Waste removal efficiency of gas-liquid biofilter reactors for waste water treatment depends on its flow regime and residence time distribution (RTD) as key parameters of bio-reactor performance. The present study reports RTD regime in a fibrous fixed bed biofilm reactor related to a fluid velocity range appropriate for biofilm operation. The data from tracer experiments are correlated in terms of the one-parameter "tanks-in-series" model. The aerated fibrous bed reactor RTD function is found to be dependent on net liquid and gas phase superficial velocity U(L) and U(G). Liquid internal recirculation exhibited small effect comparable with the effect of net liquid flow. A power law relationship relating the number of perfectly mixed cells with liquid and gas superficial velocity is elaborated. Assuming similarity of the prototype and real vessels' flow fields, the equation as well as its corresponding range of fluid velocity can be used for bio-reactor design and scale-up. Comparison over the model parameters obtained in fixed bed bubble columns at low fluid velocity shows the results of this study to be comparable with previous data of mesh wire packing. (C) 2009 Elsevier Ltd. All rights reserved.