Which process relies on microbial communities to break down organic matter in wastewater?

• A. Septic Tanks
• B. Wetland-Based Treatment Systems
• C. Secondary Sewage Treatment
• D. Tertiary Sewage Treatment

Answer: (C)

Biological breakdown of organic matter in wastewater is carried out during the secondary treatment stage, where microbial communities do the heavy lifting. In this phase, wastewater is exposed to aerobic bacteria and other microbes in systems such as activated sludge or biofilm reactors. These microorganisms consume the dissolved and suspended organic compounds as food, converting much of the organics into carbon dioxide, water, and new microbial cells. This process greatly lowers the biochemical oxygen demand and the overall organic load before the water moves on to disinfection or further polishing.

Septic tanks rely mainly on anaerobic digestion inside a sealed tank, which is not the same aerobic microbial treatment used in the secondary stage. Wetland-based systems do incorporate microbes and plants and can provide biological treatment, but they’re a distinct, often natural-type setup rather than the conventional secondary treatment in a municipal plant. Tertiary treatment involves polishing the effluent with filtration, disinfection, and nutrient removal rather than primarily degrading organics with a microbial community.