While many of us simply flush and forget about it, our pee has the potential to be a valuable resource. Urine contains the nutrients nitrogen, phosphorus, and potassium—a trio known in agriculture as NPK—in soluble forms that plants can take up, similar to synthetic fertilizers widely used in crop production.
As it’s currently managed, though, the world’s pee is not doing us nor the planet a service. Only seven percent of the world’s wastewater is fully treated to remove nitrogen, and when the urine-tinged effluent is released into water bodies it acts as a pollutant, causing algal blooms and fish die-offs. About 80 percent of wastewater globally goes back into the environment totally untreated, contributing to the spread of disease.
Making synthetic fertilizer is heavily reliant on finite resources and fossil fuels. Transforming nitrogen from a gas—the type that makes up more than 70 percent of the air—to a soluble compound that plants can use relies on fossil fuels; it’s estimated that this reaction, known as the Haber-Bosch process, consumes about one percent of all the energy produced in the world. Phosphorus and potassium, meanwhile, are minerals with limited reserves. Though estimates vary, some scientists say we could reach “peak phosphorus” around the year 2030, after which production will decrease as the mineral’s reserves diminish and decrease in quality.
That’s why momentum for recycling our own liquid gold is growing: upcycled pee could readily sub in for conventional fertilizers.
At the Rich Earth Institute in Vermont, researchers have been collecting urine from nearby residents and running field trials. The institute is tapped into a community network, where about 200 people contribute with the help of special toilets that separate urine from poo. In tests comparing urine, synthetic fertilizer, and no-fertilizer controls, they’ve found no difference in yields from hay fields fertilized with pee and conventional fertilizer. “[Urine recycling]’s addressing two problems at once,” says research director Abraham Noe-Hays: wastewater’s surplus of pee and agriculture’s need for nitrogen. “And it’s a circular economy when you’re reclaiming urine, because what you used to think of as waste is now actually the raw material.”
Urine-collecting toilets and urinals have been available for decades. However, they aren’t without their problems—they can get stinky and produce a massive volume of pee that can’t easily be stored, says Jenna Senecal, a researcher studying urine-fertilizer production at the Swedish University of Agricultural Sciences. A family of four pees about 13 gallons a week, and that can add up to millions of gallons every year in a big city. That’s why Senecal is a part of a team developing a potential solution: a urine evaporation system.
Concentrating urine through evaporation is harder than just warming it up, though. When we pee, the nitrogen is mostly contained in the compound urea. But enzymes in the pee convert that urea to ammonia, which readily volatilizes, forming a gas and wafting away. Senecal is working on an evaporating system that uses an alkaline material to raise the pH of the pee, which prevents this reaction. Once the pH is above 10, it’s possible to evaporate the water, creating a nutrient-dense pee powder that’s five percent the volume of the starting liquid. Through drying, the team says they’re able to create a fertilizer that’s just as nutrient-dense by mass as a typical conventional fertilizer.
This dry pee can be turned into pellets that are sold to farmers. Senecal recently launched the company Sanitation360 to bring dehydrated pee to the market. “We already have quite a big interest,” she says; She has already been approached by an oat milk producer and a beer company. If Sanitation360′s soon-to-debut toilet design takes off, it seems there’s a market for food producers looking to grow greener.
There are also several other ways to concentrate pee. At the Rich Earth Institute, Noe-Hays is working on a freezing-based method. When pee in a tank is chilled, the water portion freezes separately, making it possible to drain away more-concentrated urine. The final material has 90 percent less volume, while retaining 90 percent of its nutrients.
So far, research suggests that pee-cycling does indeed have a net environmental benefit compared to current conventional wastewater and fertilizer systems. In a life cycle analysis comparing a traditional wastewater system to a urine-diverting system, Stephen Hilton, a sustainable systems researcher at the University of Michigan, found that the latter performed better in a number of environmental measures, including greenhouse gas emissions, energy use, freshwater use, and the potential to fuel algal blooms in water bodies.
The analysis also accounted for the energy and materials that are needed for wastewater plants to remove nitrogen and phosphorus from sewage, and the environmental impact of producing fertilizer from pee compared to conventional processes. “Urine diversion is hopefully one way we could have a more efficient system,” says Hilton.
Perhaps in the future, we’ll not haul out not only bottles, cans, and food scraps to the curbside, but also a bucket of a concentrated pee.