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The effects of four different filtration modes (i.e. relaxation, backwash, mixed and continuous) producing the same flux productivity (time average flux) on membrane fouling were investigated in membrane bioreactors (MBRs). The fouling behaviour was found to be strongly dependent on the applied instantaneous flux rather than the filtration modes themselves. The transmembrane pressure (TMP) obtained after 24h of filtration was dominated by the fouling rates calculated within the first hour of the experiment. After the filtration experiments, the resulting fouling layers were fractionated by rinsing, backwashing and then chemical cleaning, with the foulant removal reflecting the strength of attachment to the membrane. An analysis of the three different fouling layers provided a unique insight into the composition (protein and carbohydrate) and spatial distribution of the particulate and soluble foulants. The upper fouling …