When the wastewater is processed through the secondary stage of treatment, the bacteria that remove the nutrients from the wastewater die over time and they enter the waste stream as a solid material. Just like with the primary settling stage, this material can then be settled out of the wastewater through additional settling.
The secondary settling tanks are very similar to the primary settling tanks. The secondary settling tanks also have a series of scrapers in the bottom that drag the settling solid material to a sump at the end of the tank where it can be pumped out. The solids that are captured during this process are also sent to the anaerobic digester system so they can be reclaimed.
Unlike the primary settling system, instead of four tanks there are only two as the volume of solids that can be expected to be captured here are considerably less than in the primary settling tanks.