Urban Water Quality: Sewage Overflows

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Wastewater and sewage treatment are important topics in any society, all throughout history and into today. Improperly disposed of or treated sewage can cause disease and harm the ecosystem. That is why when a sewage overflow occurs it garners negative news attention.

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Urban Water Quality: Sewage Overflows

Stormsewer flowing both storm flow and sewage overflow during a major storm.

This is a storm sewer, designed to transport stormwater runoff away from streets, that cannot handle the volume of runoff. (Credit: Alan Cressler, USGS)

In some urban areas there are combined sewer systems that are sewers that are designed to collect rainwater runoff, domestic sewage, and industrial wastewater in the same pipe. These overflows, called combined sewer overflows (CSOs) contain not only stormwater but also untreated human and industrial waste, toxic materials, and debris. 

Many sewer lines are constructed next to streams to take advantage of the continuous, gradual slopes of stream valleys. Blockages, inadequate carrying capacity, leaking pipes, and power outages at pumping stations often lead to sewage overflows into nearby streams. There are three types of sewer systems:  

  1. Storm sewers carry storm runoff from streets, parking lots, and roofs through pipes and ditches, and eventually into streams.
  2. Sanitary sewers carry raw sewage from homes and businesses to wastewater-treatment facilities.
  3. Combined sewers carry a combination of raw sewage and stormwater runoff.

This picture of a sanitary sewage overflow illustrates a common problem concerning sewage overflows that occur in urban areas. This overflow happened in Sept. 2009 during historic flooding in Atlanta, Georgia, USA. Sanitary sewer overflows occur when sewer pipes clog or pumping stations break down. As shown here, mixed sewage and rainfall runoff overflows from manholes and leaking pipes into nearby streams rather than backing up into homes and businesses.

Combined sewer overflows occur during storms when there is more stormwater flowing than the pipes leading to a treatment plant can handle. The excess runoff flushes human and industrial wastes, oil, toxic metals, pesticides, and litter into streams.