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Carrie Antlfinger/AP Carrie Antlfinger/AP

“Sewer socialists” was a pejorative label applied to the leftists who governed Milwaukee during the early 20th century and often boasted about their achievements in sanitation. One hundred years later, the city’s sewerage system is a source of pride again. Joel Brammier, the executive director of the Alliance for the Great Lakes, has hailed the “ astounding” performance of the Milwaukee Metropolitan Sewerage District (MMSD) at stopping overflows, while Adam Krantz, CEO of the National Association of Clean Water Agencies, described the leadership of MMSD Executive Director Kevin Shafer as “ the gold standard.”

Milwaukee’s current success is notable given that, a few decades back, it was struggling to manage its wastewater. The city’s reputation became so bad that in 1972, the state of Illinois sued Milwaukee and three other Wisconsin cities, alleging they were a public nuisance and asking a court to force officials to address the estimated 200 million gallons of untreated discharge flowing into Lake Michigan every day. (The MMSD reached a separate agreement with the Wisconsin Department of Natural Resources to reduce overflows.)

Then and now, most overflows are caused by heavy rainfall. Milwaukee is one of more than 700 cities around the United States that use a combined sewer system, designed to carry both sewage and rainwater. One inch of rainfall dropping over the entire 411-square-mile service area equates to 7.1 billion gallons. How much of that is discharged without treatment depends on factors like ground saturation and pipe and plant capacity. Too much rain or melting snow creates an overwhelming backup, releasing the untreated water (and whatever is mixed in with it) into nearby waterways and basements.

When the city finally addressed the problem of combined sewer overflows, it went big: It bored a tunnel 300 feet below the Earth’s surface.

When the city finally addressed the problem of combined sewer overflows in the 1980s, it went big: It bored a tunnel out of bedrock 300 feet below the Earth’s surface. The tunnel is essentially a giant bathtub, 28.5 miles long and 32 feet in diameter, constructed over three phases beginning in 1987. The first and largest phase went online in August 1993 and produced immediate results. Prior to 1994, the city averaged 50 to 60 overflows a year. Since then, it has averaged 2.4. Compare that to Milwaukee’s closest neighbor, Chicago, which endured 37 area overflows in 2017 (despite having a Deep Tunnel of its own).

“The deep tunnel is the backbone to the whole system,” says Shafer, who began working for the MMSD in 1998 after the first phase was completed. “It’s the best piece of water infrastructure in the state of Wisconsin.”

Inside Milwaukee’s Deep Tunnel. Milwaukee Metropolitan Sewerage District

Subsequent work boosted the tunnel’s capacity to 521 million gallons, and an additional 2002 legal agreement with the state of Wisconsin required the MMSD to spend $900 million in upgrades to the collection system by 2010. In the quarter century that the Deep Tunnel has been operative, the MMSD has captured and cleaned 98.4 percent of all the wastewater that’s entered the regional sewer system, far exceeding the U.S. Environmental Protection Agency’s standard of 85 percent for combined sewer systems.

“Now cities like New York and Toledo that didn’t build these large storage systems are faced with a lack of capacity,” says Sandra Mclellan, a professor at the University of Wisconsin–Milwaukee School of Freshwater Sciences. “All of those, if they did not add additional capacity, are still overflowing every time it rains.”

Yet there are still times when the deep tunnel’s volume isn’t enough to handle the torrents of water from snowmelt and thunderstorms. “If you put too much water in that bathtub, it overflows,” Shafer says. “We knew we needed to build off of what we had.”

So the MMSD expanded its priorities from increasing capacity to limiting the amount of water that reaches the sewers, by using rain barrels, buried cisterns, rooftop gardens, and porous pavement to capture and slow down stormwater. This green infrastructure complements the traditional “gray” infrastructure of pipes and treatment plants. The sewerage district recently partnered with Milwaukee Public Schools to replace asphalt playgrounds with more absorbent surfaces at 16 schools. Total completed improvements are estimated to hold an additional 40 million gallons of water, a teaspoonful compared to the green-infrastructure goal of 740 million gallons over the next 15 years.

An outdoor classroom at a Milwaukee school, on what used to be a parking lot, uses rain gardens, bioswales, and trees to manage water. Milwaukee Metropolitan Sewerage District

The district has made additional investments in flood control. When there is an overflow, water is discharged into Milwaukee’s three rivers, with the potential of them filling beyond their banks. The MMSD is building flood basins and levees in neighborhoods vulnerable to flooding, widening stretches of the Kinnickinnic River—Wisconsin’s most densely populated watershed—and ripping up stretches of concrete along its riverbanks to allow the ground to soak up more rising water.

A wetlands preservation program called Greenseams involves purchasing and protecting undeveloped land in areas that are projected to see major development in the coming decades. The land remains open to the public for recreational activities, and the only improvements are to boost its ability to store water. The Greenseams program currently covers 3,700 acres, which are estimated to hold 2 billion gallons of floodwater.

“That takes the floodwaters that would have flowing downstream and into our system and flooding our homes, and hopefully finds an open area where it can be more natural,” Shafer says.

Funnily enough, the sewer socialists of old did play a role in Milwaukee’s modern sewer success. They devised a commercial byproduct from sewage sludge that is still sold today: fertilizer pellets marketed as Milorganite (short for Milwaukee organic nitrogen). Other cities have since copied this idea, but because of Milwaukee’s head start, Milorganite enjoys the widest brand recognition, and reaped sales exceeding $10 million in the past two years. That money helps the sewerage district literally keep the lights on by covering some operational costs.

Shafer hopes the district can hit zero overflows by 2035—an ambitious goal, because the region is growing more waterlogged. “Our system is built to reduce overflows, and the problem with that is we’re seeing a changing climate,” Shafer says. In 2018, Milwaukee had six overflows, tied with 1999 as the most in a calendar year since the Deep Tunnel went online. Modeling done by the University of Wisconsin predicts that the region will experience greater storm intensities, with single events that dump larger amounts of rain. The most intense storms stress even the MMSD’s capabilities.

“Approximately two inches or more in a 24-hour period—that’s when we see our city overwhelmed in the terms of the capacity of the pipes,” Mclellan says. “We want to get to zero overflows, but unfortunately we might have more.”

CORRECTION: This article originally misstated the amount of money that the MSSD was required to spend upgrading its collection system in 2002: It was $900 million, not $900. The article has been updated.