Abstract: Based on the biological nitrogen and phosphorous removal mechanisms, the treatment effects and differences of transforming SDAO process into A²O process and MUCT process without additional building and apparent structural modification were studied. The results showed that the removal efficiency of COD_Cr and NH₄⁺-N in both processes could reach above 90.0%, but with the relatively low TN and TP removal efficiencies under the condition of influent C/N=3.25 during the early domestication period. Nitrogen and phosphorous removal efficiency could be considerably improved by supplementing additional carbon source. After improving the C/N to 6.75 and keeping the A²O process at stable operation, the TN and TP removal efficiencies of 75.6% and 78.8% could be reached with effluent TN of 15.6 mg/L and TP of 1.4 mg/L, respectively. The removal efficiency of MUCT process was not as good as A²O: the removal efficiencies of TN and TP were 62.8% and 37.3%, and the average effluent TN and TP were 25.3 and 2.8 mg/L, respectively. Step-feed could improve the removal efficiencies of MUCT: under stable operations, the TN and TP removal efficiencies could be improved to 72.8% and 86.8%, with effluent TN of 20.3 mg/L and TP of 0.9 mg/L, respectively. In conclusion, the A²O process had more obvious advantages than the MUCT process when used for the improvement of discharge standards and for the reconstruction of SDAO process. The effluent COD_Cr, NH₄⁺-N and TN of A²O process met the requirements of Class 1A discharge standards of the National Standard (GB 18918-2002); while the effluent TP could meet the Class 1B standard, and additional chemical phosphorus removal was thus required to meet the Class 1A discharge standard.