Which stage uses bacteria to break down organic matter in wastewater?

• A. Primary sewage treatment
• B. Tertiary sewage treatment
• C. Sewage treatment
• D. Secondary sewage treatment

Answer: (D)

Biological degradation by microorganisms is the key idea here. In secondary sewage treatment, bacteria and other microbes actively break down the organic compounds still present after the initial physical removal. This happens in aerated systems—like activated sludge or biofilm reactors—where oxygen is supplied so the bacteria can oxidize the organic matter. The outcome is a large reduction in the water’s organic load (lower biochemical oxygen demand) and clearer effluent before any final polishing steps. Primary treatment relies mainly on settling solids with little microbial digestion, and tertiary treatment adds advanced polishing (such as nutrient removal or disinfection) rather than focusing on bacteria breaking down organics. So the stage where bacteria break down organic matter is secondary sewage treatment.