Spatial and Temporal Distribution of Greenhouse Gas Emissions from Municipal Wastewater Treatment Plants in China from 2005 to 2014

Municipal wastewater treatment plants (MWWTPs) are considered significant artificial sources of greenhouse gas (GHG) emissions, in the forms of methane (CH4) and nitrous oxide (N2O), during their normal operations. In this study, we used an emission factor method to determine the spatial and temporal distribution characteristics of GHG emissions from MWWTPs in China during the period 2005-2014; influencing factors and uncertainties were also analyzed. Our results show that total GHG emissions from Chinese MWWTPs increased from 326.54 to 1294.03 Gg CO2-eq between 2005 and 2014, and that regional distribution was extremely variable. During this decade, the proportion of CH4 in the total GHG emissions decreased from 74.1 to 59.4% while that of N2O increased from 25.9 to 40.6%. The observed increase in N2O was probably due to the enhancement of wastewater discharge standards for nitrogen discharge, resulting in lower waterborne but higher atmospheric levels of nitrogen oxides. Our comparison of GHG emissions from wastewater discharging directly to the aquatic environment with that treated at MWWTPs in 2014 indicate that the latter disposal method resulted in an 18-fold drop in GHG emissions. Regional economic development level and wastewater treatment capacity were the factors most closely related to GHG emissions from MWWTPs; the per-capita protein supply was closely related to N2O emissions.