The relationship between Theoretical Oxygen Demand (Th. OD), Biochemical Oxygen Demand (BOD) and Chemical
Oxygen Demand (COD) is given by:
Theoretical Oxygen Demand (Th.O.D.): The maximum calculated oxygen required assuming 100% complete chemical
oxidation of all organic compounds based strictly on chemical formulas. (Highest value) Chemical Oxygen Demand
(C.O.D.): The oxygen required to chemically oxidize both biodegradable and non-biodegradable organic matter using
strong oxidizing agents. (Intermediate value)
Biochemical Oxygen Demand (B.O.D.): The oxygen required by microorganisms to biochemically degrade only the
biodegradable portion of the organic matter. (Lowest value)
Therefore, the correct decreasing magnitude hierarchy is: Th.O.D. >C.O.D. > B.O.D..
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