graphic of mmsd and city sewer lines


What is a Sewer?

If you own a house, you own part of the sanitary sewer system that helps protect public health. The section you own is called a lateral and it connects to the sewer system owned by your city or village.


Account for 3,000 miles of sanitary sewer pipe in the MMSD service area. They transport all the water you use in your home out to the street where it drains into the sewer system your city or village owns.

Do you have issues with your lateral? Please contact us.

City and Village Sanitary Sewers:

Make up another 3,000 miles of sewer pipe in the region. MMSD serves 29 communities that all own and operate sanitary sewer systems.

MMSD Sewers:

About 300 miles of regional sewer pipes that transport wastewater to one of two water reclamation facilities where the water is cleaned and returned to Lake Michigan.

illustration of a how a lateral works with your home

We also have the Deep Tunnel system, which is 28-miles long and ranges in size from 17 feet to 32 feet in diameter.

The majority of MMSD’s service area is drained by separate sewer systems, meaning that the stormwater runoff and sanitary sewage are collected in separate pipes and not mixed. In these types of systems, stormwater that is conveyed and released to nearby bodies of water is not mixed with sanitary/sewage water. However, approximately 6% of MMSD’s service area, located entirely within the City of Milwaukee and the Village of Shorewood, is serviced by the combined sewer system. Reducing stormwater inflow into the combined system with green infrastructure will reduce the risk of a combined sewer overflow.

Why Doesn't MMSD Separate the Sewers?

Separating the sanitary and storm sewers would decrease the amount of water captured and treated, however, the number of pollutants going into our rivers and Lake Michigan would increase. In urban areas, with lots of impervious surfaces (buildings, parking lots, streets), there is little opportunity for stormwater to be absorbed into green areas. Resulting in run off with a high degree of pollutants that would further erode water quality. To learn more about MMSD’s Water Quality initiative and holistic watershed approach read our water quality newsletter.

What is a Combined Sewer? What is a Separate Sewer?

A combined sewer system (CSS) collects rainwater runoff, domestic sewage, and industrial wastewater into one pipe. Under normal conditions, it transports all of the wastewater it collects to a sewage treatment plant for treatment, then discharges to a water body. The volume of wastewater can sometimes exceed the capacity of the CSS or treatment plant (e.g., during heavy rainfall events or snowmelt). When this occurs, untreated stormwater and wastewater discharge directly to nearby streams, rivers, and other water bodies. Combined sewer overflows (CSOs) contain untreated or partially treated human and industrial waste, toxic materials, and debris as well as stormwater. They are a priority water pollution concern for the nearly 860 municipalities across the U.S. that have CSSs. (Environmental Protection Agency, Combined Sewer Overflows (CSOs).)

Sanitary sewer systems collect and transport domestic, commercial, and industrial wastewater and limited amounts of stormwater and infiltrated groundwater to treatment facilities for appropriate treatment. Sanitary sewers are different than combined sewers, which are designed to collect large volumes of stormwater in addition to sewage and industrial wastewater. Occasionally, sanitary sewers will release raw sewage. These types of releases are called sanitary sewer overflows (SSOs). SSOs can contaminate our waters, causing serious water quality problems, and back-up into homes, causing property damage and threatening public health. (Environmental Protection Agency, Sanitary Sewer Overflows (SSOs).)

graphic explaining CSO and SSO

Active MMSD Sewer Projects

underground tunnel below street with equipment


Learn about MMSD's improvement project on NS-12 Collector System.

historical photo of culverts below Wisconsin State fair


Learn about MMSD's work on the State Fair Culvert Rehabilitation.

pipe being dug under road


Learn about MMSD's improvement on the West Roosevelt Sewer Project.

MMSD pipeline before cleaning.


Learn about MMSD's work to replace the valves and clean four pipelines beneath I-794 off Pryor Ave.