Which of the following statements about COD and BOD is FALSE?
- ACOD is always greater than or equal to BOD for the same sample.
- BCOD measures oxygen equivalent consumed by chemical oxidation, while BOD measures oxygen consumed by biological oxidation.
- CA high COD/BOD ratio (>3) indicates that the wastewater is easily biodegradable.Correct
- DCOD can be measured in 2-3 hours, while BOD requires 5 days.
Solution
Concept
Comparison of COD and BOD parameters
Formula
COD ≥ BOD; Biodegradability index = BOD / COD
Step-by-step solution
1. Statement A is true: Dichromate oxidizes nearly all organics, so COD ≥ BOD. 2. Statement B is true: COD uses chemical dichromate digestion while BOD uses microbial respiration. 3. Statement C is false: High COD/BOD indicates non-biodegradable refractory organics, not easily biodegradable wastewater. 4. Statement D is true: Standard COD reflux takes ~2-3 hours whereas standard BOD5 incubation takes 5 days. 5. Therefore, statement C is the false statement. Answer: C
Explanation
Statement C is FALSE. A high COD/BOD ratio (>3) indicates that a large fraction of the organic matter is resistant to biological decomposition (refractory or non-biodegradable). Readily biodegradable wastewater has a low COD/BOD ratio (typically between 1.5 and 2.0, corresponding to BOD/COD > 0.5).
Why the other options are wrong
A) COD is chemically exhaustive and always ≥ biological BOD. B) Accurately states the oxidative mechanisms of both tests. D) Accurately states the laboratory test durations.
Shortcut
Low COD/BOD = High biodegradability; High COD/BOD (>3) = Poorly biodegradable.
Common mistake
Confusing BOD/COD ratio with COD/BOD ratio; high BOD/COD means biodegradable, whereas high COD/BOD means non-biodegradable.
Correct answer: C) A high COD/BOD ratio (>3) indicates that the wastewater is easily biodegradable.