BOD

Biochemical oxygen demand (BOD) measures how much dissolved oxygen microorganisms use to break down the organic matter in a water sample. It is the classic measure of the strength of sewage and the pollution load on a river. When untreated sewage enters a stream, bacteria consume the organic matter and with it the oxygen that fish and other aquatic life need; if oxygen falls too low, the stream turns septic.

The standard test incubates diluted samples in sealed bottles in the dark for a fixed period (three days at 27 °C in Indian practice, five days at 20 °C in many other countries) and measures the drop in dissolved oxygen. Seed microorganisms are added if the sample lacks them. Because dissolved oxygen in a bottle is limited, strong samples are diluted so that oxygen is neither exhausted nor barely used. BOD is calculated from the oxygen drop and the dilution.

BOD exertion follows roughly first-order kinetics: the remaining demand falls exponentially with time, governed by a rate constant k that depends on temperature and waste type. The ultimate BOD is the total carbonaceous demand; the 3-day or 5-day value is a fraction of it. Nitrification adds a later oxygen demand. Chemical oxygen demand (COD), measured by strong chemical oxidation in a few hours, is always higher than BOD and is used for faster control; the BOD/COD ratio indicates how biodegradable a waste is. River models such as Streeter-Phelps combine BOD decay with re-aeration to predict the oxygen sag curve downstream of an outfall.