Why Bio Chemical Oxygen Demand (BOD) at 5 days??

BOD or “Bio Chemical Oxygen Demand” is an important factor which describes us how pure the given water sample is. It represents the amount of “Dissolved Oxygen“ consumed by microbes to convert the organics present to carbon dioxide and water. Higher the BOD value higher is the water polluted. A sample of water with lesser BOD value is less polluted than a one with much higher value.
* Generally as per global standards BOD value of sample at 5 days which incubated at 20 degrees Celsius is represented as the standard BOD of the sample. The reason for taking 5 days BOD as standard is due to the following fact:
The “Dissolved Oxygen” present in water gets used for the decomposition of organic matter as well as nitrogenous matter.
From the combined BOD curve, we have two oxygen demands: 1. Carbonaceous BOD (initial) , 2. Nitrogenous BOD (final).

The first 5 days demand is the “Carbonaceous Bio Chemical Oxygen Demand” which accounts for 68% of the total BOD value. Hence taking the value of 5 days BOD as a standard one represents the oxygen demand for the decomposition of organic matter only.