Identify the correct decreasing order of magnitude for Theoretical Oxygen Demand (ThOD), ultimate and 5-day Biochemical Oxygen Demand (BOD₅ and BODᵤ), and Chemical Oxygen Demand (COD):
Options: (A) BOD₅ (B) BODᵤ (C) COD (D) ThOD
Choose the correct answer from the options given below:

Correct option is D

1. ThOD (Theoretical Oxygen Demand):
ThOD is the maximum oxygen demand calculated theoretically based on complete oxidation of organic matter, considering all carbon and hydrogen atoms. It is the highest value in oxygen demand measurements.
2. COD (Chemical Oxygen Demand):
COD measures the amount of oxygen needed to chemically oxidize organic and inorganic matter in water. It is generally less than ThOD but greater than BOD.
3. BODᵤ (Ultimate Biochemical Oxygen Demand):
BODᵤ represents the total oxygen demand over an extended period, typically 20 days. It is lower than COD but higher than BOD₅.
4. BOD₅ (5-Day Biochemical Oxygen Demand):
BOD₅ measures oxygen consumption by microorganisms over 5 days. It represents only a part of the total BOD and is the smallest value in oxygen demand.
Thus, the correct order is ThOD > COD > BODᵤ > BOD₅.
Information Booster 1. ThOD Characteristics:
· Based on stoichiometric calculations.
· Assumes complete oxidation of organic compounds.
2. COD vs. BOD:
· COD includes organic and some inorganic compounds.
· BOD focuses on biologically degradable organic matter.
3. BODᵤ and BOD₅:
· BODᵤ reflects long-term biodegradation potential.
· BOD₅ is used for quick water quality assessments.
4. Usage in Wastewater Treatment:
· ThOD and COD help design chemical treatment processes.
· BOD is crucial for biological treatment processes.
5. Practical Implications:
· COD is faster to measure than BOD tests.
· BOD tests reflect the impact on aquatic life.