Sanitary sewer overflow (SSO) is a condition whereby untreated sewage

Sewage

Sewage is water-carried waste, in solution or suspension, that is intended to be removed from a community. Also known as wastewater, it is more than 99% water and is characterized by volume or rate of flow, physical condition, chemical constituents and the bacteriological organisms that it contains...

 is discharged into the environment prior to reaching treatment facilities thereby escaping wastewater treatment

Wastewater Treatment

Wastewater treatment may refer to:* Sewage treatment* Industrial wastewater treatment...

. When caused by rainfall it is also known as wet weather overflow. It is primarily meaningful in developed countries, which have extensive sewage treatment facilities. The main causes of SSO are:

• Infiltration
  Infiltration (hydrology)
  Infiltration is the process by which water on the ground surface enters the soil. Infiltration rate in soil science is a measure of the rate at which soil is able to absorb rainfall or irrigation. It is measured in inches per hour or millimeters per hour. The rate decreases as the soil becomes...
  
   of excessive stormwater
  Stormwater
  Stormwater is water that originates during precipitation events. It may also be used to apply to water that originates with snowmelt that enters the stormwater system...
  
   into sewer
  Sanitary sewer
  A sanitary sewer is a separate underground carriage system specifically for transporting sewage from houses and commercial buildings to treatment or disposal. Sanitary sewers serving industrial areas also carry industrial wastewater...
  
   lines during heavy rainfall
• Rupture or blockage of sewerage
  Sanitary sewer
  A sanitary sewer is a separate underground carriage system specifically for transporting sewage from houses and commercial buildings to treatment or disposal. Sanitary sewers serving industrial areas also carry industrial wastewater...
  
   lines
• Malfunction of pumping station
  Pumping station
  Pumping stations are facilities including pumps and equipment for pumping fluids from one place to another. They are used for a variety of infrastructure systems, such as the supply of water to canals, the drainage of low-lying land, and the removal of sewage to processing sites.A pumping station...
  
   lifts or electrical power failure
• Human operator error at treatment plant facilities.

SSO is distinct from a combined sewer overflow (CSO), a similar problem of untreated sewage discharges, but which occurs only in a municipal system specifically designed with a combined sewer.
Since medieval times rulers have been aware of the impact of raw sewage improperly discharged to the environment.