Typically found in older municipalities and communities in the Great Lakes, Northeast and Pacific Northwest areas of the United States, combined sewer systems transport sewage, industrial wastewater and storm water runoff in the same pipes to wastewater treatment plants, where it is treated and discharged into a local body of water.

Significant rainfall and snowmelt can cause the volume of wastewater traveling through a combined sewer system to exceed capacity. When this happens, the system overflows, discharging untreated storm water and wastewater, pathogens, toxins and other contaminants into area waterways.

Combined sewer overflows

These overflows, known as combined sewer overflows, contain debris, untreated and/or partially treated human and industrial waste, stormwater and other toxic materials. CSOs pose significant dangers to our waterways — and to public health — if not properly addressed.

EPA requirements

After passage of the Clean Water Act, the U.S. Environmental Protection Agency required cities to eliminate untreated discharge into bodies of water. While the EPA makes allowances for spills due to severe weather, many municipalities across the country are under a consent order from the EPA to prevent and eliminate all CSO into rivers and streams. These municipalities can either continue to incur fines — in some cases, millions of dollars — or make investments in improving sewer infrastructure.

Contents of CSOs

In addition to stormwater, CSOs also include such pollutants as grease, oil and toxic substances that are discharged directly into local waterways, compromising water quality standards.

According to a medical study on sewage (LiveSciences, Oct. 6, 2011), “unknown” viruses found in raw sewage outnumber known viruses at a ratio of 30:1. The study further shows that:

• 3,000 total viruses are found in raw sewage
• 2,766 “unidentifiable” viruses are found in raw sewage
• Only 234 viruses have been categorized and identified
• 17 of 234 identified viruses are known to be communicable

Impact on waterways

Pathogens can significantly endanger aquatic habitats and aquatic life, while also impairing the use of our local waterways. In addition to affecting fish and shellfish survival and contaminating drinking water, CSO discharge can also:

• Introduce toxic materials into the sediment at the bottom of waterways. Contaminated sediment can negatively impact aquatic life over time.
• Create financial burdens for communities, including cleanup and maintenance expenses, lost tourism revenue and medical expenses.

Impact on public health

Discharge from combined sewer overflows can also back up into basements, causing property damage and creating health problems for anyone exposed to untreated sewage and wastewater.

CSO discharge can carry bacteria, intestinal worms, protozoa and viruses. Contact, inhalation or ingestion of CSO discharge can cause diarrhea and nausea, as well as a variety of infections, including ear infections, respiratory infections and skin/wound infections. In worst-case scenarios, people exposed to these discharges can also contract life-threatening diseases, including cholera, dysentery, infectious hepatitis and severe gastroenteritis.

Proper maintenance reduces CSOs

Combined sewer overflows can be effectively reduced through routine inspection and maintenance, and improved operations. Basic maintenance programs and acceptable National Pollutant Discharge Elimination System practices include using proper street sweeping and sewer cleaning equipment to clean sewer lines and catch basins.

A sewer cleaning maintenance program maintains the stormwater and wastewater collections system, so it will function properly while also minimizing the number of stoppages per mile of sewer pipe. Sewer cleaners ensure safe, consistent movement of stormwater, wastewater and solid material to the collection system or treatment plant.

To learn more about combined sewer overflows and the National Pollutant Discharge Elimination System, please visit www.epa.gov/npdes/combined-sewer-overflow-frequent-questions. 

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