Steady state operational data from a pilot scale circulating fluidized bed bioreactor (CFBBR) during biological treatment of landfill leachate, at empty bed contact times (EBCTs) of 0.49, and 0.41 d and volumetric nutrients loading rates of 2.2-2.6 kg COD/(m(3) d), 0.7-0.8 kg N/(m(3) d), and 0.014-0.016 kg P/(m(3) d), was used to calibrate and compare developed process models in BioWin (R) and AQUIFAS (R). BioWin (R) and AQUIFAS (R) were both capable of predicting most of the performance parameters such as effluent TKN, NH4-N, NO3-N, TP, PO4-P, TSS, and VSS with an average percentage error (APE) of 0-20%. BioWin (R) under-predicted the effluent BOD and SBOD values for various runs by 80% while AQUIFAS (R) predicted effluent BOD and SBOD with an APE of 50%. Although both calibrated models, confirmed the advantages of the CFBBR technology in treating the leachate of high volumetric loading and low biomass yields due to the long solid retention time (SRT), both BioWin (R) and AQUIFAS (R) predicted the total biomass and SRT of CFBBR based on active biomass only, whereas in the CFBBR runs both active as well as inactive biomass accumulated. (C) 2011 Elsevier B.V. All rights reserved.