The application of flocculants in modern water treatment is very common. However, many peers do not fully understand the action mechanism of flocculants or have a relatively one-sided understanding. This article will analyze the action mechanism of flocculants — explaining exactly what the difference is between flocculants and coagulants!

I. Coagulation

Coagulation: mainly refers to the process of colloid destabilization and the formation of tiny aggregates. The action mechanisms of coagulation are generally explained by four theories: compression of the double electron layer, adsorption-charge neutralization, adsorption bridging, and net-trapping/sweeping.

1. Compression of the Double Layer

According to DLVO theory, when an electrolyte containing high-valent positive ions is added, the high-valent positive ions enter the surface of the colloidal particles through electrostatic attraction, replacing the original low-valent positive ions. In this way, the double layer remains electrically neutral, but the number of positive ions decreases, which means the thickness of the double layer becomes thinner, and the ζ potential on the sliding surface of the colloidal particles decreases.

When the ζ potential drops to 0, it is called an isoelectric state, at which point the repulsion barrier completely disappears. When the ζ potential drops to a certain value such that the energy barrier Emax on the total potential energy curve of the colloidal particles is 0, the colloidal particles aggregate. This ζ potential is called the critical potential ζk.

2. Adsorption-Charge Neutralization

The surface of colloidal particles adsorbs ions of opposite signs, colloidal particles of opposite signs, or polymers with opposite charges, thereby neutralizing part of the charge carried by the colloidal particles themselves, reducing the electrostatic attraction between the colloidal particles, and making them easier to aggregate and settle. The driving forces include electrostatic attraction, hydrogen bonds, coordinate bonds, and Van der Waals forces. This can explain the re-stabilization phenomenon of colloidal particles in water treatment.

3. Adsorption Bridging

Colloidal particles in a dispersed system are connected by bridging through the adsorption of organic or inorganic polymer substances, aggregating into large clusters and destabilizing for settlement. This is divided into long-chain polymer bridging and short-distance bridging. There are three types:

• Bridging between colloidal particles and uncharged polymer substances, involving adsorption forces such as Van der Waals forces, hydrogen bonds, and coordinate bonds.

• Bridging between colloidal particles and polymer substances with opposite charges, involving charge neutralization in addition to Van der Waals forces, hydrogen bonds, and coordinate bonds.

• Bridging between colloidal particles and polymer substances with the same charge, through the "electrostatic patch" effect.

4. Net-trapping and Sweeping

Coagulants such as aluminum salts and iron salts added to water form a large amount of hydrated metal oxide precipitates with three-dimensional structures after hydrolysis. When these hydrated metal oxide volumes contract and settle, they capture and sweep down colloidal particles and suspended matter particles in the water like a net. Net-trapping and sweeping is primarily a mechanical action.

II. Flocculation

Flocculation: Flocculation mainly refers to the process in which destabilized colloids or tiny suspended solids aggregate into large flocs.

1. Perikinetic Flocculation: Collision and aggregation of colloidal particles caused by Brownian motion. Brownian motion gradually weakens as the particle size increases. When the particle size grows to a certain dimension, Brownian motion no longer plays a role.

2. Orthokinetic Flocculation: Collision and aggregation of colloidal particles driven by external force. Colloidal particles move in a certain direction under the action of external force. Due to the speed difference between different colloidal particles, the collision and aggregation of particles are completed.

III. Coagulation

Coagulation includes both the action of destabilization and the action of aggregation. It is the general term for the two processes. It is the aggregation process of colloidal particles and tiny suspended solids in water.

In other words, "Coagulation" covers the entire process from the addition of chemicals to the raw water to water mixing, chemical reaction and finally to the formation of large particle flocs. While "Flocculation" refers to the stage from the formation of tiny flocs to the formation of large flocs after the colloidal particles are destabilized.

IV. Distinction Between Flocculation and Coagulation

The distinction and definition of flocculant names are very clear, based on the primary role they play in practical applications. Those that play a role in colloid destabilization are called coagulants; those that play a role in the aggregation of destabilized colloids or tiny suspended solids are called flocculants; and those that both play a role in colloid destabilization and the aggregation of destabilized colloids into large particles are called coagulants.

For example, inorganic polymer flocculants such as PAC and PFS generally play a role in colloid destabilization in coagulation-sedimentation, so in this context, PAC and PFS should be called coagulants. However, when the sludge flocs are poor and PAC or PFS are added to increase flocculation by using their bridging effect to flocculate disintegrated sludge together, they play a flocculation role and are defined as flocculants. PAM has both charge neutralization and net-trapping effects, and also acts as a flocculant, so it is generally called a coagulant.

In many international contexts, the names are not as clearly distinguished and are generally just called flocculants. In actual practice, water treatment personnel also generally call them flocculants without special distinction. Moreover, there is no need to distinguish them too clearly; a name is just a title, and if everyone is accustomed to calling them that, there is nothing wrong with it.

V. YexSensor Digital Monitoring Solution for Dosing Systems

VI. Frequently Asked Questions (FAQ)

Q1: Why is it important for a system integrator to distinguish between Perikinetic and Orthokinetic flocculation?
   A: Perikinetic is molecular and cannot be controlled. Orthokinetic is mechanical; integrators must design stirring speed and paddle types to match the flow rate, ensuring particles collide without breaking apart the flocs.

Q2: How does the ζ potential relate to sensor selection?
   A: While zeta potential meters are lab-based, the result is reflected in turbidity. YexSensor's YEX-ZS-206 can detect the onset of successful coagulation by monitoring the initial formation of tiny aggregates in the mixing zone.

Q3: Can PAC and PAM be added at the same point?
   A: No. In an integrated system, PAC requires high-speed mixing for rapid dispersion and destabilization, while PAM requires low-speed mixing to prevent the long-chain polymers from being sheared apart.

Q4: What is the risk of "over-dosing" in a coagulation system?
   A: Excess dosing can cause charge reversal, leading to the re-stabilization of colloidal particles. Real-time turbidity feedback prevents this wastage and system failure.

Q5: Why use RS485 Modbus RTU sensors for dosing control?
   A: High-power motors and dosing pumps create significant EMI. Digital signals are more robust than 4-20mA, ensuring the PLC receives precise data for the PID dosing loop.

Q6: Is net-trapping effective for all water types?
   A: It is most effective when the concentration of colloidal particles is high. In low-turbidity water, it is harder to form a "net," so the mechanism shifts more towards adsorption-charge neutralization.

Q7: How do YexSensor products handle the high viscosity of concentrated PAM?
   A: Sensors like the YEX-ZS-206 are designed with IP68 protection and automatic mechanical wipers that periodically clean the optical window, preventing PAM build-up and measurement drift.

Q8: Does "Coagulation" refer to the chemicals or the equipment?
   A: In a project context, it usually refers to the entire unit process, including the chemicals, the rapid mixers, and the slow-speed flocculation tanks.

VII. Conclusion

In conclusion, the appearance of high-turbidity industrial waste is an external manifestation of severe ecological imbalance. Whether you call it coagulation or flocculation, the goal is to stabilize the system and remove pollutants. By understanding these mechanisms and deploying high-precision YexSensor monitoring tools, system integrators can ensure efficient, cost-effective, and long-term water treatment success.

Project Inquiries:
       For detailed technical data on our RS485 turbidity sensors or support for dosing system integration, please contact the YexSensor Engineering Department.