A novel membrane-based sensor device for upstream membrane integrity monitoring has been developed and evaluated in this study The sensor is based on relative trans-membrane pressures created by two membranes in series inside the sensor device that detects deposition from the sample stream onto the first of the sensor membranes. The sensor pressure signals can distinguish between intact or damaged membranes in the upstream membrane filtration process. Studies were conducted to evaluate both stabilities and sensitivities of the relative trans-membrane pressure monitoring technique. Sensitivity, based on the response times of the membrane sensor for particle detection, was determined for a range of operating conditions, membrane sandwich configurations, and particle concentrations in both simulated membrane failures and for actual pin-hole defects on a submerged MF membrane. The results showed that both sensitivities and stability strongly depended on membrane sandwich configurations (membrane characteristics) in the sensor, and mode of operation (pressurized or vacuum). The membrane sensor detected bentonite particles with a concentration of 0.3 mg/L (turbidity similar to 0.3 NTU) in approximately 35 min in the vacuum mode. The sensor is reliable, sensitive and low cost. It has potential applications in decentralized systems or in multichannel monitoring of local conditions in a large plant. Possible applications of the membrane sensor for fouling monitoring are also discussed. (C) 2008 Elsevier B.V. All rights reserved.