"This project is about turning a waste problem into a resource opportunity," said senior author William Tarpeh, assistant professor of chemical 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, an essential fertilizer component, is typically manufactured through energy-intensive processes concentrated in wealthy nations, leaving developing regions dependent on costly imports. Human urine contains nitrogen equivalent to 14% of global annual fertilizer demand.
The prototype separates ammonia from urine through membrane chambers powered by solar-generated electricity. Waste heat from solar panels, collected via copper tubing, accelerates ammonia gas production while cooling panels to boost efficiency. The system captures the ammonia as ammonium sulfate, a widely used fertilizer.
"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 lead author Orisa Coombs, a Stanford Ph.D. student. "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."
Integrating solar waste heat boosted power output by nearly 60% and improved ammonia recovery efficiency by more than 20% compared to earlier designs. The team's model shows the system could yield up to $4.13 per kilogram of nitrogen recovered in regions like Uganda, where fertilizer costs are high and energy access is limited.
The researchers are developing a larger version with three times the reactor capacity. Beyond resource recovery, the system also enhances sanitation, removing nitrogen that otherwise pollutes waterways and contributes to oxygen-depleting algal blooms. With over 80% of global wastewater untreated, the approach could benefit communities lacking sewage infrastructure.
"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."
Research Report:Prototyping and modelling a photovoltaic-thermal electrochemical stripping system for distributed urine nitrogen recovery
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