Concrete Progress is an ongoing series of columns by Peter Brewitt devoted to exploring America’s infrastructure. It is part of Orion’s two-year Reimagining Infrastructure project.

Our poop will always be with us, but in the twenty-first century we do a pretty good job of hiding it. We flush it away and go about our business, and we rarely think, as we walk down the sidewalk, of the millions of peoples’ waste coursing beneath the concrete. It flows, of course, to municipal water treatment plants, and after thinking about it, I’d say that these plants do as much as any one thing to make urban life tolerable; just imagine life without them. But at the same time, even after the treatment process, we’re left with millions of pounds of waste. Do we need to, well, waste it? In most places they do. But in Milwaukee, Wisconsin, they transform it into a helpful substance called Milorganite.

Milwaukee’s geography demands it take its sewage seriously. The city sits at the confluence of the Kinnickinnic, Milwaukee, and Menomonee Rivers, where they run into Lake Michigan. Those streams’ watersheds cover more than 800 square miles of rainy southeastern Wisconsin. That’s a lot of water rushing toward the city and, if the sewers can’t handle it, gushing into the lake. What to do?

The city attacks its sewage in a variety of ways. Some of it is old-school gray infrastructure—they have a deep tunnel that retains overflows. Some of it is new-school green infrastructure— they subsidize green roofs and buy conservation easements on lands with hydric (absorptive) soils. This program, called Greenseams, ensures that water stays in the ground and doesn’t run into the sewer. And some of it is just smart analysis—the Milwaukee Metropolitan Sewerage District (MMSD) consults rainfall gauges and weather reports, ensuring that its operations adjust as efficiently as possible to real-world conditions. All of this allows them to release clean water into Lake Michigan. If you’re reading Orion, you’re probably pretty familiar with these kinds of tactics. But Milorganite doesn’t exactly fit with the green or gray—I’d call it brown infrastructure.

Milorganite! The name conjures up the future—it could be the raw material for light sabers, or a planet where the Serenity stops for repairs. But in fact, Milwaukee Organic Nitrogen (they picked the name, which I think you’ll agree is perfect, after a nationwide contest) is a fertilizer. It’s made out of microbes from MMSD’s secondary treatment process facilities. This is important for you to know—Milorganite is made out of the microbes that treat sewage—not the waste itself. But what, you might ask, is secondary treatment?

For a long time, many cities only put their fresh sewage through primary treatment, which means letting the solids (nice euphemism) settle out and then letting the liquid flow along to wherever. This approach had many downsides, which you can imagine and maybe, if you’re old enough, remember. Secondary treatment—biological purification of wastewater, done by adding microbes to break down the waste—came to much of American after the 1972 Clean Water Act. The spread of secondary treatment has allowed places like the Great Lakes, which were somewhat disgusting in the sixties, to be fishable and swimmable once again. It’s hard to imagine, nowadays, that much of America’s water was filled with sewage for so long, and that so many people just accepted it.

Milwaukee did not accept it. The MMSD began putting Milwaukee’s water through secondary treatment in the 1920s. Once their microbes ate up waste, though, what could be done with them? A University of Wisconsin graduate student named O. J. Noer found a way to safely process the spent microbes into fertilizer. After some tinkering, Milorganite was born in 1926. The MMSD started out selling it to golf courses and landscapers, but now private people buy it too. Milorganite is salt-free, pathogen-free, and binds to the soil without watering. As a bonus for your garden, deer seem to dislike it. The MMSD makes Milorganite on an industrial peninsula called Jones Island, south of the city. In September I went to see it myself.

It starts, of course, with wastewater, running in from Milwaukee’s sewers. Waste solids, after separating out from the liquid, are piped to another facility and anaerobically digested – this produces energy that helps runs that facility. The liquid receives MMSD’s microbes, and these digest the waste. Their work done, the spent microbes are mixed with some of the digested solids to make gross-looking brown water: the raw material of Milorganite. I expect that the plant, would smell pretty rank, but I was glad to find that in fact it doesn’t smell bad at all—more like a brewery than a latrine. The brown water is pressed dry, until it eventually looks just like a brownie (I was cautioned not to eat it). Finally, they bake the brownie at a thousand degrees, rendering it into little gray pellets. These pellets are Milorganite, ready to be bagged up and sold and applied to your lawn.

Knowing that anything coming out of a sewage plant is going to be scrutinized, Milorganite is tested for heavy metals and contaminants every day, and it beats the EPA’s “exceptional quality” standards in all categories. The pellets are non-toxic for pets, safe to handle for people. The pellets come in different sizes, and are sold as fertilizer all over the United States, Canada, and the Caribbean. One fifth of MMSD’s sewage-eating microbes transform into Milorganite, benefiting the MMSD, the city of Milwaukee, the gardens of North America, and all the people who use Lake Michigan.

Peter Brewitt has wondered about infrastructure ever since a flood kept him away from three days of kindergarten. He’s currently at work on a PhD at the University of California, Santa Cruz, where he’s researching the ways people restore and remake their environments.