The carbon emissions of three typical processes （AAO, MSBR, and oxidation ditch） were systematically analyzed from the perspective of the whole wastewater treatment process based on the annual data of eleven urban small and medium-scale WWTPs in the year 2022, and the effects of different influent characteristics （TP, TN, BOD5, COD, influent volume, and COD/TN） on the carbon emissions were studied by using the partial least squares structural equation modeling （PLS-SEM） method. The results showed that indirect carbon emissions dominated the total carbon emissions of small and medium-scale WWTPs （69.5%）, and carbon emissions from electricity consumption were the largest source （43.1%）； the evaluated carbon emission intensity of the MSBR process [（0.363±0.007） kg·m-3] was lower than that of the AAO process [（0.439±0.099） kg·m-3] and oxidation ditch process [（0.396±0.025） kg·m-3], which had a superior level of low-carbon operation. The carbon intensity of the various components of small and medium-scale WWTPs was as follows： biological treatment （58.7%） > sludge dewatering+deodorisation （17.9%） > deep treatment （17.3%） > pre-treatment （6.1%）； the biological part accounted for 51.5%, 53.9%, and 73.1% of the total carbon intensity of the three processes, respectively. In addition, the influent COD concentration and COD/TN had significantly positive （path coefficients of 0.584 and 0.5） and negative （path coefficient of -0.416） impacts on the direct carbon emissions （N2O and CH4, respectively）, and the influent concentrations of TN, BOD5, and TP had significantly positive effects on the indirect carbon emissions （electricity consumption） and indirect carbon emissions （material consumption）, with path coefficients of 0.078, 0.537, and 0.5, respectively. Carbon reduction in small and medium-scale WWTPs can be carried out by reducing carbon intensity, decreasing carbon operation levels, and strengthening government management.
Keywords: carbon accounting; carbon emission; carbon reduction; low-carbon operation; wastewater treatment plants（WWTPs）.