Answer: Secondary treatment.
- A Primary treatment
- B Tertiary treatment
- C Secondary treatment
- D Chlorination
Correct answer: C. Secondary treatment
Explanation: Secondary treatment uses aerobic microorganisms to biologically degrade organic matter in sewage, reducing BOD.
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