In this study, the impact of membrane incorporation for effluent extraction on the performance of an upflow anaerobic sludge blanket (UASB) reactor treating municipal wastewater was investigated in terms of system performance and sludge characteristics. A laboratory-scale system was operated in two different operational stages: (i) as a sole UASB reactor and (ii) as an anaerobic membrane bioreactor (AriMBR). Membrane incorporation induced an accumulation of tine particles and a decrease in extracellular polymeric substances (EPS), resulting in a decrease in particle size distribution (PSD) and, thus, a drop in sludge setheability. Deterioration of sludge setheability led to an increase in sludge washout, with a resultant increase in chemical oxygen demand (COD) and total suspended solids (TSS) in the UASB effluent. However, suspended solids-free permeate with an average COD of 42 mg/L was obtained and despite the sludge bed deterioration, the average transmembrane pressure (TMP) value was 85 mbar indicating no severe membrane fouling. Pyrosequencing analysis demonstrated the increase in microbial community indexes of the UASB sludge after membrane addition, exhibiting an increase in both richness and evenness. Considering all responses of the UASB reactor to membrane incorporation, the overall results put UASB reactor forward as a suitable alternative for coupling membranes in AnMBRs. (C) 2014 Elsevier B.V. All rights reserved.