Biochemical Oxygen Demand (BOD): It is the amount of Oxygen is taken up by micro­organism that decomposes organic waste matter in water. It is therefore used as the measure of the amount of certain types of organic pollutants in water. BOD is calculated by taking a sample of water containing a known amount of oxygen for 5 days at 20°C in a BOD incubator.

Chemical Oxygen Demand (COD): A parameter of water quality which measures the amount of oxygen in parts per million required to oxidize organic and oxidizable inorganic compounds in the water sample. COD is preferable to the BOD because it is rapidly measurable parameter for river, streams and industrial waste studies and control of water treatment plants.

In short,

1. COD or Chemical Oxygen Demand is the total measurement of all chemicals in the water that can be oxidized. Whereas, Biochemical Oxygen Demand is supposed to measure the amount of food (or organic carbons) that bacteria can oxidize.

2. COD is usually measured and the test is simple and easy to perform with the right equipment and can be done in 2 hours. BOD usually takes 5 days.

3. Permissible limit of COD =250 to 500 ppm.

        Permissible limit for BOD = 30 mg/L.