Abstract:The total design capacity of WWTP in a northern city is 18.0×104 m3/d, in which the design capacity of expansion project is 8.0×104 m3/d, and the effluent quality standard of WWTP is the first class A standard specified in Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant (GB 18918—2002). According to the influent quality and discharge requirements, the process of "primary settling tank - multi-stage AO biochemical tank - high-speed air flotation tank - denitrification deep bed filter" is adopted in the design. The technological process of the expansion project, the design parameters of the main structures and the design characteristics are introduced, and the operation effects are analyzed. The actual operation data show that various indexes of the effluent can be all stable up to the standard. The average removal rates of COD, SS, NH3-N, TN and TP are 88.90%, 96.24%, 97.31%, 78.00% and 92.87% respectively.