BOD is the indirect ...

BOD is the indirect measurement of organic pollution a wastewater stream contains in it when discharged into the aquatic resources. 

As any wastewater stream contains diverse range of Organic pollutants, quantifying them individually might not help a lot in terms of the impact it makes on the receiving water. While when the waste stream containing organic pollutants are discharged in to the aquatic body, it imposes an oxygen demand on the aquatic resources consuming the DO present in the aquatic body due to reductive nature of the organic pollutants present in the stream. As DO was considered as one of the most significant parameter to measure the health of aquatic bodies due to its impact on the aquatic life present in the aquatic bodies, tests were developed to measure the amount of OXYGEN a stream could consume when discharged into the aquatic body. The BOD test is the measure of this organic pollution using Bacteria as the basic process "workers" which consumes the Organic pollution in the presence of Oxygen. Thus by measuring the DO level at a period of interval , one can estimate how much oxygen demand the pollution event could make on the receiving water. So BOD test is indirect measurement of organic pollution present in the stream containing organic pollutants.