Question

The correct statement of comparison of Ultimate BOD (BODu), COD, Theoretical oxygen demand (ThOD), BOD\(_5\) is:

• A. BODu > COD > ThOD > BOD\(_5\)
• B. COD > ThOD > BODu > BOD\(_5\)
• C. ThOD > COD > BODu > BOD\(_5\)
• D. COD > BODu > BOD\(_5\) > ThOD

Hint:

Always remember: {Theoretical > Chemical > Biological (ultimate) > Biological (5-day)}.

Answer 1

The Correct Option is C

Step 1: Understanding oxygen demand terms.
ThOD represents the total oxygen required for complete oxidation of organic matter theoretically. COD measures chemically oxidizable matter, while BODu measures biologically oxidizable matter over infinite time. BOD\(_5\) measures oxygen demand over only 5 days.
Step 2: Comparing magnitudes.
Since ThOD assumes complete oxidation, it is the highest. COD is lower than ThOD as some matter may not be chemically oxidized. BODu is lower than COD because not all organic matter is biodegradable. BOD\(_5\) is the lowest since it measures oxygen demand for a limited time.
Step 3: Evaluating options.
(A) Incorrect ordering.
(B) Incorrect, ThOD cannot be lower than COD.
(C) Correct, it follows the correct decreasing order.
(D) Incorrect, ThOD should be the highest.
Step 4: Conclusion.
The correct comparison is ThOD > COD > BODu > BOD\(_5\).

Answer 2

This question compares four ways of expressing oxygen demand, and the cleanest way to rank them is to think about how much of the organic matter each measurement actually accounts for.

1. BODu > COD > ThOD > BOD\(_5\): This places the theoretical oxygen demand below the chemical and biological values, but ThOD is calculated assuming complete oxidation of every oxidisable atom, so it cannot be smaller than measurements that only capture partial oxidation; this ordering is inconsistent.
2. COD > ThOD > BODu > BOD\(_5\): Placing COD above ThOD is not possible, because COD measures oxidation by a strong chemical oxidant which still cannot exceed the theoretical maximum demand of the sample.
3. ThOD > COD > BODu > BOD\(_5\): ThOD represents the absolute upper bound since it assumes every oxidisable substance is fully converted; COD comes next since strong chemical oxidants oxidise most, but not quite all, of that material; BODu comes after since biological oxidation over long durations still misses some resistant organic matter; and BOD\(_5\) is lowest because it only captures five days of biological activity out of the full biodegradation timeline.
4. COD > BODu > BOD\(_5\) > ThOD: Placing ThOD last is not possible, since ThOD represents the theoretical ceiling that no measured value can exceed.

Ranking each measure by how completely it accounts for oxidisable matter places the theoretical, chemical, ultimate biological, and five-day biological values in strictly decreasing order.

Therefore, the correct answer is ThOD > COD > BODu > BOD\(_5\).