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Being able to use human feces as fertilizer could be really helpful for human colonies on other planets. It could also be useful for human colonies on THIS planet! And who doesn’t love recycling!?

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To those of you who saw the Martian and walked away wondering if you could actually use astronaut poop to help you grow potatoes on Mars, the answer is yes. You can also use astronaut poop to grow things back on Earth.

Or...just any person. And while that might sound a little gross, we humans have been using our own feces for fertilizer for thousands of years. And we’re still doing it, through what are called biosolids — solids from wastewater that have been processed to make them safer.

There are a lot of ecological benefits to using biosolids so some experts argue we should be using more of them. But questions about their safety remain, so researchers are still studying how to best recycle our poop. The danger with feces—be it cow or chicken or human—is that they can contain some nasty pathogens.

That’s because many microbes and parasites have evolved to spend their lives in other critters’ guts and move between hosts via the fecal-oral route. Which is... I don’t need to explain that, right?

If nothing is done to kill those off, then they can end up on any crops that use manure for nourishment. And that’s a good way to start an E. coli or salmonella outbreak. So if you want to safely use human poop as fertilizer, you first have to make sure that disease-causing bacteria and parasites are dead.

The traditional way to do that is composting, which is when you let a massive heap of wet waste from living things—you know, vegetable peels, chicken bones, or yes, even poop—just sit around until it’s broken down by bacteria, worms, and fungi. Well, not just sit there - you have to mix it occasionally to aerate it so our decomposing buddies can get the occasional influx of oxygen needed to continue breaking the stuff down. That process generates a surprising amount of heat.

A compost pile can get up to around 71 degrees Celsius—which is enough to at least kill off most of the pathogens, assuming you’re doing it right. Which means, in theory, you could make fertilizer from human poop on Mars, or just at home. And yes, there are people that do this.

There are special toilets for it and everything. But most people in the US leave biosolids production to sewage treatment plants. Commercial biosolids produced from wastewater are regulated by the EPA.

Anything derived from human waste that’s destined to be fertilizer must have a virtual absence of pathogens, according to the agency’s standards. So these biosolids tend to undergo both biological treatments—similar to composting—and other sterilization procedures, though there are several different methods used. A 2017 study estimated that publicly owned sewage treatment centers produce about 7.2 million tons of biosolids every year, half of which makes its way onto farms.

That’s a lot of processed poop. But it’s only about half of the total amount of solids that the facilities produce—the rest remained as sewage sludge, which has to be disposed of through incineration or in landfills. And less than one percent of U.

S. farms fertilize with them, so we could potentially ramp up the use of biosolids by quite a lot. The biggest reason to do this would be that biosolids are so green. Instead of sewage ending up in waterways or landfills, it gets repurposed to resupply depleted soil with nutrients.

Recycling for the win! And pound for pound, biosolids are also generally cheaper to make than synthetic fertilizers. Together, that means they could make the biggest difference in countries that struggle with sanitation.

By composting human waste, countries could make cheap fertilizer and improve the health of their citizens at the same time—a win-win. But they’d also be pretty great in the US, or anywhere, really, because their disposal is costly and harms the environment. So by recycling, sewage treatment plants save money and help Mother Nature.

But while the upsides are clear, there are scientists and organizations that are wary of the idea. The issue isn’t the gross factor—it’s what else ends up down the drain, like pharmaceutical compounds, cleaning agents, and heavy metals. While biosolids are considered heavily regulated, those regulations don’t cover absolutely everything that could potentially harm us.

And studies have shown that such concerns aren’t completely without merit. A 2009 EPA survey found traces of pharmaceuticals, steroids, and flame retardants in various treatment plants’ biosolid samples. And a 2012 study found that earthworms in biosolid-treated soil had taken in pharmaceuticals and other stuff like disinfectants from antibacterial hand soaps.

But it’s unknown if this actually harms the worms, or could harm us. Even the lead researcher on the worm study called the risks ‘speculative’, while the benefits of using biosolids were ‘clear.’ There’s also the risk that biosolids could promote superbugs: microbes that are resistant to multiple or all known antibiotics. Resistance is a concern whenever microbes are exposed to levels of antibiotics below what kills them outright.

And that can happen with biosolids, because many of the antibiotics we take end up in our urine and feces. We know that resistant bacteria are found in sewage and sludge prior to the treatment process to make biosolids—and sometimes, after, depending on the method used. So it’s possible that these bacteria or their genes could be spread when biosolids are used or produced.

But some methods seem to be better at killing these bugs than others, so scientists need more information about what works best and why. That way, they can find ways to recycle our poop without contributing to the growing problem of antibiotic resistance. So, we just have to collect more.

Data - not poop. We’ve got plenty of poop. There are also efforts to find other safe uses for biosolids, like a coalition of agencies in the San Francisco Bay area looking into using them to generate electricity.

Biosolids could be turned into biofuels, cutting down on our need for oil, or even be converted into building materials for some seriously eco-friendly housing. No plans in the works for Martian compost piles, yet, though. I guess we’re just going to have to wait for Elon Musk on that one.

Thanks for watching this episode of SciShow! If you want to better understand sewage treatment, you might want to check out our episode on what happens to your pee and poop after you flush. [♩OUTRO].