Answer: Greater level of organic pollution requiring more oxygen.
- A The water is unusually clean and contains abundant dissolved oxygen
- B Greater level of organic pollution requiring more oxygen
- C The presence of dissolved heavy metals such as lead and mercury
- D An unusually low level of bacterial metabolic activity in the sample
Correct answer: B. Greater level of organic pollution requiring more oxygen
Explanation: A higher BOD indicates more organic matter present in water, requiring more oxygen for microbial decomposition, signifying greater pollution.
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