An empirical correlation for the effect of a Streptomyces coelicolor biofilm on hydraulic permeability is presented. This relation was based on a two-phase logistic fit of the biofilm growth kinetics, taking into account uneven growth of the biofilm. The model was applied to a vertically orientated, pressure-controlled, membrane gradostat reactor (MGR) operated in the dead-end mode. The permeability correlation developed was further used in the investigation of the effect of bacterial growth on velocity and pressure profiles of the nutrient solution in the lumen of the MGR. The velocity profiles emanating from the application of the model were in correspondence with the observed flux decline in the pressure-controlled MGRs. The hydrostatic pressure profiles pointed to the existence of a critical hydraulic permeability of the biofilm, below which there is recirculation of the nutrient solution back to the membrane lumen. Even at hydraulic permeabilities much higher than the predicted K,,it, this recirculation was marked by the existence of the secondary metabolite, actinorhodin, in the lumen of the MGR. (C) 2009 Elsevier B.V. All rights reserved.