Biological Oxygen Demand

Biological oxygen demand (bod), or biochemical oxygen demand, is an index or measure of the concentration of biodegradable organic matter in a body or quantity of water. BOD is a natural phenomenon, but is also a test used to determine the condition of water for maintaining natural ecological processes of human use. Oxygen dissolved in water is a naturally occurring resource, provided by waterfalls, rapids, winds, and waves, without which organic materials would build up beyond the capacity of water bodies to sustain life. BOD occurs naturally as organic debris wash into streams, rivers, and lakes. These organic materials are oxidized by bacteria and other microorganisms, which decompose organic wastes (e.g., dead leaves, fish, plants, manure, sewage), consuming oxygen in the process. Should oxidation outpace the availability of oxygen, the water can become anoxic (depleted of oxygen). The lack of oxygen in water will render a lake or a sea dead. For example, most of the water of the Black Sea is anoxic water.

Pollutants in water in high levels—nitrates, phosphates, or manure—cause the rate of oxygen consumption to increase as the available bacteria consumes the waste and grows in number. Rises in water temperature can also contribute to a reduction in the oxygenation of water, which promotes the growth of algae and other plants. They also die more rapidly, which increases the amount of organic material in the water, and thus, the food supply for bacteria. If the BOD is high, the levels of dissolved oxygen will decrease, threatening the water quality. Human pollution deprives water of oxygen, and is a threat to both natural and human life.

The greater the organic matter in the water, the greater will be the oxygen consumption by the microorganisms laboring to dispose of the waste. The Royal Commission on River Pollution first used BOD as a way of measuring water pollution in England in 1865. After the work of the Royal Commission on Sewage Disposal in 1898, BOD tests were developed to define the amount of pollution in rivers. The test took five days, the length of time for water in English rivers to travel from the source to an estuary. An additional element of 30 ppm (parts per million) was added in 1912 as the maximum concentration of organic material that a sewage system could discharge into a river. The test was later refined.

Direct measurement of BOD is used to report the amount of oxygen that a biological process is consuming as it reduces organic material in water. BOD can also be used to describe an indirect measurement of the amount of waste in a solution by measuring the concentration of the biologically degradable organic material in a given unit of water. The term then applies to the amount of oxygen needed in five days if a biological process is to break down a quantity of organic waste. From the results, the quality of the water may be inferred. If the water quality is rated as poor because the level of organic material in the water is high, then the water may be considered polluted, and unsafe for humans to drink or swim in.

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