The aim was to investigate the influence of mixer design and operating conditions in an anaerobic digester designed for BioH(2) production. Distributive mixing was studied through different dual-impeller configurations with various impeller size, clearance and types. Advanced optical experimental techniques, such as Particle Image Velocimetry (PIV) and Planar Laser Induced Fluorescence (PLIF) were used to determine mixing time and the flow pattern in the bioreactor. PLIF, decolorization and conductimetric methods were compared for mixing analysis. CFD was used to achieve a more detailed analysis of hydrodynamics and mixing. Experiments and simulations were limited to power input lower than 10 W/m(3), which is a necessary condition to achieve sustainability and corresponds to 50-150 rpm in this work. Nine designs of dual-impeller configurations were compared in an unbaffled tank. Experimental results suggest that mixing was strongly modified by changing the geometry of the lower impeller and, to a lesser extent, the off-bottom clearance and the injection position for dual-impeller devices. Macromixing time t(m) derived from the conductimetric technique always agreed with PLIF, but was higher than values from chemical decolorization when an axial-tangential flow pattern validated by PIV and CFD data was reported. CFD was able to predict the mean flow and the turbulence features, so that their respective role on mixing could be compared. Finally, a non-conventional mixer dual-impeller device was shown to counterbalance lower turbulence and power input by an enhanced axial flow circulation that made t(m) weakly dependent on the position of the injection and favored dispersive mixing. (C) 2017 Elsevier B.V. All rights reserved.