Answer: Biogas plants.
- A Trickling filters
- B Activated sludge tanks
- C Biogas plants
- D Primary settling tanks
Correct answer: C. Biogas plants
Explanation: Methanogens like Methanobacterium occur in the rumen of cattle and in anaerobic sludge digesters of biogas plants where they produce methane.
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