Batch experiments with varying initial substrate concentrations and bio-particle volumes were performed on a three-phase fluidized bed biofilm reactor treating simulated domestic waste. The objective was to study the simultaneous oxidation of total organic carbon (TOC) and ammonium nitrogen (NH, ·-N) by biofilm process. The presence of autotrophic nitrifying bacteria in the biofilm, together with the heterotrophic microbes, made possible the simultaneous removal of the nutrients NH; -N and TOC. A simplified mass balance equation for the biofilm was proposed and five different kinetic rate equations were used to establish the degree of correspondence of theoretical to actual data curves. The kinetic parameters were obtained by using nonlinear regression analysis on the set of two differential equations representing the simultaneous TOC and NH -N oxidations. The competitive inhibition model, which described the effect of TOC on NH, -N oxidation rate, was the best-lit model based on average r2 and on the fit of the theoretical curve obtained from numerical simulation using the average of the best fit parameters from individual runs. The study showed that the simplified modeling method provided a fit of the actual data. This study also illustrated that the kinetic model For the biofilm process and its parameters could be easily obtained from batch-experiment results.