A wastewater sample has high COD but relatively low BOD. Which of the following is the most reasonable interpretation?
- AThe wastewater is highly biodegradable and easy to treat biologically
- BThe wastewater may contain a substantial fraction of non-biodegradable oxidizable matterCorrect
- CThe sample must necessarily have high dissolved oxygen
- DThe wastewater is likely free from industrial contamination
Solution
Concept
BOD to COD ratio and wastewater biodegradability
Formula
BOD/COD < 0.2 indicates presence of significant non-biodegradable organic matter
Step-by-step solution
1. COD reflects both biologically oxidizable and refractory organic matter. 2. BOD represents only the biologically oxidizable portion. 3. A high COD with low BOD implies a low BOD/COD ratio, meaning much of the oxidizable content cannot be digested by microbes. 4. Therefore, the wastewater contains a substantial fraction of non-biodegradable oxidizable matter. Answer: B
Explanation
Chemical Oxygen Demand (COD) measures total chemically oxidizable matter (both biodegradable and non-biodegradable), whereas Biochemical Oxygen Demand (BOD) measures only biologically oxidizable organic matter. A high COD coupled with a relatively low BOD (low BOD/COD ratio) signifies that a large portion of the organic matter is resistant to biological breakdown (non-biodegradable), commonly observed in industrial effluents.
Why the other options are wrong
A) Incorrect because low BOD indicates low biodegradability. C) High COD does not indicate high dissolved oxygen; in fact, oxidizable matter exerts oxygen demand. D) High non-biodegradable COD typically signals industrial wastewater presence.
Shortcut
High COD + Low BOD = Low biodegradability = High non-biodegradable/refractory organics.
Common mistake
Confusing COD with DO or assuming high COD implies high biodegradability.
Correct answer: B) The wastewater may contain a substantial fraction of non-biodegradable oxidizable matter