In water treatment, removal of arsenite, the most toxic arsenic inorganic form, consists often in its oxidation, followed by precipitation or adsorption step. In this work, this oxidation step was done by a biological process, using an autotrophic bacterial consortium named CAsO1, isolated from a gold-mining site. A pilot plant was built, consisting of two biological fixed beds, inoculated with the CAsO1 consortium and fed with synthetic water spiked with As(III). Firstly, the hydrodynamic parameters were determined. The residence time distribution modelling showed a plug flow distribution with low diffusion phenomena. Secondly, the kinetic analysis, considering a first order reaction, made it possible to calculate the oxidation kinetic constant (0.04 min(-1)) and showed a heterogeneous distribution of the active biomass along the reactor. Batch experiments conducted with parts of the bed-support made it possible to characterize this kind of distribution. Finally, the hydrodynamic and kinetic studies enabled us to propose a design approach for such a biological treatment process. (C) 2009 Elsevier Ltd. All rights reserved.