On Friday a group of friends and I arranged for a tour of the World's Largest Sewage Treatment Plant, in nearby Stickney, IL. This plant serves the central part of Chicago as well as nearby suburbs, a sprawling campus of aeration tanks and sludge collection ponds alongside the Sanitary & Shipping Canal.
We arrived at the plant on a steamy afternoon and waited for our bus and tour guide in the broiling sun in the parking lot. The tour rules stated that we must all wear long pants and closed shoes, though whether this was to keep a dignified appearance as guests or for safety reasons was unclear. I doubt you'd want to wear your favorite sandals or beach wear to this stink factory anyway. As we waited outside the gate a hot southern breeze carried the ripe odors downwind toward us, though it really smelled no worse than an afternoon at the Chicago River. Still, I wouldn't choose to live in one of the many little bungalows directly across the street from the sewage plant.
Finally an old school bus arrived, and our tour leader Matt, one of the engineers at the plant, and Theresa, a college intern, welcomed us aboard. They drove us to a low air-conditioned building, where we were encouraged to use the bathroom before the tour began. With that introduction to the plumbing system, we walked through a wide control room where several workers hunched over panels of switches and gauges to a conference room to take a seat. Several new handlers hovered about the back of the room, and we sat in hushed expectancy as Matt turned the lights down for a video.
The 15-minute presentation made use of a lot of aerial shots, slow transits over each convoluted network of piping and water tanks in the district, from the humble Lockport facility through the stately Skokie plant leading up to the granddaddy of treatment plants, the Stickney complex. A narrator explained the founding of what would become the Metropolitan Water Reclamation District as an organization charted by the state, separate from the tainted waters of city politics, with a clear mission of ensuring clean drinking water to the citizens of Chicago. All of the engineering wonders it has accomplished since that time: the reversal of the Chicago River, the construction of the world's largest water purification and water treatment plants, the enormous public works project of Deep Tunnel, these all serve only to guarantee our most basic right of clean fresh water.
Now that the video had us fired up about the importance of clean water, Matt brought our attention to a poster diagramming the steps of water treatment that we would see on our tour. Arrows indicated material flows, with vectors of sludge and grit and biosolids being refined off the flow of water, leaving behind only the pure goodness of fresh clean water. Our guides directed us to a stack of hard hats and we stepped back out into the heat to get on the bus.
The first stage of treatment involves screening out the big bits from the incoming wastewater, but Matt explained that on such a hot day the hydrogen sulfide fumes in that building could be downright dangerous, so we moved on to the next step.
The sewer mains which arrive at the plant are 30 feet below ground, fed by gravity from distant parts of the city, so after screening the water must be pumped up to 11 feet above ground to begin its flow through the treatment process. Entering a neat brick building we gazed down to the sunken floor below where enormous pumps roared. The level of wastewater coming down the mains was posted on a large digital readout: -17.35 feet, a sort of sewage aquafer these enormous pipes tapped into.
Outside the pumphouse we stopped by several types of settling tanks where slow moving paddles scraped solids off the bottom into troughs and floating fats and oils were skimmed off the top of the palid pools. When the Stickney plant was first built in the 1930s by the WPA, this was the extent of wastewater treatment, enough to remove 40% of the contaminants before the water was dumped into the river. Sunshine and bacteria naturally present in rivers were supposed to clean the rest.
But by the 1940s the sanitary engineers were incorporating these natural processes into their plant designs. Hundreds of new tanks were built at the plant to let the water sit longer in the open air, with aeration pumps to encourage the growth of bacteria to break down volatile organic waste into a safe sludge which could be spread on open lagoons to dry before trucking it to landfills. The second stage works well enough to remove 98% of the waste from the water before it is sent into the river.
Most of the settling ponds of the second stage water treatment are the circular tanks visible at most any small-town sewage treatment plant, where water trickles over a round weir into a central trough which leads to the next pond. Slow-moving arms rotate along the weirs to scrape them clean of particulates. Eventually the tanks fill up with six feet of sludge and they are drained to clean them out.
By this time we were behind schedule, so we only paused the bus by the newest finishing step, the third stage treatment, where sludge is turned into biosolids which can be used as fertilizer. The bus followed the path of an elevated pipeline which carries sewage sludge 20 miles underground from the Skokie plant on the northside. Wouldn't want to be around if that pipe burst! Around the far side of a metal building we were able to see a clumpy slurry of biosolids pouring into rail cars in a loading area. The MWRD has its own short-line right-of-way leading a few miles south to enormous holding ponds in old quarries where the sludge is spread out to dry further in the sun. Every day the Stickney plant produces 150 tons of fertilizer from sewage sludge.
In a more distant corner of the grounds a new sludge plant is under construction. The goal is to produce a Grade A fertilizer that can be used on farm fields, with a new plant under construction which bakes the sludge into small fertilizer pellets, similar to the process Milwaukee uses to market its own city waste as the brand name Milorganite.
As a final stop on our tour, the bus dropped us off at a laboratory testing building, where we were met by a new tour guide who took us in to several labs where water samples are tested daily for excess nitrogen, phosphorus and other chemical signs of what kind of wastewater is coming into the facility. Some of the water samples are taken at points throughout the treatment plant, but other samples come from all points upstream in the sewage system, so that engineers can monitor exactly what materials are entering the system from factories and businesses all over the city. The lab workers explained that stricter laws limiting what can be dumped into the sewer have made their job much easier in cleaning the water that makes it to the treatment plant.