1. Definition of Cationic Coagulants

Cationic coagulants (or flocculants) are a type of high-molecular-weight or inorganic-organic composite agents that can ionize in water and carry a positive charge. They are primarily used in coagulation, flocculation, and solid-liquid separation processes during water treatment.

Their core function is to neutralize the surface charge of negatively charged suspended particles, colloids, and dissolved organic matter in water, destabilizing them and causing aggregation into larger flocs. This facilitates removal through sedimentation, flotation, or filtration.

In practical engineering, cationic coagulants are widely applied in:

  • Municipal sewage treatment
  • Industrial wastewater treatment
  • Sludge dewatering
  • Papermaking white water recovery
  • Dyeing wastewater decolorization

They are essential functional chemicals in water treatment systems.

2. Working Principles of Cationic Coagulants

2.1 Charge Neutralization

Most colloidal particles and organic pollutants in water carry a negative surface charge, remaining stably dispersed due to electrostatic repulsion. After dissociating in water, cationic coagulants release positively charged active groups that rapidly neutralize these negative charges, reducing or eliminating the zeta potential of the particle surfaces. This destabilizes the colloidal system and initiates aggregation.

2.2 Adsorption and Bridging

High-molecular-weight cationic coagulants (e.g., cationic PAM, PolyDADMAC) have long molecular chains. While adsorbing onto the surface of one particle, the unadsorbed portion of the chain can extend and capture other particles, forming "bridges" between them. This leads to the formation of larger, denser flocs, accelerating sedimentation or flotation.

2.3 Enmeshment and Sweep Flocculation

When used in combination with inorganic coagulants (e.g., polyaluminum chloride, aluminum sulfate), cationic coagulants can participate in forming a three-dimensional network structure. This creates a "sweeping" and "enveloping" effect on fine particles in the water, further improving removal efficiency.

3. Main Classification of Cationic Coagulants

3.1 Inorganic Cationic Coagulants

Represented by metal salts:

  • Aluminum sulfate (Al₂(SO₄)₃)
  • Aluminum chloride (AlCl₃)
  • Polyaluminum chloride (PAC, partially hydrolyzed to exhibit positive charge)

Characteristics:

  • Low cost, high dosage required
  • Good adaptability to various water qualities
  • Generate larger sludge volumes
  • Performance significantly affected by temperature and pH

3.2 Organic High-Molecular-Weight Cationic Coagulants

Mainly include:

  • Cationic polyacrylamide (CPAM)
  • Polydiallyldimethylammonium chloride (PolyDADMAC)
  • Polyethylenimine (PEI)
  • Polyamine-based coagulants

Characteristics:

  • Adjustable charge density
  • Wide range of molecular weights
  • High flocculation efficiency, low dosage required
  • Suitable for wastewater with high organic loads, low turbidity, or difficult-to-treat characteristics

3.3 Composite Cationic Coagulants

Formed by blending inorganic salts and organic polymers:

  • Organic-inorganic composite coagulants
  • Modified PAC + cationic polymers

Characteristics:

  • Combine the rapid charge neutralization of inorganic coagulants with the strong adsorption/bridging capabilities of organic coagulants
  • Suitable for applications with highly variable water quality or stringent treatment requirements

4. Typical Application Areas

4.1 Municipal Wastewater Treatment

Used in primary sedimentation tanks, secondary sedimentation tanks, and sludge dewatering stages to improve removal rates of:

  • Suspended solids (SS)
  • Chemical oxygen demand (COD)
  • Total phosphorus (TP)

While enhancing sludge dewaterability.

4.2 Industrial Wastewater Treatment

Widely used in:

  • Papermaking wastewater (retention, deinking, white water recovery)
  • Dyeing wastewater (decolorization, surfactant removal)
  • Petrochemical wastewater (emulsified oil demulsification, flocculation)
  • Electroplating wastewater (removal of heavy metal complexes)

4.3 Sludge Dewatering

Cationic PAM is the preferred agent in sludge dewatering. It significantly reduces sludge moisture content and improves the efficiency of:

  • Belt filter presses
  • Centrifuges
  • Plate-and-frame filter presses

4.4 Raw Water and Drinking Water Treatment

In treating low-turbidity or organically contaminated raw water, PolyDADMAC is often used in combination with PAC for:

  • Enhanced coagulation
  • Reducing effluent turbidity
  • Lowering organic content

4.5 Other Specialized Fields

  • Clarification of mining and coal washing wastewater
  • Food processing wastewater treatment
  • Pretreatment of pharmaceutical wastewater
  • Retention and drainage aids in paper pulp

5. Summary

Cationic coagulants achieve efficient removal of suspended particles and organic pollutants from water through multiple mechanisms including:

  • Charge neutralization
  • Adsorption/bridging
  • Enmeshment/sweep flocculation

Depending on water quality conditions and treatment objectives, appropriate cationic coagulants can be selected from:

  • Inorganic types
  • Organic high-molecular-weight types
  • Composite types

Key to success: Scientific selection and proper dosing are crucial for optimal performance. For complex water quality or high treatment requirements, we recommend:

  • Conducting jar tests
  • Developing customized formulations

For tailored cationic coagulant solutions based on your specific water quality, please contact our technical team for professional selection and application support.