Impact of upgrading on carbon emission characteristics of township sewage treatment plants

To achieve carbon peak and neutrality targets and control watershed water pollution, township sewage treatment plants (STPs) need to collaborate in reducing pollution and carbon emissions. Based on water quality and quantity data from 2016 to 2022 at a municipal sewage treatment plant in a township in Chengdu, the temporal variability of COD, NH₃-N, TP, and TN was analyzed before and after the standard enhancement and upgrading. Using 2006 IPCC Guidelines for National Greenhouse Gas Inventories (2019 revision) and Guidelines for Carbon Accounting and Emission Reduction in the Urban Water Sector, the characteristics of direct and indirect carbon emission intensity were investigated, and the response of carbon emissions to seasons, water quality, and pollutant reduction amounts before and after the standard enhancement and upgrading was explored. The results showed: (1) After upgrading from Cyclic Activated Sludge System (CASS) to Anaerobic-Anoxic-Oxic -Membrane Bioreactor (AAO-MBR), the effluent quality met Emission Standards for Water Pollutants in Minjiang and Tuojiang River Basins of Sichuan Province (DB 51/2311-2016). Moreover, by increasing carbon source, using MBR membrane to trap sludge and other measures, the effluent concentration and water quality indexes were more stable, and the treatment efficiency of pollutants was higher. (2) After upgrading, direct and indirect carbon emission intensities increased by 41.59% and 105.70%, respectively, with values of 0.296 and 1.082 kg/m³ (in terms of CO₂-eq). In summer, the carbon emission intensity was significantly lower than that of other seasons (P<0.01), and indirect carbon emission intensities before and after the upgrade remained higher than direct carbon emission intensity. (3) After the upgrade, the total carbon emission intensity increased by 0.643 kg/m³(in terms of CO₂-eq). Due to increasing electricity consumption caused by upgrading, the indirect carbon emission intensity significantly changed. Therefore, the township STP upgrading could not only improve treatment efficiency but also increase carbon emissions. It was recommended to consider pollutant removal and energy consumption control synergistically during process transformation to achieve pollution reduction and carbon emission reduction simultaneously.