The aim of this study was to assess the effect of several operational variables on both biological and separation process performance in a submerged anaerobic membrane bioreactor pilot plant that treats urban wastewater. The pilot plant is equipped with two industrial hollow-fibre ultrafiltration membrane module; (PURON (R) Koch Membrane Systems, 30 m(2) of filtration surface each). It was operated under mesophilic conditions (at 33 degrees C), 70 days of SRT, and variable HRT ranging from 20 to 6 h. The effects of the influent COD/SO4-S ratio (ranging from 2 to 12) and the MLTS concentration (ranging from 6 to 22 g L-1) were also analysed. The main performance results were about 87% of COD removal, effluent VFA below 20 mg L-1 and biogas methane concentrations over 55% v/v. Methane yield was strongly affected by the influent COD/SO4-S ratio. No irreversible fouling problems were detected, even for MLTS concentrations above 22 g L-1. (C) 2011 Elsevier Ltd. All rights reserved.