Biochemical oxygen demand (BOD) is a measure of water pollution.

Some organic matter is always found in rivers, streams and other watercourses. It consists of dead leaves and plants, dead fish and other aquatic life, sewage effluent from septic tanks and sewage treatment plants, animal waste, food processing waste, etc.

As this organic matter decomposes, microorganisms (such as bacteria and fungi) feed on it and it oxidizes (combines with oxygen). Biochemical oxygen demand (BOD) is a measure of the quantity of oxygen used by these microorganisms during this aerobic oxidation of organic matter.

As a result of this digestion, plant nutrients, such as nitrates and phosphates, are released into the water. These feed the plants and stimulate plant growth. Eventually, these plants also die and the circle is continued. High levels of nitrogen and Phosphates can be a major reason for high BOD in rivers. A river with a high BOD has less oxygen available for high oxygen dependency plants and animals and the aquatic life suffers as a result.

BOD is also higher at dawn than at sunset. This is because the aquatic plants release oxygen during the day as part of Photosynthesis, but plants and animals continue to use oxygen during the night when there is no photosynthesis, thus reducing the oxygen level in the river. With excessive plant growth, the percentage of oxygen saturation (dissolved oxygen) often falls below 50% at this time. Large daily fluctuations in dissolved oxygen levels are typical in rivers with extensive plant growth. Many species cannot tolerate such low levels of dissolved oxygen and die, whilst other species that can tolerate low DO levels increase in population. As the BOD of the river rises, the diversity of lifeforms reduces, damaging the eco-structure of the river.

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