The amount of oxygen required by bacteria to break down the organic matter present in a certain volume of a sample of water is called biochemical oxygen demand (BOD). The amount of BOD in the water is a measure of the amount of organic material in water in terms of how much oxygen will be required to break it down biologically. Clean water have BOD value less than $5 \mathrm{ppm}$ whereas highly polluted water such as municipal sewage has BOD value more than $100 \mathrm{ppm}$.