Biochemical Oxygen Demand (BOD) is the amount of dissolved oxygen required for microbial breakdown of biodegradable organic matter.
Aerobic organisms use a lot of oxygen and as a result, there is a sharp decline in Dissolved Oxygen (DO) in the water body. This can cause death of fishes and other aquatic species. The relationship between BOD and sewage can be understood from the graph given below:
Determination of BOD is thus an important parameter in suggesting the quality of a water body. The presence of more organic waste increases the microbial activity thus decreasing the DO. BOD is higher in polluted water and lesser in clean water.