A lab-scale filtration cell was used to determine the optimum relaxation time for membrane bioreactor (MBR) membranes. Sludge from 15 plants was analysed with fixed filtration times and intermittent relaxations of decreasing durations. By determining the average flux, the optimum relaxation time was found to be 0.2-4 min; therefore, the optimal relaxation time should be determined for the actual sludge and not based on the manufacturer's standard. It may also be necessary to change the relaxation protocol as the sludge properties change. Test on the lab-scale filtration cell and a pilot MBR shows that the highest net flux was obtained with the same relaxation time, i.e., the lab-scale filtration cell can be used to optimize the relaxation time in large-scale MBR systems. The permeate flux was significantly higher in the lab-scale filtration cell than the MBR system due to sludging/irreversible fouling in the pilot MBR. Mathematical simulations indicated that approximately 85% of the membrane area in the pilot MBR plant was blocked during operation. The results indicate that the lab-scale filtration cell can be used independently to check both sludge filterability and membrane condition in full-scale MBR systems, and that the optimal relaxation time for large-scale MBRs can be determined in lab-scale systems. (C) 2016 Elsevier B.V. All rights reserved.