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Biochemical Oxygen Demand Description
B.O.D.

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Description of Biochemical Oxygen Demand:

When organic matter decomposes, it is fed upon by aerobic bacteria. In this process, organic matter is broken down and oxidized (combined with oxygen). Biochemical oxygen demand is a measure of the quality of oxygen used by these microorganisms in the aerobic oxidation of organic matter.

When aquatic plants die, they are fed upon by aerobic bacteria. The input of nutrients into a river, such as nitrates and phosphates, stimulate plant growth. Eventually, more plant growth leads to more plant decay. Nutrients, then, can be a prime contributor to high biochemical oxygen demand in rivers.

Impounded river reaches also collect organic wastes from upriver that settle in quieter water. The bacteria that feed on this organic waste consume oxygen. Percent saturation (dissolved oxygen) values in water with much plant growth and decay often fall below 90 percent.

Biochemical Oxygen Demand

There are natural sources of organic material which enter lakes and rivers from swamps, bogs, and vegetation along the water, particularly leaf fall.

There are also human sources of organic material. When these are identifiable points of discharge into rivers and lakes, they are called point sources. Point sources of organic pollution include:

• pulp and paper mills
• meat-packing plants
• food processing industries
• waste water treatment plants

Nonpoint pollution comes from many sources that are difficult to identify. Nonpoint sources of organic pollution include:

• urban runoff of rain and melting snow that carries sewage from illegal sanitary sewer connections into storm drains; pet wastes from streets and sidewalks; nutrients from lawn fertilizers; leaves, grass clippings, and paper from residential areas.
• agricultural runoff that carries nutrients, like nitrogen and phosphates, from fields.
• runoff from animal feed lots that carries fecal material into rivers.

Changes in Aquatic life

In rivers with high BOD levels, much of the available dissolved oxygen is consumed by aerobic bacteria, robbing other aquatic organisms of the oxygen they need to live. Organisms that are more tolerant of lower dissolved oxygen may appear and become numerous, such as carp, midge larva, and sewage worms. Organisms that are intolerant of low oxygen levels, such as caddisfly larvae, mayfly nymphs, and stonefly nymphs, will not survive. As organic pollution increases, the ecologically stable and complex relationships present in waters containing a high diversity of organisms is replaced by a low diversity of pollution-tolerant organisms.

(Used with permission from Mitchell and Stapp, 1992)
Courtesy of Project Green

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