Rivers frame our region and make the St. Louis area a good place to live. Every day at MSD, we work hard to protect the water environment and protect our community.

Several feet of MSD’s wastewater collection system is more than 150 years old. When heavy rains overload our sewer pipes, some of the water may flow past the wastewater treatment plant and into the river. MSD’s goal is to work with area communities to minimize the environmental impact of these overflows. The process begins with an understanding of the evolution and construction of sewer systems in the St. Louis area.

Combined Sewer Systems
In St. Louis city and some adjacent cities, wastewater and storm water flow through the same sewer pipes to a wastewater treatment plant. This type of collection system is called a combined sewer system because wastewater and storm water are combined in a single pipe.

More than 750 cities in the United States have combined sewer systems, many of them dating to the 19th century. At that time, the combined flows went through discharge structures directly to the river. When wastewater treatment plants were built in the 20th century, the combined flows were directed to treatment plants. Most of the time, when rainfall is low to moderate, both storm water and wastewater are treated at the plants before being discharged to local rivers. However, the combined system cannot always handle the extra volume of water during heavy rains.

Combined Sewer Overflows (CSOs)
To handle the extra volume, cities kept discharge structures in place to direct the flows to local waterways during heavy rains. The overflows created a system of “safety valves” that prevented basement back-ups and street flooding. 

Starting in the 1940s, St. Louis area cities recognized the area was outgrowing the combined sewer system. Cities worked to prevent CSOs by constructing separate collection systems that transported wastewater to a treatment plant and storm water to local rivers. MSD has constructed separate sewer collection systems since its formation in 1954, but we maintain all the sewers in our service area, including the 80 square miles served by combined sewers.
 

Controlling CSOs
Controlling CSOs is an environmental priority at MSD. During and after a rainstorm, CSO discharges may contain impurities that can cause water quality problems. Our goal is to reduce the health and environmental impacts of CSOs by:

• Monitoring interceptor sewer pipes to reduce overflow volume and improve the quality of water in area waterways
• Maximizing the efficiency of our treatment plant operations and collection system maintenance practices to better manage wet-weather flows
• Designing and building projects that will control overflows
• Providing public information about CSOs and steps that residents and businesses can take to keep local rivers and streams safe and clean