Carbon accounting of A2O process based on carbon footprint in a full-scale municipal wastewater treatment plant

The traditional carbon accounting model for wastewater treatment plants (WWTPs) only calculates the total amount of each greenhouse gas (GHG), which has no guiding significance for the net zero carbon transformation of WWTPs under the goal of carbon neutrality, and the system boundary is only within the WWTPs. In this paper, the accounting boundary is extended to the outside of the plant, and the CO2 produced by the pumping pump outside the plant and the sludge transport is included in the accounting, which improves the accuracy of the results. In addition, the WWTP is divided into three stages, which are the pretreatment stage, biological treatment stage, and sludge treatment stage. The emission sources in each stage are calculated in detail to determine the key emission reduction targets in each stage. The research shows that the main carbon emission source in the pretreatment stage is indirect CO2 produced by chemicals, the main carbon emission source in the biological treatment stage is N2O emission, and the main carbon emission source in the sludge treatment stage is indirect CO2 produced by electricity consumption. We also found that the carbon intensity of the biological treatment stage was 1.8 times that of the other two stages. Therefore, carbon emission reduction in the biological stage is the focus of plant emission reduction.