This work deals with the optimization of mixing in an anaerobic submerged membrane bioreactor (AnMBR) devoted to the production of biohydrogen as a 2nd generation biofuel using the dark fermentation process from lignocellulosic waste. The AnMBR consists of an unbaffled mechanically-stirred tank equipped with a two-stage radial impeller which is coupled to an external hollow fibre membrane module placed in a forced circulation loop with permeate suction. In the tank, mixing conditions must enable the suspension of solid waste, homogenize local concentrations, and enhance hydrogen desorption simultaneously. For optimizing reactor design, mixing was investigated both experimentally and numerically. The computational strategy consisted in the combination of 1D and 3D methodologies including single-phase and two-fluid CFD models. The experimental approach encompassed RTD and mixing time measurements, the analysis of vortex formation and the description of solid suspension using straw as waste. The results showed that a laminar flow pattern prevailed in the recirculation loop and the membrane unit, while turbulent flow was observed in the stirred tank in which mixing time was far smaller than residence time in single-phase flow. A compromise was defined using CFD, which prevented vortex formation and promoted homogeneous suspension, by optimizing impeller position and rotation speed. The positions of the inlet and the outlet of the forced circulation loop in the tank were deduced from CFD simulations and validated by experimental data. Finally, this work confirms the potential interest of AnMBR for biohydrogen production and the applicability of CFD for the optimization of abiotic factors. (C) 2015 Elsevier B.V. All rights reserved.