Sedimentation (primary sedimentation)

Sedimentation (primary sedimentation)

	Sedimentation (primary sedimentation) Sewage from the primary settling basin is now admitted into a huge tank called the primary sedimentation tank. It is a very large rectangular tank. Nearly 50-70% of the solids settle down under influence of gravity. A reduction between 30-40% in the number of coliforms is obtained. The organic matter which settles down is called sludge and is removed by mechanically operated devices (scrappers) without disturbing the operation in the tank.
Simultaneously a small amount of biological action also takes place in which microbes present in the sludge attack complex organic solids and break them down to simpler soluble substances and ammonia. A small amount of fat and grease rises to the surface to form a scum which is removed from time to time and disposed. Purpose of sedimentation is to remove suspended solids and thereby reduce the strength of the sewage. Sedimentation or clarification is the settling and removal of suspended impurities which occur when water stands still or flows slowly through a basin or tank. There is negligible turbulence and hence particles of high density tend to settle down (gravity) and form a sludge layer at the bottom of the tank whereas clarified water will be collected through the outlet. Hence sedimentation units play dual role:
Removal of settleable solids. Concentration of removed solids into sludge. There are three types of sedimentation tanks: Vertical flow tanks Horizontal flow tanks (most common) Circular flow tanks or radial flow tanks which receive sewage in the centre and their flow is towards the sides.

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