Public Works

350 South 5th Street
RM 203 City Hall
Minneapolis, MN  55415-1390

Combined Sewer Separation in Minneapolis

A history of separating sewage from stormwater

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. The construction of a separate storm drain system began in 1922 around the 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 storm water connections from the combined sewer, began in earnest in the 1960s in conjunction with a residential street paving program.

In 1986, the City began an accelerated program of sewer separation supplemented with federal and state funds and separated more than 4,600 acres of the City served by combined sewers. Less than 5% of the area within the City limits still requires sewer separation.

The City of Minneapolis and the Metropolitan Council are joint permit holders for a National Pollution Discharge Elimination System (NPDES) Permit that regulates combined sewer overflows into the Mississippi River. The City, represented by Minneapolis Public Works and Minneapolis Environmental Management, with the support of the Mayor and City Council, is working with the Metropolitan Council to end combined sewer overflows 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 characterized by the spread of water-borne diseases, bad smell and appearance, dead fish, and the inability to use the River for many recreational and commercial purposes.

In 1933 a joint sanitary district for the cities of Minneapolis and Saint Paul was created. A continuous "interceptor" sewer was constructed on both sides of the River to connect all existing combined sewers and convey dry weather flows to the treatment plant built in 1938 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 and 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, Minneapolis Public Works began to reconstruct almost all of Minneapolis’ 600 miles of residential streets. Included in the plans was funding to construct separate storm drains as the streets were paved. Areas prone to severe flooding were given high priority. In addition, 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, aided by state and federal funds, completed work to achieve greater than 95% of Minneapolis served by separate storm and sanitary sewers. 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. (See Current Overflow Location Map, for more information.)

1996 – Current

The remaining identified separation areas were generally difficult and more 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 has now committed to accelerated scheduling of such projects as part of a five-year plan focused on eliminating CSOs.

Until 2003, rainleader and area drains in private buildings that were connected to the sanitary sewer system were detected and addressed during site plan review when affected buildings underwent redevelopment. An ordinance was recently passed by the City Council, also as part of the five-year CSO elimination plan, authorizing inspections to identify these connections on private property within the City and requiring their removal.

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

A public education campaign continues to help inform Minneapolis residents and business owners about the CSO problem, solutions, the City's response and how they can help. The program also is providing information to property owners about environmentally healthy ways to manage stormwater runoff so that it doesn't contribute to any form of pollution in any area lake, river or stream.

Last updated Jan 20, 2012