Comparative Experimental Study on A²O and MUCT Processes for the Improvement of Discharge Standards and Reconstruction of Wastewater Treatment Plant with SDAO Process

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.