Membrane-aerated biofilms (MABs) are an effective means to achieve nitrification and denitrification of wastewater. In this research, microsensors, fluorescence in situ hybridization (FISH), and modeling were used to assess the impact of bulk liquid biological oxygen demand (BOD) concentrations on the activity and microbial community structure of nitrifying MABs. With 1 g m(-3) BOD in the bulk liquid, the nitrification rate was 1.3 g N m(-2) day(-1), slightly lower than the 1.5 g N m(-2) day(-1) reported for no bulk liquid BOD. With bulk liquid BOD concentrations of 3 and 10 g m(-3), the rates decreased to 1 and 0.4 g N m(-2) day(-1), respectively. The percent denitrification increased from 20% to 100% when the BOD increased from 1 to 10 g m(-3) BOD. FISH results indicated increasing abundance of heterotrophs with increasing bulk liquid BOD, consistent with the increased denitrification rates. Modeling was used to assess the effect of BOD on nitrification rates and to compare an MAB to a conventional biofilm. The model-predicted nitrification rates were consistent with the experimental results. Also, nitrification in the MAB was much less sensitive to BOD inhibition than the conventional biofilm. The MAB achieved concurrent nitrification and denitrification, whereas little denitrification occurred in the conventional biofilm.