Sedimentation (water Treatment) - Basics


Suspended solids (or SS), is the mass of dry solids retained by a filter of a given porosity related to the volume of the water sample. This includes particles of a size not lower than 10 μm.

Colloids are particles of a size between 0.001 µm and 1 µm depending on the method of quantification. Because of Brownian motion and electrostatic forces balancing the gravity, they are not likely to settle naturally.

The limit sedimentation velocity of a particle is its theoretical descending speed in clear and still water. In settling process theory, a particle will settle only if:

  1. In a vertical ascending flow, the ascending water velocity is lower than the limit sedimentation velocity.
  2. In a longitudinal flow, the ratio of the length of the tank to the height of the tank is higher than the ratio of the water velocity to the limit sedimentation velocity.

Removal of suspended particles by sedimentation depends upon the size and specific gravity of those particles. Suspended solids retained on a filter may remain in suspension if their specific gravity is similar to water while very dense particles passing through the filter may settle. Settleable solids are measured as the visible volume accumulated at the bottom of an Imhoff cone after water has settled for one hour.

There are four types of sedimentation processes:

  • Type 1 – Dilutes, non-flocculent, free-settling (every particle settles independently.)
  • Type 2 – Dilute, flocculent (particles can flocculate as they settle).
  • Type 3 – Concentrated suspensions, zone settling, hindered settling (sludge thickening).
  • Type 4 – Concentrated suspensions, compression (sludge thickening).

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