Combined sewer overflows

Combined sewer overflows occur when heavy rain or melting snow causes sanitary sewers to overflow into stormwater drainpipes. You can learn more about combined sewer overflows.


Combined sewer overflows (CSOs) occur when heavy rain or melting snow causes sanitary sewers to overflow into stormwater drainpipes. This allows sewage to mix with runoff from buildings, parking lots and streets and flow, untreated, into the Mississippi River.

Compared to many other cities, combined sewer overflows (CSO) events are rare in Minneapolis. The City has been actively working on sewer separation since the 1960s. Most of the City’s sewer system has been separated. When a CSO does occur, it can cause serious health and environmental problems.

The threats posed by CSOs are serious enough that federal and state mandates have been issued that require the City of Minneapolis to stopping a CSO event from entering the Mississippi River. Non-compliance with these mandates could lead to fines and other legal action.

Stopping CSOs

Fixing the remaining problems that can cause CSOs in Minneapolis won't be easy. Here is why:

  • The combined sewers that remain were left for last because they are the most difficult and expensive to separate.
  • The entire system is large, complex, and has been in place for a long time. While the City has found and addressed many of the problems that contribute to the risk of a CSO, there are some that have not yet been detected.
  • Some solution strategies require action by home and building owners -- at their own expense.

It is the goal of the City of Minneapolis, along with the Metropolitan Council Environmental Services (MCES) , to eliminate CSOs. The goal of the ordinance is to prevent discharges to the sanitary sewer system from homes and commercial buildings, that can contribute to the occurrence of a CSO event.

Commitment to eliminate CSOs

The City is committed to investing in and giving higher priority to capital improvement and repair projects that will contribute to eliminating CSOs. Examples of these projects include:

  • Sewer separation and sanitary main restoration projects
  • Use of hydraulic modeling of the sanitary sewer system to identify improvements that will maximize storage capacity
  • Locating problem areas
  • Evaluating the effectiveness of completed CSO program work
  • Continuing research to find inlets and cross-connections to storm drains, before eliminating them through maintenance repairs

Learn more about the Chapter 56 Ordinance by reading the Rainleader ordinance fact sheet.

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