The formation of a biofilm on membranes with micro-obstacles as structures has been studied. These structured membranes showed growth of the biofilm upstream of the structures regardless of their orientation relative to the flow direction. A typical net shaped spacer was fouled under similar experimental conditions as a benchmark to observe the fouling locations. The spacer junctions showed similar biofilm formation as the structured membranes with the biomass accumulating upstream. 2D CFD simulations were used to determine the flow profile around structures of different shapes as well as the local surface shear rates on the walls of these structures. The flow profiles showed recirculation zones behind star and circular shaped geometries. The local shear forces showed a minimum downstream of the structures with the maximum shear rate observed along the sides of the structures. These observations contrast earlier findings where deposition and biofilrn formation occurs downstream of the obstacles. (C) 2010 Elsevier B.V. All rights reserved.