This is a transcript of the Gastropod episode, Good Shit: How Humanure Could Save Agriculture—and the Planet, first released on June 20, 2023. It is provided as a courtesy and may contain errors.


MIKALA MINN: This is Mahele Farm. And yeah, let’s go take a walk. Go see what you guys want to see.


MINN: We won’t, we won’t even worry about the niceties here. We’re just going to go straight to the shitter.


CYNTHIA GRABER: Doesn’t smell like anything. [SNIFFING]

MINN: [LAUGHS] That’s the key.

GRABER: It smells a little bit like, earthy.

MINN: Yeah. Which is great.

GRABER: I mean a little like, woody, kind of.

MINN: Yeah, so I mean, I’m not over here like spraying it with Febreze before you guys got here. You know. It’s like, it’s just, that’s, that’s like—it’s almost like more stink in like a normal toilet, you know? [LAUGHS]

NICOLA TWILLEY: Welcome to Gastropod, the podcast where we go straight to the shitter.

GRABER: Or you know, to the contraption that’s making fertilizer to help plants grow on a farm. Because yes, Gastropod is as usual the podcast that looks at food through the lens of science and history. I’m Cynthia Graber.

TWILLEY: And I’m Nicola Twilley. And so on one level, there’s an obvious connection between food and poo—we eat, and then you know, stuff comes out the other end.

GRABER: The other, maybe, not-so-obvious connection, at least if you live in the city, is that farmers since time immemorial have used animal poop to help plants grow, and manure is still sprayed on farm fields today. So, could our poop serve the same function, would it help our food grow?

TWILLEY: Should I be taking a dump in my raised beds to fertilize my tomatoes? With apologies to my neighbors, of course.

GRABER: But then wouldn’t we get sick? I mean, Nicky, of course, your poop comes out perfectly clean. But we’re all told to wash our hands after we go to the bathroom, and we know that not washing our hands means that people could end up puking, so there’s clearly a good reason to flush it far away and treat it and hope we never see it again, right?

TWILLEY: Well, you and lots of public health experts might think that, Cynthia, but as our listeners now all know, there are lots of problems with our current fertilizer situation—which, side note, if you haven’t listened to our recent episode on phosphorus, now’s definitely the time! The idea is, if we could figure out how to use it safely, maybe our poo could be a substitute for synthetic fertilizer?

GRABER: That’s exactly what we’re getting to the bottom of this episode, pun very much intended. Just FYI, listeners, we will be using the word shit a lot this episode to refer to, well, shit. This episode is supported in part by the Alfred P Sloan foundation for the public understanding of science, technology, and economics as well as the Burroughs Wellcome Fund for our coverage of biomedical research. Gastropod is part of the Vox Media Podcast Network, in partnership with Eater.


LINA ZELDOVICH: So I had somewhat of an unconventional upbringing. I grew up on my grandfather’s farm in the former Soviet Union. And my father had two degrees, one in agriculture and one engineering, and he sort of combined both in one in terms of taking care of our farm.

TWILLEY: This is Lina Zeldovich. She’s a journalist and the author of a recent book called The Other Dark Matter, the Science and Business of Turning Waste Into Wealth and Health.

ZELDOVICH: Every fall, he would open up our septic system. He would put on his special suit, big boots, heavy gloves, take two massive buckets and unload the content of that septic system onto our farm. Some of it went underneath the fruit trees, but most of it went into his rotating compost beds.

TWILLEY: He would let it sit for three years with some kitchen scraps and dead plants. And then in the spring he would open it up.

ZELDOVICH: It was amazing. There was nothing left in there from the original stuff. No plants, no sewage, no nothing. Just rich black dirt, full of earthworms. And it smelled so good. It smelled of the classic garden dirt. Smelled of the forest floor. And it smelled of the next harvest. And I would sniff that dirt and think about how much food it was going to grow for us. My grandfather used to say, you have to feed the earth the way you feed people. And it was such a beautiful statement to me. And it made so much sense. It was that beautiful circle of life. We take from the earth. We give back to the earth.

GRABER: Lina’s family used all this human-created compost, because they grew a lot of their own food. And this was pretty important in Soviet Russia.

ZELDOVICH: Food was a real challenge in the former Soviet Union. If you wanted to have apples, you had to grow them. If you wanted to have pickles in winter, you had to grow them in summer. If you wanted to be able to have preserves, you had to grow your strawberries. It was crucial that you could harvest enough to feed your family.

TWILLEY: Lina grew up thinking this whole human waste-fertilizer system was normal.

ZELDOVICH: I thought the whole world did the same. Because otherwise what would they eat? Even people who lived in the big apartment buildings had to have a similar system in my view of life, because otherwise where would they get food? So imagine my shock when I grew up and realized, no, that’s not what people do. They just flushed the toilet.

GRABER: Most of us in wealthy countries didn’t grow up the way Lina did.

TWILLEY: I know I was one of those people who grew up thinking just flushing the toilet was normal!

GRABER: Me too, and, for toilet flushers like you and me and probably most of our listeners, human waste seems like waste, not future food. But actually, Lina’s family’s system is one of the original ways that nutrients have circulated back to plants—including the plants that become food.

TWILLEY: Let’s back up for a minute here. The first thing to understand is that plants need nutrients from the soil to survive and thrive. They make the sugars they consume to grow in-house—that’s what’s going on with photosynthesis. But they also need to take in minerals from the soil to be able to photosynthesize and build cells and stay healthy.

GRABER: The three most important minerals for plants are nitrogen, phosphorus, and potassium—NPK. If there aren’t enough of those minerals in the soil, the plant won’t grow well—it may not grow at all. And plants use a lot of them. Hungry crops like corn or wheat can quickly use up nitrogen and phosphorus and potassium. At which point, if you want to eat those crops, you have to add those nutrients back. Which is where poop comes in.

TWILLEY: So what is poo, and why is it so good for growing food?

GRABER: Well, most of it is water—more than two thirds of what comes out each time you poop is water. Plants do need water, but it’s actually what’s in that solid third that matters.

BRYN NELSON: Depending on where you are around the world, probably about 30 to 40% is microbes. Living and dead.

TWILLEY: Drink! Because as we know here at Gastropod, your gut is full of microbes and some of them exit through the rear end every time you take a shit.

GRABER: And once you’re done drinking, we should introduce Bryn Nelson. He is also a journalist, and the author of yet another recent book on human excretions. Two books on number two. Seems appropriate for this episode.

TWILLEY: Bryn’s book is called Flush: the Remarkable Science of An Unlikely Treasure. So, apart from microbes, what else does this treasure consist of?

NELSON: You also have plant material. And this is both soluble and insoluble fiber that comes from what we eat. Also a bunch of minerals. So, so phosphorus is one of those, as well as other essential nutrients and minerals and vitamins.

GRABER: We know phosphorus is important for plants, but there’s other great stuff in there too. There’s a little nitrogen, there’s a little potassium, because our bodies don’t use all the nutrients we eat, so we excrete them.

TWILLEY: Just like us today, early humans also had all of this plant food goodness in their poo, and they would have noticed, as they moved around the landscape, that wherever they relieved themselves, things grew really well

GRABER: Even Lina noticed this when she was a kid.

ZELDOVICH: When we vacationed in the middle of the forest, in, in, in cabins where all of our sanitation facilities were just latrines. Kind like a little bit on the outside of that little campsite. When we went berry picking the best and the tastiest and the hugest berries were always around the latrines. So we kind of, we just knew, you know, empirically, that that’s where the best stuff grows.

TWILLEY: And once humans noticed this, they started being more strategic about their shit. Basically, wherever people were farming crops, human waste was seen as valuable manure—humanure. And all over the world, people came up with more or less complex ways of gathering it and composting it for use in the fields.

NELSON: One of the really interesting examples is in the Amazon. And in this case there are regions they’re called terra preta de indo, or black soils, black Earth. It depends on how you interpret it. But basically these are parts of the Amazon that are unusually fertile. And so this was kind of a mystery for researchers for a long time. They couldn’t figure out why that was.

GRABER: Archaeologists eventually determined that people in the Amazon managed and fertilized parts of the rainforest for farming. A couple thousand years ago, they were mixing their poop in with food scraps and charcoal and creating these mounds of super fertile soils.

TWILLEY: Around the same time, ancient Romans used their waste, too, there was a whole profession devoted to collecting and processing sewage. But one of the most well-developed and most intricate systems to use human waste as humanure was developed in Japan, because there wasn’t a lot of great farmland there.

ZELDOVICH: It doesn’t bear fruit as easily as, let’s say, the European soils that had a lot of forests. Where land was naturally much more fertile. So these societies figured out early on that if they had to survive, and thrive, they had to perpetuate that cycle of taking nutrients out and putting that nutrients back in.

TWILLEY: By the early 1600s, in cities like Osaka, and Tokyo or Edo as it was called then, the collection of human waste was big business.

ZELDOVICH: The Japanese call it shimogoe. Which meant, literally meant, fertilizer from the bottom of a person.

TWILLEY: The non literal translation is nightsoil, that’s what English speakers generally called it.

ZELDOVICH: People produced night soil during the night; in the morning out goes the bucket. So all these shimogoe collectors, which was a business, gathered it from Osaka’s neighborhoods, brought it to the port in buckets and carts, loaded it up into boats and shipped it to the farmers who used it to grow food.

GRABER: The farmers didn’t just take the waste and dump it on crops, they composted it for a while, and this killed any pathogens, any diseases in the waste. And then they sold it.

NELSON: Basically that was a commodity that was so highly valued that people could end up in jail if they stole it.

ZELDOVICH: The price of shimogoe produced by 10 households in a year was half a of ryo of gold. And to put that in perspective, one ryo could buy all the grain needed to feed one person for a year.

TWILLEY: Because it was so valuable, there were all sorts of rules and customs about all kinds of different… shall I say, shit uations!

GRABER: Wow, they just keep coming.

ZELDOVICH: For example, if a family rented a house: who had the rights to the excrement, the tenants or the landlord? Like you’d think it’s logical that the tenants who produced it should have been the proud owners of their poo. But nope. These riches belonged to the landlord who sold it to farmers. And the rent went down if there were more people living in a dwelling because they produced more poo.

TWILLEY: Hence the traditional Japanese saying, ‘the landlord’s child is brought up on dung’

GRABER: And then there’s a particularly Japanese idea of a hostess gift…

ZELDOVICH: In the Japanese culture, there was this really interesting tidbit that was just so telling. It was polite when you would come to your friend’s house, and eat their dinner. And at the end of that dinner you would go to their toilet and leave a gift of your poo. And people who didn’t do that were considered not very polite. It was a good thing to do, a friendly thing to do.

TWILLEY: In historical documents from 1600s and 1700s Japan, there are records of big disputes over collection rights and prices, and even nightsoil heists. That said, although it was all valuable, not all poo was created equal.

ZELDOVICH: The excrement collected from the richer parts of the city usually was sold to the highest bidder. Because the people who lived there, who had more money, supposedly had better diets. So they had more nutrients in their poo. And you know, those who were considered, you know, less fortunate didn’t produce poo as good. Notably animal dung was sold also, but it’s usually a lot cheaper.

TWILLEY: This is not just shit snobbery. Modern science has shown that a more protein rich diet increases phosphorus levels in your waste, and of course it was the wealthy who could afford to eat more protein.

GRABER: This kind of shit talk didn’t just happen in Japan. Neighboring China thought that their citizen’s waste was worthy of the emperor’s personal attention.

ZELDOVICH: In 1737, the Chinese emperor issued a decree. It read, every household should collect night soil and treasure it, as if it were gold. And at the time, China had some of the largest cities on earth. So if you didn’t remove all that you’d be drowning in shit. And the land just would not be able to feed everyone.

GRABER: No one knows how far back organized nightsoil collection goes in China, but as early as nearly four thousand years ago, the ancient Chinese character for shit was literally the symbol for a human body with the symbol for rice growing beneath it.

TWILLEY: European cities also had teams of night soil collectors, but honestly, nothing reached the organizational perfection of Asian poo collection systems.

GRABER: Part of the reason Europeans weren’t as good at this as their Asian counterparts wasn’t just because they couldn’t get their shit together. It turns out they were lucky. They had better soil. Much of continental Europe also had a lot of forests, and they could cut down those forests if they needed fresh land to grow food on.

ZELDOVICH: So they had the luxury of seeing their excrement as this ultimate filth and not wanting to use it. With a very few exceptions. They would use cow manure, but they wouldn’t use their own shit.

TWILLEY: In most of Europe, people paid to have their shit taken away, rather than being paid for it. But there were exceptions, like in the Low Countries, what’s now Belgium and The Netherlands, where there were large populations being supported on small amounts of land.

GRABER: In fact, the servants in Ghent, in what’s now Belgium, they were paid for all the human waste they could collect. And that sometimes literally doubled their wages.

TWILLEY: Across the pond, North America, like Europe, was a big continent with a lot of natural bounty and fertility in its soils and forests and so shit was not generally at a premium.

GRABER: One interesting exception was New York City in the 1830s. A company in New Jersey picked up waste there and processed it in New Jersey, because of course.

ZELDOVICH: There it was dried, deodorized and turned into a fine powder, charmingly called peurette. French for fine dust. That dust was then mixed with other organic substances, including like Peruvian guano or bird shit, which is also very good fertilizer. And then it was shaped into briquettes and shipped to farmers.

GRABER: One reason this was particularly successful at the time was that Long Island, which is right next to New York City, it was mostly farmland. But it had sandy soil that was pretty crappy for growing produce, and so farmers there were kind of desperate for all that compost.

TWILLEY: Our point is: all the way through history, humans have been using nightsoil to fertilize their fields. So here’s my next, potentially stupid question. If human waste is so awesome, and has been so useful for so long, why aren’t we using it as fertilizer today? Or are we and I just don’t realize it?

GRABER: That answer, coming up after the break.


GRABER: Today, as you listeners know, for most of us in the western world, when we need to relieve ourselves, we go to the toilet, and then we flush it.

ZELDOVICH: When you push that lever or push that button, your poop goes down the drain, then flows through miles and miles of sewage pipes. Definitely for hours, and sometimes for days, depending on how close the nearest sewage treatment plant is.

TWILLEY: I have had the particular joy of touring two or three wastewater treatment plants in my time, including the legendary annual Valentine’s Day tour of New York City’s Newtown Creek Wastewater Treatment Plant. A lot of nifty stuff happens in these places—but what it boils down to is that basically water is reclaimed and treated, and then the solids are aerated so that bacteria can break down a lot of the organic matter. And then, you’re left with sludge. Sometimes affectionately known as biosolids. And that sludge meets a variety of fates, depending on where you are.

NELSON: Well a lot of our poop is either burned or buried. So we incinerate a percentage of it. We put another percentage of it into landfills. That’s probably about, I think combined, about 40%.

GRABER: In the past it was even dumped in the ocean. Today, we do use more than half of it to fertilize golf courses, parks, and farmland, but at the end only about 15 percent of it actually fertilizes food crops.

TWILLEY: Most wastewater treatments plants have to pay a contractor to take the sludge away, even if it’s going to be used as fertilizer, and most farmers don’t pay to use it—they get it for free. So even though some of our shit is still being used as fertilizer, that idea—that this is waste that has to be paid to be disposed of—that is a very different paradigm than the idea that shit is so valuable that it’s rude to go to dinner somewhere and not take a dump. What happened?

GRABER: Well, as we’ve said, if you didn’t desperately need a lot of fertilizer, poop could quickly become a problem rather than a solution. That started happening as soon as people stopped living nomadic lives and settled down in one place.

ZELDOVICH: Shit began to pile up. At first it was probably slow. In little villages it was still manageable and people went out into the bushes, into the forest. But when they started aggregating in cities, shit really hit the fan. One of the really early places when the humans probably really felt the impact of it was in Knossos, which actually predates ancient Greece. It had close to a hundred thousand people. And given the fact that an average person produces about a pound of poo a day, it’s like, a hundred thousand pounds. It’s like a mound of shit.

TWILLEY: Like we said, in some places, like Japan and China and the Low Countries, people needed this shit enough to figure out how to deal with it. In other places, like that Greek city state of Knossos, this constant flood of poo seemed overwhelming—and it was easier just to wash it out to sea.

GRABER: Another thing is you all know the saying, don’t shit where you eat! Shit has a lot of bad shit in it. The usual suspects are bacteria like salmonella and E. coli, viruses like noro, parasites, and other diseases like giardia.

ZELDOVICH: Human waste does cause disease. Left outside to its own devices, a pile of poo begins to endanger humans almost immediately. Even in the antiquity, people knew that human waste can spread disease. And then as science and medicine developed and we learned about microbes, poop got a really bad rap. And then we learned to distance ourselves from our waste as quickly and efficiently as possible.

GRABER: In places like Osaka and Edo in the 1700s, they learned to kill those diseases and parasites using time and heat: basically, by letting it sit in a compost pile until it was safe.

TWILLEY: But In cities that weren’t up to the challenge of organized waste collection, shit piled up in the streets and in pits. Sometimes it was taken away by pipe, often it was moved using water, and ultimately it usually ended up washing into rivers and then the ocean.

GRABER: In the 1800s, this became a particular issue in London. At the time, London had industrialized, and it was the largest city in the world. There were actually some toilets around to use, but there might be up to 100 people using any one toilet. Sewage overflowed into the streets and then into the Thames.

ZELDOVICH: Which eventually caused the infamous Great Stink of London. Because the river got so polluted with excrement that not only was chock full of it, but the boats run aground and stuck in it. The water was so black some newspaper editor once tried using it as ink, and it worked. It was a scandal. It was such a problem that a royal commission of the metropolitan sewage discharge was formed to deal with it. It was led by this guy, Sir George Bramwell, who together with a bunch of other sirs, got onto a boat and sailed up the Thames to see, exactly how bad was it? And it was bad.The sewage was so thick that at first they were actually looking for glimpses of water in it.

TWILLEY: One of the reasons this scandalous stink got so much attention is because the Houses of Parliament is right on the river. And so Britain’s politicians were personally invested in finding a solution to the quote unquote, “odor problem.”

ZELDOVICH: So there was a lot of talk about how to fix it and multiple solutions were discussed. The most mind-boggling thing to me was that they actually knew it was a valuable fertilizer. They even put a price tag on it. They wrote that the output of about 4 million Londoners would be worth almost 2 million pounds annually.

TWILLEY: That’s at least 250 million dollars in today’s money. For shit. There was a whole group of Victorian humanure stans. They called themselves the Sewage Doctors. Karl Marx, of Marxism fame—he was one of them, and he said that flushing feces into the river was like throwing gold sovereigns into the Thames.

GRABER: But the Great Stink was REALLY stinky. And there was another thing happening in London at the time that added to the urgency. There were continual outbreaks of cholera, and a doctor named John Snow was the one to help people understand it was spread in water, by feces, and so the British knew they really had to clean their shit up.

TWILLEY: The question was how, and the answer was a sewage system. But the debates were about what flavor of sewage system. The separatists wanted to keep the poo and pee well, separate from all the other wastewater—rainwater, everything else you washed down the plug hole from homes and factories. That way this hugely valuable resource—the pee and poo—wouldn’t get lumped in with everything else and treated like waste. So it could still be harvested and used.

GRABER: The combiners said that separating things out like that was too expensive and too complicated from an engineering perspective and we should just build one set of pipes to rule them all. Those combiners won the day, and London built an expensive new combined sewer system, at the time they described it as a cathedral of sewage.

TWILLEY: Pictures on, I’ve visited it, it really is impressive.

GRABER: This system was an amazing innovation. London smelled a lot better, and the people who lived there were healthier. People from around the world came to tour this new wonder. And they copied it, too.

ZELDOVICH: Oh, let’s make it clear. Our Western flush sanitation systems are in many ways incredibly awesome. They’re the reasons we don’t die from cholera and dysentery. They keep us healthy. Without them, we literally wouldn’t last a day.

NELSON: But I think that was probably part of the, the genesis of this shift, from useful thing to ew, gross, get it away from me.

TWILLEY: That shift—shit’s journey from treasure to trash—it took place in the West in the mid to late 1800s. But the other big thing that really undercut the appeal of human waste was that by the start of the twentieth century, Mr. Haber and Mr. Bosch had done their thing, and we had synthetic fertilizer. If you missed that story, it’s in our phosphorus episode.

ZELDOVICH: It became so easy to produce, that it just put all of this poop populations out of business. Synthetic nitrogen doesn’t leak or stink. You can package it, you can ship it anywhere. It was very convenient. And so poop plummeted from a resource to ultimate waste, it no longer had any value.

GRABER: This is maybe sad for all the nightsoil entrepreneurs but otherwise it seems like things are pretty great, right? Our cities are cleaner, we can buy synthetic fertilizer to take the place of poop, it’s all good. But it turns out, there are some problems. The nutrients in synthetic fertilizer wash out of the soil more easily than the nutrients from poop, and synthetic fertilizer doesn’t do everything that poop does. And soils are being kind of drained of the things plants need.

NELSON: And so by withholding our nutrients and basically doing something completely different than what nature has done for millions of years, we’re… interrupting the, the recovery and, you know, the re-accumulation of a lot of these nutrients in the soil.

TWILLEY: In previous episodes, we’ve talked about what happens when all that synthetic fertilizer washes out of the soil. You get toxic algal blooms and it is an environmental disaster.

GRABER: Plus it’s really energy intensive to make nitrogen out of thin air, that’s what we do to make that nitrogen. And when it comes to phosphorus, it’s a bottleneck. There’s only so much in the world, and so far we’re okay, but we could be facing shortages in the future.

TWILLEY: Meanwhile we use a ton of water and a ton of energy to flush all our shit away and treat it, and that’s expensive and not super sustainable either. So it’s not as though the modern way of doing things—using synthetic fertilizer and treating shit like shit—its not actually all that great.

GRABER: Well so could we go back to the way we used to do things? Is there a way to see shit as valuable again?

TWILLEY: This is not a scientific poll, but I have noticed that if you talk to friends about the idea of using shit to grow food—as I have been doing while we’ve been on this whole humanure journey, sorry friends!—you immediately see that there are a few challenges to reconnecting the shit to food loop. One problem that you listeners might be afflicted with personally is the yuck factor. Most people’s first reaction is that they actually don’t want to eat food that was grown on human shit.

GRABER: So if we do want to grow food with poop, can we get over the yuck factor? Kim Nace is co-founder of the Rich Earth Institute in Vermont and she’s now head of its technology spinoff Brightwater Tools, and unlike you, Nicky, she’s done actual scientific research into just this question.

KIM NACE: Well, when we started the work, we, we used to call it the “ick factor” or the “yuck factor.” And somewhere in one of the research projects we did, we realized that it was more of a giggle factor than an ick factor. It’s more of an embarrassment than an, than an ick.

ZELDOVICH: The yuck factor. The yuck factor is very much learned. If you, you learn to look at it differently and know that it actually works for you. I think the yuck factor is reasonably easily overcome.

GRABER: After all, as Lina pointed out, being so utterly grossed out is a pretty recent phenomenon—remember, for most of history and in a lot of places, nightsoil had value.

TWILLEY: Okay, but beyond any ick factor, something people are concerned about is pharmaceuticals. We take all sorts of drugs, and they’re often designed to have more of the drug than we need because pharma companies know that we’re going to pee some of them out when we go to the bathroom.

GRABER: Again, the humanure experts are one step ahead.

NANCY LOVE: We’ve done a lot of work on this. We’ve done field trials on this.

GRABER: This is Nancy Love, she’s an engineer at the University of Michigan.

LOVE: So, we know how to remove the pharmaceuticals. We know what happens to them. We’ve applied urine-derived fertilizers, with or without pharmaceutical removal on lettuce and carrots. And then we analyze the pharmaceuticals in the lettuce and carrots. Carrot peels versus carrot cores, things like that. But even that dosing you would have to eat, I don’t know how many salads a day, you know, to even get the equivalent of like one acetaminophen pill.

TWILLEY: Nancy told us this issue of drugs is our waste is not necessarily a big deal, and that’s a conclusion that’s based on quite a bit of research. But the thing you do see headlines about is these things called forever chemicals, and how they’re in our wastewater and our sludge and then they get into our food. They’re called PFAS—that’s P-F-A-S—and they’re a whole family of very long lasting, hard to break down chemicals.

GRABER: These chemicals are often used to make fabrics fire-retardant and to make all sorts of things grease- and water-repellant.

PATRICK MCNAMARA: And if you think about it, if they’re made to resist fire fighting foam. And they’re made on non-stick pans to cook on. They’re supposed to not degrade. So what happens when these really solid, sturdy chemicals go in the environment? They don’t degrade. What happens when they get in our body? They don’t degrade. So they get this name, forever chemicals, because they’re not biodegradable.

GRABER: Patrick McNamara is an environmental engineer at Marquette University, and he says these PFAS chemicals are really all around us

MCNAMARA: So if you, if you get fast food and that bag doesn’t break and it’s full of grease, it’s not made of just paper, right? It’s coated with something to hold that wet, greasy burger. If you have makeup like lipstick or foundation or if you have a rain jacket, Gore-Tex, and it beads up. That’s got PFAS on it.

TWILLEY: These chemicals have been around since soon after the second World War. We used to manufacture a lot of them in the US, now we still use a lot but they get made elsewhere. And they’re really not so great for you, which is something the manufacturers knew in the 50s and the EPA realized in the late 90s.

MCNAMARA: These are long-term chronic health effects, potentially cancer, or bladder cancer, or long-term effects to your kidneys. Which is why the EPA sets these lifetime drinking water advisory levels. Like if you drink two gallons of water a day that has this much concentration for this many decades, you might have a problem.

GRABER: Our exposure to PFAS in general is a much bigger issue than we can cover on Gastropod today, but we are interested in understanding what our exposure is when it comes to using waste to grow food. This has been an issue. There was a major problem in Maine where biosolids were spread on farmland and Maine farmers began to find high levels of PFAS in milk and meat. They even had to kill some animals because of it. This seems like a pretty cautionary tale.

TWILLEY: As it happens, the day before we spoke to her, Kim had just received the results of a study on PFAS that focused in particular on urine. She tested urine for the two PFAS chemicals that the government currently regulates.

NACE: It, it was non-detect for PFOA and PFOS in in the urine that we sent out. So we were really excited to get that result, because it’s been a question people have been asking.

TWILLEY: Bigger picture, Patrick told us, it now seems clear that most if not all of the different PFAS chemicals found in treated wastewater and sludge is not coming from our pee or our poo. It’s coming from industrial sources—companies that use PFAS and then their waste water goes to the same treatment plant as our bodily waste.

GRABER: This was true for Maine as well. Research seems to show that the PFAS on crops in Maine came from industrial sources, not from our poop and our pee.
TWILLEY: Weirdly, that’s actually good news.

MCNAMARA: So if 1% of your wastewater flow has 90% of your PFAS, just treat that 1%. Before it gets to the treatment plant. So you don’t have to treat a hundred percent of your biosolids.

GRABER: So that’s what the EPA and local wastewater treatment plants are now focusing on, making sure PFAS don’t get to the treatment plant in the first place.

TWILLEY: Meanwhile, even if there is some PFAS in that combined sludge from our wastewater plants right now, Patrick says it’s likely not as big a deal as our exposure from all of the PFAS laden products we interact with everyday.

MCNAMARA: The concentrations of PFAS are a lot higher in dust, in makeup, in fast food than it is in wastewater, drinking water, and certainly wastewater products like biosolids Or humanure.

GRABER: So the truth is, the things people are worried about with using our waste on food, these seem to be pretty solvable, and they don’t seem to be a reason to give up on this valuable resource. The bigger problem is actually figuring out how to make the whole thing work.

TWILLEY: Aloha composting toilets. We’re diving into the future of shit, after the break.


MINN: The model we have is called a Centrex 3000. It’s kind of the, like, Mercedes-Benz of composting toilets. If you will.

GRABER: When we were in Hawai’i, we took a brief pit stop at a composting toilet. It was on a farm called Mahele Farm, and Mikala Minn showed us around. It just looked like a little building, almost like an outhouse, but built up on a platform, on like stilts. Mikala gave us a tour.

MINN: And so yeah, you can do everything in here. And..

TWILLEY: Even TP, you can put down there?

MINN: Yeah, yeah. It totally disappears.

TWILLEY: We dutifully made our contributions. And then we went back downstairs to see what happened to them.

GRABER: All the waste falls down into a cylindrical container at ground level. And then whoever is assigned compost duty uses a handle to turn the drum around.


MINN: It’s exactly seven turns to, for one rotation. And we do, for, for a number two, you know, we want to rotate it about seven times. So that’s like 50 turns, you know, 49 to be exact.

TWILLEY: The pee and poo gets mixed in with rice hulls and a spritz of lactobacillus microbes to help compost it. Mikala told us he leaves the door open to lure in flies to lay their eggs, because the maggots also help break the poo down. In general, it takes about three months to turn into the final product, which Mikala urged us to sniff.

TWILLEY: So, your humanure doesn’t also doesn’t smell like shit?

MINN: Let’s find out for yourself. Yeah, I don’t think it does. I might just be like, conditioned, but you guys are fresh, so…

GRABER: It really didn’t smell like much, a little woody, a little earthy. And then Mikala spreads it on plants around the farm.

MINN: That’s why these flowers are all beautifully in bloom and, and I just, avocado trees and whatever else.

TWILLEY: Just like in the old-school nightsoil systems or Lina’s family’s set up in the former Soviet Union, the process of composting—time, heat, and some beneficial microbes doing their magic—that turns potentially dangerous, stinky crap into beautiful, safe plant food. Mikala told us that these days you can buy all kinds of models of composting toilets, and with their help you too could be making fabulous fertilizer from your own poo.

GRABER: This is fine if you own your own house and want to invest in an entirely new sewage system and have property on which to spread the compost. But a lot of the world lives in cities. So what should us city folk be doing?

TWILLEY: Yeah, this is where things do get more complicated. Because of the combiners.

GRABER: Remember London’s Great Stink.

TWILLEY: Like it was yesterday!

GRABER: Or just a few minutes ago, in this very episode. The separators—the people who wanted to keep poop and pee separate from all the other wastewater, so it could still be used as fertilizer—they lost, and the combiners won. And so nowadays, our systems dilute all that valuable plant food we’re excreting with a whole lot of other things, some of them toxic, and treat it all together. That’s why the sludge, or the biosolids, that comes out the other…end, even though some of it gets used as fertilizer, it isn’t always the greatest.

TWILLEY: Lina told us that today some cities are trying to innovate on their existing combined wastewater treatment plants to make the most of shit. They’re updating the old-fashioned composting approach with some new technologies.

ZELDOVICH: My absolute favorite was the wastewater treatment plant in Washington, D.C., which gathers the waste of about 2.2 million people, partly from DC, and partly from its surroundings. And they have this amazing system in which all that shit is loaded into these massive pressure cookers called Cambi.

TWILLEY: It gets cooked and digested and heated and all sorts, and in the process they produce a bunch of biogas that they can use for power.

ZELDOVICH: And the end result of what they produce, kind of like this dark, watery muck, is then dried, and packaged into these very cute-looking bags, as a fertilizer, as a branded fertilizer called Bloom. And anybody including you and me can buy it and use it in our backyards or whatnot.

GRABER: This is different from old fashioned biosolids. That gas is a new benefit, and the DC plant produces a higher quality fertilizer, which is why they can sell it rather than having to give it away.

TWILLEY: This all sounds great. Cleaned-up, sweet smelling poo, fresh from our nation’s capital and ready to apply to your backyard—it sounds like the recipe for a fabulous harvest. Finally, poo gets its due. But these systems are very expensive. Not everywhere can afford to install them and make it work. Basically, because combined systems weren’t originally designed with reclamation in mind, getting the nutrients out of our wastewater today is way more inefficient and energy intensive and complex than it needs to be.

GRABER: And that’s where an even more radical solution comes in. Screw the poop, just separate out the pee. Because it turns out that poop isn’t the only player in this whole fertilizer game.

LOVE: Oh gosh, 80% of the nitrogen in wastewater comes from urine,and about 65% of the phosphorus. So the urine definitely has most of the nitrogen and phosphorus. There certainly is some in what we call the blackwater, the solid residuals. But it is definitely, if you separate off the urine, you are capturing the vast majority of the nitrogen and phosphorus.

TWILLEY: When Nancy told us this, we were like: wait, what? Is shit not actually the shit?

GRABER: While she was busy blowing our minds, Nancy told us that the people at Kim’s Rich Earth Institute in Vermont only bother to collect urine.

LOVE: And they are the centerpiece, I’d say, in the United States, of urine separation and repurposing for beneficial reuse. In their case, they have the only community scale urine separation program in the United States.

GRABER: Rich Earth was founded just over a decade ago, and today they have a lot of volunteers.

NACE: We have over 200 people who bring their urine to a centralized location. It’s completely contained, but they open up their container and insert a pump system, and it goes into a really big tank. So you know, we have, you know, thousands of gallons that we’re treating and storing and treating, and then, applying through our with our farmer partners.

GRABER: The folks at Rich Earth treat the urine and they create a super concentrated fertilizer from it. Local farmers use that fertilizer for fields of hay that will eventually feed animals.

NACE: A thousand gallons of urine fertilizes, one acre of hay. That’s about enough urine from eight people if you gather their urine for a year.

TWILLEY: And that hay, when you feed it to a cow, eventually yields a bunch of ice cream, cheese, and other good things.

GRABER: In case you’re thinking, this works for crunchy hippies in Vermont, but it wouldn’t work in my city—well, you should know that the Vermont peecyclers are not alone. There are urine reclamation efforts going on all over the place. On a Swedish island, in a handful of buildings in Paris, New York, and a suburb of Zurich, and there are lots more in the pipeline.

TWILLEY: And you don’t necessarily have to pee into a jar or anything weird. These days, companies make totally normal-looking toilets that just divert the pee before it gets to the water at the bottom of the bowl. Which makes it much easier and, this is key, much cheaper to just collect, concentrate, treat, and use. More and more, cities are thinking that keeping pee out of their wastewater plants is the way forward.

GRABER: For example, Detroit has a huge expensive treatment plant, it was built when the city was a much bigger one. But after some recent upgrades, that plant still isn’t equipped to remove enough of the nutrients like phosphorus, and so those nutrients end up in Lake Erie. And as you might remember from our phosphorus episode, that then makes Lake Erie all shmeary. Or, you know, feeds toxic algal blooms in the lake.

TWILLEY: The folks who run the wastewater treatment plant know they need to remove those nutrients, but retrofitting their current system is way too expensive for Detroit ratepayers, and so now they’re starting to talk to folks like Kim at the Rich Earth Institute about how to implement urine diversion, how to get it out at the source, maybe by collecting and treating it at big apartment buildings and public buildings before it even gets to the wastewater plant. Ultimately, this would just be much more cost effective for them.

GRABER: Because if you get urine out before it even gets to the treatment center, you don’t have to upgrade your facility with expensive new treatment technologies to remove phosphorus—because most of the phosphorus isn’t even showing up. And this type of situation, where we have aging infrastructure, that maybe isn’t built for the amount of waste coming in or can’t treat it appropriately, it’s happening all over.

NACE: That’s one reason why like in San Francisco, they’re mandating that new construction over a certain size has to, has to do onsite wastewater treatment. Paris has done this. They’re actually implementing urine separation because it was the only way they could grow their city. It was the only way they could bring more people and more housing in, that didn’t add more nitrogen load to their plant.

TWILLEY: This is a super cool example: it’s an entire district of Paris in the 14th arrondissement that’s being redeveloped, 600 houses, businesses, public buildings, the whole shebang. It’s under construction now, and they’re focused on pee. Basically it will be diverted using shiny new source-separating toilets, and then concentrated on site, and then turned into fertilizer.

GRABER: Like we said, this kind of pee-focused solution can be cheaper and easier to implement for cities that already have centralized wastewater treatment plants. Collecting poop before it got flushed would be a lot harder, logistically.

TWILLEY: And fortunately, it turns out that like Nancy said, pee is gold. At least in the developed world. In the past, nightsoil was just all the excretions—it was pee and poo mixed together. But actually, pee and poo are not created equal when it comes to feeding plants.

GRABER: So, pee and poop are different in terms of how we get rid of things. Poop is what we haven’t been able to digest. Pee is, yeah, sure we’ve digested food and nutrients, but we don’t use it all, and so then we excrete whatever we don’t use.

TWILLEY: And what that means is that the exact nutrient composition of our poo and pee varies depending on how digestible our diets are. Researchers have found that in the kind of diets we tend to eat in the Western world, where the food is easy to digest, our bodies are able to extract nitrogen and phosphorus from food more easily.

GRABER: And so that means that we don’t poop out the nutrients because we’ve processed them. Instead, we pee out whatever we don’t use. Today, in the west, like Nancy said, most of the nitrogen and phosphorus left over from our food comes out in our pee and not in our poop.

TWILLEY: In other countries, more of those nutrients remain undigested in the feces. So, for example, a study on Chinese pee and poo found that in some cases most of the total phosphorus remained in poo.

GRABER: But Kim told us that, even though in rich countries most of the nitrogen and phosphorus in wastewater is in the urine, and that’s what Rich Earth collects, she doesn’t want to shit on shit.

NACE: It’s… [LAUGHS] it’s, it’s not like an either or, and this is not like a competition between the two parts of what comes out of our bodies. Like, they’re both important and they’re both useful.

TWILLEY: Urine might be about concentrated nutrients, but poo is all about soil health. For one, it contains a lot of carbon—all those dead microbes are organic matter, carbon.

NELSON: And so carbon is really important for the organisms that are living in the soil. Because those are the ones that are contributing to the soil health. So not just the microbes, but all of the other, you know, microfauna, the animals that are living in there in this whole ecosystem. So, so it’s just a much more natural boost for the soil and it’s not just this kind of like, one-off burst for the plants.

ZELDOVICH: The other thing is human shit is naturally sticky. Which means that it becomes part of the soil and it doesn’t wash away. Like, let’s say, synthetic nitrogen does. Human shit has this ability to hold water in it. It has the ability to generally hold different soil particles together and keep all of these fertilizers, nitrogen, phosphorus, potassium, all together, rather than letting it wash out with the nearest big rain.

TWILLEY: So in an ideal world, we’d separate and collect and use both pee *and* poo. Pee is just the low-hanging fruit so to speak. But that’s only true in the developed world.

GRABER: In the developing world, there are many regions that don’t have a functioning centralized sanitation system, and Lina and Bryn told us about humanure innovations taking place all over. In Kenya, in India, in rural Mexico. People are trying out new toilets that are more sanitary than what’s there now and that in a variety of ways can turn human waste into something useful. Sometimes it’s compost, sometimes it’s fertilizer, sometimes it’s a charcoal briquette to cook food.

TWILLEY: In a lot of these places, where there isn’t already a Western-style waste treatment system, with miles of sewage pipes and expensive centralized treatment centers, we don’t need to make the same mistake that the combiners did. If you’re building from scratch, these kinds of new, more sustainable systems, where you can keep pee and poo separate—they make economic and environmental sense.

GRABER: But Nancy said that actually, these kinds of systems make sense in the West too.

LOVE: Here’s how I think about it. We’re going to replace that infrastructure in the next… three decades. We are. Because our infrastructure is beyond age, it’s beyond design life, and we’re replacing it anyway.

TWILLEY: So why not replace it with something better?

GRABER: As we said, it is already starting to happen, but we’re kind of at the pilot stage. Different places are trying out different systems, different technologies—what’s the best option for, say, Detroit, might not be perfect for another city or region. And Nancy says that’s just the right stage for us to be in.

LOVE: We have to be doing the work now to be there in 30 to 40 years. Because what we don’t want to do is replace—when we replace our infrastructure and we have big things like the Infrastructure Act, now we’re putting in 20 year old technology in some cases. We have to advance these more sustainable approaches, so that they can become part of the, the options on the table when decisions are made on, what we’re going to replace that technology with. What we’re going to replace those pipes with. What form are we going to put in place? How are we going to reduce our water use? How are we going to do all these things that make our cities and our livability more sustainable and green?

TWILLEY: And that’s exactly what Kim at Brightwater Tools, as well as a lot of her colleagues around the world, are focused on—super practical designs for new technology that can separate out urine and poo and reclaim the nutrients from them without people needing to make any major change to their normal bathroom habits.

NACE: My vision for how this is going to move forward is that things are going to happen in parallel. So that as we adopt the resource-oriented sanitation, the resource recovery systems, however they look, that will, we’ll still be using the systems that we have now. They’ll just phase out as we get really good at the resource-capturing ones.

GRABER: And the good news is, if we help advocate for these changes, it’s something we can do to help the environment and make our communities more resilient in the face of climate change. And it’s pretty painless, too. We’re just reconnecting a loop — we don’t have to give up anything but our shit, like people have done for centuries, like Lina’s family did in Soviet Russia. And that has rewards beyond just next season’s harvest.

NACE: I mean, the thing you have to think about is not what it would cost to get it done, but what is it costing us now? And the cost now of centralized treatment systems and plants is, is astronomical.

GRABER: And to bring it back to fertilizer, Nancy and Kim have worked with their colleagues to figure out a back-of-the-envelope estimate of how much farm fertilizer our urine could replace—it’s just urine they’ve looked at, not our poop as well, but even just that is pretty solid. Looks like it’s about 30 percent, and that’s significant.

NACE: I think it feels like this is something we can do. We’re always thinking about what can we do less, like you know, consume less or travel less or all those kinds of things to be better stewards of the planet. But with this one, we realized something that’s coming out of our body that we produce every day actually has a benefit to agriculture. And I think that’s very hopeful for people.


TWILLEY: A special thanks this episode to our listener Matt Dupak, a grad student at Marquette University in environmental engineering. He got in touch with us after we mentioned PFAS in our compost episode, and he recommended we talk to his teacher, Dr. Patrick McNamara. Thanks Matt—Patrick is great, and that was a great suggestion! And thanks of course to Patrick McNamara for talking to us!

GRABER: Thanks also to Mihala Minn, Lina Zeldovich, Bryn Nelson, Nancy Love, and Kim Nace, we have links to their books, institutes, and research on our website, We’ll be tweeting and putting links to those resources on our social media too—we’re @gastropodcast.

TWILLEY: Check those links out because there’s lots of practical ideas that you can help make happen. Your new building could have source separating toilets! Your city could require that in new developments! This is an issue where you can really make a difference, so get out there and let’s make shit happen!

GRABER: Oh my god, enough already. Finally, and as always, thanks to our superstar producer Claudia Geib. We’ll be back in two weeks with a brand new episode, ‘til then!