OVERFLOWS

2019 CSO Notification Report

2020 Overflows YTD:

Here's MMSD's overflow record since 1994, the first full year of operation for the Deep Tunnel system:

mmsd sewer overflow graphic

Why Do 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. 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 2025 calls for investing $1.5 billion in clean water infrastructure, flood management, and debt financing to help protect public health and Lake Michigan.

Thanks to the Deep Tunnel and many other improvements, we average 2.4 overflows per year (down from 50-60) and have captured and cleaned 98.4% of all the water that's entered the regional sewer system since 1994. The goal nationally is to capture and clean 85% for the more than 700 cities with systems like ours. Please note, that combined sewer overflows are 90 to 95% stormwater and groundwater. We continue to strive for zero overflows by investing in our treatment plants, sewers and implementing green infrastructure in our region to help manage water where it falls. View MMSD's overflow volume since 1994 below.

What is a Combined Sewer Overflow? What is a Sanitary 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, discharges 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).)

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?

Water Drop Alert

Text: WATERDROP to 797979

Receive Water Drop Alert text messages when large storms threaten the area. When a Water Drop Alert has been issued a reminder is sent to use less water until the storm passes. 

What do you do when an alert is issued?
Try to use less water until the storm passes.

  • Hold off on washing dishes and laundry
  • Take a shorter shower 
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

Every downspout on your home can send 12 gallons of water a minute to the sewer system, which increases the risk of basement backups and sewer overflows. Disconnect and help keep excess water out of sanitary sewers.