Activated sludge from a secondary clarifier contains large numbers of which microbial groups? (Select the most complete characterization of the mixed liquor biomass.)

Introduction:
Activated sludge is a mixed microbial community maintained in suspension to oxidize organic pollutants and, in advanced systems, remove nutrients. This question checks whether you recognize the diversity of organisms present in healthy sludge flocs.

Given Data / Assumptions:

• Bacteria dominate by number and drive BOD removal and nitrification.
• Protozoa graze on dispersed bacteria, improving effluent clarity.
• Fungi/yeasts may appear under specific conditions (e.g., low pH, high BOD, filamentous bulking).

Concept / Approach:
Well-formed flocs consist of bacteria embedded in extracellular polymeric substances, with protozoa and sometimes metazoa (e.g., rotifers) indicating good settling and stable operation. Diversity supports resilience and process performance across variable loads.

Step-by-Step Solution:
Consider each group’s role: bacteria (biodegradation), protozoa (clarification), fungi/yeasts (niche conditions). Recognize that all are observed in routine microscopy of mixed liquor. Choose the option that encompasses the full microbial assemblage. Confirm that “all of these” best describes activated sludge biomass.

Verification / Alternative check:
Process control manuals recommend microscopic surveys to assess balances of floc formers, ciliates, flagellates, and filamentous organisms.

Why Other Options Are Wrong:

• Bacteria / Yeasts and molds / Protozoa (alone): Each is present, but none alone fully characterizes activated sludge.

Common Pitfalls:
Equating activated sludge solely with bacteria; trophic interactions with protozoa are vital for good effluent quality.

Final Answer:
all of these accurately reflects activated sludge composition.