PLATE 51 Slow-moving coastal plain stream choked with algal bloom caused by nitrogen and phosphorus from up-stream farmland.
	Fish kill caused by hypoxic conditions

Biological Oxygen Demand (BOD): a measure of oxygen consumption required by the microbial oxidation of readily degradable organic substrates and ammonia in liquid wastes such as sewage.

Chemical Oxygen Demand (COD): oxygen consumed by a total oxidation of the liquid waste by strong chemical oxidants.

BOD < COD: since biological oxidation is often not complete, and there is production of microbial biomass, the difference is often due to the yield factor (Y).

X(HCO) + X(HCO-NH₂) + O₂CO₂+ H₂O + NO₃-(chemical)
X(HCO)+ X(HCO-NH₂) +O₂cells + CO₂+H₂O+NO₃-+residues (microbial)

Natural water has low O₂ solubility, dissolved oxygen in natural water seldom exceeds 8 mg per liter. Because of heterotrophic microbial activities, O₂ concentration in water is often lower than 8mg/L. So organic substances in sewage will often consume large amount of oxygen.

Once oxygen is depleted, self-purification processes slow down drastically. Fermentation or anaerobic processes may begin to operate, which will produce fermentation products with noxious odors and colors, the system becomes anaerobic or septic.