Wastewater Clarifier Sedimentation Tank

Wastewater Clarifier or Sedimentation Tank plays an important role either after or before biological treatment processes to remove heavier sludge solids by means of settling and separation from the liquid phase. When it is used ahead of biological treatment, the main advantage is that it could help towards significant reduction in BOD level and thus, reducing the load feed into the aeration pond. These types of primary sedimentation tanks were designed to deal with higher rate of loading and also having shorter retention time for the water.

The basis on the design of a wastewater clarifier relies heavily on the 1) surface area of the tank and 2) the time the water is retained in the system. The depth however, would have little impact and basically as long as the horizontal flow velocity is low enough to prevent settled sludge solids from being scoured from the bottom of the tank, that should be good enough. In fact the removal efficiency were mainly calculated based on the assumption that all the sludge solids present in the wastewater has uniform size, density, specific gravity and also shapes. However, in reality, such condition would not exist as solids in wastewater will sometimes appear in highly dispersed mode and therefore the settling time will be prolonged and this should be taken as a main consideration to come up with the design.

A normal wastewater treatment system with a typical capacity that ranges between few thousand gallons per day should have the sedimentation tank designed to retain water for at least 2 to 3 hours. Although temperature is not an important consideration here, it could have major impact especially in cold climate countries whereby increase in water viscosity will slow down the rate of settling. On the feed or loading rate, usually the calculation is based on gallons of wastewater input determined based on per square meter of surface area of the tank. The main idea for the whole system to operate efficiently is to control the overflow of wastewater to achieve high reduction in BOD level at any defined loading rate.

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