Biochemical Oxygen Demand as Explained by the Organic Carbon Cycle

Biochemical Oxygen Demand as Explained by the Organic Carbon Cycle
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If nontoxic organic pollutants get discharged into a river, lake or stream, they should be pretty harmless, right?


Organic pollutants, even if they have no toxicity, are one of the causes of water pollution because they will have an effect on the dissolved oxygen level in the water. This effect is called BOD or biochemical oxygen demand. Dissolved oxygen is essential to much of the aquatic life, so anything that affects the dissolved oxygen in water should be of concern. This article will delve into the organic pollutants/oxygen demand issue in a bit of detail. The organic carbon cycle and the reactions in that cycle will be discussed. Then the part of the carbon cycle of concern for pollution of water will be considered.

The Organic Carbon Cycle

The organic carbon cycle is the set of natural processes by which inorganic carbon is converted to organic carbon, is then cycled through different forms of organic carbon, and finally is converted back to inorganic carbon. Before proceeding further, a definition of organic matter would be in order. A commonly used definition is that organic matter is any material that is part of a living organism or came from a living or once living organism. Although there are many elements present in small amounts, organic matter is made up predominantly of carbon, hydrogen, and oxygen. Organic carbon is a term used for the carbon that is in organic matter, while inorganic carbon is carbon in other compounds, such as carbon dioxide, carbonate and bicarbonate compounds, diamonds, or graphite.

The two major reactions that take place in the organic carbon cycle are photosynthesis and biological oxidation of organic matter. Photosynthesis is the process by which green plants convert carbon dioxide and water into organic matter in the presence of sunlight. Biological oxidation is the process by which aerobic bacteria in the presence of oxygen break down

Organic Carbon Cycle Image

organic matter to carbon dioxide and water as end products. These two reactions can be represented by the following two equations:

Photosynthesis: CO2 + H2O + sunlight → organic plant matter + oxygen (catalyzed by the chlorophyll in green plants)

Biological Oxidation: waste organic matter (primarily C, H & O) + O2 → CO2 + H2O + energy (This reaction is carried out by aerobic microorganisms; it is the ‘death and decay’ shown in the diagram.) This reaction causes biochemical oxygen demand (BOD) when organic pollutants are present in water and biological oxidation uses dissolved oxygen from the water.

You may notice that these two reactions are each just the opposite of the other. Together they make up a major part of the organic carbon cycle, as shown in the diagrams. The organic plant matter may be eaten by an animal and converted to organic matter in the animal, or the plant may die resulting in decay and biological oxidation of the plant matter. The animals will give off waste organic material and will die at some point. The waste and dead organic matter will decay and be converted to CO2 and water by biological oxidation. If organic pollutants are present in water, their effect as pollution of water is to cause a reduction in dissolved oxygen level in the water.

BOD and Water Pollution

With an understanding of the organic carbon cycle, it becomes clear that even nontoxic organic pollutants are indeed one of the causes of water pollution when discharged into a body of water. The aerobic microorganisms that carry out biological oxidation of organic pollutants are present almost everywhere, because waste organic matter (food for the microorganisms) is present almost everywhere. Thus, when organic pollutants enter a body of water, aerobic microorganisms will be present, and biological oxidation of the organic matter will occur. This will use dissolved oxygen from the water.

Unfortunately, oxygen is only slightly soluble in water, yet it is vital for most aquatic life, so reduction of the oxygen level in a body of water can do anything from causing stress on the aquatic life to killing off the aquatic life, depending on how low the oxygen level becomes.

Biochemical oxygen demand or BOD is a measure of the effect that organic pollutants in a water sample will have on the dissolved oxygen in that water. Specifically, biochemical oxygen demand (BOD) is the mass of oxygen required to react with the organic pollutants in a specified volume of water. Usually BOD is given in mg/L, meaning mg of oxygen to react with the organic pollutants in one liter of the water sample.

For an introduction to wastewater treatment processes for removal of BOD, see the articles, “Biological Wastewater Treatment Processes” and “Learn About Activated Sludge Systems in the Treatment of Wastewater.”

References and Image Credits


The Organic Cycle - drawn by H. Bengtson

Organic Carbon Cycle Image -

REFERENCES for further information:

U.S. EPA -

The Water Environment Association -

National Oceanic and Atmospheric Administration, Earth System Research Laboratory -