The Full form of BOD is Biochemical Oxygen Demand. The amount of oxygen required by the biological organisms like bacteria for the breakdown of organic matter by oxidation process in a given water sample is called as Biochemical Oxygen Demand. There are standardized laboratory procedures used to measure this relative oxygen needed, and it is usually measured after 5 days using a redox titration called Winkler method. Depending on the Biochemical Oxygen Demand measured, the degree of how bad is the water quality can be determined. It is expressed in milligrams of oxygen consumed per liter of sample. More the BOD of a given water sample, more polluted is the water. It is usually measured on a scale of 1-9 and more than 100. BOD of 1-2 indicates very good quality, 3-5 indicates fair quality, 6-9 indicates poor quality, and more than 100 indicates very poor quality of water.