In the growing discourse surrounding climate change, wastewater treatment plants (WWTPs) have long been overlooked as significant contributors to greenhouse gas emissions. Recent research, however, is shedding new light on the crucial but underestimated role these facilities play in global warming. Traditionally regarded as mere sanitation infrastructure, WWTPs actually emit substantial quantities of methane (CH₄), nitrous oxide (N₂O), and notably, fossil-derived carbon dioxide (CO₂), the latter frequently omitted from global climate inventories. A groundbreaking review, meticulously integrating systematic literature analysis with field data, now reveals that current emission accounting methods severely underestimate the total greenhouse gas footprint of these plants, calling for a fundamental re-evaluation of monitoring practices worldwide.
Wastewater treatment plants operate at the intersection of urban infrastructure and environmental management, processing billions of liters of domestic and industrial wastewater every day. Methane and nitrous oxide emissions during various biological treatment stages have been recognized but are typically approximated using broad emission factors advocated by the Intergovernmental Panel on Climate Change (IPCC). Such generalized factors lack the granularity to address site-specific variables including treatment design, influent wastewater composition, regional climate conditions, and operational practices. This deficiency creates a generalized, often inaccurate picture that fails to capture the dynamic nature of emissions, limiting policymakers’ capacity to design effective mitigation strategies.
Adding further complexity to the issue is the presence of fossil carbon dioxide emissions, arising from the breakdown of synthetic detergents and industrial effluents introduced into wastewater streams. Unlike biogenic greenhouse gases, fossil CO₂ is anthropogenic and results from non-renewable carbon sources, yet it has largely escaped the purview of emission inventories. This omission is critical; emerging evidence demonstrates fossil CO₂ emissions from WWTPs can constitute a significant portion of total greenhouse gas output, with their exclusion leading to systemic underreporting and thereby influencing global climate models and policy frameworks.
Published in Environmental Science and Ecotechnology in July 2025, a critical review spearheaded by researchers at Harbin Institute of Technology presents the first comprehensive appraisal of greenhouse gas measurement methodologies specifically tailored to wastewater treatment facilities. Through an innovative approach combining exhaustive literature synthesis and direct on-site experimentation, the authors scrutinize the precision, scalability, and practicality of existing monitoring techniques. The study unearths substantial gaps between theoretical emission estimations and actual measured values, with fossil CO₂ emissions—quantified using advanced radiocarbon tracing—identified as a particularly underrepresented category.
Measurement methods examined in the study are broadly categorized into unit-based and plant-integrated approaches, each possessing distinct strengths and limitations. Unit-based techniques, including flux chambers and optical gas imaging, facilitate granular analysis at individual treatment units such as aeration tanks or sludge digesters. While invaluable for pinpointing specific emission hotspots, these localized assessments fail to capture the integrated emission profile of entire facilities. Conversely, plant-integrated methods harness aerial drone surveys, mobile lab assessments, and aircraft-based remote sensing to monitor emissions on a facility-wide scale. These methods, although varying in resolution and cost, present a more holistic view of emission dynamics across the treatment plant.
Intriguingly, aerial measurement approaches consistently registered the highest methane fluxes, underscoring substantial fugitive emissions that unit-level monitoring might overlook. Simultaneously, off-gas analyses within enclosed WWTP systems revealed elevated nitrous oxide concentrations, suggesting complex microbial and chemical interactions unique to specific operational conditions. These findings emphasize the necessity of employing multi-method monitoring frameworks that integrate both micro and macro scales to derive accurate emission inventories.
A major breakthrough of the review lies in its detailed investigation of fossil carbon contributions to CO₂ emissions. Utilizing radiocarbon analysis techniques, the researchers detected that fossil carbon compounds constitute between 4% and 28% of the organic load entering wastewater facilities. The subsequent microbial and chemical degradation of these fossil-derived substances results in CO₂ emissions that have eluded standard accounting practices. Particularly in plants that employ sludge incineration or energy recovery, inclusion of fossil CO₂ could elevate their reported greenhouse gas emissions by as much as 22.8%, highlighting a significant underestimation in current reporting frameworks.
This revelation underscores the pressing need for customized emission factors, tailored not only to the geographic and climatic context of the WWTP but also to the specific technologies employed. The universal application of generic IPCC emission factors is thus inadequate, especially for nations navigating the challenges of infrastructure modernization and climate compliance. Accurate quantification demands site-specific data that reflect real operational diversity, enabling more reliable carbon accounting and informed climate policy.
Dr. Haiyan Li, the study’s corresponding author, encapsulates this paradigm shift by emphasizing the transition from viewing wastewater treatment solely as a public health function to recognizing its integral role in climate mitigation. According to Dr. Li, “By overlooking fossil CO₂ and relying on outdated estimation methods, we significantly underreport a major source of greenhouse gases. Our review advocates for adoption of smarter, site-specific monitoring technologies that mirror actual emissions and support robust climate strategies.”
Looking ahead, the study offers a roadmap to transform WWTPs into climate-smart infrastructure hubs. By integrating continuous, multi-gas monitoring systems and incorporating fossil carbon into emission inventories, WWTPs could become model facilities in urban climate initiatives. This approach promises to refine national greenhouse gas inventories, enhance emission reduction policy precision, and foster innovative technological solutions rooted in real-time data.
The integration of automated measurement systems also hints at the future of wastewater management—an era where emission data are directly linked to plant operations, enabling dynamic control and optimization for reduced carbon footprints. Cities around the globe could leverage such technologies to achieve low-carbon, high-efficiency wastewater treatment benchmarks, further aligning urban infrastructure with sustainability goals.
In summary, this pioneering review compellingly redefines the narrative around greenhouse gases from wastewater treatment plants. It exposes critical blind spots in current accounting methods, especially regarding fossil CO₂, and urges the global scientific and policy community to embrace refined, technologically advanced monitoring strategies. Such progress not only advances climate science but also champions one of our planet’s most essential yet hidden contributors to climate change—wastewater treatment facilities.
Subject of Research: Not applicable
Article Title: Quantifying greenhouse gas emissions from wastewater treatment plants: A critical review
News Publication Date: 25-Jul-2025
Web References:
https://www.sciencedirect.com/science/article/pii/S2666498425000845
References:
DOI: 10.1016/j.ese.2025.100606
Image Credits:
Environmental Science and Ecotechnology
Keywords:
Greenhouse gases
