Biochemical Oxygen Demand (BOD) of Sewage

Biochemical oxygen demand (BOD) is a measure of the amount of oxygen that is used by microorganisms to break down organic matter in water. The BOD of sewage is an important indicator of the quality of water, as it can indicate the presence of pollutants.

If a 2% solution of a sewage sample is incubated for 5 days at 20°C and depletion of oxygen was found to be 5 ppm, what is the BOD of the sewage?

The answer is 250 ppm.