Several methods have been proposed over the last decade for the monitoring of fouling processes in membrane bioreactors (MBRs). Many of these methods aim to acquire information measuring the transmembrane pressure (TMP) and permeability (K) directly on the membrane modules used in wastewater treatment plants. This study assessed in a critical light the use of TMP and K for membrane fouling monitoring. TMP and K were directly measured on a pilot-scale submerged MBR operated continuously for approximately 450 d at different solids retention times and fed with real municipal wastewater. The results showed that under standard operating conditions such as those usually applied in full-scale MBR and with the monitoring instruments typically employed in industrial applications, the identification of specific TMP trends within a filtration phase was often problematic and the results were unreliable, mainly because of the noise and the variability of real wastewater and environmental conditions. Moreover, a non-linear trend of the pressure behaviour within a filtration phase was observed when fouling phenomena were taking place. This study also proposes the use of the ratio between filtration and backwash permeability in order to assess whether the membrane flux is deviating from the "pressure controlled region" and, consequently, fouling processes are occurring. (c) 2013 Elsevier B.V. All rights reserved.