To measure oxygen demand, the biochemical oxygen demand (BOD) method relies on bacteria to oxidize readily available organic matter during a five-day incubation period. In contrast, chemical oxygen demand (COD) methods use strong chemicals to oxidize organic matter.

BOD simulates the actual treatment plant process by measuring the organic material microorganisms can oxidize. Although COD is comparable to BOD, it actually measures chemically oxidizable matter. The COD test is not a direct substitute for the BOD test; however, a ratio usually can be correlated between the two tests. This requires COD versus BOD testing over a specified period of time.

Generally, COD is preferred to BOD in process control applications, because results are more reproducible and are available in just a few hours rather than five days. In many industrial samples, COD testing may be the only feasible course because of the presence of bacterial inhibitors or chemicals which would interfere with a BOD determination. Many industrial and municipal laboratories find that parallel COD and BOD testing is beneficial because the COD test can be used to target a specific BOD range, eliminating the need for multiple BOD dilutions.

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