Combined Sewer Overflows

From Beachapedia

By Caroline Gleason
Last Updated: 8/4/21

Introduction to Combined Sewer Overflows

The majority of sewage infrastructure in the United States consists of centralized wastewater systems, in which sewage and wastewater from homes, businesses, medical facilities and more is discharged to a network of sewers, and transported to a central wastewater plant for varying levels of treatment.

Combined sewer systems (CSSs) are designed to collect rainwater runoff, domestic sewage, and industrial wastewater all in the same pipe for transport to a central wastewater treatment plant. Combined sewers were some of the first centralized wastewater infrastructure installed in the United States, and are one of the oldest types of wastewater infrastructure still in use today.[1] The volume of stormwater and wastewater carried by these combined sewers can fluctuate greatly, so these systems are designed to overflow at specific discharge locations when their capacities are exceeded. These discharges of diluted wastewater are called combined sewer overflows, and are a primary pollution concern for the 40 million people served by combined sewer systems in the US. [2]

In most US cities, stormwater and wastewater are separated into two different sewer systems: a stormwater sewer system and a sanitary sewer collection and treatment system. Similar to combined sewer systems, separated sewer systems can overflow in unplanned flooding events called sanitary sewer overflows (SSOs). An important distinction, though, is that SSOs are not part of the design of separated sewers, while CSOs are a planned feature of combined sewer systems. Read more about separated sewers and SSOs here.

How Do Combined Sewer Overflows Happen?

During dry weather and periods of light rain, combined sewer systems are able to transport all of their wastewater to a central treatment plant, where it is treated and then discharged to a water body. When they were installed, this was considered an advantage of a combined sewer system: both stormwater and wastewater would receive treatment before being discharged into the environment, as opposed to only treating wastewater. However, this advantage is increasingly outweighed by the public health and environmental problems caused by combined sewer overflows, or CSOs.

CSO’s happen when combined sewer systems are overloaded with stormwater, rainwater, or other forms of inundation. CSSs are designed to collect stormwater runoff, household wastewater (sewage), and industrial wastewater all in the same pipe. When the amount of stormwater increases during periods of heavy rain or snowmelt, the volume of input often exceeds the capacity of the sewer system or treatment plant. To keep the sewer system from backing up and flooding streets, yards, and homes, combined sewer systems are designed to overflow at specific discharge points (often referred to as CSO locations), sending excess (untreated) wastewater directly to nearby streams, rivers, or other water bodies. [3] [4]

Across the United States, CSOs release around 850 billion gallons of diluted yet untreated sewage into surface waterways every year.[5] As more extreme storms and flooding events are expected due to climate change, CSOs will likely occur more frequently, releasing even more untreated sewage into our waterways.

Current Use

40 million people in approximately 772 communities around the US use combined sewer systems for their wastewater management, mostly in older cities in the Northeast, Great Lakes and Pacific Northwest. [6] As mentioned, most combined sewer systems in the US were built in the mid- to late-1800s (the first was built in Chicago in the 1850’s).[7] At the time, combined systems made sense; a system that collected stormwater and wastewater together into one sewer network required less piping and thus were considered more affordable than sewer systems that separated stormwater from wastewater. [8] However, most CSSs were built before cities started treating their sewage at all, and many also predate the advent and popularization of cars, which brought about now-ubiquitous paved roads that reduce the amount of rain that can seep into the ground during precipitation events, increasing volumes of runoff during storms. CSSs were built to serve cities that were vastly different than the ones we live in today.

Centuries later, combined sewer systems are no longer a reliable wastewater management solution. Because of population growth and increased stormwater input from precipitation, sea level rise and urban runoff from impervious surfaces, CSSs are dealing with volumes of stormwater and wastewater that frequently exceed their capacities. Combined systems are easily overwhelmed, causing over 40,000 combined sewer overflows that discharge 850 billion gallons of untreated wastewater into surface waterways every year.[9] These discharges contain a host of contaminants and disease-causing pathogens that put human health and the environment at risk (more on this later).

Though the overall proportion of combined sewer systems in the United States is quite small (772 out of over 20,000 central wastewater systems), they serve several large, generally older cities like New York City, Boston and Washington, D.C.[10] Additionally, while CSSs and the combined sewer overflows they cause are a significant pollution concern for some of the largest cities in the US, most communities with CSO problems have populations of fewer than 10,000 people.[11] See a map of US cities with combined sewer systems here.

Problems with Combined Sewer Systems

How Do Combined Sewer Systems Fail?

When their capacity is exceeded, combined sewer systems are designed to fail as a wastewater treatment system. Today, it doesn’t take much rainfall to overwhelm a combined system and trigger a CSO. In New York City, as little as one-twentieth of an inch of rain can overwhelm the system. Because of this incredibly low threshold, more than 27 billion gallons of untreated wastewater are discharged each year into New York Harbor alone, coming from the city’s 460 CSO locations specifically designed for these overflows to happen.[12]

In addition to CSOs, combined systems can also suffer unplanned failures. Just as separated sewer system failures can cause sanitary sewer overflows (SSOs), build-ups of fat and grease (from cooking oils, petroleum-based cleaning products, and more) cause blockages in combined sewers that can prevent the normal flow of wastewater to the treatment plant, and result in sewage backing up into streets, yards and even houses. Likewise, flushing foreign items like wipes and sanitary products can clog sewer lines and cause sewage backups. Even wipes that are labeled ‘flushable’ can cause problems; all products aside from toilet paper should be discarded in the garbage and not flushed (note that any hazardous products, like chemicals or batteries, should be disposed of at a local hazardous waste station). Tree roots can also grow into sewer lines, causing blockages similar to the grease build-ups described above. Or, over time, a very old sewer pipe can simply corrode and crack.

Both separated and combined sewer systems can also suffer mechanical failures from cracks or blockages that lead to raw sewage releases, or breakdowns of poorly maintained equipment at pump stations, which are often reported as the cause of sewage spills (e.g. Los Angeles, CA’s July 2021 spill which released 17 million gallons of sewage into Santa Monica Bay).

Read more about sewer line failures and SSOs in our Sewer Systems Beachapedia article.

How Do CSOs Pollute Coastal Waters?

CSOs are a significant pollution concern, particularly for the East Coast and Great Lakes regions of the US. The state of Pennsylvania is currently dealing with the largest CSO problem in the US, with 152 communities including Pittsburgh, Philadelphia and Harrisburg (the state capital) experiencing CSOs.[13]

While it likely took a significant rain event to overwhelm combined sewer systems when they were first designed and installed, in highly urbanized environments like New York City, it now takes as little as one-twentieth of an inch of rain for sewage to overflow into the harbor.[14] Climate change will only make this worse as more frequent and more severe coastal storms will occur, dumping high volumes of rain, causing more flooding and combined sewer overflows (more on this later).

Overloaded combined sewer systems discharge 850 billion gallons of diluted and untreated sewage into our waterways each year.[15] CSOs contaminate the water with not only pollutants typical of sewage (including pathogens and excess nutrients), but also anything else that stormwater collects as it flows across the urban landscape, meaning that CSOs give heavy metals, pesticides, cleaning agents, trash and debris direct transport into our surface waterways.

Human health impacts from sewage spills, including combined sewer overflows, are a serious and growing concern as spills become more frequent. Sewage pollution contains many harmful contaminants, including several types of disease-causing pathogens that can lead to lung and intestinal infections, symptoms like fever, diarrhea and vomiting, as well as even more dangerous diseases like typhoid fever and cholera. High concentrations of bacteria like enterococcus, which is often used as an indicator of sewage pollution in water quality sampling, can cause serious infections when exposed to open wounds. These are just a few examples of the health risks that sewage pollution poses. In the words of Marilu Flores, Surfrider’s regional manager for Florida and Puerto Rico: “If you have underlying health conditions, if you are immunocompromised, the sky’s the limit on how some of this bacteria can affect you.”[16]

Sewage spills, and the high concentrations of nitrogen and other nutrients they contain, also cause a suite of environmental problems in coastal ecosystems. Nitrogen acts as a fertilizer, causing algae populations to skyrocket in events called ‘’algal blooms’’. These increased populations of algae eventually die and sink to the bottom of the water body to decompose, which depletes the dissolved oxygen in the water and often causes mass die-offs of fish, turtles, manatees and other aquatic life from the lack of oxygen and food sources. When this happens on a large scale, affected areas are called ‘’dead zones’’.

Drops in dissolved oxygen are not the only consequence of ‘’eutrophication’’, or the over-enrichment of nitrogen and other nutrients in waterways. In tropical regions, the algae cover reefs, starving corals of sunlight and oxygen. The subsequent, sudden loss of reef habitat has repercussions for fish, with one study finding 83% of the most abundant species either severely reduced or completely eliminated following an algal bloom in the Gulf of Oman.[17] In more temperate regions, high levels of nitrogen in the water lead to the decline of seagrass beds that provide nursery habitat for many important fisheries, as well as provide critical storage for atmospheric carbon as a ‘’blue carbon’’ ecosystem. Eutrophication can also lead to an increase in harmful algal blooms and rred tides that produce toxins that contaminate shellfish, cause fish kills and other sea life to die, and even threaten human health with a variety of mild to severe symptoms.

Contributions to CSOs from Climate Change/SLR/Exacerbated Flooding

We are becoming increasingly familiar with the consequences of climate change and sea level rise, including changing weather patterns like the increased intensity and frequency of strong coastal storms and hurricanes. Combined sewer systems are already particularly sensitive to these climate-driven changes, and increased pressure on wastewater infrastructure will likely mean more sewage spills and failures from both combined and separated sewer systems in the future. We must make significant investments now to make our wastewater infrastructure more resilient in the face of climate change.

Flooding associated with extreme weather events, which are expected to increase in intensity as our climate warms, introduce additional rainwater to combined sewers and cause them to overflow even quicker. What’s more, these overflows may become more common even in dry weather. Many coastal regions in the US are already experiencing recurrent flooding at high tide during events aptly called high-tide flooding or sunny day flooding. Research shows that high-tide flooding events will become more common in areas already experiencing high-tide flooding, and will expand to currently unaffected areas as sea levels rise.[18] Increased incidence of flooding means increased frequency of combined sewer overflows as coastal sewers take on saltwater and exceed their capacities.

Likewise, in low-lying coastal areas, sea level rise can cause seawater to infiltrate old pipes, through storm drains or compromised sewers left in disrepair.[19] Projected levels of sea level rise are also set to raise water tables, elevating the depth of permanently saturated soils and potentially flooding sewers with groundwater. This is already happening in Honolulu, Hawai’i, where a study has found direct evidence of tidally-driven groundwater flooding of the city’s wastewater infrastructure.[20] According to a study conducted across California, a coastal region with diverse topography and climate, 1 meter of sea-level rise is expected to expand the reach of areas flooded from below by approximately 50 to 130 meters inland, with low-lying coastal communities most at risk.[21] [22] The end result of groundwater flooding is the same as during rain or storm events: sewer systems get overwhelmed and discharge untreated sewage into local waterways. The risks to human health and the environment from sewage pollution remain the same as well, and aging, neglected infrastructure is only exacerbating those risks.


Personal Actions

There are many actions we all can take to care for our wastewater systems, regardless of type. Good practices include:

  • Only flush the three P’s: pee, poop, and (toilet) paper.
  • Conserve water inside the house.
  • Don’t pour cooking grease or oils down the drain; instead, collect it in a container, freeze it, and throw it in the trash.
  • Check the ingredients and opt for natural personal care products (like soaps and sunscreen) when possible.
  • Try to avoid cleaners with petroleum additives or fillers, as they act like grease to cause blockages once they enter a wastewater system.
  • Skip the powder detergents for the same reason.

Reducing stormwater and dry-weather urban runoff can go a long way to reduce combined sewer overflows. Use as little water as possible. Don't over-water your lawn, and adjust your sprinklers to avoid watering the pavement (it won't grow). Consider installing an Ocean Friendly Garden to increase infiltration (rainwater that is absorbed into the ground) and reduce runoff entering sewer systems. Find more tips on how to reduce runoff here.

Alternative Technologies

Due to the highly developed, urbanized locations some of the nation’s combined sewers serve, solutions for reducing or eliminating CSOs can be both complicated and costly. Proposals fall under three categories: separating stormwater conveyances from sewer pipes; increasing the storage capacity of combined sewer systems, and reducing the amount of rain that enters the system through something called “green infrastructure.”

More and more communities are looking to green infrastructure to increase the capacity of their watershed to absorb rain and reduce polluted runoff. According to the EPA, Vegetated swales, rain gardens, permeable pavement, rain barrels and green roofs are all types of green infrastructure that have been successfully implemented in cities to reduce CSOs.

The benefits of green infrastructure go way beyond only improving local water quality conditions. Green infrastructure can reduce the urban heat island effect, improve local air quality, reduce impacts of climate change through carbon sequestration of trees, plants and healthy soils, and overall improve the quality of life for urban residents and visitors.

Check out an in-depth example of these kinds of green stormwater infrastructure (GSI) projects in the Southern California Stormwater Management Coalition’s 2019 report here.

Policy Actions

Combined sewer systems and CSOs are regulated by the US EPA through their CSO Control Policy, which sets standards and offers guidance for communities on how they can most effectively meet the water quality standards set in the Clean Water Act. EPA works closely with states and regional authorities to achieve water quality goals in, as the agency website says, “a flexible and cost-effective manner.” [23] One example of this kind of collaboration was between EPA and the Massachusetts Water Resources Authority (MWRA). In 2006, when EPA reached a settlement with MWRA to reduce combined sewer overflows discharging into the Charles River, reducing CSO outflows from 1.7 billion annual gallons in 1988 to 6.88 million gallons per year following the settlement.

A growing number of municipalities are moving beyond EPA’s regulations to further address pollution from CSOs by implementing stormwater fees. Acre-by-acre, developed areas cause more stormwater runoff than undeveloped areas where rainwater is allowed to infiltrate (seep into the ground) and recharge groundwater reserves.[24] Developed areas have lots of paved surfaces (called impervious cover) like roads, parking lots, and driveways that serve as a barrier and prevent stormwater from seeping into the ground. Instead, increased amounts of stormwater runoff travels along roads and paved surfaces to storm drains, and has to be managed by sewer systems.

Stormwater fees are an attempt to correct the imbalance between the commercial properties with lots of impervious cover that generate disproportionate amounts of stormwater runoff, and community members who have historically paid more to operate their combined sewer systems while still suffering the consequences of CSO pollution. The fees provide a stable revenue source for local governments to invest in maintaining, and perhaps even upgrading, their stormwater infrastructure. In Pennsylvania, the state considered to have the largest CSO problem in the United States, at least 27 municipalities have already put some form of stormwater fees in place.[25] Read more here.

More broadly, significant investments in repairing and rebuilding wastewater infrastructure are required to address the harms associated with sewage pollution. Our sewer systems have been neglected for years, decades even, resulting in approximately 900 billion gallons of under-treated sewage being discharged into surface waters every year.[26]

US EPA’s Clean Water State Revolving Fund (CWSRF) is one source of federal funding for states to upgrade their wastewater infrastructure. Funding available through CWSRF is determined by Congress, which is responsible for EPA’s budget. The 2021INVEST in America Act, which passed the House of Representatives on July 1, 2021, would authorize much-needed funding for wastewater infrastructure investments. Included in the House version of the bill was the Water Quality Protection and Job Creation Act, which designates $8 billion annually over five years to the EPA CWSRF, and a Climate Resilience Provision that requires any CWSRF-funded infrastructure projects to complete a climate resiliency assessment and be designed and constructed to withstand climate change impacts.

Surfrider created our SStop Sewage Pollution campaign to help improve our wastewater infrastructure and ensure that all sewage in the US is adequately collected and treated to protect public and environmental health. This effort includes tracking state and federal policies like the CWSRF and BEACH Act, as well as operating the Blue Water Task Force, a national network of volunteers monitoring bacteria levels at more than 450 ocean, bay, estuary and freshwater sampling sites across the US. Read more about how to get involved in our Stop Sewage Pollution campaign here.

Case Study: Camden, NJ

The state of New Jersey and it’s 21 CSO cities deal with a significant number of pollution issues from combined sewer overflows. Approximately 17% of the state’s population lives in cities with combined sewer systems, and the populations in those cities are growing. Pollution from CSOs is also an environmental justice issue: 36% of New Jerseyans living below the poverty line live in cities served by CSOs. Camden, NJ provides one example of how these cities are addressing problems with CSOs.[27]

Camden County Municipal Utilities Authority (CCMUA) is responsible for providing wastewater services to around 500,000 people in the county’s 37 municipalities, which amounts to around 80 million gallons of wastewater per day. [28] Camden County uses a combined sewer system which, due to age and lack of adequate funding, can be overwhelmed by an inch of rain to cause flooding of roads, parks, homes, and waterways with sewage. An unusually wet summer in 2013 caused several flood events that forced the shutdown of the city’s public transportation and cut off road access, effectively stranding anyone in town - residents, workers, students and visitors. [29] CCUMA and the city of Camden have recognized that these precipitation events will only become more common with climate change, and have taken steps to adapt accordingly.

One major way that the city is addressing CSOs is through the Camden Stormwater Management and Resource Training Initiative, or Camden SMART Initiative, founded in 2011. Under the initiative, local government worked with the EPA, the NJ Department of Environmental Protection, Rutgers University and local non-profits to promote green stormwater infrastructure (GSI) projects to help Camden reduce CSOs and adapt to future, wetter conditions driven by climate change. As the initiative’s website describes, “the Camden SMART Initiative is a community-driven movement to protect human health, improve conditions for economic development, improve water quality, and enhance the quality of life.”

Examples of [ initiative successes] include the construction 50 GSI projects that have allowed over 60 million gallons of stormwater to be captured, treated, and infiltrated into the ground each year since its inception. That is 60 million gallons of stormwater that would have contributed to CSOs and flooding of streets, homes, parks, and waterways with contaminated wastewater. On top of these larger GSI projects, Camden SMART has distributed over 200 rain barrels to residents for free, planted over 1,500 trees, and engaged thousands of community members through events and workshops alongside both private and public businesses and organizations.

Through all of this, city stakeholders have taken steps to keep equity at the forefront. The Camden County Municipal Utilities Authority used the blueprint of an AmeriCorps-led youth employment program in nearby Philadelphia to develop their own program, PowerCorps Camden. As an AmeriCorps program, PowerCorps crew members receive a living stipend, health insurance, professional development and job training, as well as an education stipend. Corps members work on stormwater management projects that both help with CSO pollution and “green” the community, providing vital community space and parks that deliver on the suite of benefits described earlier in this article. Learn more about PowerCorps here.

Learn more about Camden and CCMUA’s greenwater infrastructure work here.

Additional Resources

Map of US Combined Sewer Systems


  1. PennState Extension. (2021, April 8). What are Combined Sewer Overflows?
  2. US EPA. (2021, May 3). Combined Sewer Overflows (CSOs)
  3. US EPA. (2021, May 3). Combined Sewer Overflows (CSOs)
  4. Riverkeeper. (2021). Combined Sewage Overflows (CSOs)
  5. US EPA. (2015, September 16). EPA Needs to Track Whether Its Major Municipal Settlements for Combined Sewer Overflows Benefit Water Quality.
  6. US EPA. (2021, May 3). Combined Sewer Overflows (CSOs)
  7. PennState Extension. (2021, April 8). What are Combined Sewer Overflows?
  8. PennState Extension. (2021, April 8). What are Combined Sewer Overflows?
  9. US EPA. (2015, September 16). EPA Needs to Track Whether Its Major Municipal Settlements for Combined Sewer Overflows Benefit Water Quality.
  10. Riverkeeper. (2021). Combined Sewage Overflows (CSOs)
  11. US EPA. (2020, August 24). Combined Sewer Overflow Frequent Questions
  12. Riverkeeper. (2021). Combined Sewage Overflows (CSOs)
  13. PennState Extension. (2021, April 8). What are Combined Sewer Overflows?
  14. Riverkeeper. (2021). Combined Sewage Overflows (CSOs)
  15. US EPA. (2015, September 16). EPA Needs to Track Whether Its Major Municipal Settlements for Combined Sewer Overflows Benefit Water Quality.
  16. Everitt Rosen. (2021). Major spills from Florida’s sewage treatment plants are on the rise - and so are the storms that can cause aging pipes to burst.
  17. Ella Davies. (2010, October 8). Toxic algae rapidly kills coral.
  18. Thompson, P. R., Widlansky, M. J., Hamlington, B. D., et al. (2021, June 21). Rapid increases and extreme months in projections of United States high-tide flooding.
  19. McKenzie, T., Habel, S., Dulai, H. (2021, March 12). Sea-level rise drives wastewater leakage to coastal waters and storm drains.
  20. McKenzie, T., Habel, S., Dulai, H. (2021, March 12). Sea-level rise drives wastewater leakage to coastal waters and storm drains.
  21. Befus, K. M., Barnard, P. L., Hoover, D. J., et al. (2020, August 17). Increasing threat of coastal groundwater hazards from sea-level rise in California.
  22. US Geological Survey. (2020, September 30). New Model Shows Sea-level Rise Can Cause Increases in Groundwater Levels along California’s Coasts]
  23. US EPA. (2021, May 3). Combined Sewer Overflows (CSOs)
  24. Penn State Extension. 2021. What is a municipal stormwater fee?
  25. Penn State Extension. 2021. What is a municipal stormwater fee?
  26. Surfrider Foundation. (2020). Clean Water Report
  27. New Jersey Future .2015. New Jersey: Combined Sewer Systems by the Numbers
  28. New Jersey Future .2015. New Jersey: Combined Sewer Systems by the Numbers
  29. New Jersey Future .2015. New Jersey: Combined Sewer Systems by the Numbers