Historically speaking . . . . .
In 1912 the Royal Commission on Sewage Disposal took the view that the five day Biochemical Oxygen Demand (BOD) test was the most reliable chemical index of river water quality. The BOD figures recommended by the committee became known as the ' Royal Commission river classification'. It is perhaps reasonable to note that the BOD test was not introduced to measure the strength of raw wastewater, rather the effect upon the receiving waters. Interestingly, the 5-day duration for BOD determination has no theoretical grounding but is based on historical convention (Tchobanoglous & Schoeder, 1985).
"In a report prepared by the Royal Commission on Sewage Disposal in the United Kingdom at the beginning of last century, it was recommended that a 5-day, 18.3°C, BOD value be used as a reference in Great Britain. These values were selected because British rivers do not have a flow time to the open sea greater than 5 days and average long-term summer temperatures do not exceed 18.3°C. The temperature has been rounded upward to 20°C, but the 5-day time period has become the universal scientific and legal reference."