Thoughts From Engineers: When a Deep Tunnel Isn’t Enough: Milwaukee Wrestles with Combined Sewer Overflows
The City of Milwaukee was on the cusp of a victory lap. After several expansions to the Deep Tunnel storage system and other significant district-wide improvements to grey and green infrastructure, Milwaukee appeared more prepared than ever for the “mother of all rain events” to strike.
The city’s extensive capital investments and long-term planning initiatives seemed to have paid off: combined sewer overflows (CSOs) were at a near-record low. Milwaukee appeared to be on a trajectory to move from its standing as a city with 50 to 60 CSOs per year in the Great Lakes basin to a city with no more than one, as was the case in 2017.
The Cream City Classic, a swim event designed to showcase the “fishable, swimmable” water quality achieved in parts of the Milwaukee River as well as promote use of the city’s downtown waterways for recreational uses, was organized for the first time in August 2018. The race led qualified open-water swimmers through a 1.5-mile course through the lower reaches of the Milwaukee River, a stretch of water that had for a long time functioned as a type of open sewer. Some 72 swimmers participated, and although the race was scheduled for a specific day and time to minimize risk, few found swimming conditions objectionable.
But then the historic rains of 2018 struck. In the rural countryside of Wisconsin, this same storm system knocked out bridges and several roads in Cross Plains, Wis., and swamped entire towns. In Milwaukee, the historic rainfall sent city managers scrambling. Milwaukee Metropolitan Sewerage District (MMSD) operators tried to strike a careful balance between managing the water coming into the Inline Storage System (ISS) or “Deep Tunnel” system, sometimes at the rate of 1.5 inches per hour, and keeping an eye on the wastewater moving in from the separated sewer systems of suburban Milwaukee.
In the end, to open up capacity in the Deep Tunnel and prevent a mix of raw sewage and stormwater from percolating back into basements, MMSD released 295 million gallons of the Deep Tunnel’s brew from outfalls along the Kinnickinnic, Milwaukee and Menomonee rivers. By October 2018, MMSD had five additional CSOs, releasing more than 750 million gallons of sewage overflow into the area’s waterways. This volume was equal to the record set in 1999 for the most CSOs since the Deep Tunnel was completed in 1994.
CSOs in the Great Lakes
The term CSO refers to the untreated wastewater discharged into receiving bodies of water when combined systems reach capacity due to high-volume storm events. Combined sewer systems (CSSs) represent the earliest sewer systems constructed in the United States. Combined systems, unlike separated systems, are designed to move raw sewage and wastewater from domestic, industrial and commercial sources as well as stormwater runoff from streets and other surfaces into publicly owned treatment facilities. Given normal conditions, this blended wastewater can be managed and treated to acceptable federal standards before it’s discharged.
During extreme storm events, however, when incoming volumes of water are above average, these combined systems can reach capacity quickly, resulting in the discharge of millions of gallons of untreated waste into the receiving waterway. For this reason, CSOs can be a major source of pollution as well as a management challenge for facility operators across the country. The “2016 Report to Congress: Combined Sewer Overflows into the Great Lakes Basin” (bit.ly/2LJiJPQ) found that the 184 combined sewer systems in the Great Lakes basin produce a significant number of CSO events, with tens of billions of gallons of untreated sewage and polluted stormwater entering the basin annually.
All Bets on the Deep Tunnel
The City of Milwaukee has had many “firsts” when it comes to wastewater management. It was the first to create a Sewerage Commission in 1913 and the first in the nation to build a secondary wastewater treatment facility. The Jones System, built in 1925, was the first to use microorganisms (i.e., activated sludge) to break down waste. (Milwaukee also was the first to market the byproduct of the treatment process as lawn fertilizer, “Milorganite.”)
Milwaukee’s two plants provide wastewater services to 28 municipalities in Milwaukee County, which serves a 411-square-mile area. Of that area, 23.5 square miles of Milwaukee and the neighboring community of Shorewood have combined sewers. The other areas within MMSD’s service area are covered by separate sanitary sewers. In these systems, one set of pipes moves raw domestic and industrial sewage to treatment, and another set of pipes discharges stormwater runoff directly into the waterways.
Completed in 1993, the Deep Tunnel was originally 300 feet deep and 19.4 miles long. Following court-mandated expansion due to ongoing CSO violations, the tunnel became 30 miles long with a capacity to hold 521 million gallons of wastewater by 2005. Completion of the Deep Tunnel did result in a lower frequency of CSOs. According to the Alliance for the Great Lakes’ 2012 report, “Reducing Combined Sewer Overflows in the Great Lakes: Why Investing in Infrastructure is Critical to Improving Water Quality,” Milwaukee reduced the number of CSOs by 75 to 80 percent (bit.ly/2YDnUEC). The extreme rain events of 2018, however, reset historic benchmarks; Milwaukee released 1.3 billion gallons of untreated sewage last year.
Focus on Green Infrastructure
Undeterred, the City of Milwaukee presses on and continues to innovate and strategize. Milwaukee’s “green infrastructure” presently traps nearly 40 million gallons of stormwater runoff. The city projects a starring role for green infrastructure in the coming years, with ambitious goals to use green strategies to capture and soak up nearly 760 million gallons of water by 2035.
The City of Milwaukee’s “water-centric” website (city.milwaukee.gov/WCC) outlines a number of initiatives designed to grow the city sustainably and manage water resources responsibly. Through its Green Infrastructure Plan, the city evaluates road or vacant-lot reconstruction projects with an eye toward enhancing bioinfiltration with use of bioswales, “stormwater trees” and permeable pavement. Roof gardens are multiplying across the city, and strategic land-acquisition efforts are on the uptick. Several grant programs support innovative efforts to implement and maintain green infrastructure projects. An initiative called BaseTern converts the basements of abandoned houses to massive cisterns for temporary water storage. StormGUARDen uses an innovative water-filtration technique to store the equivalent of six rain barrels of stormwater before allowing it to slowly percolate through the soil some distance from the building’s foundation.
Our ability to reduce the frequency of CSOs is a telling indicator of how equipped our cities are to manage the looming and likely formidable storm events of the 21st century. Growing urban populations and large ratios of paved areas combined with storm events that appear to crush historic benchmarks is the new norm. Cities need to maximize the use of a broad variety of tools: larger-capacity and more-robust infrastructure, smart data-monitoring methodologies (see EPA’s recent guidelines on “Smart Data Infrastructure” at bit.ly/30t1NBr) to detect leaks and variable flow rates, as well as solutions that are indisputably low-tech, such as grass installations and porous substrates where there was previously cement.
There’s no one way to tackle the epic problem of CSOs, but success hinges on an iterative approach that builds on diverse strategies. In the meantime, downtown Milwaukee doggedly prepares to host the Cream City Classic on Aug. 10, 2019, and many swimmers seem game to take the dive