What term describes the biological process using bacteria to treat sewage?

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

Answer: (B)

Secondary sewage treatment is the biological stage where bacteria break down the organic matter in wastewater after the solids have been removed in primary treatment. In this stage, microbes such as bacteria metabolize dissolved and suspended organics—often in aerated systems like activated sludge or other biofilm setups—reducing pollutants before any polishing steps. This is why it best describes using bacteria to treat sewage: it specifically refers to the microbial, BOD-reducing processes that follow initial solids removal.

Septic tanks rely on anaerobic digestion in a single tank and aren’t the standard municipal secondary treatment. Wetland-based systems use plants as well as microbes and can serve different roles depending on design, not the classic bacterial degradation stage. Tertiary treatment is the advanced polishing step that removes remaining nutrients and pathogens beyond secondary treatment.