What is the main difference between Chemical Oxygen Demand (COD) and BOD?

The main distinction between Chemical Oxygen Demand (COD) and Biochemical Oxygen Demand (BOD) lies in the speed at which they measure the organic pollution strength of water samples. COD measures the total amount of oxygen that is required to chemically oxidize organic material in water, and this process can be completed relatively quickly, typically within a few hours. This rapid assessment is an advantage when immediate results are necessary for wastewater treatment processes.

BOD, on the other hand, measures the amount of oxygen that microorganisms will consume while decomposing organic matter in a water sample over a specific time period, usually 5 days at a standardized temperature. This makes it a slower method compared to COD, as it relies on biological activity to gauge organic matter levels.

The other responses could lead to misunderstandings. The assertion that BOD is always greater than COD is not accurate, as BOD can be lower than COD depending on the nature of the organic compounds present. Moreover, COD does not exclusively deal with solid waste but encompasses all the oxidizable organic materials, both solid and dissolved. Finally, while BOD is indeed measured over 5 days, this detail doesn't highlight the core difference between the two measurements in the context provided.