Secondary treatment is a biological treatment process used to stabilize the dissolved solids.
Microorganisms (e.g., bacteria) feed on the organic solids (food) in the wastewater and convert the organics into a cellular or biological mass that can later be removed.
These biological processes are aerobic processes. Oxygen must be provided for these aerobic organisms to work properly and efficiently. An integral part of secondary treatment processes is another set of settling tanks or clarifiers.
These secondary clarifiers (final clarifiers) remove the biological mass that has grown during biological treatment.