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Municipal Storm Water: Combined Sewer Overflows, Sanitary Sewer Overflows Compliance Monitoring
CWA Topics
- National Pollutant Discharge Elimination System
- Pretreatment of Wastewater
- Biosolids
- Oil Spill Prevention
- Industrial Storm Water
- Concentrated Animal Feeding Operations
- Municipal Storm Water
- Wetlands
- Wastewater Trading Program
- Discharge Monitoring Report - Quality Assurance
Combined Sewer Overflows
Combined sewer systems are sewers that are designed to collect rainwater runoff, domestic sewage, and industrial wastewater in the same pipe. Most of the time, combined sewer systems transport all of their wastewater to a sewage treatment plant, where it is treated and then discharged to a water body. During periods of heavy rainfall or snow melt, however, the wastewater volume in a combined sewer system can exceed the capacity of the sewer system or treatment plant. For this reason, combined sewer systems are designed to overflow occasionally and discharge excess wastewater directly to nearby streams, rivers or other water bodies.
These overflows, called combined sewer overflows (CSOs), contain not only storm water but also untreated human and industrial waste, toxic materials, and debris. They are a major water pollution concern for the approximately 772 cities in the U.S. that have combined sewer systems.
CSOs may be thought of as a type of "urban wet weather" discharge. This means that, like sanitary sewer overflows (SSOs) and storm water discharges, they are discharges from a municipality's wastewater conveyance infrastructure that are caused by precipitation events such as rainfall or heavy snow melt.
EPA develops national priorities that focus on significant environmental risks and noncompliance patterns. For Fiscal Years 2005 to 2007, the CSO national priority strategy aims to protect public health and the environment by increasing the number of communities that implement the nine minimum controls and have a long-term CSO control plan in place.
Sanitary Sewer Overflows
Properly designed, operated, and maintained sanitary sewer systems are meant to collect and transport all of the sewage that flows into them to a publicly owned treatment works (POTW). Overflow from sewers occur from municipal sanitary sewers in almost every system occasionally as unintentional discharges of raw sewage. These types of discharges are called sanitary sewer overflows (SSOs).
SSOs have a variety of causes, which include:
- severe weather
- improper system operation and maintenance
- vandalism.
EPA estimates that there are at least 40,000 SSOs each year. The untreated sewage from these overflows can contaminate our waters, causing serious water quality problems. SSOs can also back-up into basements, causing property damage and threatening public health.
EPA develops national priorities that focus on significant environmental risks and noncompliance patterns. For Fiscal Years 2005 to 2007, the SSO national priority strategy aims to protect public health and the environment by minimizing the number of overflows from SSOs.
Compliance Monitoring
EPA conducts inspections of POTWs with combined sewer and sanitary sewer overflows. EPA inspections at CSOs involve:
- reviewing the National Pollutant Discharge Elimination System (NPDES) permit and any enforcement orders
- verifying the permittee is preventing CSOs during dry weather
- reviewing compliance with the nine minimum CSO controls
- verifying the permittee is adhering to a schedule in the long term control plan
- eliminating overflows from sensitive areas
- implementing a monitoring program
EPA inspections at SSOs involve:
- reviewing the NPDES permit and any enforcement orders
- verifying that the permittee is in compliance with the NPDES standard permit conditions to mitigate and institute proper operation and maintenance
- determining there are no unpermitted discharges, or discharges from a location other than the discharge point specified in the permit
Details on the CSOs and SSOs can be found in Chapters 12 and 13 of the NPDES Compliance Inspection Manual.