BACKGROUND: Both laboratory-scale and pilot-scale liquid-solid circulating fluidized bed (LSCFB) bioreactors have demonstrated excellent biological nutrient removal (BNR) from municipal wastewater. In this study, a model for the LSCFB for biological nutrient removal has been developed, calibrated, and validated using pilot-scale experimental results. RESULTS: An efficient reactor arrangement predicted anoxic-anaerobic and aerobic biofilm thicknesses of 150-400 and 70-175 mu m in the riser and downer, respectively. Furthermore, distribution of chemical oxygen demand (COD), NH4-N, NOX-N, and dissolved oxygen in the biofilm, as well as nutrients removed in the aerobic and anoxic zones, reflect nitrification, denitrification and enhanced biological phosphorus removal in the LSCFB. The model predicted both anoxic effluent and final effluent COD, SCOD, SBOD, NH4-N, NO3-N, TKN, TN, PO4-P, and TP were within the 95% confidence intervals of the experimental data. Model-predicted simultaneous nitrification/denitrification occurring in the aerobic downer. CONCLUSION: This model developed for LSCFB using the AQUIFAS biofilm diffusion model successfully evaluated the process performance. It is an efficient tool for further research, design, and optimization of the fixed film bioreactor. (C) 2010 Society of Chemical Industry