Princeton Study Identifies Major Emissions Gap In Global Wastewater Reporting

Undercounted methane and nitrous oxide could weaken national climate baselines

Countries worldwide are underestimating greenhouse gas emissions from wastewater systems, according to research from Princeton University published Feb. 11 in Nature Climate Change. The study finds that national climate inventories underestimate methane and nitrous oxide emissions from wastewater by 19 to 27 percent, raising concerns about the accuracy of emissions baselines used in international climate policy.

After reviewing National Inventory Reports from 38 countries across five continents, researchers found that methane and nitrous oxide emissions from wastewater are underestimated by 19 to 27 percent. Across the countries analyzed, the discrepancy represents 52 to 73 million metric tons of CO_₂-equivalent per year. When extrapolated globally, the gap could reach 94 to 150 million metric tons annually.

In a policy paper that accompanies the research article, Z. Jason Ren, who led the research, said that accurate estimates are a foundation for sound public reporting and practical decision making in the wastewater sector. He urged the IPCC and policymakers to take note of these differences and move countries toward the most accurate wastewater standards when developing national inventories.

“Wastewater systems are essential infrastructure, yet their climate impacts have not been fully reflected in many national inventories,” said Z. Jason Ren, professor of civil and environmental engineering and the Andlinger Center for Energy and the Environment at Princeton University. “Accurate accounting is fundamental to credible mitigation planning.”

Why the Gap Exists
The analysis shows that underreporting is systemic rather than isolated. Many national inventories omit decentralized systems such as septic tanks and latrines, limit accounting of emissions from effluent discharge and untreated releases, and rely on outdated emission factors that underestimate methane and nitrous oxide formation in biological treatment processes. Sewer networks and sludge management pathways are also frequently excluded due to limited monitoring data. More recent measurement based studies indicate higher and more variable emissions than earlier guidance assumed.

The researchers said wastewater systems also present significant opportunities for emissions reduction because they have historically received less attention than sectors such as transportation and power generation, where carbon dioxide emissions dominate. Because wastewater infrastructure often operates for many decades, the emissions consequences of today’s technology choices can persist for generations.

“A municipal treatment plant built today may still be operating at the end of the century,” Ren said. “If accounting is inaccurate, it becomes harder to make the right decisions about technologies and investments that will shape emissions for decades. As a researcher, I want to understand the process better to put in better technologies and avoid, I don’t want to say disastrous, but very unfortunate scenarios.”

Why It Matters
National inventories underpin climate commitments and inform international assessments of progress. Incomplete wastewater accounting may distort emissions baselines, affect mitigation planning, and obscure cost-effective reduction opportunities.

“As electricity related emissions fall, methane and nitrous oxide from treatment processes will account for a larger share of wastewater’s climate footprint. Accurate inventories are essential to ensure decarbonization strategies reflect the sector’s real emissions profile. Without accurate accounting, it is difficult to develop effective policies, technologies, or strategies to reduce emissions,” Ren said.

To address these gaps, the authors recommend countries:

• Expand inventory coverage to include all wastewater pathways, including septic systems, effluent discharges, and decentralized systems.
• Harmonize reporting methods using consistent, up-to-date emission factors.
• Improve data collection and transparency, particularly in developing countries.
• Incorporate accurate wastewater emissions into national mitigation plans.

About the Study
The study, Discrepancy of National Inventories Reveals Large Emissions Gap in the Wastewater Sector, was published Feb. 11 in Nature Climate Change (DOI: 10.1038/s41558-025-02540-6). A companion policy brief was also released (DOI: 10.1038/s41558-026-02555-7). The research was led by Cuihong Song, now at the University of New Hampshire, and includes Wei Peng and Z. Jason Ren of Princeton University and David Ponder of the U.S. Water Alliance. Support for the research was provided by the Grantham Foundation and the Water Research Foundation through the Paul L. Busch Award.

About the Princeton Water and Energy Technologies Lab
The Princeton Water and Energy Technologies Lab, part of the Department of Civil and Environmental Engineering and the Andlinger Center for Energy and the Environment at Princeton University, focuses on the intersection of water infrastructure, energy systems, and decarbonization. Led by Professor Z. Jason Ren, the lab develops technologies and policy-relevant analysis to reduce greenhouse gas emissions, advance energy and material recovery, and accelerate the decarbonization of wastewater treatment and related environmental systems.