Activated sludge consists of which organisms and what factors affect its performance?

• A. A mixture of bacteria, protozoa, and metazoa; performance is affected by BOD, pH, and dissolved oxygen
• B. A pure culture of bacteria; performance depends on light
• C. A film of fungi; performance depends only on temperature
• D. An algae mat; performance unaffected by DO

Answer: (A)

Activated sludge relies on a mixed community of organisms, with bacteria doing the actual breakdown of organic matter and protozoa and metazoa grazing on bacteria and helping with settling and keeping the sludge well flocculated. The system’s performance is governed by the amount and quality of the wastewater’s organic load (measured as BOD or COD), the pH, and the dissolved oxygen available for aerobic metabolism. When oxygen is plentiful, bacteria oxidize organics efficiently, which supports stable flocs and effective treatment. If the organic load is high, or DO is too low, or pH drifts outside the favorable range, microbial activity drops and treatment efficiency suffers, potentially leading to poor effluent quality or bulking sludge. The other options don’t fit real activated-sludge biology: a pure culture isn’t representative of the mixed community, light isn’t a controlling factor for aerobic digestion, a fungal film isn’t the typical dominant system, and an algae mat with DO being irrelevant contradicts the aerobic nature of the process.