Abstract: The effects of applying the cyclic activated sludge technology (CAST) process embedded with moving bed biofilm reactor (MBBR) on pollutant removal, electricity consumption, chemical consumption, and carbon emission were investigated by comparing and analyzing the operational data of a wastewater treatment plant (WWTP) in Zhejiang Province before and after upgrading and retrofit. The results showed that MBBR could strengthen the biological removal of nitrogen and phosphorus and improve the anti-shock loading capacity of the system. After retrofitting, the water quality of the effluent could meet the Zhejiang Provincial Standard for Discharge of Major Water Pollutants for Municipal Wastewater Treatment Plant (DB 33/2169-2018) stably, with the annual average values of COD, NH₃-N, TN and TP being 13.7, 0.2, 4.3 and 0.05 mg/L, respectively. There was no significant change as to the distribution of electricity consumption within the WWTP after retrofitting. The electricity consumption per ton water increased by 19%, while the annual consumption of external carbon source, coagulants and disinfectants decreased by varying degrees, and the total chemical consumption per ton water decreased by 44%. The carbon emission of the WWTP decreased from 1.04 kg/m³ to 0.79 kg/m³ after retrofitting, and the direct methane emission and material consumption in the treatment process contributed greatly to the overall carbon emission of the WWTP.