Course: BFM 203 Water Treatment and Control

Lecture

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Introduction to Sedimentation

Note: All photos courtesy of the RRCC Water Quality Management Program.

The sedimentation process follows the coagulation/flocculation process. The purpose of sedimentation is to remove suspended solids that are denser than water. Suspended solids may be in a natural state (bacteria, clay, silt, etc.), floc, or precipitate (hardness and iron precipitates when oxidation chemicals are added). Sedimentation should be able to remove a very high percentage of suspended solids, increasing efficiency of the subsequent filtration step.

How It Works

When water is almost still in sedimentation basins, settleable solids will move toward the bottom of the basin. Sedimentation is accomplished by decreasing the velocity of water to a rate in which it can no longer transport suspended material. Gravity pulls particles to the bottom of the sedimentation basin where it can be removed. Plate and tube settlers increase the efficiency of the settling process and require much less space than traditional settling basins.

Tube settlers (as shown in the photo to the right) force water to change directions and flow up through the tubes. Some particles change directions less easily than others and settle to the bottom of the basin. Lighter particles begin to stick to the tube walls and form a film that eventually sloughs off and falls to the bottom as well.

Factors Affecting Sedimentation

A number of factors affect sedimentation, including

- Particle size and distribution
- Particle shape
- Particle density
- Water temperature (viscosity and density)
- Particle charge
- Substances dissolved in water
- Suspended material flocculation characteristics
- Environmental conditions
- Sedimentation basin hydraulic and design characteristics

Proper selection and application of coagulant chemicals can overcome may of these factors by forming particles into a settleable floc.

Nature of Particles

Particle nature is dictated by source water condition. Sand and silt greater than 10 microns in size can generally be settled. Smaller particles must be treated into floc to be settled. Spherical particles may settle more quickly than irregularly shaped particles because there is less friction (drag) on spherical particles. Particle charge may also affect settling. Remember that particles may have some sort of electrical charge. Particles with the same charge tend to repel each other, keeping them stirred up. Thus, they do not congregate into floc and settle.

Observing floc condition and water clarity in settling basins is important when evaluating effectiveness of coagulation, flocculation, and settling processes.