Identification and quantification of the separate roles of compression and of cake build-up in membranes under different fouling conditions can provide a better understanding of filtration and MBR behavior. A simple and rapid flux-step method for evaluation of critical flux, subcritical and supracritical fouling rates and in situ determination of internal and external irreversible fouling resistance and compressibility, without provoking alteration of the system and without stopping the treatment process is proposed. The main novelty of the modified method is the inclusion of a new stage, termed pre-compression, by means of a flux increase at the beginning of each filtration step. The incorporation of pre-compression phases suppresses the time dependent compression of the irreversible fouling that interferes with the accurate determination of fouling rate, even in highly compressible fouled membranes. Hysteresis tests confirmed that extra fouling clue to the pre-compression stages is reversible. The compressibility index of external irreversible fouling rose as high as 0.59, pore blocking compressibility was significantly small and reversible fouling compression effect was not observed in short-term test. A novel method was employed to determine the scouring effect and to measure the efficiency of in situ chemical cleaning methods applied to an AnMBR. (C) 2013 Elsevier B.V. All rights reserved.