Combined sewer overflow discharge volume
| Object type | Damage Signal |
|---|---|
| SIGNAL Earth ID | DS-00761 |
| Observable type | Untreated wastewater overflow volume released to the environment |
| Unit | m3/year (cubic meters of untreated wastewater released to the environment per year) |
| Temporal structure | Annual |
| Monitoring backbone | Utility overflow / bypass reporting |
refers to the total amount of untreated or partially treated wastewater and stormwater released into the environment during combined sewer overflow (CSO) events. These events occur when combined sewer systems, which collect both sewage and stormwater in a single pipe, exceed their capacity during heavy precipitation or snowmelt, resulting in the discharge of excess flow directly into nearby water bodies. This phenomenon is significant because it introduces pollutants, pathogens, and nutrients into aquatic ecosystems, potentially impacting water quality and public health.
Understanding and quantifying the volume of combined sewer overflows is essential for urban water management, environmental monitoring, and regulatory compliance. It provides insight into the frequency and magnitude of untreated discharges, which can inform infrastructure improvements and pollution mitigation strategies. The discharge volume is typically expressed in cubic meters per year, reflecting the annual total volume of overflow events.
Within the broader context of urban hydrology and wastewater management, combined sewer overflow discharge volume is interconnected with precipitation patterns, sewer system design, and wastewater treatment capacity. Monitoring this volume helps assess the environmental impact of urban runoff and wastewater systems under varying climatic and operational conditions.
Geographic / System Context
[edit]Combined sewer overflow discharge volume is relevant primarily in urban areas served by combined sewer systems, which are prevalent in many older cities in North America and Europe. These systems integrate sanitary sewage and stormwater runoff into a single pipeline network. During periods of intense rainfall or rapid snowmelt, the capacity of these systems can be exceeded, leading to overflow events. The geographic scope of this signal is not confined to a specific region but applies wherever combined sewer systems exist. The environmental medium affected by these discharges is primarily surface water bodies such as rivers, lakes, and coastal waters receiving the overflow. The impact and frequency of CSO events can vary widely depending on local climate, urban infrastructure, and watershed characteristics.
Monitoring and Measurement
[edit]Monitoring of combined sewer overflow discharge volume relies on utility overflow and bypass reporting systems maintained by municipal water and wastewater agencies. These entities typically record the occurrence, duration, and estimated volume of overflow events using flow meters, telemetry systems, and event logs. Measurement methods may include direct flow measurement at overflow points, modeling based on rainfall data and sewer system hydraulics, and estimation through combined sewer system capacity analyses. Data collection is generally aggregated on an annual basis to capture the total volume of untreated or partially treated wastewater released over time. This monitoring is essential for regulatory reporting, environmental compliance, and infrastructure planning.
Within the SIGNAL system, this phenomenon is treated as a defined environmental signal whose boundaries and measurement conventions are described below.
Signal Definition
[edit]The combined sewer overflow discharge volume signal quantifies the total volume of untreated or partially treated wastewater and stormwater discharged into the environment through combined sewer overflow events. It is measured in cubic meters per year (m3/yr) and reflects the annual aggregate volume of overflow released from combined sewer systems during periods when system capacity is exceeded.
Boundary Conditions
[edit]Boundary inclusions for this signal encompass all volumes of untreated or partially treated wastewater and stormwater discharged through combined sewer overflow events from municipal combined sewer systems. This includes discharges occurring during rainfall, snowmelt, or other events that cause the sewer system to exceed its design capacity. Boundary exclusions involve discharges from separate sanitary sewer overflows, fully treated effluent releases from wastewater treatment plants, and stormwater discharges from separate storm sewer systems not combined with sanitary sewage. Additionally, non-urban runoff and groundwater infiltration are excluded from this signal's volume measurements.
Aggregation Semantics
[edit]Aggregation of combined sewer overflow discharge volume is performed primarily on an annual temporal scale, summing overflow volumes over the course of a calendar year to provide a comprehensive measure of discharge magnitude. Geographic aggregation is generally conducted at the utility service area level or watershed scale, depending on data availability and management boundaries. Cross-signal aggregation may involve integrating this signal with related environmental indicators such as extreme precipitation intensity, urban flood inundation extent, and wastewater contaminant overflow load to assess broader urban water quality and hydrologic impacts. These aggregation approaches facilitate analysis of temporal trends, spatial distribution, and interactions with other environmental stressors.
Observational Status
[edit]Monitoring of combined sewer overflow discharge volume is ongoing through municipal utility reporting systems, with data typically compiled and reported annually. While comprehensive datasets exist for many urban areas with combined sewer systems, variability in monitoring methods and reporting standards can affect data comparability. Future SIGNAL releases may incorporate enhanced spatial resolution, integration with precipitation and water quality data, and improved characterization of overflow event drivers. Continued development of monitoring technologies and data sharing will support more detailed assessments of CSO impacts on water quality and ecosystem health.
Related Signals
[edit]- Extreme precipitation intensity
- Freshwater oxygen depletion pressure index
- Untreated wastewater overflow and release to the environment
- Urban flood inundation extent
- Urban stormwater pathogen load
- Wastewater contaminant overflow load
- Wastewater nutrient overflow load
- Wastewater service disruption ratio
Key People
[edit]- William Bernard Perry
- Reza Ahmadian
- Max Munday
- Owen D. Jones
- Isabelle Durance
Key Associated People
[edit]- R. Berndtsson — Lund University [Researcher; High]
- William Hogland — Linnaeus University [Researcher; High]
Inclusion reflects material contribution to the scientific understanding of this damage signal; it does not imply review, endorsement, or affiliation with SIGNAL Earth.
Sources
[edit]- A Screening Assessment of the Potential Impacts of Climate Change on Combined Sewer Overflow (CSO) Mitigation In the Great Lakes and New England Regions (Final Report) — U.S. Environmental Protection Agency, 2008. [Assessment; Supporting; High]
- Three-dimensional model to capture the fate and transport of combined sewer overflow discharges: A case study in the Chicago Area Waterway System — Science of The Total Environment, 2017. DOI: 10.1016/j.scitotenv.2016.08.191. [Paper; Supporting; High]