In this study, an extended activated sludge model was established to describe the transformation of nitrite (S-NO2), nitrate (S-NO3) and other components in A 20 process when mixed liquid recycling ratio (MLRR) varied. The significant differences between the extended model and other models are listed as follows: (1) the contribution and processes of heterotrophs (X-H) which used different carbon sources to reduce S-NO2 and SNO3 under anoxic conditions, (2) the contribution and processes of phosphorus accumulating organisms (X-PAO) which reduce S-NO2 and S-NO3 under anoxic condition and (3) the contribution and two-stage nitrification processes of ammonia oxidizing bacteria (X-AOB) and nitrite oxidizing bacteria (X-NOB) which oxidized ammonia (S-NH4) and S-NO2 under aerobic condition. The results showed that the variation of S-NO2 and S-NO3 could be modeled successfully using the extended model. The mu(AOB) and mu(NOB) were 0.8 and 0.4 day(-1), respectively. Y-AOB value was 0. 18, and Y-NOB value was 0.06. According to model simulation, X-H, X-PAO, X-AOB and X-NOB concentrations were 1081-1203, 377-407, 19-21 and 11-12 mg L-1 in three test runs, respectively. From anoxic tank to aerobic tank, X-AOB and X-NOB increased from 20 to 21 mg L-1 and from 11 to 12 mg L-1 when MLRR was equal to 0.5 but their concentrations maintained at 20 and 11 mg L-1 in both tanks when MLRR was equal to 2.0. In the aerobic effluent, XH decreased from 1203 to 1164 mg L-1 and X-PAO increased from 397 to 407 mg L-1 when MLRR increased from 0.5 to 2.0. The ratio of total nitrifying species to total active biomass was about 2% in each tank. (C) 2007 Elsevier Ltd. All rights reserved.