The evaluation of GHG emissions from Shanghai municipal wastewater treatment plants based on IPCC and operational data integrated methods (ODIM)

Due to the rapid wastewater treatment plants (WWTPs) development and the China's 2060 goal of carbon neutrality, the contribution of greenhouse gas (GHG) emissions from WWTPs should not be overlooked. Thus, the GHG accounting method, featured with site-specific and real operation data targeting city-level WWTPs, is scientifically and pragmatically called for. In this paper, we compared IPCC (IPCC 2006 and IPCC 2019), optimized IPCC and operational data integrated methods (ODIM) for the direct N2O and CH4 emissions for total 50 WWTPs in Shanghai. Data for all WWTPs were collected for ODIM and indirect GHG emissions evaluation, covering frequently used wastewater treatment processes in China such as anaerobic anoxic oxic, oxidation ditch, sequencing batch reactor, anoxic oxic, biofilm and membrane bio-reactor. The factors affecting the total GHG emissions intensity were also analyzed. The results showed that, in comparison with IPCC 2019, IPCC 2006 underestimated both direct N2O and CH4 emissions for a total percentage of 87%. What's more, IPCC 2019 overestimated by a percentage of 355% compared to ODIM due to the general default values. ODIM considered the significant differences in emission factor values of specific treatment process, making use of the local operational data to achieve more accurate city-level estimation with direct GHG emissions of 243,320 tCO2-eq in 2016. While indirect emissions were 511,851 tCO2-eq, accounting for 68% of total direct and indirect emissions. Average GHGs emission intensity for Shanghai WWTPs was evaluated as 0.290 kgCO2-eq/m3 while the specific WWTP intensity varied between 0.268 and 0.738 kgCO2-eq/m3. In detail, WWTPs with AAO process, large-scale (>100 k m3/day), high load rate (80%–100%), low theoretical oxygen consumption (0.2– 0.4 kgO2/t), influent COD of 150– 250 mg/L, NH3-N of 15– 25 mg/L and discharge limits B demonstrated lower amounts of total GHG emissions than other WWTPs. Shanghai cases in this study provided a reference for future city-level WWTPs GHG management in China.