Earlier this year I had the pleasure of touring an incredible part of Chicago that most residents know nothing about. Deep below the city – 297 feet to be exact, lies 110 miles of of massive tunnel, effectively managing the waste water that Chicago’s 3.7 million residents and the weather generate. An additional 570 miles of intercepting sewers merge with the deep tunnel, making for a massive spiderweb of infrastructure.
The TARP (Tunnel and Reservoir Plan) deep tunnel is 33 feet in diameter, has a 12 foot thick reinforced concrete liner, and is one of the largest civil engineering projects on Earth. Construction on the tunnel began in 1975 and was put into service in 1985. By the end of 2006, all of phase 1 was complete which includes a 2.3 billion gallon volume capacity. Phase 2 of the project will increase total capacity to 18.5 billion gallons. Many Chicago construction union workers have the privilege to brag about their involvement of this impressive structural feat. A 22 foot diameter mining bore was used to drill the tunnel through thick layers of limestone.
(Inside a pumping station situated over the tunnel)
Some photos from construction:
Here are a few images and diagrams to help better illustrate how the system works:
Much of the water waits for treatment in unused quarries turned to reservoirs. Remember my tour of the massive quarry in Thornton, IL owned and operated by Lehigh Hanson? One section of that quarry is now a designated reservoir for the MWRD.
What happens to that waste water once it reaches a water treatment facility?
The MWRD is recognized worldwide as the leader in water treatment. One billion gallons of water is treated per day! The process can be quite overwhelming for visitors like me who have no prior understanding of the treatment process. I found the following summary on the MWDR website which does a great job of explaining:
Local sewers from each of the 125 municipalities within MWRD boundaries connect to the large MWRD interceptors, which gather the wastewater and convey it to one of the treatment plants. Here it undergoes a number of cleaning processes.
Primary Treatment
Primary treatment consists of removing contaminants by some physical mechanism:
- Screens remove debris which can clog the machinery. The wastewater flows into chambers where heavy solids such as sand and grit sink to the bottom; these solids are washed before being deposited in a sanitary landfill.
- It then goes to a primary settling tank where a significant portion of the organic solids settle to the bottom while fats, oils and grease rise to the top.
- Revolving “arms” simultaneously scrape the primary (untreated) solids from the bottom and skim the grease from the top.
Secondary Treatment
Secondary treatment usually employs a biological process whereby a large population of micro-organisms help convert the remaining organic material into other forms which can be easily separated into solids and a clear liquid.
The primary affluent flows through a series of large rectangular aeration tanks which have been seeded with bacteria and other microbes (tiny organisms which exist naturally in plant and animal life). Filtered air is pumped through the liquid to enable the microbes to breathe and grow. In the constantly churning water, these microbes flourish and multiply, eating the remaining organic materials and nutrients in the wastewater.
This mixture of microbes and water flows into a secondary settling tank. The microbes, now stabilized, clump together and settle to the bottom of the tank where they become part of the organic residuals and are removed. Approximately eighty-five percent of these microbes are recycled to the start of the aeration tanks to begin the biological treatment process for the primary effluent.
The cleaned water flows out of the top of the secondary settling tank to be returned to the waterway or to the tertiary treatment process.
Tertiary Treatment
Tertiary treatment is only required when the final effluent must be so clean that 95% or more of the contaminants must be removed by wastewater treatment. Tertiary treatment may include:
- Filtration
- Removal of Ammonia and other specific contaminants
- Disinfection to destroy bacteria which can cause disease in humans.
Within a few hours, the cleansing action of hundreds of miles of flowing river has been replicated within the series of tanks. The reclaimed water has more than 95% of the impurities removed and can be deposited into a river or stream without any adverse environmental impacts. This “effluent” is often cleaner than the water of the stream. The entire process from the time wastewater reaches the treatment plant to the time it is cleaned and “reclaimed” takes less than 12 hours.
A great video of the process can be found on their website – https://mwrd.org/water-reclamation-plants
Interesting Tidbits
- Each treatment plant is connected to its own power source as well as multiple back up sources, so that it is guaranteed to run no matter what.
- Chicago’s process is one of the most environmentally friendly in the world due to only having to “lift the water” once. All of the water is funneled to one place before being pumped to the surface for the treatment process. Most other treatment facilities are built and added as populations grow which causes them to be located at multiple sites around town. Each site requires the water to be lifted to the surface. The amount of energy needed to “lift” water is astronomical.
- Largest electricity user in the state – Costs $5,000 to start one pump!
- The actual filtration process is all done by gravity – allowing the water to flow downwards through the plant.
- Even the residual “biosolids” produced from the process are used as mulches and fertilizers.
- Waste water maintains a constant temperature of 50ºF and because it does not freeze in the winter, the agency experiences a lot of issues with birds.
- An entire lightning mitigation system is needed to protect susceptible plant parts from strike damage. Spline Ball Ionizers can be found by the hundreds around a plant.
- 2nd largest land owner in the state after the park districts.
- Compressed natural gas is a byproduct of the digester process, which is then turned into fuel. Enough is produced to create 11,000gal of gasoline per year or to heat 900 homes per year.
- A large hydroelectric dam is situated over the sanitary shipping canal generating lots of green energy.
Where does oil come into play?
Aside from the more obvious oil needed in the vehicles and equipment used to build and maintain the facilities, there are many applications for oil within the entire water reclamation system. A key place is in the electric motors used to power the water pumps.
Hydraulic oils and turbine oils are also used in the pumps; antifreezes to prevent water lines from freezing in cold temperatures, and various greases. The MWRD also has its own rail system, so rail specific products are needed in the locomotives. Compressor oils can also be found.
This was one of the most interesting adventures of my career thus far and I am so appreciative of the opportunity to see it all! Thanks to Tim Delathouwer for being a great tour guide and inviting me in!
