The membrane-aerated biofilm reactor (MABR) is a promising technology for wastewater treatment, especially for simultaneous organic and nitrogen removal. The mass transfer phenomena induced by flow velocity and flow pattern is required in order to improve the reactor design and the removal pollutants efficiency. Tracer experiments and residence time distribution (RTD) theory were used to characterize the flow in a special MABR. The liquid phase flow patterns were investigated through tracer pulse stimulus-response technique using dextran blue as model tracer. RTD curves were analyzed by cold-model tests (axial dispersion model ADM, tanks in series model TIS and mixing cell model MCM). The detailed flow pattern of the reactor was obtained from computational fluid dynamic (CFD) simulation. According to experimental results of RTD studies and CFD simulation, the flow patterns were demonstrated to be analogous to completely mixed flow with deviations of the ideal hydrodynamic behavior; stagnant zones (low fluid velocity) in 85% of its volume, being the remainder a channeling trouble (high fluid velocity). These deviations were quantitatively described (macromixing level-global mixing) with a minimum quadratic error function value of (S) 0.01. The local mixing flow pattern (micromixing level) obtained by CFD allowed determining the location of each zone; the stagnant zone is situated in the area where membranes are located; therefore it is possible to assume that degradation reactions of pollutants would take place in this area. (C) 2015 Elsevier Ltd. All rights reserved.