At this point, the bugs and other solids from the wastewater stream must be separated from the wastewater. This is accomplished by circulating the water through another set of settling tanks (the secondary clarifiers). At the end of this process, 99% of the contaminants and solids are gone, and the remaining water is clear. Most of the solids from this settling process are returned to the aeration basins, where the bugs continue their work. A small portion of solids (2%) is sent for solids handling, to keep the population of bugs at the right level.

Secondary Clarifier

Here is a simple diagram of how the activated sludge process works.

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