Why Do Sewer Overflows Happen?

Preventing sewage from backing up into your basement is our highest priority when too much rain gets into the sanitary sewer system. Unfortunately, our only weapon in that battle is a relief valve that sends excess water to the nearest river or lake, an overflow pipe. Thanks to the Deep Tunnels and many other improvements, we average 2.2 overflows per year (down from 50-60) and have captured and cleaned 98.6% of all the water that's entered the regional sewer system since 1994. The national goal is to capture and clean 85% of the more than 700 cities with systems like ours. 

As a region, we've invested more than $4 billion to reduce sewer overflows, a massive effort that is paying off. Our current financial forecast through 2033 calls for investing $3.5 billion in clean water infrastructure, flood management, and debt financing to help protect public health and Lake Michigan. 



To view MMSD's 2023 Annual Combined Sewer Overflow (CSO) Notification Report. To see how MMSD complies with Federal requirements for public notification, view the MMSD CSO Public Notification Plan.


Gallons Treated: 48,911,000,000

Gallons Overflowed: 357,000,000

Total Gallons: 49,268,000,000

MMSD % Treated: 99.28%


One Inch of Rain on MMSD's Service Area = 7.1 Billion Gallons of Water


MMSD's Overflow Volume Since 1992:

Click photo to enlarge

mmsd overflow gallons

What is a Combined Sewer Overflow?

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).)

What is a Sanitary Sewer Overflow?

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).)

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.

graphic explaining CSO and SSO

How Can You Help Reduce Overflows?

MMSD Water Drop Alert

Text: WATERDROP to 414-296-4422 for Española texto GOTADEAGUA a 414-296-4422

Receive Water Drop Alert text messages when heavy rain threatens the area. When a Water Drop Alert has been issued, a reminder is sent to use less water.

What do you do when an alert is issued?

  • Hold off on washing dishes and laundry
  • Take a shorter shower 
  • Empty your rain barrel
Install a Milwaukee Rain Barrel

Install a Rain Barrel

Get FREE water by the barrel from your roof and use it when it’s dry outside to use in your landscape.  Rain barrels help keep excess water out of the sewer system and help reduce water pollution. 

disconnect your downspout in milwaukee

Downspout Disconnection

During heavy rain, every downspout on your home can send 12 gallons of water a minute to the sewer system, which increases the risk of sewer water backing up into your basement and overflowing into our rivers and lake Michigan. Disconnect your downspout and help keep excess water out of sanitary sewers and into your yard or a rain garden.