Methane emissions from sewer networks are an important source of anthropogenic greenhouse gases (GHGs) but are not currently reflected in the national GHG inventory. We found significant CH4 emissions of approximately 573 [395–831] CH4 t y−1 from sewer networks in the old residential and commercial areas of Seoul (Gwanak district) using an electric vehicle-based atmospheric GHG monitoring platform. The majority of ethane-to-methane ratios (〈0.005) from the observations further suggest that distinctive CH4 emissions from sewer networks are likely related to microbial activity rather than to simple natural gas leakage. Because over 90% of the sewer network in Seoul is a gravity drain type of combined sewer network, where both wastewater and stormwater flow through the same pipes, resulting in the generation of methane emissions from the microbial activity and the manholes and rain gutters, which are directly connected to the combined sewer networks are major sources of atmospheric methane emissions. This study suggests that appropriate treatment of sewer networks can mitigate missing methane emissions in cities that were not originally included in GHG inventory of South Korea.

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