Why Test for Oxygen Demand?
The biological oxygen demand (BOD) water test is used to determine how much oxygen is being used by aeorbic microorganisms in the water to decompose organic matter. If these aeorbic bacteria are using too much of the dissolved oxygen in the water, then there will not be enough left over for the fish, insects, and other organisms that rely on oxygen. The rich diversity of life on a healthy river is then reduced to a low diversity (but sometimes high volume) of pollution tolerant organisms.
There are two ways that humans inadvertently drive up biological oxygen demand. First, too much organic material is dumped into a river or lake from paper mills, food processing plants, wastewater treatment plants, urban runoff, etc. Second, fertilizers in the form of nitrates and phosphates flow into a river from agricultural and urban runoff and then stimulate the overgrowth of plants and algae. However, once this organic matter (plants, algae, human, food, and animal waste, yard clippings and saw dust) begin to decompose, it begins to suck the oxygen out of the water.
The BOD test is done by taking a water sample and keeping it cool and dark for five days so as not to stimulate algal growth. Then take a water sample from the same site after five days. Perform a dissolved oxygen test on both samples and subtract the two results so see how much oxygen was used during the time period.
Refer to the Field Manual for Water Quality Monitoring for details on performing this test.
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