Question 2 of 19: TOD, COD, BOD u and BOD 5 — General Relationship (Bar Graph)

Question 2: TOD, COD, BOD_u and BOD₅ — General Relationship (Bar Graph) (3 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

For a given wastewater, these four oxygen-demand measures are nested inside one another because each test captures a progressively narrower slice of the waste's oxidizable content: TOD ≥ COD ≥ BOD_u > BOD₅. TOD (total oxygen demand, measured by high-temperature catalytic combustion) is the largest because it oxidizes essentially everything — carbon, nitrogen and sulphur species alike. COD (chemical oxidation, typically dichromate reflux) is usually somewhat smaller than TOD because it does not fully account for ammonia-nitrogen oxidation and some refractory compounds resist chemical oxidation even though they combust. BOD_u (ultimate biochemical oxygen demand) is smaller still because it only measures the oxygen consumed by biological (microbial) oxidation of the biodegradable fraction — any non-biodegradable ("refractory") organic content that is captured by COD is excluded. Finally, BOD₅ is the smallest because it is only a 5-day snapshot of the first-order BOD reaction curve, typically capturing roughly 60–70% of BOD_u for a domestic-type waste (with the exact fraction set by the deoxygenation rate constant k₁).