Clarifiers: They are essentially a type of settling tank that has mechanical means for continuous sludge removal. 

A moving example of a secondary clarifier

In this process shown on the map, the clarifier is shown in the secondary treatment step because it is used to collect microbes from the aeration tank step. A portion of the collected microbes will be returned to the aeration tank to replenish the microbe population.

More information for the clarifiers can be found on the sedimentation tank page earlier in the module.