Life Cycle Greenhouse Gas Emissions Analysis of Wastewater Sludge Management Practices in the United States

Abstract

Publicly owned treatment works in the United States generate more than 12 million dry tonnes of wastewater sludge annually, underscoring the need to evaluate current management practices, emissions burden, and sludge-to-energy potential. In this work, we developed a first-of-its-kind facility-level database tracing ∼75% of the national wastewater sludge flow from treatment through disposal and used R&D GREET to quantify life-cycle greenhouse gas (GHG) emissions for 25 pathways with Monte Carlo uncertainty analysis. Emissions vary widely: landfilling untreated or limed sludge is the most GHG-intensive pathway (2,030 kg CO2e dry t–1), driven by methane release during decomposition, while several pathways ending in land application, including composting plus land application (34 kg CO2e dry t–1), thermal drying plus land application (110 kg CO2e dry t–1), and anaerobic digestion with combined heat and power plus digestate land application (−71 kg CO2e dry t–1), exhibit markedly lower emissions. Some pathways achieve net-negative emissions through soil carbon sequestration, electricity export, and fertilizer displacement. This study establishes standardized business-as-usual counterfactuals for life-cycle analyses of sludge-derived fuels, identifies key drivers such as landfill gas capture and flaring, soil carbon sequestration, and methane emissions from digestate management, and highlights the GHG mitigation potential of improved sludge management.