Human urine could help feed the world. A Stanford-led team has built a prototype that converts urine into crop fertilizer using solar energy, while also generating electricity and improving sanitation.
The system, described in Nature Water, captures nitrogen in the form of ammonium sulfate, a widely used fertilizer, through a process that integrates waste heat from solar panels. This innovation could reduce reliance on fossil-fuel-based fertilizer production and expand access to both food and power in resource-limited regions.
A Circular Approach to Waste
Nitrogen fertilizer is typically made using the Haber-Bosch process, an energy-intensive method that drives up costs and concentrates production in wealthy countries. Meanwhile, human urine already contains enough nitrogen to meet about 14 percent of global fertilizer demand. Without recovery, much of it pollutes waterways and worsens climate change through nitrous oxide emissions.
The Stanford prototype channels urine through membranes that separate and trap ammonia. Powered by photovoltaic panels, the system uses both electricity and captured waste heat to accelerate ammonia recovery. The end product is ammonium sulfate fertilizer. The team reports that this integration improved solar power generation by nearly 60 percent and boosted nitrogen recovery efficiency by more than 20 percent compared with earlier versions.
“This project is about turning a waste problem into a resource opportunity,” said senior author William Tarpeh, assistant professor of chemical engineering at Stanford. “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.”
How the Solar-Urine Reactor Works
The prototype operates as a photovoltaic-thermal electrochemical stripping (solar-ECS) system. It consists of:
- Three chambers separated by membranes to move ions and trap ammonia
- Solar panels that provide electricity for ion movement
- A copper plate system to capture solar heat and warm the reaction
- An output of ammonium sulfate, ready for agricultural use
Heating improves the process because ammonia volatilization is the rate-limiting step. By keeping the reactor warm, more nitrogen moves across membranes, reducing losses and speeding up fertilizer production. At the same time, removing heat from the solar panels prevents efficiency drops in electricity generation.
Economic and Global Potential
The research team modeled performance across different regions and market conditions. In Uganda, where fertilizer is costly and grid electricity is limited, the system could generate up to $4.13 per kilogram of nitrogen recovered—more than double the potential value in the United States. Because the process is self-powered, it could provide both fertilizer and electricity in rural communities that lack infrastructure.
Lead author Orisa Coombs, a mechanical engineering Ph.D. student, emphasized its versatility: “Each person produces enough nitrogen in their urine to fertilize a garden, but much of the world is reliant on expensive imported fertilizers instead. 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.”
Scaling Up and Future Applications
The team is now building a prototype with triple the reactor capacity to process larger volumes of urine. They also suggest that similar heat integration strategies could benefit other industries, such as wastewater treatment plants that lose vast amounts of thermal energy.
Beyond fertilizer, the system provides sanitation by removing nitrogen from urine, making wastewater safer to discharge or reuse for irrigation. This could be transformative in low- and middle-income countries, where more than 80 percent of wastewater remains untreated and only a small share of people have access to centralized sewage systems.
A Path Toward Sustainable Cycles
By recovering nutrients and producing power at the same time, the solar-ECS approach addresses multiple sustainability challenges. It supports UN Sustainable Development Goals related to hunger, clean water, energy, and responsible consumption. Future versions could become modular units that farmers or communities install wherever sunlight is available.
As Coombs concluded, “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. It’s clean, it’s scalable, and it’s literally powered by the sun.”
Journal: Nature Water
DOI: 10.1038/s44221-025-00477-w
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