Methane, a potent greenhouse gas second only to carbon dioxide in its impact on global warming, has been a focal point of climate research due to its high efficacy in trapping heat in the earth’s atmosphere. According to the Climate and Clean Air Coalition, human activities contribute nearly 45% of the net warming effect attributed to methane emissions. This alarming statistic places methane reduction at the forefront of strategies aimed at mitigating climate change. However, a significant but hitherto underexplored source of methane emissions has recently come under scientific scrutiny: sewer networks.
An international team of researchers, spearheaded by Professor Yuan Zhiguo from the School of Energy and Environment at City University of Hong Kong (CityUHK), has conducted a groundbreaking study over two decades to illuminate the overlooked methane emissions emanating from urban sewer systems. This comprehensive research culminated in the development of the first globally applicable methodology and tool designed expressly for estimating methane emissions from sewer networks. Such an innovation not only challenges prevailing assumptions but also heralds a paradigm shift in how urban wastewater management’s environmental impact is assessed and addressed.
Historically, the scientific consensus has largely dismissed urban sewers as significant methane sources due to the presumed insufficient residence time of wastewater within pipes to foster substantial methane production. Consequently, global greenhouse gas inventories, including those compiled by the Intergovernmental Panel on Climate Change (IPCC) and numerous national agencies, have traditionally assigned a zero-emission status to sewer methane outputs. This study rigorously contests that assumption through both empirical data and advanced modeling techniques.
Central to the team’s approach is the SeweX model, a sophisticated simulation tool originally developed in 2008 under Professor Yuan’s guidance. SeweX uniquely integrates the physical, chemical, and biological dynamics occurring within sewer systems, enabling it to predict the generation of hydrogen sulfide and methane accurately. Given the scarcity of real-world methane flux data from sewers, the team undertook extensive data collection campaigns across Australian sewer networks, leveraging custom-designed online sensors to provide vital calibration and validation inputs for the model.
This methodological rigor allowed the researchers to simulate nearly 3,000 disparate pipeline conditions, encompassing variations in pipe dimensions, slopes, flow rates, and ambient wastewater temperatures. Their analyses revealed that methane emissions are intricately linked to the wetted surface area inside sewer pipes, a finding that facilitated the refinement of a streamlined estimation model. By integrating easily accessible parameters such as pipe size, slope, actual versus designed flow rates, and temperature, this model provides a practical and scalable means to estimate methane emissions from sewer systems globally.
The study’s validation phase extended across 21 cities internationally, including locales in Australia, the United States, China, and Belgium. This broad data set affirmed the robustness and predictive accuracy of the developed estimation framework. Applying the model globally, the research estimates that sewer methane emissions range from approximately 1.18 to 1.95 million tons annually. These figures translate into an augmentation of the recognized waste sector methane emissions by 1.7% to 3.3%, and a substantial 16% to 38% increase in the carbon footprint attributed to wastewater management.
This revelation carries profound implications for climate policy and urban environmental management. As urban areas expand and sewer infrastructure proliferates, ignoring methane emissions from these networks risks underestimating overall greenhouse gas outputs significantly. Professor Yuan emphasizes the urgency of incorporating sewer methane into national and international emissions accounting, a move essential not only to enhance inventory accuracy but also to unlock new opportunities for emissions mitigation within urban infrastructure systems.
Beyond policy, the study catalyzes new research trajectories in wastewater management technologies. The integration of methane emission controls into sewer design and operation could be transformative. Existing anaerobic conditions within sewers, once overlooked, may become focal points for bioengineering interventions aimed at methane capture or suppression, potentially converting sewers from passive emitters into active greenhouse gas management sites.
The CityUHK-led initiative exemplifies interdisciplinary collaboration, bringing together experts from The University of Queensland, The Hong Kong Polytechnic University, Tianjin University, and Tongji University. This collective expertise across environmental biotechnology, water engineering, and atmospheric science has been instrumental in overcoming the complex challenges posed by methane quantification in sewer environments.
In summary, this pioneering research overturns longstanding assumptions about urban sewers as negligible methane sources, establishing them instead as significant contributors to global emissions. The implications extend across climate science, urban infrastructure design, and policy frameworks. Accurately recognizing and addressing these emissions can play a crucial role in meeting global climate mitigation targets and advancing sustainable development goals in increasingly urbanized societies.
Subject of Research: Estimation and quantification of methane emissions from global urban sewer networks.
Article Title: Estimating methane emissions from global sewer networks.
News Publication Date: 2-Feb-2026.
Web References: http://dx.doi.org/10.1038/s44221-025-00574-w.
Image Credits: City University of Hong Kong.
Keywords: Methane emissions, sewer networks, greenhouse gases, climate change mitigation, wastewater management, environmental biotechnology, SeweX model, urban infrastructure, global warming, methane quantification.
