Three different methods for fi ltration characterization in Membrane Bioreactor (MBR) systems were compared. These were the Delft Filtration Characterization Method (DFCm), the Berlin Filtration Method (BFM) and an ex situ side-stream fi ltration test cell for the determination of the critical fl ux. The ex situ fi ltration test cell and the DFCm fi lter activated sludge from a tank, while the BFM works in situ with a test cell directly submerged into the biological tank at similar operational conditions to a typical MBR plant. The mixed liquor of four different MBR units was characterised several times with the three fi ltration methods. The three tested methods seemed to agree in the classifi cation of the tested mixed liquors in terms of fi lterability except for one of the tested activated sludges. Additionally, three critical fl ux protocols were studied using the BFM fi ltration test cell. The fi rst consisted in the classical fl ux-step method, the second included relaxation between fi ltration steps and in the third protocol, 2 min fi ltration at a fi xed fl ux were performed before every fi ltration step. The last protocol was selected as the most representative of full scale MBR operation and the most interesting one for giving valuable information about the irreversibility of the fouling.