This stage utilizes the power of microorganisms to further purify the water. The process involves:

• Aeration: Sewage liquor is directed into an aeration tank where oxygen is introduced. This creates an ideal environment for microorganisms to thrive.
• Biodegradation: Microorganisms consume dissolved organic particles and interact with pollutants like heavy metals, effectively breaking them down.
• Secondary Settling: The resulting colonies of microorganisms settle at the bottom of a secondary settling pond.

The combined efforts of the mechanical and biological treatment stages result in the removal of approximately 90% of pollutants, leaving the effluent significantly cleaner.