Urban communities also face the problem of sewage in their stream water. These communities typically have water treatment plants where sewage is chemically treated then piped to nearby rivers and streams. The untreated sewage, along with stormwater runoff, is often carried to the sewage treatment plant in combined sewage outflows (CSOs).
During heavy downpours, stormwater runoff overwhelms the treatment plant. Consequently, a system of gates opens and releases the excess runoff -- along with the untreated sewage -- into nearby waterways.
Sewage carries excessive nutrients such as nitrogen and phosphorus to stream water. The excessive nutrients can lead to decreased levels of dissolved oxygen as a result of the process of eutrophication. Sewage also contributes to increased total solids , including fecal coliform, as well as increased water temperature.
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