What is coagulation?

Definition of coagulation

Coagulation is a process which allows different particles to stick together and form a compact block, known as a floc.

Small particles contained the same charge (negative) and therefore repel each other. With coagulation, a chemical (called a coagulant) removes fillers from the particles to facilitate their agglomeration.

Coagulation for water treatment

Coagulation process

Coagulation is the most common chemical process used in water treatment. It is mainly used to remove particles (suspended solids) from water, by manipulating the electrostatic charges of particles suspended in water. This process introduces small, highly-charged molecules into the water to destabilize the charges of suspended particles, colloids or oily matter. Coagulation is used in many wastewater treatment plants, as its mechanism is highly effective for primary treatment. This process involves a chemical treatment of particles that do not sediment and that can be found in water. This involves the addition of a coagulant to induce agglutination of suspended particles. These coagulants are often charged with different energy(s) to those of the particles in suspension. This will reduce the electronegative character of the particles and thus their repulsive capacity. This way, they’ll be able to get in touch. This process allows you to extend the limits of settling. In fact, some materials increase water turbidity and are not sensitive to settling (clay). This coagulation phase is therefore a prerequisite. This coagulation is often followed by a mechanical phase called flocculation. Flocculation will force the neutralized particles to meet. This second phase is essential to ensure the effectiveness of the first treatment phase.


There are many different coagulants. They all have one thing in common: they have a charge opposite to that of suspended solids. These are frequently inorganic aluminum or iron salts. These include aluminum sulfate, ferric sulfate and aluminum chloride. Ferric and aluminum sulfates are the most commonly used.

Ferric chloride

Ferric chloride is the coagulant of choice for many industrial and sanitary wastewater treatment applications, due to its high clarification efficiency and usefulness as a sludge dewatering agent. Although it leaves a slight residual color, this chemical has excellent turbidity removal properties.

It can be found in many applications, both biological (blood elimination) and environmental (industrial and sanitary wastewater treatment, sludge dewatering, metal precipitation, etc.).

Here are a few of the coagulant’s physico-chemical properties. Available in liquid form, it has a slightly pungent odor, a reddish color and a pH below 1, making it highly acidic.

Alumina sulfate

Alumina sulfate’s strength is based on its flexibility. It is also the most versatile chemical used in municipal and industrial markets. It is found in both water cycles:
  • Wastewater for clarification and phosphorus removal
  • Lakes and ponds to remove excess nutrients, such as phosphates, which contribute to algae growth.
It can also be found in many factories:
  • paper reprocessing
  • textile industry used to fix colorants to fabrics without altering their quality.
There are many other uses for this coagulant, such as the production of synthetic catalysts or the regulation of ammonia in poultry houses. Alum is the most widely used coagulant in drinking water and wastewater applications. Combined with the flocculation process, alum eliminates TSS, BOD, turbidity and total organic carbon.
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