Anaerobic digester is the most widespread technology for the treatment of biomass to produce biogas. Mixing is an important parameter to have a highly efficient anaerobic digestion. Computational fluid dynamics can provide detailed modeling about the hydrodynamics and mixing of an anaerobic digestion. In this research, a computational fluid dynamics analysis is developed to establish the better geometric configuration and mixing system for an anaerobic digester producing biogas. Three different scenarios as continuous stirred tank reactors are considered. External pumps for liquor recirculation are used as mixing system. Flow patterns, media, variance, standard deviation of average velocity and velocity gradient are used as parameters to evaluate mixing. Simulation results suggest that the innovative configuration in which the fluid enters tangent to the lateral surface of the anaerobic digester is the best solution. Media, variance, standard deviation of average velocity and velocity gradient are equal to 0.01522 m/s, 0.00006 m/s, 0.00758 m/s, 75 s(-1) respectively. These ensure to have dead zones equal to the 0.2% of total volume. Shear rate at the bottom is 1.1.10-3 s(-1) in all plane while the power of recirculating pump is 1.9 kw/m(3). Computational fluid dynamics is then a promising technology for analyzing mixing. (C) 2018 Elsevier Ltd. All rights reserved.