suitable for a wide pH range, with stable sourcing and controllable costs.
No risk of aluminum ion residue, suitable for drinking water treatment.

However, when used alone, ferrous sulfate exhibits significant limitations: its floc particles are small, structurally loose, and settle slowly. Efficiency drops markedly under low-temperature and low-turbidity conditions. Higher dosages increase sludge production and subsequent dewatering pressure.

1.2 Structure and Functional Characteristics of Poly(diallyldimethyl)ammonium Chloride

Poly(diallyldimethylammonium chloride) (PDADMAC) is a cationic organic polymer flocculant characterized by high charge density and a linear structure. Its molecular chain contains numerous quaternary ammonium cations, offering excellent water solubility, strong stability, non-toxicity, and biodegradability. Key advantages include:

Outstanding adsorption and bridging capacity, rapidly aggregating microflocs into large flocs.
Excellent molecular chain extensibility, maintaining stability across a wide pH range (4–10).
Strong chlorine and salt tolerance, suitable for high-salinity wastewater and complex industrial water quality.
Low dosage requirement, with minimal increase in sludge volume.
Compliant with drinking water treatment standards, ensuring high safety.

When used alone, PDADMAC exhibits limited total phosphorus removal efficiency and insufficient cost-effectiveness for highly turbid or colloid-laden wastewater. Therefore, it is more suitable for use in combination with inorganic coagulants.

2. Synergistic Mechanism of Poly(diallyldimethylammonium chloride)–Ferric Sulfate

2.1 Electrostatic Neutralization

Colloidal particles, bacteria, and minute suspended particles in wastewater typically carry negative surface charges, causing mutual repulsion that maintains their stable dispersion. The hydrolysis of ferric sulfate generates abundant hydroxyl iron complexes with high positive charges, which rapidly neutralize colloidal surface charges, reduce zeta potential, destabilize colloids, and promote aggregation to form initial microflocs.

2.2 Adsorption and Bridging

PDADMAC is a long-chain linear cationic polymer. Its abundant positive charges on the molecular chain can adsorb multiple destabilized particles or microflocs, linking them into a three-dimensional network floc structure. This bridging action rapidly increases floc particle size and density, serving as the core mechanism for enhancing sedimentation velocity.

2.3 Entrapment and Sweeping Action

Fe(OH)₃ flocs exhibit strong sweeping, enveloping, and adsorption capabilities during sedimentation, enabling the co-precipitation of suspended particles, organic matter, colloids, colorants, and some dissolved pollutants. PDADMAC-enhanced flocs feature denser structures and more pronounced entrapment effects.

2.4 Complexation Precipitation and Advanced Phosphorus Removal

Ferric sulfate reacts chemically with phosphate ions in water to form insoluble iron phosphate precipitates, achieving advanced phosphorus removal. PDADMAC further enhances the settling efficiency of phosphate flocs through adsorption bridging, preventing the loss of fine flocs and ensuring stable compliance with total phosphorus (TP) standards in effluent.

2.5 Floc Structure Optimization

Flocs formed by iron salts alone are fine, fragile, and settle slowly. Adding PDADMAC significantly increases floc particle size, strength, and compactness, accelerating settling velocity by 40%–80%. This reduces sedimentation tank load and stabilizes effluent SS levels.

3. Process Control for the Compounding of Poly(diallyldimethyl)ammonium Chloride and Ferric Sulfate

3.1 Dosage Sequence (Engineering Core)

Correct Sequence: Add ferric sulfate first → Rapid mixing → Reaction → Then add PDADMAC

Reason:

Ferric sulfate first completes electrostatic neutralization, phosphorus removal, and microfloc formation.
Adding PDADMAC afterward maximizes its bridging effect.
Reversing the sequence causes PDADMAC to prematurely occupy binding sites, preventing effective coagulation by iron salts and significantly reducing efficacy.

3.2 Dosage Ratios (Engineering Experience Range)

Municipal Sewage / Drinking Water: Ferric sulfate : PDADMAC = 20:1 ~ 50:1

Industrial Wastewater (Chemical, Textile Dyeing, Electroplating, Mining): Ferrous sulfate : PDADMAC = 10:1 ~ 30:1

Sludge Dewatering Conditioning: Ferrous sulfate : PDADMAC = 12:1 ~ 20:1

Excessive PDADMAC may cause deflocculation, increased water viscosity, and suspended solids floating to the surface. Therefore, the optimal dosage range must be strictly controlled.

3.3 Dissolution and Dosage Concentration

Ferric sulfate: Prepare a 5%–10% aqueous solution
PDADMAC: Prepare a 0.1%–0.5% dilute solution

Dilute solutions facilitate uniform mixing, preventing localized high concentrations that cause performance fluctuations.

3.4 pH Control

Optimal operating pH for composite system: 6.5–8.0. Within this range:

Iron salts undergo complete hydrolysis, achieving maximum phosphorus removal efficiency
PDADMAC molecular chains remain stable, delivering optimal bridging effects
Floc structure becomes compact, yielding superior settling performance

3.5 Agitation Intensity and Time Control

Rapid agitation: 200–350 r/min, 20–60 seconds (iron salt mixing).
Medium-speed agitation: 80–120 r/min, 1–3 minutes (microfloc formation).
Low-speed agitation: 30–50 r/min, 3–10 minutes (PDADMAC bridging).
Settling time: 5–15 minutes.

Excessive agitation breaks flocs; insufficient agitation causes uneven mixing and low chemical utilization.

4. On-Site Effect Evaluation and Troubleshooting

4.1 Ideal Conditions

After adding ferric sulfate: Fine brown microflocs appear.
After adding PDADMAC: Flocs rapidly enlarge, densify, and settle quickly.
Supernatant becomes clear and transparent, with no viscous sensation.
Effluent SS, TP, COD, and color simultaneously decrease.

4.2 Common Issues and Solutions

The flocs are fine and settle slowly.

Causes: Insufficient PDADMAC dosage.
Countermeasure: Slightly increase PDADMAC concentration or dosage.

Water becomes viscous, supernatant appears cloudy, suspended solids float to the surface.

Causes: Excessive PDADMAC causing deflocculation.
Countermeasure: Reduce PDADMAC dosage.

Poor removal efficiency for total phosphorus.

Causes: Insufficient iron sulfate, pH deviation, inadequate reaction time.
Countermeasure: Increase iron salt dosage, adjust pH to 6.5–8.0.

Flocs are fragile, and sludge escapes from the sedimentation tank.

Causes: Excessive stirring intensity, mismatched PDADMAC molecular weight.
Countermeasure: Reduce slow-speed stirring speed, replace with PDADMAC of appropriate molecular weight.

5. Poly(diallyldimethyl)ammonium chloride–ferrous sulfate in engineering applications across various industries

5.1 Municipal Wastewater Treatment

Advanced phosphorus removal, consistently achieving TP ≤ 0.3 mg/L or 0.5 mg/L.
Enhanced secondary sedimentation tank efficiency, reducing sludge bulking.
Lowered sludge moisture content, minimizing chemical consumption for dewatering.
Improved system resilience against shock loads.

5.2 Industrial Wastewater Treatment

Textile Dyeing Wastewater

Strong decolorization, removal of colloidal and anionic dyes.
Enhanced COD removal efficiency, reduced biochemical load.

Chemical Wastewater

Removes emulsified oils, suspended colloids, and dissolved organic matter.
Highly adaptable to complex saline water quality.

Electroplating Wastewater

Co-precipitates with heavy metal ions to enhance effluent stability.
Reduces suspended solids and heavy metal residues.

Mining and Mineral Processing Wastewater

Rapidly clarifies tailings water for reuse.
Produces dense flocs with excellent filter press performance.

Paper Mill Wastewater

Captures anionic impurities to improve white water recovery efficiency.
Minimizes fiber loss and reduces membrane system fouling.

5.3 Raw Water Purification for Drinking Water

Algae removal, turbidity reduction, and elimination of natural organic matter.
No aluminum residue for enhanced safety.
More stable filter operation with reduced backwash frequency.

5.4 Sludge Dewatering Conditioning

Improved sludge filterability and reduced cake moisture content.
Minimized filter cloth clogging for extended service life.
Increased dewatering machine throughput

5.5 Membrane System Pretreatment

Reduces membrane fouling rate, extends chemical cleaning intervals.
Improves membrane flux and system stability.
Lowers membrane maintenance costs.

6. Economic Efficiency and Comprehensive Advantages of the Composite Flocculation System

6.1 Lower Chemical Costs

PDADMAC dosage is extremely low, only 1–10 mg/L.
Reduces ferric sulfate consumption by 10%–40%.
Total cost per ton of treated water decreases by 15%–40%.

6.2 Reduced Sludge Volume

Organic polymers partially replace inorganic chemicals.
Form denser flocs with lower moisture content.
Significantly lower sludge dewatering and disposal costs.

6.3 Enhanced System Stability

Strong adaptability to temperature, turbidity, and salinity fluctuations.
Outperforms single iron salts under low-temperature conditions.
More stable effluent parameters and simplified operation/maintenance.

6.4 Safety and Compliance

PDADMAC complies with GB/T 22905-2022 drinking water standards.
Ferrous sulfate meets GB 14591-2006 requirements.
Non-toxic, formaldehyde-free, no acrylamide monomer risk.
Suitable for municipal, tap water, food processing, and other sectors.

Conclusion

The composite flocculation system combining polydiallyldimethylammonium chloride (PDADMAC) and ferric sulfate represents a synergistic technology with a well-understood mechanism, remarkable efficacy, outstanding cost-effectiveness, and broad applicability in contemporary water treatment engineering. Ferrous sulfate delivers efficient electrostatic neutralization, advanced phosphorus removal, and entrapment capabilities, while PDADMAC significantly enhances floc structure and settling performance through strong adsorption bridging. Their synergy achieves high-efficiency pollutant removal and optimized operational costs.

In practical applications, strict control of four core parameters—addition sequence, dosage ratio, pH value, and mixing intensity—ensures stable and efficient system operation across municipal wastewater, industrial effluent, drinking water, sludge dewatering, and membrane pretreatment scenarios.

As a specialized chemical enterprise, we continuously invest in flocculant R&D, formulation optimization, and field service support to deliver more efficient, stable, and economical water treatment solutions. The PDADMAC-ferrous sulfate composite flocculation technology will continue to provide critical support for water environment management and green industrial development.