The main objective of this study was to investigate the influence of seasonal temperature fluctuations on raw domestic wastewater composition and MBR sludge filterability. The influent and sludge samples were collected from a full-scale MBR in Heenvliet. All samples were analyzed in terms of filterability, particle size distribution, respirometry, fractionation and were further characterized by an extensive set of physicochemical measurements. Typical seasonal fluctuations and deterioration of activated sludge filterability during low temperature periods were observed. Filterability deterioration at low temperatures was linked to the incoming organic load, reduced MBR biomass concentrations, increased VSS/TSS ratio and reduced biodegradation of wastewater in the mixed liquor. Biological activity of the biomass was concomitantly decreased with temperature to reach a minimum during the winter period. Results show that the colloidal and soluble fraction ( < 1 mu m) represents the majority of the constituents, contains most of the COD, and plays an important role in the membrane resistance increase. (c) 2012 Elsevier B.V. All rights reserved.