Answer: Aspergillus niger.
- A Lactobacillus
- B Saccharomyces
- C Aspergillus niger
- D Streptococcus
Correct answer: C. Aspergillus niger
Explanation: Aspergillus niger, a fungus, is used for the commercial production of citric acid.
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