What is the biological treatment where bacteria break down organic matter called?

• A. Secondary sewage treatment
• B. Primary sewage treatment
• C. Tertiary sewage treatment
• D. Final chemical treatment

Answer: (A)

Secondary sewage treatment is the stage where microorganisms break down organic matter in wastewater. In this biological process, aerobic bacteria in aeration tanks or biofilm systems consume the dissolved and fine organic materials as food, converting them into microbial biomass, carbon dioxide, and water. This greatly reduces the pollutant load, especially the biochemical oxygen demand, before the water moves on to polishing steps or discharge. That’s why this option is the best choice—the term specifically describes a biological treatment driven by bacteria breaking down organics.

Primary treatment, by contrast, relies on gravity and screens to remove settleable solids—physical separation rather than biological decay. Tertiary treatment refers to advanced polishing that follows secondary treatment, targeting nutrients or pathogens and further improving water quality. Final chemical treatment isn’t a standard stage in wastewater treatment.