Environmental Engineering: Fundamentals and Design - Chapter 10

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TOC > Ch. 10 > 10.4 Coagulation and flocculation

# Chapter 10 Drinking Water Treatment

David W. Hand and Qiong Zhang

## Coagulation and flocculation

### Coagulation

Solids are removed by sedimentation (settling) followed by filtration. Small particles are not removed efficiently by sedimentation because they settle too slowly; they may also pass through filters. They would be easier to remove if they clumped together (coagulated) to form larger particles, but they don't because they have a negative charge and repel each other (like two north poles of a magnet).

In coagulation, we add a chemical such as alum which produces positive charges to neutralize the negative charges on the particles. Then the particles can stick together, forming larger particles which are more easily removed.

The coagulation process involves the addition of the chemical (e.g. alum) and then a rapid mixing to dissolve the chemical and distribute it evenly throughout the water.

### Flocculation

Now that the particles have a neutral charge and can stick together. The water flows into a tank with paddles that provide slow mixing and bring the small particles together to form larger particles called flocs. Mixing is done quite slowly and gently in the flocculation step. If the mixing is too fast, the flocs will break apart into small particles that are difficult to remove by sedimentation or filtration.

#### Credits

Treatment illustrations: © 2007 State of Michigan.
Coagulation animation: © 2007 State of Michigan.
Flocculation animation: © 2007 State of Michigan.

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