BACKGROUND: This study investigated utilization kinetics of ammonia-nitrogen induced by chronic sulfamethoxazole (SMX) exposure on biomass with an enriched nitrifying community. A lab-scale activated sludge system was supplied with 100 mg COD L-1 of peptone mixture and 50 mg N L-1 of ammonia nitrogen at a SRT of 15 days. At steady-state, the reactor was operated with additional daily SMX dosing of 50 mg L-1 for 35 days. Profiles of oxygen uptake rates and nitrogen fractions obtained in respirometric/batch experiments were used for estimation of nitrification kinetics. RESULTS: Acute inhibitory impact of SMX was expressed with high levels of half saturation constants and endogenous decay rates. After 35 days, half saturation constants significantly increased while higher active biomass fractions and recovered endogenous decay rates were estimated. At the end of the acclimation phase, 92% nitrification was achieved with 10 mg L-1 SMX utilization. CONCLUSION: Interpretation of modeling results with the outcomes of molecular analysis facilitated explanation of autotrophic/heterotrophic bacteria behavior in the course of SMX exposure. Removal of SMX may be attributed to co-metabolism with ammonia oxidation and/or activity of SMX utilizing heterotrophic bacteria. (C) 2017 Society of Chemical Industry