The aim of this study was to evaluate the long-term performance of hollow-fibre (HF) membranes used to treat urban wastewater in a submerged anaerobic MBR when operating sub-critically. To this end, a demonstration plant with two industrial scale HF ultrafiltration membrane modules was operated under different conditions. The main factor affecting membrane performance was the concentration of mixed liquor total solids (MLTS). The reversible fouling rate remained low even when MLTS levels (about 25 g L-1) in the membrane tank were high. No chemical cleaning was conducted whilst operating the plant for more than one year because no irreversible fouling problems were detected. The almost complete absence of irreversible fouling was mainly attributed to: operating at sub-critical filtration conditions; establishing a proper membrane operating mode; and to the characteristics of the anaerobic environment. No chemical precipitation problems were observed in the membranes due to the relatively low operating pH (always below 7) of the sludge. The biogas sparging encouraged high levels of dissolved CO2 in the sludge, resulting in pH levels below 7 and alkalinity values around 600 mg CaCO3 L-1. (C) 2012 Elsevier B.V. All rights reserved.