National Pollutant Discharge Elimination System (NPDES)

Sanitary Sewer Overflows (SSOs)


Sanitary sewer systems collect and transport domestic, commercial, and industrial wastewater and limited amounts of stormwater and infiltrated ground water to treatment facilities for appropriate treatment. Sanitary sewers are different than combined sewers, which are designed to collect large volumes of stormwater in addition to sewage and industrial wastewater. Occasionally, sanitary sewers will release raw sewage. These types of releases are called sanitary sewer overflows (SSOs). SSOs can contaminate our waters, causing serious water quality problems, and back-up into homes, causing property damage and threatening public health.

Possible causes of SSOs include:

  • blockages,
  • line breaks,
  • sewer defects that allow stormwater and groundwater to overload the system,
  • power failures,
  • improper sewer design, and
  • vandalism.

EPA estimates there are at least 23,000 - 75,000 SSOs per year (not including sewage backups into buildings) in the U.S.

SSOs that reach waters of the U.S. are point source discharges. Like other point source discharges from municipal sanitary sewer systems, SSOs are prohibited unless authorized by a NPDES permit. Moreover, SSOs, including those that do not reach waters of the U.S., may be indicative of improper operation and maintenance of the sewer systems, and may violate NPDES permit conditions.