Degree of impurity of water due to organic matter is measured in terms of BOD (Biochemical oxygen demand). BOD is the oxygen in milligrams required for five days in one litre of water at $20^{\circ} \mathrm{C}$ for the microorganisms to metabolise organic waste. It is measured in the milligrams of oxygen taken up in one litre of sewage. The effluent that is legally allowed to be discharged into a river or stream should be such that $B O D$ at $20^{\circ} \mathrm{C}$ should not be more than $20 \mathrm{ppm}$, i.e., 20 mg per litre.