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\(_5\).