Wastewater treatment plants (WWTPs) are indispensable to urban life, processing billions of liters of wastewater daily. Yet their environmental footprint remains underexamined. While it is known that methane (CH4) and nitrous oxide (N2O) are released during treatment processes, estimating their volumes has relied heavily on broad emission factors recommended by the Intergovernmental Panel on Climate Change (IPCC). These generic methods often fail to capture local variation in treatment design, climate, and influent composition. Adding to the complexity is fossil carbon dioxide (CO2)—originating from synthetic detergents and industrial effluents—which remains largely invisible in existing climate reports. Due to these gaps, a more nuanced understanding of WWTP emissions is urgently needed.
In a new review (DOI: 10.1016/j.ese.2025.100606) published in Environmental Science and Ecotechnology (July 2025), researchers from Harbin Institute of Technology and collaborators present a critical review of greenhouse gas quantification methods in WWTPs. Through a critical review and their own on-site experiments, the authors examine the accuracy, feasibility, and scalability of current monitoring approaches. The work highlights discrepancies between estimated and actual emissions, especially for fossil CO2, and suggests practical strategies to make carbon accounting in wastewater more precise, actionable, and globally relevant.
The study divides existing measurement tools into unit-based and plant-integrated approaches. Unit-based methods—like flux chambers and optical gas imaging—track emissions at specific treatment steps and are useful for locating hotspots. However, they fall short in assessing overall emissions. In contrast, plant-integrated techniques—including drone mapping, mobile laboratories, and aircraft surveys—capture facility-wide emissions with varying levels of resolution and cost. Aerial methods reported the highest methane fluxes, while off-gas measurements in enclosed WWTPs revealed the highest nitrous oxide levels.
A breakthrough aspect of the study is its focus on fossil CO2—emitted when fossil-based chemicals break down during treatment. Using radiocarbon analysis, researchers found that fossil carbon accounts for 4–28% of incoming wastewater. This carbon is largely converted to CO2 and released into the atmosphere, yet goes uncounted in most inventories. The team found that including fossil CO2 could raise reported emissions by more than 20%, especially in systems with sludge incineration or energy recovery. These results make a strong case for customized, technology-specific emission factors in both developed and developing contexts.
“Wastewater is not just a sanitation issue—it's a climate issue,” said Dr. Haiyan Li, corresponding author of the study. “By overlooking fossil CO2 and relying on outdated estimation methods, we're underreporting a major source of greenhouse gases. Our review urges the scientific and policy community to adopt smarter, site-specific tools that reflect real-world emissions and inform better climate action.”
This review lays the foundation for transforming wastewater management into a climate-smart sector. By integrating real-time, multi-gas monitoring and including fossil CO2 in emission inventories, governments can dramatically improve the accuracy of national climate data. These insights empower decision-makers to tailor emission reduction policies based on local treatment technologies, operational practices, and urban-industrial profiles. The study also paves the way for continuous, automated monitoring systems that link greenhouse gas (GHG) emissions with plant operations, offering an effective path toward low-carbon, high-efficiency wastewater treatment in cities around the world.
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References
DOI
10.1016/j.ese.2025.100606
Original Source URL
https://doi.org/10.1016/j.ese.2025.100606
Funding information
This work was supported by the National Natural Science Foundation of China (No. 52470071 and 52321005), Shenzhen Science and Technology Program (No. RCBS20210609103731062, KCXST20221021111404011, JCYJ20240813092008011, and KQTD20190929172630447), Scientific Research Project of Shenzhen Polytechnic University (No. 6024310035K) and Natural Science Foundation of Guangdong Province (No. 2021A1515110887 and 2023A1515012063).
About Environmental Science and Ecotechnology
Environmental Science and Ecotechnology (ISSN 2666-4984) is an international, peer-reviewed, and open-access journal published by Elsevier. The journal publishes significant views and research across the full spectrum of ecology and environmental sciences, such as climate change, sustainability, biodiversity conservation, environment & health, green catalysis/processing for pollution control, and AI-driven environmental engineering. The latest impact factor of ESE is 14, according to the Journal Citation ReportTM 2024.