The aim of this work was to study an externally-submerged membrane bioreactor for the cyclic extraction of volatile fatty acids (VFAs) during anaerobic fermentation, combining the advantages of submerged and external technologies for enhancing biohydrogen (BioH(2)) production from agrowaste. Mixing and transmembrane pressure (TMP) across a hollow fiber membrane placed in a recirculation loop coupled to a stirred tank were investigated, so that the loop did not significantly modify the hydrodynamic properties in the tank. The fouling mechanism, due to cake layer formation, was reversible. A cleaning procedure based on gas scouring and backwashing with the substrate was defined. Low TMP, 10(4) Pa, was required to achieve a 3 L h(-1) m(-2) critical flux. During fermentation, BioH(2) production was shown to restart after removing VFAs with the permeate, so as to enhance simultaneously BioH(2) production and the recovery of VFAs as platform molecules. (C) 2015 Elsevier Ltd. All rights reserved.