In addition to being used as a measure of the amount of organic pollutant in streams or lakes, the BOD is used as a measure of the strength of sewage. This is the most important parameters for design and operation of water pollution control plant. A strong sewage has a high concentration of organic material and a correspondingly high BOD. The complete decomposition of organic material by microorganisms takes time, usually 20d or more under ordinary circumstances. The amount of oxygen used to completely decompose or stabilize all the biodegradable organics in a given volume of water is called Ultimate BOD, or BODL for example, if a 1-L volume of municipal waste requires 300 mg of oxygen for complete decomposition of the organics, the BODL would be expressed as 300mg/L. One liter of waste from an industrial or food processing plant may require as much as 1500 mg of oxygen for complete stabilization of the waste. In this case, the BODL would be 1500mg/L, indicating a much stronger waste than ordinary municipal or domestic sewage. In general, then, the BOD is expressed in terms of mg/L of oxygen.
The BOD is a function of time. At the very beginning of a BOD test, or time = 0, no oxygen will have been consumed and the BOD = 0. As each day goes by oxygen is used by the microbes and the BOD increase. Ultimately, the BODL is reached and the organics are completely decomposed. A graph of the BOD versus time has the characteristic shape called the BOD Curve.