Science

The Planet Has a Human Poop Problem

Our waste kills coastal marshes and fuels algal blooms. We need to harness it as a resource.

a cartoon illustration of planet earth, with stick-figure hands, feet, and a face. the earth has a poop emoji on top of it. the earth is blushing and putting its palm to its face in embarrassment.
Illustration by Slate, Getty Images Plus.

This article is adapted from the book The Other Dark Matter: The Science and Business of Turning Waste into Wealth and Health by Lina Zeldovich. Copyright © 2021 by Lina Zeldovich. Published by the University of Chicago Press. All rights reserved.

The next time you go grocery shopping, take a look at where your food came from.  If you live in colder climates, most of it isn’t local most of the time. In the northern United States, your strawberries likely come from California or Florida, your asparagus from Mexico or Chile, and your bananas from Ecuador or Costa Rica. Your salmon probably comes from Alaska, your beef likely originates in Texas, and your pork sausages aren’t stuffed by your local butcher either. Most of the food that gets put on our tables these days is shipped, trucked, flown and in some cases even helicoptered to us from far away.

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As it grew, our food had to extract the nutrients from the soil, in which it was planted. Then it was shipped to us—using fossil fuels. Next, we eat the food—and we excrete the unused nutrients, which ultimately end up in a local body of water, probably closer to your house than you think. The marvels of modern engineering, our industrial sewage plants separate the so-called biosolids from the wastewater and clean that wastewater from germs—but not from the nutrients such as nitrogen, phosphorus and potassium, all of which are potent fertilizers. The sewage plants release that nutrient-rich effluent into the lakes and the rivers, which carry it down to the ocean, where it fuels algal blooms and destroys coastal marshes.

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Canadian epidemiologist David Waltner-Toews calls this a “redistribution of nutrients on the planet,” that ruins the nutritional balance of ecosystems. “You’re taking all this biodiversity out of one ecosystem and creating these piles of shit somewhere else,” Waltner-Toews says—hence farm soils turn to dust while waterways suffocate from toxic algae blooms and marshes fall apart. As a result, our continuously depleted farmlands will require more and more fertilizer to produce food. With our changing climate, unpredictable weather, droughts, floods and heat waves, barren soils only add to the already pressing food security problems. We solve this problem by putting in more fertilizer, but we don’t have a good way of trapping the excess of nutrients on the other end. Instead, we release nitrogen-rich effluent into the water while the remaining biosolids, aka sludge, is burned or put in landfills, rather than into the fields. So even though your poop doesn’t physically pile up in the ocean, its nitrogen-rich vestiges are ruining the ecology.

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Nitrogen is the worst offender of the three fertilizing elements, scientists say. Linda Deegan, who had studied coastal ecology at the Marine Biological laboratory in Woodshole, Massachusetts found that overbalance of nitrogen makes marsh plants grow taller and leafier, but with fewer, weaker roots. Roots is what normally holds the marsh grounds together, but because there is too much food around, the plants just don’t bother investing in them. As a result, the marshland starts to fall apart, with large chunks caving into the tidal creeks. And as the ocean waves batter the weakened soil, the marsh eventually turns into stinky, dying mudflats.

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“We discovered that nutrient enrichment was turning the marsh into a muddier place,” Deegan says. “Instead of near continuous bands of marsh grass, we now had marsh edges that were lumpy, with large patches of bare mud where marsh grass had been just a few years ago.”

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Collapsing marshes present not one but three problems. First, marshes act as storm and flood protectors. They absorb the incoming currents—the waves sink into the marsh rather than smash into houses and structures, which is becoming an ever-more important aspect of life because of climate change and extreme weather events. Overfertilized, nitrogen-polluted marshes turn to mud flats that can’t absorb the tides. In the era of sea level rise and increasingly severe hurricanes, the marsh destruction problem is more dangerous than ever—because destroyed marshes exacerbate coastal flooding. The second problem is that marshes serve as nurseries for fish and other creatures, and when these populations collapse so do our food sources—no more shrimp cocktails, crunchy crabs or fried fish that feeds on marsh-dwellers. And the third problem is that healthy coastal marshes are a very efficient carbon sink—more efficient per square inch than forests, thanks to all the thick heavy grass and other plants that sprout up, sequestering carbon in their biomass. But if the plants die out leaving behind dead muddy banks, all that carbon quite literally goes “puff.” The dead plants rot, decompose and dissolve into carbon dioxide, the greenhouse gas that’s heating up our planet.

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So when we continuously take nutrients from some parts of the planet, and discharge them in others we cause two problems at once. We deplete soils in some places and over-fertilize waterways in others. What we completely forget in the equation is that our waste, the potent fertilizer we produce regularly, goes to fertilize all the wrong places—not farm fields, but rivers, lakes, marshes and the ocean. And because we don’t ship our shit back to where the food came from this problem will only get worse. Your poop may not immediately endanger you germ-wise, but it still comes back to bite you in the rear.

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There is no doubt that sewage disposal methods in modern industrialized countries are more hygienic overall than they were centuries ago. Our waste doesn’t decay in pit latrines.  It doesn’t leech into drinking water. We no longer have cholera epidemics. But this modern, sophisticated sewage disposal methods introduced new, more nuanced problems. Can the solution be found in growing food locally and supplying city residents with poop composting bins? Or should we load our sewage into barges and ship it to Florida and California to scatter on farmland? If oil tankers can ship oil across the ocean, why can’t shit tankers ship shit? Or can we do something even better?

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That’s exactly what the visionary shit thinkers are trying to come up with. In 2011, the Bill and Melinda Gates Foundation issued a “Reinvent the Toilet Challenge” asking the world’s brightest minds to redesign our privies to recover valuable resources from the waste. Several universities won the funding, and the challenge triggered a wave of innovations in the sanitation realm. Scientists, engineers and entrepreneurs all over the globe began looking at excrement not as waste, but a resource.

There’s no single one-size-fits-all remedy for our sewage problem, but some poop-upcycling efforts are already happening on the industrial levels. A Canadian company called Lystek blends sludge into a sewage smoothie that can be dispersed on fields because the process kills all the pathogens. Lystek is exporting this “humanure” tech across the border, to California. In New York City, a leading wastewater treatment plant loads up the Big Apple’s output into massive egg-shaped digestors where bacteria chew through it, generating methane. That output will one day be channeled into Con Edison’s natural gas pipeline. In Washington DC, a plant kills two birds with one poop-stream. It burns the sewage-derived biogas to generate electricity, and then converts the sludge into garden dirt neatly packaged into square bags with cute logos, which local gardeners can buy as organic fertilizer. Meanwhile, Metro Vancouver treatment plant is testing a different approach altogether: converting sludge into a form of crude oil.

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There’s really no end to what this versatile and fully renewable resource can do. So why hasn’t every municipality upcycled its residents’ excrement into something that benefits humans or the planet? The problem quite often is in the humans themselves. We like to act as if our shit doesn’t stink—or that we even don’t produce it. Most of us are happy to flush and forget. The sooner we admit that our daily output is a cheap and fully renewable resource, the sooner we’ll put it to good use globally, as a society. Scientists, innovators, and ecologists are already making it happen. The rest of us just need to stop holding our noses and dive in.

a light blue book cover, featuring 
an almost-empty roll of toilet paper
University of Chicago Press
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