the amount of oxygen used by microorganisms in the process of breaking down organic matter in water. The more organic matter there is (e.g., in sewage), the greater the number of microbes. The more microbes there are, the greater the need of oxygen to support them; consequently, less oxygen is available for higher animals such as fishes. The BOD is therefore a reliable gauge of the organic pollution of a body of water. One of the main reasons for treating sewage or waste water prior to its return to a water resource is to lower its BOD—i.e., reduce its need of oxygen and thereby lessen its demand from the streams or rivers into which it is released.