Answer: Methanogenic archaea.
- A Aerobic heterotrophs
- B Sulfate-reducing bacteria
- C Methanogenic archaea
- D Lactic acid bacteria
Correct answer: C. Methanogenic archaea
Explanation: Methanogenic archaea (methanogens) are responsible for the final step of anaerobic digestion - converting acetate and H<sub>2</sub>/CO<sub>2</sub> into methane (CH<sub>4</sub>) in sludge digesters.
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