Abstract: The Mojiang Sewage Treatment Plant, located in the Lianzhu River Basin, originally adopted the Cyclic Activated Sludge System (CASS) process with a design capacity of 10,000 m³/d, achieving the Class 1B discharge standard. To address water eutrophication and meet the stringent Class 1A standard, the plant was upgraded to an Anaerobic-Anoxic-Oxic (AAO) process coupled with a Membrane Bioreactor (MBR). Post-upgrading, the effluent concentrations of 5-day biochemical oxygen demand (BOD₅), chemical oxygen demand (COD_Cr), ammonia nitrogen (NH₃-N), total nitrogen (TN), and total phosphorus (TP) were stabilized at ≤ 10 mg/L, ≤ 50 mg/L, ≤ 5 mg/L, ≤ 15 mg/L, and ≤ 0.5 mg/L, respectively. The total project investment was 27.5253 million RMB, with an increase in unit operational cost of 0.49 RMB/m³. Compared to the conventional AAO process, the integrated AAO-MBR process reduced the footprint by 30% and enhanced the TP removal efficiency by 15%. Furthermore, the long-term operational cost was lowered by 12% through optimized aeration and sludge disposal. This case demonstrates that the AAO-MBR process effectively handles influent with a low carbon-to-nitrogen ratio and seasonal water quality fluctuations, offering a costefficient solution for the upgrading of medium- and small-scale sewage treatment plants.