What is a Sanitary Sewer Overflow?

Average, daily flow of water through a community’s Sanitary Sewer system can be very different than the flow seen during a large rain storm or in Spring during high snowmelt runoff. This increase in water does not mean that residents are suddenly creating more wastewater (water that goes down drains and toilets). It usually means that “extra” “clear” water is getting into the sanitary sewer system through the process of Inflow and Infiltration.

WLSSD dataWastewater plants are sized to handle much more than “average” daily flow. In Duluth, average daily flow to the Western Lake Superior Sanitary District is about 48 million gallons per day. The treatment facility can handle up to about 120 million gallons per day. On rainy days, this flow may rise to well over 100 million gallons per day, and in extreme situations, considerably more. See what happened at WLSSD from a huge storm on July 3, 2003. How can this happen?

When a lot of “extra” water that should be handled by the storm sewer system, that drains water from parking lots and roads, seeps into the sanitary sewer system, the capacity of the pipes in the collection system can be overloaded. Water cannot reach the treatment plant, as the pipes become pressurized and the system overflows before the water is delivered to the treatment plant. This is called a sanitary sewer overflow (SSO). This problem exists in many cities around Lake Superior and throughout the U.S.. This can also occur during rain events in cities that have a combined sanitary sewer and storm sewer system. Combined sewer overflows (CSOs) are perhaps a more difficult problem because the systems were never separate to begin with.


These overflows mean that untreated or partially treated wastewater and sewage can back up into basements, run down streets or directly enter water bodies such as creeks and rivers or even Lake Superior. Untreated wastewater contains pathogens that can make humans sick, and contain high levels of nutrients, organic matter and solids that can cause algae blooms and other eutrophication impacts in the water bodies.

Do Overflows cause Beach Closures?

Sanitary Sewer Overflows are undesirable situations that all residents and agencies want to see stopped. It is true that overflows contribute many pollutants including disease causing microorganisms (pathogens) to water bodies. However, it is not evident that the indicator bacteria seen during beach monitoring water testing always come from sanitary sewer overflows. In fact, many if not most beach closures occur during times when there have been no sanitary sewer overflows. It is believed that bacteria seen during beach tests comes from a variety of sources, including animal feces (gulls, geese, pets), improperly disposed of diapers, children not properly cleaned after using the bathroom, a vomiting or fecal accident in the water, swimmers with diarrhea, malfunction in septic systems in the vicinity, nearby boat moorings or marinas releasing sewage into the water, and stormwater runoff into streams and from direct sheet runoff that potentially includes all of these other sources, as well as sanitary sewer overflows or treatment plant malfunctions. Find out more about pathogens here and about how to minimize your risks at the MPCA Beach Website.

What is being done about Overflows?

The Environmental Protection Agency and state environmental agencies such as the Minnesota Pollution Control agency are working with cities to address this serious issue. The City of Duluth and the Western Lake Superior Sanitary District have developed a Plan of Action (2.5 MB pdf) to prevent overflows. Find out more about the progress of the plan at I&I or WLSSD.

NewMarch 2009 View a presentation describing the City of Duluth's Inflow & Infiltration reduction project.