This paper sets out to characterize the sludging dynamics in a MBR hollow fibre system. Rates of sludge;accumulation inside the membrane bundle and rates Of Sludge removal by aeration are evaluated in order to reach the optimal use of membrane aeration, avoiding any irreversible sludging. Experiments were performed in a submerged membrane bioreactor. Capillary membrane modules were placed oil the outside of the bioreactor ill a side stream position. Air was injected below each membrane module, ensuring vertical air lift circulation. During experiments, two air flow rates were compared (100 and 300 L/h). Some membrane air bubbling characteristics are described in terms of bubble size and bubble penetration inside the HF bundle. Results showed that membrane air flow rates appeared as a determining criterion to minimize sludging when filtering under intensive conditions (30 LMH permeate flux, 8-10 g/L TSS) with all air flow rate of 300 L/H allowing continuous functioning, and all air flow rate of 100 L/h which did not. Specific experiments to evaluate sludge removal rates from the HF bundle showed the high capacity of aeration to remove Sludge from the HF bundles as soon as no dead zones appeared. These dead zones corresponded to local dried sludge accumulation (10-12% DM). The compactness of such dried deposit appeared non-reversible even when practising the higher 300 L/h membrane air flow rate, inducing a more intensive filtration condition in the working zone and more intensive irreversible sludging.