A single night of record-breaking rain dumped billions of gallons of untreated sewage into Lake Michigan and Milwaukee’s rivers, overwhelming a system designed to prevent exactly this kind of disaster.

Extreme flooding from an Aug. 9-10 downpour resulted in an estimated 5.14 billion gallons of untreated waste water being released in Lake Michigan and nearby waterways. The Milwaukee Metropolitan Sewerage District initiated the overflow of wastewater on the evening of the ninth, and it continued for more than three days. 

After even greater levels of untreated wastewater were released—9 billion gallons worth—in 1992, Milwaukee constructed what is called the Deep Tunnel. The tunnels are an underground network of sewage overflow storage tunnels, which began operating just two years after the 1992 disaster. During periods of heavy rainfall, if the deep tunnel and treatment plants reach capacity, the sewage district must release untreated water into rivers and Lake Michigan in order to prevent backups into homes and businesses.  

Despite these measures, one Milwaukee woman experienced a sewage backup in her apartment and said she is still being expected to pay September’s rent despite her vacating the home.

The rainfall, which is being described as a 1,000-year flood dropped 10 inches of rain in some parts of the county. (RELATED: FEMA to Conduct Damage Assessment in the Aftermath of Devastating Floods)

In a post to X, the Milwaukee Metropolitan Sewage District detailed the impact one inch of rain has on the whole system. 

“To put the enormity of the storm into perspective, when one inch of rain falls on the MMSD’s 29 communities, it equates to 7 billion gallons of water flowing through the pipes to be treated. According to the sewerage district’s rainfall data from Aug. 9 to 13 − the 78 hours in which the overflow occurred − an average of 9 inches of rain fell on the service area. That would mean roughly 63 billion gallons of water flowed through the pipes − or enough to fill the three Mitchell Park Domes more than 3,700 times.”

The event underscores that even with decades of investment, Milwaukee’s system still struggles to manage extreme rain. When storage and treatment reach capacity, billions of gallons of untreated water are still forced into area waterways—and sometimes into residents’ homes—leaving taxpayers to deal with the aftermath. (RELATED: Sluggish Flood Response from Evers Roils Wisconsin Residents)