Biochemical Oxygen Demand also known as biochemical oxygen demand it is chemical procedure to determine the amount of oxygen required organism which is requird during aerobi oxidation, refers to the amount of oxygen that would be consumed if all the organics in one liter of water were oxidized by bacteria and protozoa.
`C_6H_12O_6 + 6 O_2 -> 6CO_2 + 6 H_2O`
`NH_3 + 2 O_2 -> NO_3^- + H_2O + H^+`
Microorganisms are responsible for decomposing organic waste like dead plants, leaves, sewage, or even food waste is present in a water. When bacteria decompose waste by using the dissolved available oxygen it effect the oxygen need of aquatic organisms
If there are large quantity of organic waste in the water then large number of bacteria present will decompose waste that inturn decrease the oxygen in the water in this case oxygen demand increases so BOD level will be high. Waste of Nitrates and phosphates water body will lead to high BOD levels.
Thus by measuring the amount of oxygen in water we can measure the health of aquatic organism.