[Effects of Upgrading Wastewater Treatment Plants on Carbon Emissions]

Sewage treatment systems are a major source of urban carbon emissions. As sewage discharge standards increase, identifying the impact of upgrading sewage treatment plants on carbon emissions is essential. In this study, a sewage treatment plant with the typical anaerobic/anoxic/aerobic (A²/O) biological treatment process in Xi'an City was selected. Based on the IPCC National Greenhouse Gas Inventory Guide and carbon emission factor method, the carbon emissions of the sewage treatment system before and after upgrading were estimated. The results showed that the removal rates of COD, BOD₅, NH₃-N, and TP reached 95% or more after upgrading the wastewater treatment plant. The sewage treatment system's annual carbon emissions (measured by CO₂-eq, the same below) increased by 15 065.65 t, the carbon emissions per ton of water increased by 0.103 kg, and the cost per ton of water treatment increased by 0.14 yuan. The direct carbon emission intensity of the sewage treatment was 0.083 kg·m^-3, which increased by 9.19% compared with that before the transformation. The indirect carbon emission intensity was 0.412 kg·m^-3, which increased by 25.8% compared with that before the transformation. Direct and indirect carbon emissions from the sewage treatment accounted for 16.82% and 83.18%, respectively. The direct carbon emission intensity of the sludge treatment was 0.246 kg·m^-3, which increased by 4.98% compared with that before the transformation. The indirect carbon emission intensity was 0.004 7 kg·m^-3, which increased by 4.73% compared to that before the transformation. Direct and indirect carbon emissions from the sludge treatment accounted for 98.11% and 1.89%, respectively. The purpose of carbon reduction in sewage treatment plants can be achieved by establishing photovoltaic power stations in sewage treatment plants, controlling the concentration of DO and NO₂^--N within a certain range, or establishing sludge digestion ponds and using of biogas cogeneration.