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Since 1994, a more than 26-mile- (42-kilometer-) long tunnel has been keeping Milwaukee's sewage from spilling into Lake Michigan. This deep water tunnel—a holding tank on steroids—comprises two legs roughly 300 feet (90 meters) belowground that can hold nearly 500 million gallons (1.9 billion liters) of sewage and storm water during a downpour. And for the last 14 years it has kept 74 billion gallons (280 billion liters) of wastewater out of Lake Michigan, according to Bill Graffin, a spokesman for the Milwaukee Metropolitan Sewerage District.
That's a good thing, not only for water pollution but also for the drinking water plants that must pull H20 from the same lake and spend millions in money and energy cleaning it up. A breakdown in Milwaukee's clean water system in 1993 caused more than 100 deaths as a result of drinking water contaminated with cryptosporidium, a microbe which causes diarrhea, primarily in the young, elderly or infirm.
The deep water tunnel is just one part of a $3-billion water pollution initiative that has also upgraded 400 miles (645 kilometers) of sewer infrastructure in Milwaukee and surrounding communities—and ends with a project to turn dried sewage sludge into fertilizer. But Milwaukee has a long history of good sewers: the Jones Island Water Reclamation Facility connected to the deep water tunnel was one of the first wastewater treatment plants built in the U.S.
Milwaukee's sewers still face challenges, however, from a growing population to climate change. "Weather patterns have changed," Graffin says. "Recently, we've been getting fewer rain events but more intense rain events."
In fact, even the deep water tunnel can't prevent all overflow situations, though it has cut them from as many as 60 a year down to just one or two. "We had an overflow in June of 3.5 billion gallons [13.2 billion liters]," without any health impacts, Graffin notes. "To capture that we would have needed seven more deep tunnels just for one event."
And there isn't a sewer in the world that doesn't need periodic cleaning to keep it functioning reliably—especially in the face of climate change. So this past August, divers ventured into the tunnel to clear the path to the pumps that move all that wastewater back up to the surface and through the treatment plant. "It was more of a preventive measure to make sure that we didn't have a problem," says Scott Royer, general manager at Veolia Water North America's Milwaukee unit, the private company that manages the area's sewer system. "The whole tunnel system relies on these pumps to empty it and empty it quickly so we can bring in more water. It's important to its reliability to go in and clean this material out."