The influence of the sludge treatment histories on the cross-how microfiltration fluxes of submerged membranes was examined using 20 types of sludges such as intermittently aerated, continuously aerated activated sludges and anaerobically digested sludges. The fluxes of the sludges varied from 0.5 to 1.7 m(3).m(-2).d(-1) under the same MLSS and filtration conditions. The variation of the flux was not explained by the different physical properties of the sludges such as mean particle sizes, viscosities and specific filtration resistances. Instead, the variation of the flux was quantitatively interpreted by considering the concentration of particles with sizes from 8 to 15 mu m, which cause the lowest lift velocity values. The assumption, that the particles of this specific size range controlled the flux, was rationalized by the filtration model proposed for the filtration of the mixture of differently sized particles.