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 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."
Co-authors 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 researchers 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.
Source: Liquid gold: Prototype harvests valuable resource from urine
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