Abstract: The carbon-emission-factor approach was used to conduct the total carbon emissions and carbon emission intensity of 4 medium-sized sewage treatment plants with similar scale but different A²O processes in Shantou. The results show that the total carbon emissions with different A²O processes are different. The total carbon emission intensity of the 4 sewage treatment plants ranged from 0.317 to 0.402 kg/m³(CO₂-eq). The carbon emissions in order from smallest to largest based on the processing are: multi-stage AO, A²O+anaerobic/hypoxia regulating tank, traditional A²O, A²O+micro-aeration oxidation ditch. However, the carbon emission categories of different processes are similar, and electricity consumption is the most important carbon emission source for each process. Compared with the traditional A²O process, the other three improved A²O processes have advantages in reducing carbon emission intensity of chemicals consumption, CH₄ emission, N₂O emission and sludge disposal. And the multi-stage AO process shows the greatest ability to reduce the total carbon intensity. Based on the above results, some suggestions for the carbon emission optimization of the improved A²O processes were put forward.