A study from one of the world’s leading universities identifies major emissions gap in global wastewater reporting.
Countries worldwide are underestimating greenhouse gas emissions from wastewater systems, according to research from Princeton University in the US, published in February in Nature Climate Change.
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 per cent.
Across the countries analysed, the discrepancy represents 52 to 73 million metric tons of CO₂-equivalent per year. When extrapolated globally, the gap could reach 94 to 150 million metric tons annually.
“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.”
The analysis shows that underreporting is systemic rather than isolated. Many national inventories omit decentralised 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as septic tanks and latrines, limit accounting of emissions from effluent discharge and untreated releases and rely on outdated emission factors that underestimate methane and nitrous oxide formation in biological treatment processes.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.”
To address these gaps, the authors recommend countries:
- expand inventory coverage to include all wastewater pathways, including septic systems, effluent discharges, and decentralized systems
- harmonise 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.
