To find chemical engineering problems to solve, William Tarpeh uses a simple formula.
“Name a wastewater, either where it comes from or something about it. Name a pollutant you want to get rid of, and then name a product you’d be interested in making,” said Tarpeh, an assistant professor of chemical engineering in the Stanford School of Engineering.
This combination has fueled Tarpeh’s interests since he was a Stanford undergraduate. Now, it shapes his vision for finding innovative ways to extract value from wastewater, including new research that involves designing and refining ways to reclaim ammonia from nitrate-contaminated wastewater streams.
To Stanford and back
Tarpeh, who grew up outside Washington, D.C., was a curious kid who loved science and wanted to solve big problems. During a high school service-learning trip to Ethiopia, he became interested in water infrastructure.
He brought that interest to Stanford, where he took three Introductory Seminars that set him on the path to his career: Chemical Engineering and Environmental Policy, Water Engineering and Public Health, and History of South Africa.
Water Engineering and Public Health was taught by Jenna Davis – a professor of civil and environmental engineering – who further inspired Tarpeh. He was sharpening his awareness that the endless flow of wastewater – and the systems around it – was like an untapped resource waiting to be transformed, sparking his fascination with the potential to turn something disposable into something valuable.
“I didn’t realize that people could study sanitation and toilets and wastewater for their whole lives,” Tarpeh said. “When I found out, I was thrilled. I knew I could do it for the rest of my life.”
History of South Africa led him to study abroad in Cape Town, which deepened his interest in wastewater.
“Lots of people were focused on water, on wells. The more I learned about toilets, the more I realized these are connected,” Tarpeh said. “If you don’t have a clean place to go to the bathroom, your water is very likely to get polluted all over again, even if you draw it from a beautiful, pristine-looking well.”
Tarpeh earned his master’s and doctoral degrees in environmental engineering at the University of California, Berkeley, and conducted postdoctoral research at the University of Michigan, but he was drawn back to Stanford as a faculty member in 2018. Here, he’s found his niche at the interface of chemical and environmental engineering.
“All of our lab’s applications are environmental, because that’s what motivates me and gets me excited,” he said. “And all of our mechanistic work is with a chemical engineering perspective.”
Recovering ammonia from nitrate
Tarpeh loves that chemical engineering involves zooming out to look at system-scale problems and zooming in to see what’s happening on a molecular scale. His recent research publications focus on a step in between: innovation at the process level.
For the first paper, published in June in the journal Energy & Environmental Science, Tarpeh and his co-authors – including first author Dean Miller, a Stanford PhD candidate in chemical engineering – used wastewater from a Palo Alto treatment plant to test a process for converting nitrate into ammonia. For the second paper, published in October in the same journal, Tarpeh’s team tested another process that produces high-purity ammonia from agricultural runoff. Jinyu Guo, a Stanford PhD student in chemical engineering, is the first author.
Nitrate is an extremely common water pollutant that can cause harmful algal blooms and human health problems such as low blood-oxygen levels in newborns. Ammonia is used in a multitude of products, primarily fertilizer.
The two are interconnected: Humans manufacture ammonia for fertilizer from hydrogen and nitrogen in the air. Bacteria in the soil oxidize ammonia into nitrate, which runs off fertilized fields and into waterways.
In their research for the first paper, Tarpeh and his colleagues created an “electrocatalyst-in-a-box” – an electrically driven process that both extracts nitrate from wastewater and converts that nitrate into ammonia.
For the second paper, Tarpeh’s team used a two-step process of electrodialysis and nitrate reduction to take nitrate and ammonia from wastewater and convert them together into high-purity ammonia. They successfully concentrated ammonia about 12 times compared to the nitrate and ammonia in the original wastewater, Tarpeh said.
“Basically, we can take polluted water and turn it into high-purity products that are pretty indistinguishable from products that are made today, but that are derived from wastewater,” Tarpeh said.
The reclaimed ammonia could then be used to make fertilizer again, rather than drawing more chemicals from the air, or for other products, including some used in disinfection and sanitation.
Both publications are part of a National Science Foundation grant awarded to Tarpeh and four Stanford collaborators that aims to bring balance to the nitrogen cycle, and his electrocatalyst-in-a-box work is also funded by his NSF CAREER Award.
“We made all this reactive nitrogen, so why not interconvert it into these forms and then save ourselves energy, emissions, and costs,” Tarpeh said. “Why not take some of that nitrate and just turn it straight back into ammonia?”
Restoring balance to the nitrogen cycle could also decentralize ammonia production. Fewer than 100 facilities around the world produce ammonia today.
“What that means is that some people in the poorest countries end up paying the highest amounts for fertilizer because they’re far from a production plant, and then we all pay the emissions cost of transporting ammonia very long distances,” Tarpeh explained.
But if nitrate can be converted into ammonia directly from wastewater, it could change the economics of the system by enabling on-site manufacturing, he said. People could have better access not only to fertilizer and other commodity chemicals, but also to sanitation.
A hopeful future
One of the next steps in Tarpeh’s research involves zooming out: He’s working with collaborators at the Stanford Doerr School of Sustainability to apply one of the electrocatalyst-in-a-box techniques on runoff from a USDA field in Salinas, California, where the researchers from the school are doing a study on cover crops. This will require scaling up the process and running it continuously.
He’s also zooming in, studying how pulsing the electricity used to control the catalyst changes its effectiveness. Pulsing can alter the local environment around electrocatalysts and lead to major changes in production rates and products generated.
In addition, he’s continuing his partnership with a company in Senegal that focuses on resource recovery from septic tanks, as well as helping a company in Kenya that provides urban sanitation solutions find options for creating valuable materials from urine. He’s also started an environmental justice project in Alabama, where he’s studying problems with failing septic tanks, particularly in low-income Black communities, to see if some of his lab’s processes might be a solution.
There’s no shortage of wastewater problems to address, Tarpeh said: “I joke with my students we’ll be in business for a long time, because wastewater is going to be around for a while.”
But he also imagines a time when technologies like theirs have changed the world.
“I think about my kids or grandkids saying, ‘Wastewater? That’s a weird misnomer. Why would you waste water? What does that even mean?’” he said. “Because they would see any waste stream being plugged into some other industry, ‘wastewater’ would be obsolete.”