top of page
Biological Oxygen Demand
A critical test that we performed was for BOD. When organic matter decomposes, aerobic bacteria feeds off it. As it is broken down, it is oxidized (combined with oxygen). As a result, the bacteria uses up a certain amount of oxygen. BOD is the measure of the amount of oxygen that is used up by these microorganisms in the aerobic oxidation of organic matter. It is similar to DO, however, it instead measures the oxygen used up. Less BOD means that there is not that much oxygen being used up by bacteria, so there is more oxygen present for other organisms.
BOD is important because high levels of BOD means that bacteria is consuming the majority of the oxygen present in the water. Other organisms that need oxygen to live are being robbed of a crucial component of life. Many organisms are unable to live in water where there is a low amount of DO present. Therefore, certain species may experience a die off if the bacteria is continuing to consume the oxygen and produce a high amount of BOD. The diversity in the water will decrease, and this could cause devastating effects on the ecosystem.
BOD occurs naturally through organic matter entering the body of water from vegetation along the water. Such example could include leafs. This could alter the BOD because bacteria would feed off if the organic matter. However, there is nothing that humans can do to control it since it is natural.
Humans have a very heavy impact on BOD levels in water. Point sources (easily identifiable) are big producers of waste affecting the BOD. These sources include paper mills, meat packaging plants, food processing industries, and wastewater treatment plants. All of these sources produce waste that enter the water and affect the BOD since bacteria will eat the waste and use up a large amount of oxygen. Nonpoint sources (hard to identify) include urban runoff, agricultural runoff, and runoff from feedlots. All of these sources also contain waste products that enter the water and result in eutrophication. The bacteria uses up the oxygen in that process. Human impacts are crucial to raising the BOD and affecting wildlife.
In our area that we tested in the Chicago River, we acquired 5 mg/L. 5 mg/L gave us a Q value of 55/100. This represents a relatively medium quality of water concerning BOD. The water is neither bad quality nor good quality.
We got the results we did because there is probably some source of contamination near the river. Despite the fact that there are little sources of industry or human waste, there might have been something in the water that caused bacteria to use up the oxygen. This also reveals that there is a large amount of bacteria in the water that is active and eager to consume any sort of waste that enters the water. There could also be vegetation, such as certain plants, that fall into the water, causing the BOD to increase from a natural source.
To test BOD, we used a test kit. We added a small amount of water into test tubes and added a drop of solution from the BOD kit. We then wrapped it in tin foil, put it on a rack, and let it sit for 3 days. After 3 days, we unwrapped it and looked at the color and changes of the water. We compared this to a chart given and determined the BOD.
bottom of page