Answer: Methane, CO 2 , and H 2 S.
- A Methane and oxygen
- B Methane, CO<sub>2</sub>, and H<sub>2</sub>S
- C Hydrogen and nitrogen
- D Carbon monoxide and methane
Correct answer: B. Methane, CO<sub>2</sub>, and H<sub>2</sub>S
Explanation: Biogas produced by anaerobic digestion of sewage sludge contains methane (CH<sub>4</sub>), carbon dioxide (CO<sub>2</sub>), and hydrogen sulfide (H<sub>2</sub>S).
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