What is flocculation?

Flocculation is the next step after coagulation in wastewater treatment. Once the waste particles have agglomerated through coagulation, flocculants can be used to remove the agglomerates.

Definition of flocculation

Flocculation is mainly a mechanical process. This process is designed to promote the mixing of particles in a liquid. We use polymers to create a bond between the flocs, thus speeding up handling. These flocs, or clusters of particles created by flocculation, can precipitate after resting and are easy to remove.

Flocculation is an important step in water treatment, always followed by coagulation. This process is followed by settling to separate the water from impurities.

Flocculation for water treatment

Flocculation process

Flocculation is a process, mostly mechanical, designed to bring together clusters of suspended matter that have already coagulated. This process therefore requires a coagulation step before it can function. After flocculation, we obtain the formation of large flocs (clusters of materials bound together). Eventually, these flocs will become precipitates and settle to the bottom. This is the process used for particle filtration in water treatment plants. It is combined with coagulation in the first step of water treatment.

During flocculation, the solution is gently mixed to bring the particles to be removed into contact with each other. The particles, often invisible to the naked eye, will combine until they become visible and therefore easy to remove.

Heavy organic polymers (with a high molecular weight) are added to make this operation even more efficient. Among other things, this addition allows :

  • make the operation faster and more efficient by creating bridges between flocs, while increasing their weight
  • make the settling process easier

When the floc reaches its ideal size (one hour in WWTPs), the flocculation process comes to an end. This is where the water is separated, with processes such as settling.


Flocculants are water-soluble chemical agents that ensure the bond between flocs. They often have a long carbon chain to give them a high molecular weight. This characteristic makes them ideal for sedimentation of fine particles in water. They can be of different types:

  • Natural flocculants are increasingly popular for wastewater treatment due to their environmentally-friendly properties. The current situation and trends suggest good prospects for this type of flocculant. These are combinations of proteins and polysaccharides with carboxyl, hydroxyl and amine groups. They have a high molecular weight, making them promising flocculants. They are produced from biodiesel or bioethanol.
  • On the other hand, petroleum-based synthetic flocculants will undergo, and are already undergoing, a gradual decline in interest.

There is also another distinction concerning polymer loading. This can be positive or negative, resulting in anionic or cationic polymers respectively. And both play an indispensable role in flocculation.

It’s also important to know that water characteristics (temperature, PH, etc.) will influence polymer performance.

Anionic polymers

Anionic polymers are water-soluble molecules involved in the flocculation process. They have high molecular weights and are negatively charged.

These properties will help them bind and neutralize positively charged particles. These particles include minerals, which are subsequently eliminated by a softening process.

Cationic polymers

Like anionic polymers, cationic polymers are soluble in water, which is involved in the flocculation process. They share the same property of high molecular weight. However, cationic polymers are positively charged, unlike anionic polymers. These properties enable them to bind and neutralize negatively charged particles in the water, such as :

  • seaweed
  • bacteria
  • limon
  • These properties enable them to bind and neutralize negatively charged particles in the water, such as :
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