The St. Louis Metropolitan Sewer System transports waste water and storm water for approximately 1.4 million people in a 535-square-mile service area covering St. Louis city and about 80% of St. Louis County. It includes over 9,600 miles of pipe, making it the fourth largest in the United States, with 7 treatment facilities processing 330 million gallons of sewage daily.

The system has Combined Sewers and Sanitary Sewers


Combined sewers dominate in St. Louis City and older inner suburbs. These sewers combine household and industrial sewage with rainwater runoff and route it all to treatment plants where it is treated before being discharged into our large rivers. Our combined sewers are some of the oldest in the country – giant brick subterranean tunnels that underlay our historic neighborhoods and downtown. During wet weather – especially during heavy rains – the volume of sewage and rainwater can overwhelm the capacity of the combined sewers. Under these circumstances, the mixture of sewage and rainwater often bypass the treatment facility entirely and are discharged directly to our area rivers. These events are known as Combined Sewer Overflows (CSOs) and they increase the level of pollutants and disease-causing pathogens (viruses, bacteria, and parasites) in our rivers. Wet weather events can also overwhelm sewers in a neighborhood and cause the mixture of sewage and rainwater to back up into basements. These basement backups cause distress, damage and expense.

Sanitary sewers are common in the St. Louis suburbs. These systems, many of them formerly local systems built for neighborhoods or small municipalities, are now under the jurisdiction of MSD. In a sanitary sewer system, the household and industrial sewage is handled in separate pipes than rainwater runoff and routed to a treatment plant. Theoretically, a sanitary sewer system needs to have enough capacity only for the sewage of its service area – not for the hundreds of millions of gallons of runoff from a rain event. Rain water is instead routed through a separate stormwater drainage system and bypasses the sewage treatment plant. However, sanitary systems can overwhelm their capacity when more households or businesses are added to the system without expanding the capacity of the pipes, if the pipes are not routinely cleaned and maintained or if the pipes have leaks from cracks that allow rainwater to infiltrate. Sanitary sewers that are overwhelmed with rain infiltration and excess sewage can also cause basement backups. In some circumstances where sanitary sewer capacity is exceeded, pipes discharge excess sewage into local streams. From Mehville to Ladue, Webster Groves to Chesterfield, these sanitary sewer overflows have been discharging untreated sewage into our local creeks, adding pollutants and pathogens that can make waters unhealthy for fish, aquatic life, and for people. Combined Sewer Overflow discharges into streams are regulated through National Pollution Discharge Elimination System (NPDES) permitting and should be heavily monitored so that the health of the stream and local residents can be protected. EPA requires communities to minimize impacts of CSOs to water resources. Although common, Sanitary Sewer Overflows are unpermitted and illegal under the federal Clean Water Act.

Fortunately, MSD is conducting an expansive overhaul of the city’s sewer infrastructure in compliance with the Consent Decree that became effective in 2012. Sewer system updates will make overflows much less common, reducing pollutants in local waterways and threats to public health in our community. (see further information on the specifics of the Consent Decree available here). Grounds for filing the suit, the Environmental Protection Agency found that:

–Combined Sewer Overflows discharge 26 billion gallons  of untreated waste and storm water into St. Louis area streams yearly

–Sanitary Sewer Overflows discharge 226 million gallons of untreated sewage into St. Louis area streams yearly

The Clean Water Act aims to maintain the physical, chemical and biological health of America’s waters. In 2007, The Missouri Coalition for the Environment filed a Notice of Intent to Sue MSD because of its sewer overflows. The U.S. Environmental Protection Agency stepped in and filed its own suit that alleged:

1. Violations of section 301 of the Clean Water Act

•Discharge of pollutants from SSOs violating the CWA

•Discharge of pollutants from CSOs violating the CWA

•537 Overflow Locations

2. Violation of section 308 of the Clean Water Act

•MSD failed to report pollutant discharges that endanger human health or the environment

The Missouri Coalition for the Environment joined as an intervenor in the EPA’s Clean Water Act enforcement lawsuit and became full participating members in the litigation because we shared a close and common interest in the enforcement of federal and state laws against MSD to protect our region’s water quality.

After more than two years of mediation, the EPA, MSD, and the Missouri Coalition for the Environment negotiated a Consent Decree. The Consent Decree requires the elimination of illegal sanitary sewer overflows and limits the number of combined sewer overflows. It requires a 23-year timeline to achieve these goals, as well as periodic reporting requirements, sewer improvement and maintenance plans, and expansion of sewer capacity. All of this work will help prevent basement backups and some areas of localized flooding. It also requires a $100 million investment in “green” stormwater technologies in order to reduce rainwater volume in the combined sewer system.