Repurposing sewage can be environmentally friendly, but it's objectionable to some, and has to be carefully managed

Old-time ad for Milwaukee's processed sewage sludge

Recycle. Reuse. Repurpose. Repeat. Or so goes the ever-living mantra. "The precepts of circular economy—reusing water, using biodegradable plastic, composting food leftovers so they can be returned to the soil rather than become landfill—are among the hottest ideas of our century," reports Lina Zeldovich for JSTOR Daily. "But what about recycling what our food becomes after we eat it? Yes, that means excrement, a critical substance in the thoroughly functioning circular economy, seems to be left out of the equation. . . . Our industrial treatment systems separate the so-called biosolids out of the sewage stream, purify the water, and release it back to nature, but they rarely extract these essential nutrients in the process. Instead of fertilizing the fields then, the nutrients fertilize the waterways—and wreak havoc in them."

A cycle of environmental problems ensues, Zeldovich reports: "The nutrients overfertilize the water, causing what scientists call eutrophication or nitrogen overloading, which fuels toxic algae blooms, destroys coastal marshes, kills mangrove forests, and smothers corals. What happens with the remaining biosolids—a euphemism for the malodorous, pathogen-infested black muck—is equally ugly. Sometimes it’s burned, releasing carbon dioxide in the atmosphere and contributing to greenhouse gas emissions. "

Zeldovich writes, "We must figure out how to return our sludge back to land. The necessary tools already exist to do so Even better, there are technologies that turn human waste into humanure. Zeldovich provides an surprising old example: "Milorganite: Human-Made in Milwaukee Since 1925. In 1914, the Milwaukee Sewerage Commission began to experiment with a new approach for wastewater treatment." A treatment plant that opened in 1925 was the first in the nation "to tap into nature’s power to clean wastewater. But while these bacteria decomposed excrement, the gooey and gunky end product left behind still needed to be dealt with. Agronomist Oyvind Juul Noer realized that the muck contained high levels of nitrogen, phosphorus, and potassium . . . the key fertilizers plants need to grow. Compared to manure and chemical fertilizer, this microbe-processed sludge produced superior growth results. Milwaukee sewage authorities figured out how to dry and process the muck into pellets and began churning out this human-made fertilizer."

Milwaukee's process has been copied in many places, and there have been many public objections to farmers' use of it. "The presence of potentially toxic metals is the main factor that limits such use," says a 2016 paper in the journal Circular Economy and Sustainability. "Many techniques have been employed in attempt to remove these toxic metals, including physical, chemical, and biological treatments, but the high cost of the physical and chemical treatments, as well as the risk of causing secondary pollution, makes this type of sewage-sludge treatment an unsatisfactory option. Therefore, removing toxic metals through biological treatments has become an increasingly popular choice, as such treatments have been shown to be the most economically and environmentally beneficial methods."

There are other challenges to using human waste as fertilizer, including "the number of pathogens it contained—an issue modern tech addresses," Zeldovich writes. "A newer concern is the problem of hormones, antibiotics, and PFAS. . . . But the sewage industry is already working on ways to address these. Even the most stable chemical compounds fall apart under high temperatures and pressures; called pyrolysis, one such process may be the path to their destruction." Environmental experts say proper management, including careful application, is essential to use of sewage sludge on farms.

JSTOR Daily says it is "an online publication that contextualizes current events with scholarship." Its parent platform is a "digital library of academic journals, books, images, primary sources, research reports, and other material."