What is a CSO?

A combined sewer system (CSS) is a wastewater collection system that collects and transports sanitary wastewater (domestic sewage, commercial and industrial wastewater) and stormwater to a treatment plant in one pipe. During wet weather, when capacity of the system is exceeded, it discharges untreated wastes directly to surface waters—resulting in a combined sewer overflow (CSO; Fig 11).

Because CSOs release untreated wastewater, they can contribute pathogens, nutrients, organic carbon, toxic substances and other pollutants to surface waters (Fig 12).

How prevalent are CSOs in the U.S.?
[from U.S. EPA 2004]

• CSSs serve approximately 40 million people, in 772 communities (Fig 13).
• 828 NPDES permits authorize discharges from 9,350 CSO outfalls.
• U.S. EPA estimates that CSOs release approximately 850 billion gallons of untreated wastewater and stormwater each year.

Figure 13. Prevalence of combined sewer systems (CSSs) in the United States.
Courtesy of U.S. EPA.

CSSs generally have not been constructed since the mid-20th century, and efforts are underway to reduce CSOs in many existing systems (e.g., by separating wastewater and stormwater sewer systems).

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