Combined sewer separation in Minneapolis

A history of separating storm drain pipes from the combined sanitary sewer.

History of combined pipes 

The City of Minneapolis owns and maintains approximately 830 miles of sanitary sewers within public rights of ways, easements or City-owned land. The oldest sanitary sewers in Minneapolis were laid in 1870, along Washington Avenue through downtown Minneapolis. These sanitary sewers were designed to carry both sewage and stormwater in one pipe.

History of separating pipes

The construction of a separate storm drain system began in 1922 around Minneapolis lakes and for new developments, but older areas continued to be served by combined sewers.

Sewer separation, which involves building separate storm drain pipes and removing all stormwater connections from the combined (sanitary) sewer, began in earnest in the 1960s in conjunction with a new residential street paving program.

In 1986, the City began an accelerated program of sewer separation supplemented with federal and state funds, separating more than 4,600 acres of the City served by combined sewers. A small percentage of the area within the City limits still requires sewer separation.

The City of Minneapolis and the Metropolitan Council Environmental Services (MCES) are joint permit holders for a National Pollution Discharge Elimination System (NPDES) permit that regulates Combined Sewer Overflows (CSOs) into the Mississippi River. Minneapolis, represented by Public Works and Environmental Services, with the support of the Mayor and City Council, are working with the MCES to end CSOs occurring within City limits.

Prior to 1938

Minneapolis was authorized as a town by the Minnesota Territorial Legislature in 1856 and the first town council was organized in 1858. In 1872, St. Anthony and Minneapolis merged under the name of Minneapolis.

As Minneapolis prospered, homes and businesses were connected to a single sewer designed to transport both sewage and stormwater directly into the Mississippi River without any treatment. This resulted in the Mississippi River suffering from poor water quality, the spread of water-borne diseases, bad smell and appearance, dead fish, and a general inability to use the Mississippi River for recreational and commercial purposes.

In 1933, the Minneapolis/St. Paul Sanitary District was created. In 1938, a continuous "interceptor" sewer was constructed on both sides of the Mississippi River. This connected all existing combined sewers and convey dry weather flows to the Metropolitan Treatment Plant (then known as "Pig's Eye) in Saint Paul. Along the Minneapolis interceptor, 34 regulators were constructed to allow relief overflows into the Mississippi River during heavy rains. These flow regulators were needed to prevent sanitary backups into homes and system structural damage due to pressure surges.

1939 – 1960

The combined sewers continued to serve most of Minneapolis, but separate drainage systems were placed in new developments; one for sewage and the other for stormwater runoff. As Minneapolis grew and new first ring suburbs began to expand, extreme stress was being placed on the older combined sewers, as well as the new interceptor. Small rainstorms were causing overflows to the Mississippi, and increased street and basement flooding was reported across the City.

1960 – 1985

In 1960, Public Works began to reconstruct almost all 600 miles of residential streets in Minneapolis. Included in these plans was funding to construct separate storm drains as these streets were repaved, with areas prone to severe flooding given high priority. Additionally, all arterial, highway, and interstate construction projects included storm drains designed to either separate combined area, or to add sufficient capacity for future separation of upstream sewers.


In 1986, Minneapolis Public Works began an accelerated program of sewer separation construction. This program was aided by state and federal funds, whose goal was to complete separation of greater than 95% of storm and sanitary sewers in Minneapolis. The City also identified and removed more than 2,500 commercial and residential rainleaders from the sanitary sewer system. This enabled the safe elimination of all except 8 of the original 34 overflow regulators. 

Current overflow location map

1996 – present

The remaining identified CSO areas were more difficult and costly to separate. Since 1996, these projects have been deferred until they could be included as part of scheduled street improvements or flood mitigation projects. The City is committed to scheduling CSO separation projects to eliminate CSOs.

Until 2003, rainleader and area drains in private buildings connected to a sanitary were detected and addressed during site plan review as these buildings were being redeveloped. At that point, an ordinance was passed by the City Council, authorizing inspections to identify connections on private property within the City, as well as requiring removal of that connection.

Completed Flood Mitigation Program projects have been partially responsible for reductions in CSOs in Minneapolis. More capacity in the storm drain system means floodwaters will be less likely to find their way into the sanitary system.

Minneapolis Public Works continues to educate and inform Minneapolis residents and business owners about the what the CSO problem is, what solutions are available and how they can help. The City provides information to property owners about environmentally healthy ways to manage stormwater runoff, minimizing contributing to polluting lakes, rivers and creeks in Minneapolis.

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Surface Water & Sewers

Public Works




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250 South 4th St, Room 300
Minneapolis, MN 55415

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