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: ThOD is the maximum possible oxygen demand, calculated from the sample's chemical composition assuming complete oxidation of every oxidisable element, so it forms the upper bound for all the other measures.
Step 2: COD uses a strong chemical oxidant in the laboratory and oxidises most organic matter, but a small fraction can remain resistant even to that oxidant, so COD sits just below ThOD.
Step 3: BODu relies on microorganisms to oxidise the waste over an extended period, and biological action is generally less complete than strong chemical oxidation, so BODu falls below COD; BOD\(_5\) captures only the first five days of that same biological process, so it is the smallest of all four.
Step 4: Combining these three comparisons in sequence gives the full ordering.

\[ \text{ThOD} > \text{COD} > \text{BOD}_u > \text{BOD}_5 \]

Answer 2

Another way to approach this ranking is to think about the physical or chemical basis of each test and how strict or lenient the oxidation condition behind it is.

1. BODu > COD > ThOD > BOD\(_5\): This makes ThOD smaller than a chemically or biologically measured value, which cannot happen because ThOD is derived from stoichiometry and always represents the maximum attainable demand.
2. COD > ThOD > BODu > BOD\(_5\): No chemical test, however strong the oxidant, can push the measured oxygen demand past the value calculated from complete stoichiometric oxidation, so COD cannot sit above ThOD.
3. ThOD > COD > BODu > BOD\(_5\): A strong chemical oxidant used in COD testing is more aggressive than the microbial action behind BODu, and BOD\(_5\) is a truncated snapshot of that microbial action, so the strictness of oxidation decreases in exactly this order, from theoretical, to chemical, to full biological, to five-day biological.
4. COD > BODu > BOD\(_5\) > ThOD: This again puts a measured value above the theoretical ceiling, which contradicts what ThOD represents.

Looking at how strict or complete each oxidation condition is, from the theoretical maximum down to the most time-limited biological test, confirms the same decreasing order.

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