The oxygen demand of a sample of water is the amount of elemental oxygen required to react with biodegradable material that is dissolved and suspended in the sample. This amount is expressed as milligrams of oxygen per liter of sample. When a population of bacteria causes the oxidation reaction in a population of bacteria, the oxygen required is called the biological oxygen demand. When the oxidation is carried out with a chemical oxidizing reagent such as potassium dichromate, the oxygen equivalent is called the chemical oxygen demand. In a sample of water, oxidation can also be caused by heating of the sample in a furnace in the presence of a catalyst and oxygen. This is called total oxygen demand. If the heating in a furnace occurs in the presence of carbon dioxide, the result is called total carbon dioxide demand (TCO 2 D). The BOD test is perhaps the most important oxygen demand measurement for the analysis of effluents and receiving waters (streams, lakes, and rivers). Basically, the BOD test measures the amount of oxygen used by microorganisms that feed on organic pollutants in the water under aerobic conditions. In this test, a bacterial culture is added to the sample under well-defined conditions and oxygen utilization is measured. Although test procedures are carefully defined, 1,2 it is difficult to obtain highly reproducible results, and the procedure is subject to the influence of many variables, particularly when the wastewater contains a variety of complex materials.