As a scientist bent on addressing the global sanitation crisis, William Tarpeh is not fazed by urine or excrement.
But even he was struck by the sheer scale of fecal sludge arriving by the truckload at a waste treatment center he visited recently in Senegal.
“It was a conveyor belt,” says Tarpeh, a Stanford assistant professor of chemical engineering and a faculty affiliate at the Stanford King Center on Global Development. “Every minute, trucks were pulling up to empty their loads.” It was also a stark reminder of the world’s daily sewage output: three trillion 2-liter bottles, or the equivalent of 200 gallons per person. Of that daily volume, 80 percent is discharged without treatment.
To Tarpeh, the world’s wastewater factory has the potential to alleviate a multitude of global challenges — from environmental sustainability to hunger. Human urine contains nitrogen, phosphorus, and potassium, which are vital nutrients for plant growth. Feces, too, contain vitamins and minerals. What, says Tarpeh, if human effluent could be recycled efficiently and profitably — not just at waste treatment centers, but also in homes, office buildings, and factories?
As audacious as it may sound, it is a vision that Tarpeh is working hard to advance.
That is how he and two Stanford students ended up at the waste treatment center in Dakar, Senegal’s capital, that is already converting sludge into compost. They had traveled there late last year with King Center support to demonstrate Tarpeh’s novel method for extracting nutrients from urine to make liquid fertilizer, disinfectant, and other industrial products. Next, Tarpeh and his team plan to set up a demonstration lab at the site, operated by Delvic Sanitation Initiatives (a Stanford Seed grantee), where they will conduct a field study on his process once COVID-19 travel restrictions are lifted.
Recognizing the opportunity
According to the World Health Organization, 2.4 billion people globally do not have access to bathrooms. This puts them at risk of contracting potentially fatal diseases that keep them from work and an education.
There are secondary effects as well. For example, young girls drop out of school when they reach puberty because of the lack of bathroom privacy. This, in turn, exacerbates the gender divide in business and politics. Food quality also suffers as polluted water is used to grow crops.
“For most of us, sanitation is something we take for granted. We flush and forget,” Tarpeh says.
“But for people who don’t have that luxury, it’s life-changing. Something as seemingly simple as the toilet can affect everything from a country’s economy to its political leadership to environmental quality.”
For too long, he says, sanitation has “played second fiddle” to water and concerns about water safety.
Awareness, however, of wastewater’s potential is growing. A 2017 United Nations report singled out its “vast potential as a source of resources” and called it “underexploited.”
Innovating for global scale
The crux of the research in Tarpeh’s lab centers on a process called electrochemical stripping. It requires a device with three chambers that converts nitrogen in urine to ammonia, which is known as a commodity chemical for its large-scale use in industrial products like disinfectant, coolant, and fertilizer. Energy and chemical solutions are used to separate ammonia from other nutrients in the urine and turn it into liquid nitrogen, which can be used to make fertilizers or disinfectants.
A second approach, which is called an ion exchange column, involves pouring urine into a tube that contains small, negatively-charged resin beads. The nitrogen, part of the positively-charged ammonium, attaches to the beads as the remaining urine passes through. Acid regenerates the resin beads, leaving just the nitrogen solution.
Either method can be deployed with a simple handheld device or a larger apparatus placed on a mobile truck bed.
“Most water treatment approaches require a treatment site,” says Tarpeh. “We want to do the opposite, which is bring the treatment to the water. It gives you a lot more flexibility.”
Still, challenges remain. Among them is figuring out how to scale both technologies while ensuring they can achieve efficiencies at a reasonable price. To that end, Tarpeh’s research group recently figured out a way to customize the electrochemical stripping device so users can choose how much fertilizer or disinfectant gets produced. A batch of urine, for example, can be filtered to generate 75 percent fertilizer and 25 percent disinfectant.
“The King Center has been integral in helping us to develop this powerful, technically fascinating process,” says Tarpeh. Much work remains to optimize the technique, but students and partners alike are on board to bring the process to scale.
Combining passion and altruism
Tarpeh, whom students call the “waste wizard,” became intrigued with the idea of urine as a valuable resource while earning his 2017 doctorate in environmental engineering at UC-Berkeley.
“My approach to science, even from a young age, has been to identify where a field is going, distill it down as distinctly as possible and then, in an informed way, run in the other direction,” says Tarpeh. He also understood that urine would be a good way to get people to pay attention to his mission.
The roots of Tarpeh’s work are also personal. Born in West Africa to American and Liberian parents, Tarpeh and his family moved to America after a civil war broke out when he was a toddler. “Where I grew up made a big difference in my life,” says Tarpeh, who was raised in Connecticut and Virginia. “It got me to think about closing the gap between opportunities a kid has in sub-Saharan Africa and the opportunities a kid has in the United States.”
A career in academia, he concluded, would allow him to do his part to change the world for the better while satisfying his passion for science. Tarpeh, who holds a B.S. in chemical engineering from Stanford, is also an assistant professor by courtesy in civil and environmental engineering.
Closing economic and ecological gaps
Tarpeh is quickly gaining recognition for his work. Last year he was named to Forbes’s “30 Under 30” list of up-and-coming scientists and to Chemical and Engineering News’s “Talented Twelve” list of young scientists tackling the world’s toughest problems through chemistry. Tarpeh has also received numerous research awards, including a prestigious early-career fellowship. A video about his research on the YouTube science channel Seeker has racked up 5 million views.
These are other signs that Tarpeh’s ideas are taking hold, notably in Africa. With additional funding from the King Center, he is working with the United Nations to deploy his method to help rehabilitate arable land in southeastern Africa that was devastated by flooding from a cyclone in 2019. He has also partnered with Sanergy, a private sanitation company in Kenya to reduce by as much as 80 percent the costs of importing fertilizer by producing a local variety made with nitrogen extracted from urine.
“My goal,” says Tarpeh, “is to get sanitation to a point where it pays for itself. And the advantages of starting in places like sub-Saharan Africa is that there isn’t as large a sunk cost when it comes to treating polluted water. Conditions that we see as problems now can provide opportunities for reinventing and reimagining waste.”
Tarpeh is also thinking now about resource recovery beyond urine and feces. Industrial processes, he says, have created an imbalance in the Earth’s natural nitrogen cycle. As a result, algae blooms fed by excess nitrogen are destroying aquatic ecosystems and causing severe human health impacts like blue baby syndrome.
“I no longer have a laser-like focus on recovering nitrogen only to make fertilizer or disinfectant,” says Tarpeh, who credits support from the King Center for enabling to think more expansively about the problems he’s trying to solve. “My vision now is to reimagine completely how the world produces industrial products — so that pollutants in one setting become valuable products in another. We can close the ecological loop to provide industrial products and pristine environments for future generations.”
Tarpeh is also an assistant professor, by courtesy, of civil and environmental engineering and a center fellow, by courtesy, at the Stanford Woods Institute for the Environment.
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