The influence of a pre-existing biofilm on RO membrane mineral scaling was evaluated using gypsum as a model scalant. The biofilm was established using microfiltered secondary treated wastewater effluent on-site in a wastewater treatment facility. Mineral scaling was then monitored via direct visual observation of crystal growth on the membrane surface in a transparent plate-and-frame RO cell. SEM imaging of the membrane surface and sectioned biofilm revealed gypsum crystals within the biofilm matrix, including the protrusion of crystal rods from within the biofilm. Both mineral scale surface coverage and crystal number density were greater in the presence of the biofilm. Moreover, individual mineral crystal growth rate within the biofilm was significantly higher relative to growth in the absence of a biofilm. Analyses of crystal growth and nucleation rates, within the biofilm and in its absence, suggest concentration polarization enhancement within the biofilm. The present study suggests that mineral scaling may not be restricted to tail elements in RO plants, but could also occur in lead elements where biofilms can enhance concentration polarization. Therefore, understanding of the coupling of biofouling and mineral scaling may be of particular significance to the operation and design of RO plants in water reuse applications. (c) 2012 Elsevier B.V. All rights reserved.