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
The treatment process
Coagulation is the chemical process widely used in water treatment. It is primarily used to remove particles (suspended solids) from water, by manipulating the electrostatic charges of particles suspended in the 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. It is a chemical process that targets 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. Indeed, some materials increase the turbidity of the water 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.
Coagulants
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 efficiency in clarification and its usefulness as a sludge dewatering agent. Although it leaves a slight residual color, this chemical has excellent turbidity removal properties.
It is found in many applications, both biological (blood disposal) and environmental (industrial and sanitary wastewater treatment, sludge dewatering, metal precipitation, etc.)
Here are a few of the coagulant’s physico-chemical properties. It is available in liquid form, it has a slightly pungent smell, reddish color, a pH of less than 1, so it is very acidic.
Alumina sulfate
- Wastewater to ensure phosphorus clarification and removal
- Lakes and ponds to remove excess nutrients, such as phosphates, which contribute to algae growth.
- paper reprocessing
- textile industry used to fix colorants to fabrics without altering their quality.
FAQ
What is the difference between an organic coagulant and a mineral coagulant?
Organic coagulants are generally synthetic or natural polymers that promote the agglomeration of fine particles without producing residual metals. They are often used to enhance floc formation in addition to a mineral coagulant. Mineral coagulants, such as aluminum sulphate or ferric chloride, act by neutralising charge and form insoluble precipitates that carry away suspended particles. The choice depends on the characteristics of the water to be treated, the pH, and the desired performance.
Why is the pH adjusted during coagulation treatment?
The pH strongly influences the effectiveness of coagulants. Some coagulants like aluminum salts are most effective in a specific pH range (usually between 5.5 and 7.5). Too high or too low a pH can prevent the proper formation of flocs, reduce the removal of heavy metals, or increase the solubility of compounds. For this reason, the pH is sometimes adjusted with acids or bases before coagulation.
How to choose the optimal dose of coagulant for water to be treated?
The ideal dose of coagulant is usually determined by a jar test in the laboratory. This test involves simulating different doses on small water samples to observe floc formation and final water clarity. An overdose can generate unnecessary chemical residues or unstable flocs, while an underdose will not be effective. The test helps to strike a balance between performance, cost and environmental impact.
What is the role of mechanical equipment in the coagulation-flocculation process?
Equipment such as dosing pumps, agitators or static mixers are crucial to ensure good processing efficiency. The pumps ensure precise dosing of the chemical reagents, while the agitators ensure homogeneous mixing in the different phases: fast mixing during coagulation and slow mixing during flocculation. Improper sizing or speed can compromise the entire treatment.
Is coagulation only used in wastewater treatment?
No, coagulation is also an essential step in the treatment of drinking water. It removes natural organic matter, fine particles, pathogens and turbidity. By reducing the pollutant load in the early stages, it facilitates filtration efficiency and limits the formation of toxic by-products during disinfection (such as trihalomethanes).
