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This particular step helps in the removal of solid particles of organic material by gravity settling at the tank bottom. After that, the primary sludge that is formed is taken to the center portion of the tank where it is concentrated as well as pumped away for more treatment. The subsequent step is to treat the wastewater biologically which is referred to as the activated sludge process. In this particular step, naturally occurring micro-organisms are utilized for breaking down the suspended and dissolved organic solids.