Biofilm systems have been extensively used for treating different types of wastewater. Difficulty in determination of microbial mass in fixed-film reactors has been always the greatest problem in evaluating effects of loading rates on the microbial population in such reactors. For this reason, the effect of operating parameters such as organic loadings on the available microbial mass in the system and solids retention time (SRT) have not been discussed in detail. In this study an innovative methodology was developed to simulate the quantity of microbial mass in an aerated submerged fixed-film reactor (ASFFR) reactor. After determination of kinetic parameters, a dynamic model was developed using STELLA, popular dynamic modeling software, to simulate the microbial mass in the reactor at run time. The pilot plant study was performed with two different surface media and at different loading rates from 2.37 to 19.56 g m(-2) d(-1). Furthermore, the effect of different organic loadings on the accumulation of microbial mass and SRT have been studied and the relevant mathematical relationships were presented. This method makes the evaluation of biofilm system simple and practical without taking samples to quantify microbial mass in reactors. (c) 2006 Society of Chemical Industry.