Sedimentation is the tendency for particles in suspension to settle out of the fluid in which they
are entrained, and come to rest against a barrier. This is due to their motion through the fluid in
response to the forces acting on them: these forces can be due to gravity, centrifugal acceleration
or electromagnetism.
Classification of sedimentation:
1. Unhindered settling. Sedimentation is characterized by particles that settle discretely
at a constant settling velocity. They settle as individual particles and do not flocculate or stick
to other during settling.
Example: sand and grit material.
2. Settlement of flocculent particles. Sedimentation is characterized by particles that
flocculate during sedimentation and because of this their size is constantly changing and
therefore their settling velocity is changing.
Example: alum or iron coagulation
3. Zone sedimentation. In this process the particles are at a high concentration (greater than
1000 mg/L) such that the particles tend to settle as a mass and a distinct clear zone and sludge
zone are present. Zone settling occurs in lime-softening, sedimentation, active sludge
sedimentation and sludge thickeners.
4. Compression settling. The settling particles can contact each other andarise when approaching
the floor of the sedimentation tanks at very high particle
concentrations. In compression zone, the settled solids are compressed by gravity.
