The coupling of biological reactor with membrane dissolution processes has gained increasing attention in the wastewater treatment community because of their effective delivery of gaseous products and high treatment efficiencies. Turbulent flow may be an important factor for optimizing performance of these systems, both by reducing plug-flow conditions and associated axial gradients within the reactor and increasing mass transfer across the biofilm attached to membrane fibers. While turbulence is typically enhanced via increased flow velocity, rotation of the membranes themselves is an alternative mechanism for increasing turbulence that might eliminate problems associated with high flow (e.g., increased shear, mal-distribution of fluid flow). The effects of variable rotational frequencies (0, 5, 10 and 20 rpm) and membrane module packing densities (62, 82, 92%) on residence time distributions (RTD) curves of a bromide tracer were investigated to assess the potential for membrane rotation as a means of increasing turbulence. The resulting RTD curves were compared to a dispersive plug-flow model to determine the Peclet numbers (Pe) and subsequent axial dispersion coefficients. These finding indicated that increased dispersion is observed with increasing rotational frequency, with the effects of module packing density on dispersion characteristics decreasing with increased rotation. This study demonstrated that module rotation can cause turbulent flow prompting enhanced dispersion and allowing for low operational flow rates and reduction of mass transfer constraints associated with membrane bioreactors (MBR). (c) 2005 Elsevier B.V. All rights reserved.