Combined sewers drain to the Mississippi River (1870-1938)

The oldest City sewer was built around 1870 and runs under Washington Avenue South. Early sewers discharged both stormwater and sewage directly to the Mississippi River. From 1870 to 1895, 124 miles of egg-shaped sewers were built in Minneapolis. While the bottom section of the pipe was for sanitary flow, the top section was for larger flows during rainstorms. Smaller pipes were built from concrete, but larger pipes were built with brick.

Around 1896, clay became the preferred material for pipes up to 33" in diameter, and remains so today. In 1922, the first separate storm drain system was built, initially in new developments and near lakes to prevent pollution and flooding at these lakes, but the older areas were still served by combined sewers. Brick construction was replaced by reinforced concrete pipe around 1930.

Combined sewers did improve sanitary conditions at homes and businesses, but raw sewage was still flowing into the Mississippi River. In 1930, the Mississippi River had floating islands of sewage, scum on the water surface and dead fish present. Typhoid fever outbreaks were frequent because the water supply got contaminated.

In 1933, the Minneapolis - St. Paul Sanitary District was created to build the region’s first sewage treatment plant along with interceptor sewers to move the wastewater to the plant.

Combined sewers drain to a treatment plant (1938-1960)

The Pig’s Eye Sewage Treatment Plant began operations in 1938. Combined sewer flow were diverted from the Mississippi River to the plant by large interceptor tunnels on either side of the river. Overflow regulators improved the water quality in the Mississippi River by diverting normal dry weather flow to the interceptor sewer, but occasionally relief overflows into the Mississippi during heavy rainstorms still happened.

A Combined Sewer Overflow (CSO) happens when stormwater and sewage from a combined system discharge to a body of water. During times of heavy rainfall, the regulators kept the treatment plant from getting overloaded, prevented sanitary backups in homes and pressure surges that might have damaged the pipe system.

Combined sewer separation starts (1960 - 1985)

The newer areas in Minneapolis had separate storm and sanitary sewers, but older areas still had combined sewers. With the population and businesses in Minneapolis growing, so did the amount of hard surfaces. As a result, more storm runoff was discharging into combined sewers, which resulted in more overflows and basement back-ups.

In 1960, Minneapolis began reconstructing 600 miles of residential streets, including new storm drains for older areas. Areas that had flooded in the past were given high priority. Also, other county and state projects being built in Minneapolis also included new storm drains.

Continued separation of storm and sanitary (1985-1996)

In spite of the new storm drains, large amounts of combined stormwater and sewage still overflowed into the Mississippi River. In 1986, aided by state and federal funds, Minneapolis began an aggressive program to continue separation of storm and sanitary sewers.

More than 2,500 commercial and residential rainleaders were removed, which allowed eliminating 26 of the original 34 overflow regulators. Rainleaders are downspouts that discharge stormwater to the sanitary sewer system.

A separated drainage system (1996 - present)

While CSOs have been greatly reduced, they can still occur. In 2002, a study identified remaining CSO sources and solutions to fix these problems. The City of Minneapolis and the Met Council fund initiatives to eliminate CSOs, including:

• Requiring rainleader and area drain disconnections
• Drainage system improvements
• Community outreach and education programs
• Stormwater management practices that minimize runoff and pollution to our waters, from both public and private property

See combined sewer overflows for more information.