The Calumet plant, where MWRD will expand its anaerobic digestion process to save energy. The digesters are in the foreground. Metropolitan Water Reclamation District

Chicago’s wastewater authority plans to slash its energy bill by using bacteria to convert sewage into natural gas.   

There are a lot of things in the 1.2 billion gallons that pour through the world’s largest water-treatment plant every day: grime swept off Chicago sidewalks, sewage scoured from thousands of miles of pipes—and enough energy to cut an annual $50 million electricity bill to zero.

That’s if engineers at Chicago’s Metropolitan Water Reclamation District can achieve their goal to become energy neutral in the next eight years. The public agency has pledged, by 2023, to slash its energy consumption and produce whatever remaining power it needs onsite, becoming the largest wastewater treatment authority in the country to do so.

MWRD isn’t the only sewage treatment agency doing this. DC Water and Hampton Roads Sanitation District in Virginia are also taking aggressive steps to recast wastewater as energy.

“There’s more energy in the sewage that comes into a wastewater treatment plant than is required to treat it,” according to Paul Kohl, a project manager for the Water Environment Research Foundation, an independent research organization. “We think we ought to be able to go ahead and get that back out.” By some estimates, there’s two to 10 times as much energy contained in wastewater as an agency needs to use to treat it.

While water treatment accounts for just 3 percent of total energy use in the United States, WERF points out that drinking water and wastewater plants can be a municipal government’s largest energy consumers, eating up more than a third of the energy used by the public sector. Many treatment processes are energy intensive, from aerating huge vats of liquid with 3,000-horsepower blowers to pumping sludge out of the system so it can be recycled as “biosolids.”

And that economic incentive is a big reason why typically slow-moving public agencies like MWRD—which was sued by environmental groups before it started disinfecting the water it discharges into the Mississippi River system—are eager to invest in energy neutrality. Not only does it negate a $50 million electricity bill (MWRD’s biggest expense after payroll), it helps them hedge against future spikes in the cost of energy.

“Some people do it for the environmental benefit, but I was raised as a business person,” says David St. Pierre, MWRD’s executive director. “Having your own energy source, we can convert it to anything we need. So if the market drives costs, we can really kind of control that unforeseen cost of energy in the future.”

The district already gets almost a third of its energy through anaerobic digestion: harvesting gas produced by special bacteria that help break down the noxious components of wastewater. But it plans to double down on that strategy, investing $10 million to expand anaerobic digestion efforts at its Calumet plant on Chicago’s South Side.

It’s even in late-stage talks with local haulers who will pay MWRD to take their waste—mostly spent grain that’s left over from local breweries—which has a multiplying effect on the amount of gas produced when mixed in with sewage and stormwater. The plan is to build a pipeline system that could scrub some of the methane produced in MWRD’s digesters and sell it back to the grid.

Incorporating organic waste from outside the wastewater treatment process is an unusual, and logistically difficult, step. Earlier this year, the city of Gresham, Oregon, celebrated achieving net-zero-energy status at its wastewater treatment plant, largely thanks to the fats, oils, and grease it trucked in from Portland-area restaurants. Rather than just add incrementally to the gas produced, mixing organic wastes fires up the microbial activity that produces biogas, exponentially improving a digester’s output.

One of the massive, methane-producing digesters at the Calumet plant (Metropolitan Water Reclamation District)

If it works at Calumet, MWRD will bring the whole system to its flagship Stickney plant, the world’s largest, and generate enough power on-site to offset three-quarters of its energy use. It will get the rest of the way to energy neutral by reducing demand, St. Pierre says, and should pay off the initial investment in just a few years.

Anaerobic digestion isn't new—more than 1,200 wastewater treatment agencies across the country already use it—but just a few sell gas or electricity back to the grid, as MWRD plans to. Fewer still produce enough energy onsite to offset their energy consumption. But as more of them see wastewater as a potential source of revenue that could hedge against climbing energy costs, the aggressive steps taken in Hampton Roads, D.C., Chicago and elsewhere should become more widespread.

Still, St. Pierre says, their program isn't wholly cost-driven. “We're an environmental agency,” he says, “and our plants should be able to support themselves.”

About the Author

Most Popular

  1. A stained glass artwork depicting two owls and geometric patterns
    Design

    The Brilliant Artist That Chicago, and the World, Nearly Forgot

    The idiosyncratic art of Edgar Miller (1899-1993) has long been hidden behind closed doors. Finally, Chicagoans are getting more opportunities to see it.

  2. A rendering of Elon Musk's Chicago Express Loop, which would transport passengers from downtown to O'Hare in 12 minutes.
    Transportation

    The Craziest Thing About Elon Musk's 'Express Loop' Is the Price

    The $1 billion construction estimate is a fraction of what subterranean transit projects cost.

  3. POV

    To Build a Better Bus System, Ask a Driver

    The people who know buses best have ideas about how to reform the system, according to a survey of 373 Brooklyn bus operators.

  4. Sunlight falls on a row of graves through tree branches.
    Environment

    ‘Aquamation’ Is Gaining Acceptance in America

    Some people see water cremation as a greener—and gentler—way to treat bodies after death, but only 15 states allow it for human remains.

  5. Equity

    Mapping Childish Gambino’s Atlanta

    A transplant to Atlanta from Pakistan (via London) has mapped the story of Atlanta as told through the songs of some of the city’s most famous rappers.