Answer: Methane.
- A Carbon dioxide
- B Hydrogen sulfide
- C Methane
- D Nitrogen
Correct answer: C. Methane
Explanation: Methane (CH<sub>4</sub>) is the major component of biogas produced during anaerobic digestion of sewage sludge.
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