Youth water quality tests – Part 3 – Biochemical oxygen demand

Teach students about science by playing in the river! You can learn about the pollution in the river by studying the biochemical oxygen demand.

In a series by Michigan State University Extension, this third article will focus on testing water quality with students.  Students can learn about how clean or polluted nearby streams are by conducting some simple tests.  By doing further investigation, the students can identify the problem and help to improve the river. There is an international program called Global Rivers Environmental Education Networks (GREEN) which began in the spring of 1984 with a group of concerned students at a high school located along the polluted Huron River in Ann Arbor, Mich.  Their teacher contacted Dr. William Stapp at the University of Michigan, and together they developed a comprehensive educational program called GREEN. This article will continue to take a closer look at water quality tests conducted as part of GREEN.

Young people can partner with local schools, watershed groups, lake associations, drain commissioner offices, conservation districts, nature centers or other groups to conduct these tests. 

The background
This test measures the oxygen being used by living creatures and chemical processes in the sample when there is an absence of photosynthesis.  During five days, in the dark, living “critters” use oxygen to survive.  By subtracting the biochemical oxygen demand (BOD(5)) value from the dissolved oxygen (D.O.) value, oxygen usage by life in the water can be calculated.  When little oxygen is removed during the five days there is an indication that there are very few organisms using the oxygen.  The main contributor to BOD is organic waste.  Organic waste can come from manure (animal or human), decomposing plant matter (such as leaves or grass clippings dumped near a stream), discharge from food processing plants or agricultural runoff.  High BOD can also come from natural sources, such as runoff from swamps.  Certain cleaning and other chemicals can also increase the BOD.

The test
The BOD test is essentially the same as the dissolved oxygen test, done five days later from a dark bottle.  Read through the previous article in this series to learn about the dissolved oxygen test.  Any bottle can be used to conduct this test, as long as light cannot penetrate the bottle.  A water bottle wrapped in duct tape or electrical tape will work fine.  Keep the bottle in a drawer as another way to keep the light out. You subtract the dissolved oxygen level in parts per million (ppm) on day one from the level on day five, giving you the BOD level.

Note that for dissolved oxygen, ppm is the same as milligrams per liter.

Looking at the data and what kids can do to make it better
If the difference in BOD between day one and day five is greater than about 4 ppm, there is cause for concern.  Usually this can mean there is too much dead plant and animal material getting into the stream.  Work with students to see if they can identify the source.  Is there a tributary entering the stream in that area?  Are there large overhanging trees that are dropping their leaves?  Is there a large amount of animal waste?  Any of these can lead to a higher BOD level.  If students identify the source of the BOD, they can work with local property owners to minimize or eliminate those sources.

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