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Sedimentation - Wastewaters as well as raw water contain suspended matter as impurities. Sedimentation or clarification is the settling and removal of suspended particles that take place when water stands still in, or flows slowly through a basin or tank.
Since the velocity of water flow is very low, turbulence will generally be absent or negligible. So particles that have a density higher than that of water will settle down under the influence of gravity. The settling particles will form a sludge layer at the bottom of the tank and the clarified water will be collected through the outlet.
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Thus sedimentation units play dual role - the removal of settleable solids and the concentration of the removed solids into a small volume of sludge. Different types of sedimentation tanks are available for the clarification of water.
The main types are rectangular horizontal flow tanks, circular redial flow tanks and vertical flow tanks. Rectangular flow tanks are the most commosnly used ones. In circular radial flow tanks the influent water is introduced at the centre of the tank surface and the water flows radially to the periphery of the tank.
The vertical flow tank, provided with a hopper bottom, is often used in water treatment plants. A sludge blanket at the middle of the tank serves to remove particles smaller than that would be removed by Sedimentation alone.
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