USGS - science for a changing world

The USGS Water Science School

The effects of urbanization on water quality:
Sewage overflows

Picture of an manhole overflowing with sewageMany 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.

Related topics:

Effects of urbanization  Groundwater quality  Wastewater-treatment water use

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