Biochemical Oxygen Demand (BOD) and Water

Biochemical oxygen demand (BOD) represents the amount of oxygen consumed by bacteria and other microorganisms while they decompose organic matter under aerobic (oxygen is present) conditions at a specified temperature. 

When you look at water in a lake the one thing you don't see is oxygen. In a way, we think that water is the opposite of air, but the common lake or stream does contain small amounts of oxygen, in the form of dissolved oxygen. Although the amount of dissolved oxygen is small, relative to the amount in the air, up to about ten molecules of oxygen per million of water, it is a crucial component of natural water bodies. 

The presence of a sufficient concentration of dissolved oxygen is critical to maintaining the  aquatic life and aesthetic quality of streams and lakes. Biochemical oxygen demand measures one very important control on the amount of dissolved oxygen present in the water. Water bodies contain organic matter (particulate organic carbon (POC) and dissolved organic carbon (DOC)), which aerobic (oxygen using) bacteria metabolize (use as food), consuming oxygen in the process. The amount of oxygen consumed (BOD) is generally proportional to the amount of organic matter in the water body. This allows us to use BOD as a surrogate measure of the amount of organic matter. Surrogate measurements are measurements of one thing that tell us about something that is difficult to measure directly. Where organic matter concentrations are low, bacteria do not completely consume the available dissolved oxygen, and thus BOD is relatively low. However, under some conditions, for example, summer warm water temperatures, or higher organic pollution, bacteria may consume oxygen faster than it can be replenished, leaving little or no dissolved oxygen for fish and invertebrates. This can have devastating effects on aquatic life and the ecology of the water body. 

Now that we understand a little bit about biological oxygen demand, lets learn about some of the things that may affect biological oxygen demand. Fertilizers are often applied to farm fields, golf courses, and even our lawns to help plants grow. However, the fertilizer does not usually all stay where it’s supposed to and, when that fertilizer runs-off into streams, lakes, and wetlands, it can lead to increased aerobic activity and high BOD which correlates which often leads to low dissolved oxygen values. Another possible source of nutrients is wastewater. If the organic matter that flows to wastewater treatment plants is inadequately treated before release into streams, one of the harms can be decreased dissolved oxygen in the stream. BOD is used as an index of the degree of organic pollution in water. In wastewater treatment plants when the waste organic matter is stabilized or made unobjectionable through its decomposition by living bacterial organisms, which need oxygen to do their work, a BOD sample is taken and assessed to make sure that sufficient organic matter has been removed from the water.