Answer: Aeration tanks.
- A Settling tanks
- B Sludge digesters
- C Primary clarifiers
- D Aeration tanks
Correct answer: D. Aeration tanks
Explanation: In secondary treatment the primary effluent is passed into large aeration tanks where it is agitated mechanically and aerated to promote microbial growth as flocs.
Sewage passes through physical removal (primary), then aerobic biological treatment where microbes form flocs that consume organic matter (secondary); the settled activated sludge is then digested anaerobically to produce biogas - a deliberate switch from aerobic to anaerobic conditions between stages.
Concept context
Microorganisms in food, medicine, sewage treatment, biocontrol, and biofertilizers