William Tarpeh, a chemical engineer of Liberian and American heritage, is developing sustainable and practical methods to recover valuable chemical resources from wastewater. Tarpeh’s research focuses on recycling nitrogen, sulfur, and phosphorus. His resource recovery technologies require minimal infrastructure and energy, and they offer opportunities for sustainably sourcing agricultural nutrients and industrial metals in a range of contexts. His work is also inspired by a desire to develop technologies that can improve sanitation and resource access in communities around the world, including across Africa.
Nitrogen-, sulfur-, and phosphorus-containing compounds are ubiquitous in municipal, agricultural, and industrial wastewater. In wastewater, these elements are harmful to the environment and human health, but they are also the basis of essential chemical products including fertilizer and household cleaners. Conventional methods of ammonia stripping, which removes nitrogen from wastewater, require significant energy and chemical inputs.
Tarpeh has developed electrochemical-based processes to isolate nitrogen from wastewater using electricity and ion-selective membranes. His electrochemical reactors convert nitrogen in urine waste streams into ammonia-based products, and they can be tuned to produce ammonium sulfate, which is used in fertilizer, or ammonium hydroxide, which is used in household cleaners and industrial chemical production.
Unrefined and mixed-source waste streams present additional challenges for resource recovery. The relatively low concentration of nitrogen amidst many other impurities makes it more difficult to efficiently extract and convert it. Tarpeh and colleagues have demonstrated an electrocatalyst-in-a-box (ECaB) that recovers ammonia directly from municipal wastewater. The two-stage reactor assembly first separates nitrogen compounds from wastewater. Electrocatalysis applied in a separate reactor compartment converts the concentrated nitrogen into ammonium sulfate. This technology offers the potential for a closed-loop system that allows local sanitation systems to manufacture needed chemicals.
Tarpeh is also developing other technologies, including ligand-exchange adsorbents (materials that bind molecules to their surface) and polymer membranes, to recover resources with industrial applications, such as lithium and phosphorus.
Currently, Tarpeh is working to implement his technologies at scale, particularly in regions where infrastructure is limited. His pilot project in Kenya produced urine-derived fertilizers at a lower cost than synthetic fertilizer. He and his colleagues are also testing an ECaB system with agricultural runoff in California farm fields. If successful, the system would prevent nitrate pollution—which causes devastating algal blooms in waterways—while generating a sustainable, cost-effective source of fertilizer.
Born in West Africa to a Liberian parent and an American parent, Tarpeh’s family relocated to the United States after civil war erupted in Liberia during his early childhood. His Liberian heritage has helped shape his commitment to developing practical environmental technologies that can improve public health and resource sustainability in underserved communities around the world.
Waste generation from humans, agriculture, and industry is an ever-growing challenge for environmental and public health. Tarpeh’s work is helping reduce the environmental harms of wastewater while transforming waste into valuable resources, creating innovative solutions that support a more sustainable future.
