Biochemical Oxygen Demand, the amount of dissolved oxygen used by aerobic microorganisms when decomposing organic matter in water. BOD is an important water quality parameter because it provides an index to assess the effect discharged wastewater will have on the receiving environment. One of the main reasons for treating wastewater prior to its discharge into a water resource is to lower its BOD—i.e., reduce its need of oxygen and thereby lessen its demand from the streams, lakes, rivers, or estuaries into which it is released.