Each year, wastewater treatment facilities across the United States manage the enormous task of cleaning trillions of gallons of water—ranging from household sewage to gray water that flows from sinks and showers. While these operations are essential for public health and environmental protection, new research reveals that they come with a significant, previously underappreciated climate cost. A groundbreaking study led by teams from Northwestern University and the University of Illinois Urbana-Champaign has quantified greenhouse gas emissions from over 15,000 wastewater treatment plants nationwide. The findings expose a latent climate burden, emphasizing emissions far beyond carbon dioxide alone and underscoring new areas for urgent technological advancement.
The comprehensive study, recently published in the prestigious journal Nature Water, estimates that U.S. wastewater treatment plants alone are responsible for emitting the equivalent of approximately 47 million metric tons of CO2 annually. Crucially, the research highlights that methane (CH4) and nitrous oxide (N2O)—two greenhouse gases far more potent than CO2 in terms of global warming potential—account for a disproportionately large share of these emissions. Methane and nitrous oxide contributions exceed current governmental estimates by about 41%, reshaping our understanding of the sector’s true environmental footprint.
Jennifer Dunn, a professor of chemical and biological engineering at Northwestern University’s McCormick School of Engineering and the study’s senior author, remarked on the significance of these emissions. She noted that detecting methane and nitrous oxide as dominant factors was both surprising and critical, given that previous assessments underestimated their prevalence. These potent greenhouse gases derive largely from the biological and chemical processes that wastewater treatment plants use to purify water, revealing emission sources that were insufficiently accounted for in traditional carbon-centric climate models.
This reassessment of wastewater treatment’s environmental impact opens a new window into climate mitigation potential. Rather than being solely a constraint, the study suggests the sector contains “low-hanging-fruit” opportunities to reduce emissions. Some emissions result from relatively addressable issues like leaks in anaerobic digesters, while others require innovative technology development to fundamentally transform nitrogen treatment and energy harvesting approaches within plants. Dunn emphasized that identifying these leverage points is crucial for aligning wastewater treatment with broader decarbonization goals.
Wastewater treatment involves multiple stages, within which wastewater’s solids, or sewage sludge, are broken down using various biological processes. A common method involves anaerobic digestion, where microorganisms metabolize organic material without oxygen and produce biogas dominated by methane. However, the process carries a significant drawback: methane leakage. Despite biogas’s potential as a renewable energy source, unintended emissions from leaks can negate the environmental benefits of onsite energy recovery systems.
The study brings to light the troubling reality that many anaerobic digesters leak significant amounts of methane into the atmosphere. Dunn explained that while these leaks can be severe, they are fundamentally fixable through improved monitoring, maintenance, and design enhancements. Such mitigation strategies represent immediate and cost-effective emissions reduction options that wastewater treatment operators can implement without requiring major infrastructure overhauls.
Another critical but often overlooked greenhouse gas associated with wastewater treatment is nitrous oxide. This gas primarily arises from the processes used to remove nitrogen from wastewater, especially nitrification-denitrification. Nitrogen removal is crucial to prevent eutrophication—a phenomenon where excess nutrients cause harmful algal blooms and oxygen depletion in freshwater ecosystems. However, conventional nitrogen removal technologies inadvertently release nitrous oxide, a greenhouse gas with nearly 300 times the global warming potential of CO2.
While nitrification-denitrification remains the dominant method for nitrogen removal, it is energy-intensive and presents climate trade-offs due to its nitrous oxide emissions. Alternative technologies that aim to recover nitrogen before it escapes into the atmosphere offer promise. For example, methods that can capture nitrogen directly from wastewater and convert it into valuable products such as fertilizer or animal feed could simultaneously reduce greenhouse gas emissions and support circular economic models. Such innovations would close the nitrogen cycle, turning wastewater treatment plants from emission sources into carbon and nutrient resource hubs.
To achieve these advances, the research team is collaborating extensively with wastewater treatment facilities to gather high-resolution, plant-specific emissions data. They are also refining an open-source modeling tool designed to help operators quantify and manage their greenhouse gas emissions across the entire wastewater treatment lifecycle. This tool integrates emissions from onsite biological processes, energy and chemical input production, and waste disposal stages, providing a holistic evaluation framework that can guide decarbonization strategies tailored to individual plants.
The team’s approach not only aids municipalities with climate action plans looking to reduce their carbon footprints but also establishes a scalable methodology adaptable to treatment plants worldwide. Despite the study’s focus on U.S. facilities, its underlying principles and modeling tools can be applied globally, aiding regions with growing populations and expanding sanitation infrastructure. As cities and countries strive to meet ambitious climate targets, addressing emissions from wastewater systems emerges as a vital yet often neglected sector in the decarbonization landscape.
Given the expanding scale of wastewater treatment services—with public sanitary coverage reaching billions of people globally—the environmental impact and mitigation potential of the sector cannot be ignored. Dunn underscored the urgency, stating that wastewater treatment is a substantial sector “that needs attention.” She highlighted the pressing need for continued research, innovation, and policy support focused on reducing methane leaks, minimizing nitrous oxide emissions, and developing sustainable nutrient recovery technologies.
Ultimately, this landmark study reshapes the conversation about wastewater treatment’s role in climate change. It moves beyond simplistic CO2 metrics to account for potent methane and nitrous oxide emissions, directly linking operational processes to climate outcomes. The integration of new data, innovative modeling, and practical mitigation strategies provides a pathway toward a more sustainable and resilient future for water infrastructure worldwide.
As wastewater treatment plants pivot toward cleaner, more efficient operations, their evolution could serve as a blueprint for industrial sectors tackling indirect emissions and resource circularity. Investing in targeted research, adopting best practices, and deploying cutting-edge technologies will be instrumental in minimizing the concealed climate costs of one of society’s essential public services. With coordinated global efforts, the environmental legacy of wastewater treatment can shift from a climate liability to a model of sustainable environmental stewardship.
Subject of Research: Greenhouse gas emissions from wastewater treatment plants and their implications for climate change mitigation
Article Title: Benchmarking greenhouse gas emissions from US wastewater treatment for targeted reduction
News Publication Date: 8-Oct-2025
Web References:
- Original article: https://www.nature.com/articles/s44221-025-00485-w
- Northwestern Center for Engineering Sustainability and Resilience: https://www.engineeringsustainability.northwestern.edu/
- QSDSAN open-source tool: https://qsdsan.com/
- Northwestern fertilizer research: https://news.northwestern.edu/stories/2023/09/hybrid-catalyst-produces-critical-fertilizer-and-cleans-wastewater/
Keywords: Wastewater, Water, Climate change, Greenhouse gases
