QuestionEnvironment / water quality
The biological oxygen demand (BOD) of unpolluted river water is:
- (a) 10 - 20 mg O₂/dm³/5 day.
- (b) 5-10 mg O₂/dm³/5 day.
- (c) less than 5 mg O₂/dm³/5 day.
- (d) more than 20 mg O₂/dm³/5 day.
Answer: (c) less than 5 mg O₂/dm³/5 day.
BOD = O₂ used by microbes to oxidise organic matter in 5 days at 20 °C. Clean, unpolluted river water has BOD < 5 mg/L (often 1–2). Untreated sewage: 100–400 mg/L. Remember: higher BOD = more pollution.