Nowadays, waste activated sludge is more and more regarded as an energy source (through subsequent anaerobic digestion) or even as a potential source of fine chemicals (through subsequent fermentation processes). To this end, the organic content of the activated sludge should remain as high as possible implying short sludge residence times (SRT). However, the settling characteristics of the resulting sludge are most often very poor. This fact is overcome by performing the water-sludge separation by low pressure membrane filtration instead of sedimentation. This contribution focuses on the impact of the sludge retention time (SRT) on membrane fouling in such low pressure membrane filtration processes. While the disadvantage of a high SRT is indeed the mineralization of the sludge and the higher biomass concentrations in the reactor, often impairing an efficient oxygen transfer, lower membrane fouling rates are observed at higher SRTs. Indeed, a recent review on membrane fouling in MBRs pinpoints the sludge retention time as an important factor influencing membrane fouling (Drews, 2010). At higher SRTs, the activated sludge appears to be more robust and fouling rates are less influenced by variations in temperature or in polysaccharide fractions of soluble microbial products. Apart from the SRT, also the particle size distribution is known to have a major influence on membrane fouling (Meng and Yang, 2007; Van den Broeck et al., 2010, 2011). Foulants with smaller sizes than the membrane pores may enter the pores and pore blocking can occur. However, MBR activated sludge flocs and free bacteria are considerably larger than membrane pores of membranes used in MBRs, i.e., 10-50 ?m and 1-2 ?m (Jiang et al., 2003), respectively, compared to 0.01-0.5 ?m. The effect of activated sludge particle size distribution can thus be expected at the level of the cake layer resistance rather than pore blocking. Combining the above findings, this contribution focuses on the effect that the SRT has on the flocs’ particle size distribution, and, hence, on membrane fouling.