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History of Stormwater and Wastewater Drainage Systems in Minneapolis

The evolution of sewage and stormwater drainage systems in Minneapolis reflects its development into a modern urban community. The oldest City sewer on record is over 130 years old! It runs under Washington Avenue through downtown and was built around 1870. Let's see how this gravity-driven underground network that transports all the waste and stormwater of Minneapolis has become the vast system it is today…

Combined Sewers in the Mississippi River (1870-1938)

Early Minneapolis sewers were combined, carrying both stormwater and sanitary sewage directly to the Mississippi River. From 1870 to 1895, 124 miles of egg-shaped sewers were constructed within Minneapolis. The smaller section at the bottom conveyed sanitary flows, while the larger section at the top provided extra capacity for the larger flows that occurred during rainstorms. These sewers were constructed of either non-reinforced concrete or brick. Smaller sewers, 12 to 24 inches in diameter, were concrete, and larger sewers, 24 to 96 inches in diameter, were built with brick.

Around 1896, clay, rather than non-reinforced concrete, was used. Brick construction was replaced by Reinforced Concrete Pipe (RCP) around 1930. Clay (VCP) remains the preferred material today for sewers up to 33 inches in diameter. Larger sewers are seldom needed since the pipes no longer need to convey both storm and sanitary flows.

Combined sewers improved sanitary conditions at homes and businesses, but they created another problem. Raw sewage from the combined sewers began polluting the Mississippi River. The Mississippi was described in the 1930’s as having floating islands of sewage solids, scum on the water surface, and an abundance of dead fish. The offensive odors were noticeable from more than two blocks away! Typhoid fever outbreaks were frequent because of a contaminated water supply.

In 1933, the Minnesota State Board of Health ordered the creation of the Minneapolis-St. Paul Sanitary District. Its task was to build the region’s first sewage treatment plant along with interceptor sewers needed to convey the wastewater to the plant.

Combined Sewers Drain to a Treatment Plant (1938-1960)

The Pig’s Eye Sewage Treatment Plant began operating in 1938. Flows from the combined sewers were diverted from the Mississippi River to the treatment plant by a system of interceptor sewer tunnels located on either side of the Mississippi River. As part of this system, 34 overflow regulators were constructed to divert normal dry weather flows to the interceptor sewer. They also allowed relief overflows into the Mississippi during heavy rainstorms.

Combined Sewer Overflow (CSO) is the industry term used to refer to releases of mixed stormwater and sanitary sewage from a combined sewer system to surface waters. The result of this modification was a significant improvement in the water quality of the River, except for brief periods during heavy rainfall. During these peak flow periods, the regulators prevented overloading of the treatment plant, sanitary backups into homes, and pressure surges that could cause structural damage to the pipe system.

The construction of a separate storm drain system began in 1922, but older areas continued to be served by combined sewers. These drains were built mainly in new developments and around Minneapolis lakes to relieve local flooding and prevent pollution of these lakes. Extensive storm drain construction took place from 1936 to 1940.

Begin Combined Sewer Separation: Residential Paving Projects Include Storm Drains (1960 - 1985)

Although newer areas in Minneapolis now had separate storm and sanitary sewers, combined sewers were still serving the older areas. Minneapolis’s core was becoming increasingly populated, businesses were growing. Paved and impervious surfaces were increasing in number.

As a result, more storm runoff was draining to these combined sewers. The added burden to capacity lead to more frequent overflows into the Mississippi River, as well as basement back-ups.

In 1960, Minneapolis began reconstruction of almost all of its 600 miles of residential streets. Included in the construction plans were new storm drains for those areas still served by the combined sewers. Areas prone to severe flooding were given high priority. Hennepin County and MnDOT construction projects also included storm drains, meant to either separate combined areas or to add capacity for future separation of upstream combined sewers.

Completing the Separation of Storm and Sanitary Systems: Great Reduction in CSOs (1985-1996)

In spite of the new storm drains, millions of gallons of combined stormwater and sewage were still overflowing directly into the Mississippi River. In 1986, Minneapolis Public Works (MPW) began an accelerated 10-year program of sewer separation construction. This program, aided by state and federal funds, completed work to provide separate storm and sanitary sewers for more than 95% of Minneapolis.

The City has also identified and removed more than 2,500 commercial and residential rainleaders (downspouts draining directly into the sanitary sewer system) from the sanitary sewer system. This removal effort enabled the safe elimination of all except 8 of the original 34 overflow regulators.

A Minneapolis Separated Drainage System (1996-Present)

The 1997 Flood - The Birth of the Minneapolis Flood Mitigation Program

In July 1997, Minneapolis experienced torrential rainstorms that severely overburdened the existing storm drain system. The rainstorms caused flooding at many locations throughout Minneapolis, resulting in physical damage to homes, businesses and automobiles.

In November 1997, the Minneapolis City Council adopted a nine-year flood mitigation program aimed at preventing future flooding caused by undersized storm drains. The City’s goal is to provide equal levels of drainage protection throughout Minneapolis.

In order to complete all projects without increasing sewer rates, it may be necessary to extend the program to 2009. Additional project areas identified since the original program’s inception could begin in 2010. The projected target date for completion all projects currently identified would then be 2015.

Last updated Oct 3, 2017



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