The relation between activated sludge filterability and mixed liquor suspended solids (MLSS) concentration in membrane bioreactors (MBRs) is framed in a single hypothesis, explaining results seemingly contradictory. A total of 44 activated sludge samples were collected and analyzed On a variety of parameters, i.e. filterability, MLSS concentration, soluble microbial products (SMP) concentrations and particle size distribution in the range of 2-100 mu m and of 0.4-5.0 mu m. The sludge filterability was assessed by using the Delft Filtration Characterization method (DFCm). In order to investigate the impact of MLSS concentration, identical samples were diluted with permeate. Results showed that dilution of the samples led to an increased activated sludge filterability, but only when the starting MESS concentration was below the apparent critical value of 10.5 g/L. As opposed, the filterability of sludge with MLSS concentrations above 10.5 a, and which was characterized by a moderate to good filtration quality, i.e. Delta R-20 < 1 [ x 10(12) m(-1)]. worsened when diluted. The specific resistance times the particle concentration of a cake layer obtained when filtrating sludge of moderate to good filterability and MLSS concentration above the apparent critical value was 5.5 times smaller compared to the cake layer of sludge with MLSS concentration below the critical value. Results from SMP assessment and particle counting in the range 2-100 mu m showed that reduction in sludge mass and de-flocculation occurred. upon dilution of all samples. However, when diluting sludge samples with MLSS concentrations exceeding 10.5 g/L and which were characterized by a moderate to good filtration characteristics, there was also release of particles below 0.4 mu m, opposite to dilutions of samples with MLSS concentrations below 10.5 g/L. We postulate that sludge, which is characterized by a moderate to good filterability, having an MLSS concentration above the apparent critical value of 10.5 g/L, is likely to retain particles smaller than 0.4 mu m in its mass, as opposed to sludge with MLSS concentration below the apparent critical value. Our work indicates that there are optimal MLSS concentration ranges in MBR technology, to promote good filterable sludge quality in order to avoid fouling. (C) 2014 Elsevier B.V. All rights reserved.