BIOCHEMICAL OXYGEN DEMAND: Bacteria and other microorganisms use organic substance for food. As they metabolize organics are broken down into simpler compounds, such as CO2 and H2O, and the microbes use the energy released fro growth and reproduction.
When this process occurs in water, the oxygen consumed is the DO. If oxygen is not continually replaced in the water by artificial or natural means, then the DO level will decrease as the organic are decomposed by the microbes. This need for oxygen is called Biological Oxygen Demand. In effect, the microbes “demand” the oxygen for use in the biochemical reactions that sustain them. Organic waste in sewage is one of the major types of water pollutants. It is impractical to isolate and identify each specific organic chemical in these wastes and to determine its concentration. Instead, the BOD is used as an indirect measure of the total amount of biodegradable organics in the water. The more organic material there is in the water; the higher the BOD exerted by the microbes will be.