By Kelly A. Reynolds, MSPH, PhD

August rains sent a reminder to Detroit, MI residents regarding the dangers of sanitary sewer overflows as nearly 10 billion gallons of raw, untreated sewage seeped into the city’s lakes and rivers.1 Local beaches and drinking water supplies were suspected of being tainted. These events are not unique to Detroit—every community is vulnerable to the possibility of raw sewage exposures from overflows, backflows, spills or other types of discharges of sanitary sewer systems into the environment. Each year, these events impair drinking and recreational water supplies, resulting in significant public health risks.

What are CSOs and SSOs?
Sanitary sewer systems are designed to collect and carry all sewage and wastewater from their sources to municipal wastewater treatment plants, where contaminants can be controlled and treated in a centralized fashion. In many cities, these sewer systems are also combined with stormwater collection systems that serve to collect rainwater runoff during storm events. In these systems, stormwater and community sewage wastes are collected, transported to municipal treatment facilities and treated together prior to discharge to the environment. Maintaining a treatable volume of water in sanitary systems can be challenging, as community waste generation volumes vary considerably over space and time, and storm events are unpredictable. When the wastewater volume exceeds the capacity of the wastewater system, the untreated waste typically overflows directly into the environment.

Combined sewer overflows (CSOs) are typically caused by severe weather, including heavy rainfall events or snow melt. In many areas, populations have increased beyond the capacity of sewer systems, resulting in periodic overflows at peak volumes. Overflow conditions not only result in the discharge of raw, untreated sewage into the environment but also can impact areas of humans exposure, such as drinking water-sourced or recreational water venues. Sewage contaminants include microbial pathogens, industrial/medical wastes, heavy metals, grease, pesticides, oils, debris and more. While not all cities have combined stormwater/sewage collection systems, this condition affects approximately 772 US cities in 32 states, including older cities with more aged infrastructures.2

Sanitary sewer overflows (SSOs) are indicative of additional backflow events occurring due to improper system operation and maintenance, potentially from operator error or periodic pressure drops following electrical power failures. Over time, sewer lines may deteriorate or become blocked, resulting in sewage spills from the collection system. Acts of intentional vandalism are another potential cause. No community is immune to the risks of SSOs.

Enormous burdens
There are approximately 19,500 sewer systems in the US that handle an average daily flow of 50 billion gallons of raw sewage.3 Faced with an aging infrastructure and tens of billions of dollars in needed improvements over the next two decades, sewage overflow events are expected to continue. Much of the nation’s sewage collection infrastructure is in operation beyond its life expectancy, resulting in an ever-increasing risk of breaks, leaks, blockages and malfunctions due to deterioration. Environmental conditions of seismic activity and cycles of freezing and thawing, combined with years of use, increase a system’s vulnerability to impairment.

US EPA estimates that at least 23,000 to 74,000 SSOs occur each year, not including sewage backups into buildings, resulting in contaminated drinking and recreational water, costly household backups and extensive damage to private property and public infrastructure.4

Likewise, US CSOs discharge 1.26 trillion gallons of sewage and stormwater a year into the environment; that’s enough to keep Niagara Falls roaring for 18 days! Furthermore, US EPA estimates there are at least 400,000 basement backups annually, resulting in significant economic burdens. The cost of cleaning up a basement sewage spill is estimated at $700 to $4,000, resulting in a national economic burden of $280 million to $1.6 billion per year.3

Other economic burdens mount due to the closure of beaches and impact of harvesting and consumption of shellfish and molluscs. Sewage overflows also contribute to turbidity increases in waterways, which can also have an adverse effect on aquatic fish, plants and the ecosystem as a whole. Burdens extend beyond dollars and inconvenience to include human health impacts.

Researchers estimate that thousands of US residents are sickened due to exposure to untreated sewage overflows.5 Diseases such as diarrhea, cholera, hepatitis and cryptosporidiosis can be acquired from exposure to untreated wastewater. Every two years the Centers for Disease Control and Prevention publish reports on recreational and drinking waterborne illness incidence. In the latest report published in February, 13 outbreaks from 2009-2010 resulting in 965 cases of illness were reportedly due to contamination of groundwater and/or the drinking water distribution system. Experts estimate reported illnesses are 10 to 1,000 times lower than the actual, unreported disease burden. While the majority of these illnesses are not life-threatening, they can result in significant discomfort, hospitalization and a small number of fatalities, especially in immunocompromised individuals.

Minimizing risks
In two reports to Congress, US EPA has developed a set of national priorities to address the substantial environmental and human health risks associated with continuing sanitary/combined sewer overflows. The best course of action to minimize the risks, however, is to increase compliance of states and territories in reducing the number of SSOs and CSOs.

In 1994, US EPA established a CSO Control Policy aimed at providing a national framework for controlling CSOs. The framework provided a list of nine minimum controls to be implemented by January 1, 1997.6 These minimum controls are:

  1. Proper operation and regular maintenance programs for the sewer system and the CSOs
  2. Maximum use of the collection system for storage
  3. Review and modification of pretreatment requirements to assure CSO impacts are minimized
  4. Maximization of flow to the publicly owned treatment works for treatment
  5. Prohibition of CSOs during dry weather
  6. Control of solid and floatable materials in CSOs
  7. Pollution prevention
  8. Public notification to ensure that the public receives adequate notification of CSO occurrences and CSO impacts
  9. Monitoring to effectively characterize CSO impacts and the efficacy of CSO controls

Unfortunately, only 32 percent of impacted communities had implemented the minimum controls by 2004.7 Reportedly, since then, US EPA has been working with individual states to develop long-term control plans but quantitative metrics of the success of this program have not been reported in publicly available literature. In June, US EPA organized a forum of public health and engineering experts to discuss the public health impact of SSOs/CSOs and how to best manage wet-weather flows using clean-water regulations, monitoring and engineered controls. A summary of this meeting has not yet been posted.

Despite an effort by US regulatory agencies to address the problem of CSOs/SSOs, sewage contamination of drinking and recreational water sources will continue to occur. Treatment failures occur periodically and without warning, justifying the need for increased control at both centralized and POU water supplies.


  1. “Possible WaterContamination from Sewage Overflow,” WILX 500, 26 October 2014. Online. Available:–280475772.html. Accessed 11 November 2014.
  2. US EPA, Combined Sewer Overflows (CSO) Home, US EPA, 9 September 2014. Online. Available: Accessed 11 November 2014.
  3. US EPA, Why control sanitary sewer overflows?, US EPA Office of Wastewater Management, Online. Available: Accessed 15 November 2014.
  4. US EPA, Sanitary Sewer Overflows and Peak Flows, Washington, D.C., 2012.
  5. US EPA, “Report to Congress: Impacts and Control of CSOs and SSOs. Executive Summary,” Document No. EPA-833-R-04-001, 2004.
  6. US EPA, “Combined Sewer Overflows: Guidance for the Nine Minimum Controls,” Document No. EPA 832-B-95-003, Office of Water, 1995.

US EPA, “Report to Congress: Impacts and Control of CSOs and SSOs,” Office of Water, Document No. EPA 833-R-04-001, 2004.

About the author
Dr. Kelly A. Reynolds is an Associate Professor at the University of Arizona College of Public Health. She holds a Master of Science Degree in public health (MSPH) from the University of South Florida and a doctorate in microbiology from the University of Arizona. Reynolds is WC&P’s Public Health Editor and a former member of the Technical Review Committee. She can be reached via email at [email protected]


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