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Civil Engineering QUESTION #2130
Question 1
The Biochemical Oxygen Demand (BOD) test measures:
  • The total dissolved oxygen concentration in a water sample
  • The amount of dissolved oxygen consumed by biological organisms in oxidizing organic matter in a water sample at 20°C over 5 days (BOD₅) — used as an indirect measure of organic pollution load in wastewater✔️
  • The chemical oxygen demand using potassium dichromate oxidation
  • The concentration of pathogenic bacteria in sewage effluent
Correct Answer Explanation
BOD₅ (standard 5-day, 20°C test) measures DO consumed by aerobic microorganisms decomposing carbonaceous organic matter in a diluted, incubated sample. It is the most widely used parameter for characterizing wastewater strength and assessing treatment plant performance. BOD₅ captures ~60–70% of ultimate BOD. Typical BOD₅ values: raw domestic sewage 200–300 mg/L; treated effluent <20 mg/L (secondary); <10 mg/L (tertiary).