Chicago's Deep Tunnel Project Holds 17.5 Billion Gallons of Sewage Underground

Trevor English

Management of wastewater and effective treatment of liquid waste has been a problem in infrastructure since the first cities. The Romans were perhaps the first civilization to design and implement water management systems with aquifers and waste collection, but it wasn't really until the mid 20th century that engineers started taking waste management seriously. Before the early 1900s, open sewers were common throughout cities, which left the population open to disease and awful environmental conditions. Even still, wastewater and sewage effluent were often pumped directly into rivers and streams with the mentality that nature will take care of the treatment. Chicago has been a major metropolis since the 1850s, and their sewer system was designed as a combined stormwater and wastewater collection system.

Connecting_Tunnel_Thorton_m[Image Source: MWRD]

This combined system meant that rainwater and sanitary sewage flows were combined into one pipe network. This was incredibly common practice at the time, and engineers largely didn't understand the effects that faulty waste management could have on a city's population, or let alone environment. In combined systems, which are not seen in new construction today, when heavy rains come, there is often an overflow of sanitary waste into surrounding rivers and drainage basins. Many large cities still operate on combined wastewater collection systems, with Chicago being one of them. In the 20th century, overflow events became common with Chicago's wastewater, which resulted in sewage being spilled into Lake Michigan nearly 100 days out of every year.

To overcome this problem, engineers devised a solution with a scale seen nowhere else in the world. Commissioned in 1975, Chicago's Tunnel and Reservoir Plan began construction, with a goal of eventually holding 17.5 billion gallons of wastewater deep underground in storm events.

thorton gap dam[Image Source: MWRD]

This was an innovative solution, and the best one the City of Chicago had. The project would encompass an initial phase of 109 miles of underground storage tunnels along with above ground reservoir storage. Chicago's problem wasn't necessarily that their pipe networks couldn't handle extreme storm flow, but rather their wastewater treatment plants could not treat the water fast enough. During a storm event, water will overflow into these underground reservoirs, after which can be pumped over 300 feet upwards into the city's treatment plants. The video below from the Chicago Tribune will give you a little scale and background of the project.

"Imagine yourself in a garden hose that is as big in diameter as your house"

This gives you a little idea of the scale of these underground tunnels. The diameters vary from 9 to 33 feet in diameter with depths upwards of 350 feet under the city. All of this construction work isn't being done in vain. In fact since parts of the project have been implemented, more than 50 species of fish have returned to the Chicago River, and ultimately Lake Michigan.

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The final phases of the project aren't set to be completed until 2029, and when they are, the entire system of tunnels and above-ground reservoirs will hold 17.5 billion gallons of wastewater underground during high-flooding events.

deep water tunnel[Image Source: MWRD]

Above, you can see a drill head breaching the wall between two tunnels, in result connecting the tunnel systems together. As of June 2015, the project has already cost 3.9 Billion US dollars, but this cost is justified by the savings presented from future aversion to flooding and pollution disasters. The Majewski Reservoir, an above ground reservoir in this project, was completed in 1998 at a cost of US$40 million. Since its implementation in the collection system, it has saved the surrounding areas US$250 million in flood damage reduction benefits.

deep water tunnel reservoir[Image Source: MWRD]

This is perhaps the largest water detention project in the world, and quite possibly one of the largest civil engineering projects ever undertaken in terms of scope, cost and timeframe. Spanning a projected time of 1975 to 2029 or 54 years with a total estimated cost of US$10.4 billion, this project is like no other. The willingness of municipalities and local governments to invest such a large amount of capital demonstrates how important effective waste and pollution management has become in modern infrastructure.

You may say that all of this could have been avoided if the city was not designed on a combined sewer system, but the problem is, that was the best thing engineers knew how to do in that day. You might be surprised when studying the past of waste engineering that modern day practices really weren't developed but in the last 50 or so years. Many places around the world have combined sewer systems, mostly stemming from successive waste management developments. As cities transitioned from open channel sewage systems, many places simply covered the channels with metal plates or concrete arches, creating 'closed channel' systems. For a long time, no engineers saw the need to manage and treat wastewater or stormwater, as the effects of maltreatment were largely unknown.

menin tunnel[Image Source: MWRD]

In the United States alone, 40 million people are still serviced by a combined sewer system, which is manageable as long as sufficient treatment techniques are in place. Part of why combined systems are shied away from today is because normal flow can be many magnitudes less than treatment flow during storm events. This, of course, means high strain on treatment plants during storms, meaning dirtier water discharged in the long run. Chicago's innovative, yet expensive solutions are helping restore environmental clarity to the ecosystem and are succeeding in effective wastewater management.

SEE ALSO: World’s Longest Tunnel Set to Open this Summer in Switzerland

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