COD (Chemical Oxygen Demand)
The chemical oxygen demand is a test for the estimation of the organic matter. However COD i.e the oxygen consumed does not differentiate the stable organic matter form the unstable form. The amount of organic matter in water is estimated by their oxidability by chemical oxidant such as potassium permanganate or potassium dichromate (the constituent carbon and hydrogena are oxidized and not the nitrogen).
BOD (Biochemical Oxygen Demand)
The biochemical oxygen demand is a way of expressing the amount of organic compounds in sewage as metabolize it under aerobic condition. It is an index of the organic pollution. If the amount of organic matter in a sewage is more, the more oxygen will be utilized by bacteria to degrade it.