Sewers have been hiding a climate problem in plain sight

Methane is the second-largest greenhouse gas after carbon dioxide. According to the Climate and Clean Air Coalition, anthropogenic methane emissions account for nearly 45% of current net warming, making it an important factor in global warming. An international research team led by a scholar from City University of Hong Kong (CityUHK) has broken through ¹ the overlooked role of sewers as a source of methane, developing the first-ever globally applicable estimation tool and offering a new perspective on mitigating climate change.

Sewers’ overlooked role in warming

The team estimates that sewers worldwide emit 1.18 to 1.95 million tons of methane annually. This reveals that global wastewater management has a far more significant impact on greenhouse gas emissions and global warming and is crucial for improving greenhouse gas emissions accounting and promoting global emissions reduction.

Challenging assumptions about sewer emissions

It is generally assumed that the limited residence time of wastewater in sewer networks is not conducive to methane formation, and that the overall emissions are difficult to monitor and quantify. Therefore, in current greenhouse gas inventories by the IPCC and various countries, urban sewers are considered a negligible source of methane emissions, assumed to be zero.

However, sewage is rich in biodegradable organic matter, and anaerobic conditions are prevalent in sewer networks, providing a viable environment for methane formation.

From SeweX model to global tool

Professor Yuan’s research team has been dedicated to developing innovative solutions for wastewater systems and environmental biotechnology. As early as 2008, the team successfully developed the SeweX model, which simulates in-sewer physical, chemical and biological processes, including predicting the generation of hydrogen sulfide and methane.

To address the lack of field data for calibrating the methane prediction components of the SeweX model, the team collected data from sewer networks in Australia using a customized online sensor for model calibration and validation.

With the calibrated SeweX model, the team simulated nearly 3,000 different pipeline scenarios with varying structures and operating conditions. Their findings ultimately confirmed that methane generation in sewers is closely related to the wetted pipe surface. They proposed a simplified methane estimation model for sewer systems. With data such as pipe size, slope, designed and actual average dry weather flows, and wastewater temperature, the model can estimate methane emissions from sewers.

The team validated the model using real-world data from 21 cities in Australia, the United States, China and Belgium, thus successfully developing this comprehensive sewer methane emission estimation tool.

Global impact and climate implications

Using this innovative tool, the team estimates that global sewer systems emit approximately 1.18 to 1.95 million tons of methane annually, adding 1.7% to 3.3% to the currently estimated global methane emissions by the waste sector, and approximately 16% to 38% to the estimated overall carbon footprint of wastewater management.

Professor Yuan remarked, “Our research confirms that sewers are not a zero-emission source; rather, they represent a quantifiable source of methane emissions with significant global climate implications. As urban sewers continue to expand, their potential methane emissions will also increase. Therefore, including them in the greenhouse gas accounting system will help improve national greenhouse gas inventories and provide a new entry point for emissions reduction, further advancing the global goal of sustainable development.”