Which organisms drive the biological breakdown in secondary sewage treatment?

• A. Fungi
• B. Protozoa
• C. Algae
• D. Bacteria

Answer: (D)

Biological breakdown in secondary sewage treatment is driven by bacteria. In activated sludge systems, a large population of bacteria forms flocs and uses oxygen to metabolize organic pollutants found in wastewater. This aerobic digestion rapidly converts dissolved and colloidal organics into carbon dioxide, water, and new bacterial biomass, which is the main mechanism that reduces the wastewater’s organic load before final polishing. Protozoa and fungi do supportive work: protozoa graze on bacteria and help clarify sludge by removing suspended organisms, while fungi can break down larger, tougher organic compounds in waste with fibrous content. Algae can contribute oxygen in lighted, pond-based systems, but in typical secondary treatment setups the principal agents carrying out the breakdown are bacteria.