William Tarpeh

People don’t normally look at what they flush down the toilet and see something worth recovering. But William Tarpeh does.

The Stanford University environmental and chemical engineer is developing ways to extract resources from human waste, such as nitrogen in urine, to make valuable products, like fertilizers and cleaning products. This idea of looking at waste as a resource could help us meet the environmental challenges of a growing human population and changing climate, he says.

Our urine is a concentrated solution of nutrients, Tarpeh points out. It contains nitrogen, phosphorus, and potassium, all crucial elements found in fertilizers. But normal wastewater treatment strategies often end up dumping those nutrients into waterways, where the nitrogen and phosphorus can feed algal blooms that choke aquatic ecosystems, leading to oxygen-deprived dead zones. Meanwhile, industrial methods for making nitrogen fertilizers require massive amounts of energy and emit metric tons of greenhouse gases. So pulling the nitrogen out of our urine could combat several environmental problems, Tarpeh says.

He also thinks it could help improve sanitation and access to clean water in the developing world. “If we can turn human waste into a valuable product, then we can generate revenue from toilets,” he says. “And then toilets in areas like sub-Saharan Africa and Southeast Asia become a revenue-generation center instead of a cost sink.”

Tarpeh has developed multiple methods for extracting nitrogen from urine. In graduate school at the University of California, Berkeley, he developed ion exchange resins that can bind the ammonium in urine. He envisions people inserting cartridges filled with these resins into toilet pipes. Special collection services would then pick up the cartridges, treat them to extract the captured ammonium, and then send them back to consumers.

Another method Tarpeh is working on is called electrochemical stripping, which first involves separating the positively charged species in urine from the negatively charged ones. As a result of this electrochemical process, pH increases around the positively charged ions, converting ammonium to ammonia, which can then diffuse through a gas-selective membrane, away from the other cations. Ammonia then reaches an acid trap, where the gas converts back to ammonium for collection.

“William is stepping outside of dogma and thinking more broadly,” says Krista Wigginton, one of his postdoc advisers at the University of Michigan–Ann Arbor. For example, she says, environmental engineers tend to think about centralizing treatment of municipal wastewater. “William’s PhD work flipped that idea upside down.”

Also, Tarpeh doesn’t focus on just the science behind his ideas; he’s also concerned about their implementation in real-world settings, says Kara Nelson, his graduate adviser at UC Berkeley. For example, he brings his methods out into the field, including in resource-limited areas in Africa, to test them.

Tarpeh hopes his work will help tackle an enormous problem that humanity faces in the 21st century: scarcity. “We don’t have enough to go around, and so we have to develop methods to really squeeze the most value out of every chemical,” he says. “We have to figure out what is the maximum number of times we can use a molecule before we deem it as waste.”