Coagulation
- Also known as
- coagulant
Coagulation is a water treatment step in which chemicals called coagulants neutralize the electrical charges that keep fine suspended particles apart, allowing them to begin clumping together. It is usually followed by flocculation, which grows the clumps into flocs that can settle or be filtered.
What is coagulation?
Very fine particles, such as clay, silt, organic matter and microorganisms, stay suspended in water because they are tiny and usually carry a negative surface charge that makes them repel each other. They cause turbidity and will not settle on their own in any practical time.
Coagulation adds a chemical that neutralizes those charges. With the repulsion removed, particles can stick together when they collide.
Common coagulants
- Aluminum salts, such as aluminum sulfate (alum) and polyaluminum chloride.
- Iron salts, such as ferric chloride and ferric sulfate.
- Cationic organic polymers, sometimes used alone or with metal salts.
Metal salt coagulants also form metal hydroxide precipitates that sweep particles out of the water as they form.
Coagulation and flocculation
The two steps run in sequence:
| Coagulation | Flocculation | |
|---|---|---|
| Purpose | Neutralize particle charges | Grow particles into large flocs |
| Mixing | Rapid, intense mixing to disperse the coagulant | Slow, gentle mixing to promote collisions without breaking flocs |
| Typical chemicals | Metal salts, cationic polymers | High molecular weight polymers (flocculants) |
The flocs are then removed by sedimentation, dissolved air flotation or filtration.
Factors that affect coagulation
Coagulant dose, pH, water temperature, alkalinity and the type of particles all matter. Operators commonly use jar tests, small-scale trials of different doses, to choose the right conditions.
Frequently asked questions
Yes. It is used in wastewater treatment, industrial process water, and in mining for clarifying water from tailings and thickeners.
Overdosing can reverse the particle charge and restabilize the suspension, as well as wasting chemicals and increasing sludge.
Sources
- Water treatment process overview, U.S. Environmental Protection Agency
- Water Quality and Treatment, American Water Works Association (AWWA)