Sewer overflow leads to 380 million gallons of untreated wastewater flowing into area rivers, Lake Michigan

Heavy rains led to an overflow of Milwaukee's combined sewer system early Sunday morning, resulting in millions of gallons of untreated water flowing into local waterways. 

About 380 million gallons overflowed out of the sewer system, according to a report from the Milwaukee Metropolitan Sewerage District, released Friday. The untreated water, which could contain sewage, was released into the Menomonee, Kinnickinnic and Milwaukee Rivers, Lincoln Creek and Lake Michigan in different amounts. 

The overflow started at 2 a.m. Sunday after heavy rainfall across the area, which resulted in more storm water than the system could handle. The overflow ended at 11 p.m. Tuesday. 

The storms on Aug. 6 triggered the overflow, according to the report, but the storms on Aug. 7 and 8 were more intense, prolonging the amount of untreated water that flowed out of the system. Between Aug. 6 and Aug. 11, as much as 7 inches of rain fell in some areas of Milwaukee. For reference, one inch of rain across the 28 communities MMSD serves creates 7.1 billion gallons of water flowing into the sewer systems. 

In an attempt to free up more space at the treatment plants and allow more storm water in, operators at Jones Island sewage treatment plant diverted some wastewater around the full treatment process from 3 a.m. on Aug. 8 until 2 p.m.

During that time 16.5 million gallons of wastewater were pumped out of the deep tunnel to the plant's disinfection facility, where it was disinfected with chlorine to kill bacteria before being discharged to the lake. 

MMSD has the ability to allow overflows to happen in order to prevent sewage backups into homes and businesses across the area. Overflows are used after the deep tunnel fills during heavy rain events, and more water is flowing into the treatment plants than can be processed.  

"The only way to relieve of too much water is a relief valve, which allows the untreated water, the groundwater, sewage and rain water, to go into the nearest creek, stream or river untreated," said Bill Graffin, the district's public information manager. 

Though there is a risk of untreated sewage being released into water bodies during overflow events, most of the water released — over 90 percent — is just the rainwater flowing out of the system, as soon as it rushes in. 

This is the first overflow of the year for MMSD, one of six allowed yearly by the DNR under the district's state discharge permit. In 2020, there were three overflows, resulting in over 2 billion gallons of untreated wastewater flowing out of the system. 

Though the goal of an overflow is to prevent sewage backups into homes, the district did receive a few reports of instances, Graffin said. 

Along with the overflow alert, MMSD issued a water drop alert, encouraging residents to use less water in their homes to prevent excess water from flowing into the sewers with the rain water. The alert expired on Thursday. 

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Green infrastructure did keep some water out of sewers

Even though there was enough rain to force an overflow during the last week of storms, some water was kept from entering the system at all through green infrastructure, said district director Kevin Shafer. 

Green infrastructure, like rain gardens or undeveloped land along rivers in the area, helped to keep about 38 million gallons of water out of the system, in addition to any rain caught in rain barrels across the city. 

The district so far has invested about $400 million so far in flood management programs, including green infrastructure, which has both saved water from flooding into the sewers, but has also added other benefits for community members. 

"It provides a greening of the urban environment, so it's going to help us reduce the heat island impact that urban cities are facing, and it also provides habitat and recreational benefits," Shafer said of the green infrastructure program. "So it's got multiple benefits."

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Shafer said new programs and updates are necessary, especially when estimates show Wisconsin getting more floods due to the impacts of climate change in coming years. 

"We've got this system that's in place that will handle a lot of the changing climate," he said, "But I do think we've got to do more, and that's why we're doing green infrastructure, and we're constantly upgrading treatment plants and trying to increase treatment capacity of those plants."

Laura Schulte can be reached at leschulte@jrn.com and on Twitter at @SchulteLaura.