Which one of the following is defined as the oxygen required for the micro-organisms to carry out biological decomposition of dissolved solids or organic matter in the water under aerobic conditions at standard temperature?
- ABiochemical oxygen demandCorrect
- BChemical oxygen demand
- CTotal oxygen demand
- DTheoretical oxygen demand
Solution
Concept
BOD is the oxygen used by micro-organisms to break down organic matter under aerobic conditions at a standard temperature.
Formula
BODt = L0 (1 - 10-k t)
Step-by-step solution
1. The question says oxygen is used by micro-organisms, so it is biological. 2. It is under aerobic conditions at a standard temperature (usually 20°C or 27°C). 3. This is the definition of Biochemical Oxygen Demand (BOD). 4. So the answer is (A).
Explanation
BOD measures the oxygen that bacteria use to break down organic matter in water when oxygen is present. The test is done at a fixed temperature for a fixed time, for example 5 days at 20°C. So the definition in the question matches BOD.
Why the other options are wrong
B: COD uses a strong chemical oxidant (dichromate), not micro-organisms. C: TOD is oxygen used in high-temperature combustion with a catalyst, not biological action. D: ThOD is calculated from the chemical formula of the organic matter, not measured biologically.
Shortcut
Micro-organisms + aerobic + standard temperature = Biochemical = BOD.
Common mistake
Choosing COD because it also measures organic matter. COD is chemical oxidation, not biological.
Correct answer: A) Biochemical oxygen demand