Carbon footprint analysis and comprehensive evaluation of municipal wastewater treatment plants under different typical upgrading and reconstruction modes

The issue of greenhouse gas (GHG) emissions resulting from the upgrading and reconstruction of municipal wastewater treatment plants (MWWTPs) along with improved water quality is receiving attention and research. There is an urgent need to explore the impact of upgrading and reconstruction on carbon footprint (CF) in order to address concerns that the upgrading and reconstruction will increase GHG emissions while improving water quality. Here we accounted for the CF of five MWWTPs in Zhejiang Province, China, before and after three different upgrading and reconstruction models - "Improving quality and efficiency" ("Mode I"), "Upgrading and renovation" ("Mode U") and "Improving quality and efficiency plus Upgrading and renovation" ("Mode I plus U"). The upgrading and reconstruction was found to not necessarily result in more GHG emissions. In contrast, the "Mode I" had a more significant advantage in terms of CF reduction (1.82-12.6 % reduction in CF). Overall, the ratio of indirect emissions to direct emissions (indirect emissions/direct emissions) and the amount of GHG emitted per unit of pollutant removed (CFCOD、CFTN、CFTP) decreased, while both the carbon and energy neutral rates increased significantly (up to 33.29 % and 79.36 % respectively) after all three upgrading and reconstruction modes. In addition, the wastewater treatment efficiency and capacity are the main factors that affect the level of carbon emission. The results of this study can provide a calculation model that can be used for other similar MWWTPs during the upgrading and reconstruction processes. More importantly, it can provide a new research perspective as well as valuable information to revisit the impact of upgrading and reconstruction in MWWTPs on GHG emissions.