Which term describes the phase of wastewater treatment focusing on microbial digestion of organic matter at a municipal scale?

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

Answer: (A)

The phase described is the secondary sewage treatment. This stage is defined by biological, microbial digestion of organic matter in wastewater, which is the main activity in municipal-scale plants after the initial physical removal of solids in primary treatment. In secondary treatment, microbes—often in systems like activated sludge, trickling filters, or biofilm reactors—consume dissolved and colloidal organic material, significantly reducing the biochemical oxygen demand (BOD) and overall organic load of the effluent. This biological polishing occurs before any tertiary treatment, which targets nutrients and pathogens for further reduction, or other on-site/alternative systems.

Primary treatment, by contrast, mainly removes settleable solids and grit through physical processes, not microbial digestion. Septic tanks operate at a smaller, on-site scale and rely on anaerobic digestion within a tank rather than the centralized aerobic/biological processes of municipal secondary treatment. Wetland-based systems can provide biological treatment as well but are not the standard municipal secondary stage in most wastewater plants, which is why the secondary phase best matches the description.