Closing the phosphorus loop: A spatial analysis of potential for wastewater-derived fertilizers to meet US...

Phosphorus (P) is an essential, finite resource, and establishing a circular phosphorus economy is critical for sustainable management. A foundational step is identifying spatial hotspots of P fluxes and reuse potential. We developed a comprehensive phosphorus budget for wastewater treatment plants (WWTPs) across the contiguous United States to pinpoint where sewage-derived P can meet local agricultural fertilizer demand. Approximately 2 million metric tons of P (MtP/yr) are applied annually to US agricultural fields. Our analysis of ~17,000 municipal WWTPs-serving ~294 million people (~85% of the U.S. population)-revealed that just 1.1% of WWTPs (the 200 largest) account for nearly 50% (0.30 MtP/yr) of total sewage P. Among these, 187 WWTPs already land-apply about 120,000 tons P/yr in sewage solids. The remainder (~52%) is lost to landfills (67,000 tons), incineration (34,000 tons), or other disposal routes (30,000 tons). To assess recovery potential, we developed the Phosphorus Use Ratio (P_{Use Ratio}), the ratio of wastewater P to commercial fertilizer P used. Urban counties often have P_{Use Ratio} exceeding 1.0, indicating P exceeds P fertilizer demand. Conversely, rural agricultural regions-particularly in the Midwest (e.g., Illinois, Iowa, Nebraska, Minnesota)-show P_{Use Ratio} below 0.25, indicating greater fertilizer needs than can be met locally from sewage. This spatial mismatch highlights the logistical challenges in closing the phosphorus loop. While other studies note the theoretical value of wastewater P, our findings show much of it is already reused through existing land application. Nevertheless, coordinated strategies are needed to maximize recovery and address persistent spatial disconnects in the circular P economy.