Computational Fluid Dynamics (CFD) applied to the study of wastewater treatment processes is a tool that is being explored in order to optimize the reactor’s configuration. This study aimed to find the best configuration to mimic the effect of the biogas produced in UASB bioreactors, in order to validate CFD models using PIV techniques. For this purpose three different configurations of gas inlet were simulated using the software Fluent. The simulations were carried out in a two phase, laminar flow model, using eulerian-eulerian approach and two different geometries. In the first geometry the gas inlet was placed at the top of a liquid distribution system. Two configurations were simulated in the second geometry: the gas being fed through the same entrance of the liquid and the gas inlet through concentric rings placed at the bottom of the reactor. The velocity profiles showed that the use of concentric rings at the bottom was the be best option, once it creates the best mixing.