Emission of greenhouse gases from sewer networks: field assessment and isotopic characterization

Sewer networks play a vital role in sewage collection and transportation, and they are being rapidly expanded. However, the microbial processes occurring within these networks have emerged as significant contributors to greenhouse gas (GHG) emissions. Compared to that from other sectors, our understanding of the magnitude of GHG emissions from sewer networks is currently limited. In this study, we conducted a GHG emission assessment in an independent sewer network located in Beijing, China. The findings revealed annual emissions of 62.3 kg CH₄ and 0.753 kg N₂O. CH₄ emerged as the primary GHG emitted from sewers, accounting for 87.4% of the total GHG emissions. Interestingly, compared with main pipes, branch pipes were responsible for a larger share of GHG emissions, contributing to 76.7% of the total. A GHG emission factor of 0.26 kg CO₂-eq/(m·yr) was established to quantify sewer GHG emissions. By examining the isotopic signatures of CO₂/CH₄ pairs, it was determined that CH₄ production in sewers primarily occurred through acetate fermentation. Additionally, the structure of sewer pipes had a significant impact on GHG levels. This study offers valuable insights into the overall GHG emissions associated with sewer networks and sheds light on the mechanisms driving these emissions.