A groundbreaking new study shines a spotlight on a critical blind spot in global greenhouse gas emissions accounting: the wastewater sector. Despite its undeniable contribution to methane (CH4) and nitrous oxide (N2O) emissions—two potent greenhouse gases—national inventories have understated the sector’s true environmental footprint. The research meticulously dissects the gaps and discrepancies in how countries report fugitive emissions stemming from various wastewater treatment and discharge pathways. It is a revelatory insight which could significantly recalibrate our understanding of global emissions and influence future climate policies.
The researchers begin by delineating the complex system boundaries of wastewater treatment and associated emissions. They categorize the sector into five primary groups based on types of infrastructure and services: latrines, septic systems, centralized water resource recovery facilities (WRRFs), treated effluent discharge, and untreated wastewater from sources like scattered households or sewer overflows. Notably, the study excludes onsite sludge disposal emissions integrated within WRRF operations from external sludge disposal pathways, which are accounted for in other sectors such as waste management or agriculture.
One of the most underappreciated sources highlighted is the sewer collection system itself. Intriguingly, the Intergovernmental Panel on Climate Change (IPCC) provides emission factors mainly for open or stagnant sewer systems, designating flowing sewers as negligible emitters due to limited measurement data. Contrastingly, Switzerland stands as an outlier, reporting emissions from flowing sewers using a carefully derived mean emission factor based on peer-reviewed empirical measurements. This points to an urgent need for expanded global research to better quantify emissions from sewer networks, which may be key methane and nitrous oxide sources often overlooked in inventories.
In scrutinizing the IPCC’s evolving emission factor (EF) methodologies, the research reveals a marked progression from the 2006 guidelines through the 2013 Supplement to the more comprehensive 2019 Refinement. The earlier 2006 guidelines relied heavily on expert judgement for methane correction factors and excluded emissions from well-managed aerobic WRRFs entirely, assuming negligible methane release. Nitrous oxide methodologies in 2006 were primitive, limited to treated effluent discharges and nitrification-denitrification processes based on scant data from a single U.S. facility. By contrast, the 2019 Refinement integrates extensive new field data, differentiates discharge-related factors by receiving waterbody types, and incorporates constructed wetlands—phases missing or underrepresented in prior methodologies.
Crucially, this study does not just stop at assessment but compares national inventory reports (NIRs) submitted to the United Nations Framework Convention on Climate Change (UNFCCC) from 38 countries. With language barriers addressed through open-source translation tools, and data rigorously extracted and validated, the researchers provide a comprehensive cross-country review, spotlighting how wastewater emission reporting varies widely in scope and methodology. These countries represent diverse economic contexts—ranging from G7 developed economies to developing and emerging nations—and collectively account for up to two-thirds of reported wastewater sector emissions globally.
The study further employs wastewater service data from the World Health Organization and UNICEF Joint Monitoring Programme (JMP), which tracks safely managed sanitation services worldwide. By meticulously mapping JMP sanitation categories such as improved sanitation, basic, limited, and unimproved facilities against wastewater pathways, the researchers derive population ratios served by each treatment type. Importantly, they reconcile discrepancies between JMP data and national reports, manually adjusting when inventories reported emissions for pathways lacking corresponding population data, ensuring more reliable estimates.
An in-depth quantitative framework underpins the emissions estimation. Methane and nitrous oxide emissions are calculated using country-specific biochemical oxygen demand (BOD) and total nitrogen (TN) loads, adjusted by the fraction of the population served by each wastewater pathway and applying the latest IPCC 2019 emission factors. For treated effluent discharge, removal rates of these pollutants within WRRFs are explicitly considered. This rigorous methodology enables quantification of emissions gaps—both emissions unaccounted for because certain pathways are omitted in inventories and underestimated emissions due to outdated emission factors.
One particularly striking finding is the systematic underestimation of emissions from well-managed aerobic WRRFs in many national inventories that still apply the IPCC 2006 default emission factors. Field measurements show that both methane and nitrous oxide emissions from these facilities are significantly higher than previously assumed. As a consequence, countries such as Austria, France, Spain, Turkey, and Ukraine underestimate methane emissions, while Belgium, Italy, South Africa, Turkey, and Ukraine similarly underreport nitrous oxide emissions. This revelation demands urgent updates to national accounting methods and highlights the imperative for increased monitoring and data collection.
The study’s impact extends beyond technical refinement—it holds profound implications for global climate strategies. Wastewater treatment has long been a neglected piece of the emissions puzzle, often overshadowed by the energy, transportation, and agriculture sectors. This research exposes the wastewater sector as a significant, yet underaccounted, source of greenhouse gases. By addressing methodological gaps and encouraging standardized reporting practices, countries can better target mitigation efforts, potentially harnessing existing infrastructure upgrades to slash emissions.
Moreover, the study advocates for the inclusion of sewer collection systems in future IPCC guidelines. Given emerging evidence that even flowing sewers emit methane and nitrous oxide, expanding the system boundaries to incorporate these overlooked elements is critical. This would align inventories more closely with real-world emissions, enabling policymakers to design interventions that reduce leaks and breakdowns in sewer networks—often aging and poorly maintained in many regions.
Population and wastewater characteristics also feature prominently in the analysis. Country-specific per capita BOD and protein consumption data serve as proxies to approximate organic and nitrogen loads, respectively, which fuel methane and nitrous oxide production during wastewater treatment and discharge. These nuanced inputs are combined with emission factors to generate national and pathway-specific emission estimates, revealing large variability driven by sanitation infrastructure and consumption patterns worldwide.
The authors’ robust use of peer-reviewed data to supplement gaps in Chinese national inventories exemplifies an important global perspective. Given China’s substantial wastewater emissions and less detailed reporting in public submissions, integrating literature estimates thematic to latrines, septic tanks, WRRFs, and collection systems enhances accuracy. This cross-validation approach underscores the need for transparent, detailed country submissions to the UNFCCC and the benefits of bridging academic research with policy frameworks.
While uncertainties remain—particularly around coarse estimates of treated wastewater volume, variation in treatment technologies, and discharge receiving environments—the comprehensive dataset provided by the JMP remains the most granular and globally consistent tool presently available. Further refinement and integration with emerging datasets will sharpen emission inventories and underpin more targeted mitigation policies.
By stacking methodical literature review, rigorous data analysis, and transparent accounting principles, this study offers a much-needed recalibration of the wastewater sector’s greenhouse gas contributions. Its findings should serve as a clarion call to policymakers, environmental agencies, and researchers alike. Improving wastewater emissions inventories is not just an academic exercise but a critical pathway to achieving global climate commitments and safeguarding the planet for future generations.
In sum, this investigation reveals a vast, previously underappreciated emissions gap rooted in national wastewater inventories worldwide. The path forward demands concerted efforts to incorporate overlooked pathways, update default emission factors, and enhance transparency. As the climate crisis deepens, no emission source—especially one as globally ubiquitous as wastewater—can be left uncharted. Only through comprehensive and harmonized accounting can the true environmental cost be understood and effective climate action succeed.
Subject of Research:
Discrepancies and gaps in national greenhouse gas inventories for the wastewater sector, focusing on undercounted methane and nitrous oxide emissions from wastewater treatment pathways.
Article Title:
Discrepancies in national inventories reveal a large emissions gap in the wastewater sector.
Article References:
Song, C., Ponder, D., Peng, W. et al. Discrepancies in national inventories reveal a large emissions gap in the wastewater sector. Nat. Clim. Chang. (2026). https://doi.org/10.1038/s41558-025-02540-6
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