Liquid gold: Prototype harvests valuable resource from urine

A newly developed system transforms human waste into a powerful tool for profitable and sustainable energy and agriculture in resource-limited regions. The prototype, outlined in a Stanford-led study published Aug 19 in Nature Water, recovers a valuable fertilizer from urine, using solar energy that can also provide power for other uses. In the process, the system provides essential sanitation, making wastewater safer to discharge or reuse for irrigation.

“This project is about turning a waste problem into a resource opportunity,” said study senior author William Tarpeh, an assistant professor of chemical engineering in the Stanford School of Engineering. “With this system, we’re capturing nutrients that would otherwise be flushed away or cause environmental damage and turning them into something valuable—fertilizer for crops—and doing it without needing access to a power grid.”

Nitrogen is a key component of commercial fertilizers. Traditionally, it's produced using a carbon-intensive process and distributed globally from large industrial facilities, many of which are located in wealthier nations resulting in higher prices in low- and middle-income countries. Globally, the nitrogen in human urine is equivalent to about 14% of annual fertilizer demand.

The prototype separates ammonia – a chemical compound made up of nitrogen and hydrogen – from urine through a series of chambers separated by membranes, using solar-generated electricity to drive ions across and eventually trap ammonia as ammonium sulfate, a common fertilizer. Warming the system—using waste heat collected from the back of photovoltaic solar panels via an attached copper tube cold plate—helps speed up the process by encouraging ammonia gas production, the final step in the separation process. Solar panels also produce more electricity at lower temperatures, so collecting waste heat helps keep them cool and efficient.   

“Each person produces enough nitrogen in their urine to fertilize a garden, but much of the world is reliant on expensive imported fertilizers instead,” said Orisa Coombs, the study’s lead author and a Ph.D. student in mechanical engineering. “You don’t need a giant chemical plant or even a wall socket. With enough sunshine, you can produce fertilizer right where it’s needed, and potentially even store or sell excess electricity.”

The study shows that integrating the heat generated by the solar panel to warm the liquid used in the electrochemical process and managing the current supplied to the electrochemical system increased power generation by nearly 60% and improved ammonia recovery efficiency by more than 20%, compared to earlier prototypes, which did not integrate these functions. The use of this waste heat is especially promising because there is a lot of it: about 80% of the sun energy that hits solar panels is lost, which could otherwise cause system overheating and efficiency slowdowns.

The researchers also developed a detailed model to predict how changes in sunlight, temperature, and electrical configuration affect system performance and economics. The model showed that in regions such as Uganda, where fertilizer is expensive and energy infrastructure is limited, the system could generate up to $4.13 per kilogram of nitrogen recovered—more than double the potential earnings in the U.S.

The researchers believe the approach could scale to help farmers and communities around the world. Lessons learned about integrating solar panel waste heat could also be applied to industrial facilities, such as wastewater treatment plants, capable of capturing heat produced during electricity generation to power a range of applications.

Coombs is working on a prototype that will have triple the reactor capacity, be capable of processing significantly more urine, and will process faster when more sunlight is available.

Beyond the potential for harvesting a valuable product and generating energy, the approach holds the promise of effective sanitation. More than 80% of wastewater goes untreated – much of it in low- and middle-income countries, according to the UN. Nitrogen in wastewater can contaminate groundwater and drinking water sources, and cause oxygen-depleting algal blooms that kill aquatic plants and animals. By removing nitrogen from urine, the prototype system makes the remaining liquid safer to discharge or reuse for irrigation. The ability to do this with a self-powered system could be a game changer in many countries where only a small percentage of the population is connected to centralized sewage systems.

“We often think of water, food, and energy as completely separate systems, but this is one of those rare cases where engineering innovation can help solve multiple problems at once,” said Coombs. “It’s clean, it’s scalable, and it’s literally powered by the sun.”

Coauthors of the study also include Taigyu Joo, a postdoctoral scholar in chemical engineering at Stanford; Amilton Barbosa Botelho Junior, a postdoctoral research fellow in chemical engineering at Stanford and the University of Sao Paulo, Brazil at the time of the research; and Divya Chalise, a postdoctoral scholar in mechanical engineering at Stanford.

Tarpeh is also an assistant professor, by courtesy, of civil and environmental engineering in the Stanford School of Engineering and the Stanford Doerr School of Sustainability; a center fellow at the Precourt Institute for Energy; and a center fellow, by courtesy, at the Stanford Woods Institute for the Environment.

The study was funded by the Knight-Hennessy Fellowship, the National Science Graduate Research Fellowship, a Global Health Seed Grant from the Stanford Center for Innovation in Global Health, the Camille Dreyfus Teacher-Scholar Award, the Stanford Sustainability Accelerator, and the Fundação de Amparo à Pesquisa do Estado de São Paulo and Capes.

The researchers’ work to convert urine into fertilizer was supported by the Stanford Sustainability Accelerator in its first round of grants in 2022. The team built a lab-scale electricity-driven reactor that extended to 40 days of operation, which inspired and enabled work on pairing electrochemical water treatment with solar panels. The earliest iterations of this project focused on recovering nitrogen and sulfur from wastewater to enable water reuse and fertilizer production, and was supported by the Stanford Woods Institute for the Environment’s Environmental Venture Projects program.