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Michigan Environmental Education Curriculum
Water Quality - Pollutant Sources and Impacts

Earlier, we learned that the addition of phosphorus to lakes stimulated the growth of algae and higher plants which settle to the bottom and use up oxygen when they are decomposed.

In rivers, the primary source of oxygen-consuming organic matter is the effluent from municipal and industrial wastewater treatment plants. The animation shows how the level of treatment influences the dissolved oxygen sag curve. You can estimate the level of wastewater treatment (percent organic matter removed) required to meet the oxygen standard (red line) which protects aquatic life. Note that less treatment is required in winter because bacteria respire more slowly when it is cold. Cost is always a consideration in pollution control. How do you think families would react to having to pay the cost of protecting the river?

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