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 the water, in terms of much oxygen will be required to break it down biologically. Clean water would have BOD value of less than 5 ppm whereas highly polluted water could have a BOD value of 17ppm or more. It means BOD value less than 5 ppm indicates a water sample to be rich in dissolved oxygen.