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How do microorganisms clean wastewater during secondary treatment?

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Aerial view of wastewater treatment basins with circular clarifiers; overlaid toilet graphic and arrow with text reading: “Where does it go?” — wastewater treatment All the sewers in your neighborhood end up here.

During secondary wastewater treatment, microorganisms play a critical role in removing soluble organic matter and suspended solids that remain after primary treatment. This process relies on biological activity where microbes consume organic impurities as a food source, converting them into carbon dioxide, water, and energy to support their own growth and reproduction.1

By metabolizing these organic materials, microorganisms help protect the dissolved oxygen balance of receiving water bodies, such as rivers or lakes, by reducing the wastewater's biochemical oxygen demand. Common methods used to facilitate this biological action include:12

Activated sludge process: Bacteria-rich sludge is mixed with incoming wastewater and agitated in the presence of an ample air supply. Microorganisms oxidize the organic matter, while suspended solids are absorbed or adsorbed by the sludge.3
Trickling filters and oxidation ponds: These are alternative biological treatment methods that provide controlled environments for microbial activity.1

Ultimately, these engineered systems exploit the natural metabolic capacity of bacteria to stabilize organic waste before the treated water is discharged into the environment.2