1. The vibrating MBR consists of hollow fiber membranes placed vertically in a bundle. MF hollow fibers made of a polyethersul- phone/polyvinylpyrrolidon (PES/PVP) blend are used with pore sizes around 0.5  m. The module is vibrated vertically at a frequency of 25 Hz with an 0.7 mm amplitude. Bakers yeast suspensions (4 g/l dry weight) are tested. Constant flux experiments are conducted by sucking permeate through the fibers from the outside, where the skin-layer is located, towards the inner volume of the fibers. A suction pump that creating a lowered pressure on the permeate side is used. The feed suspension recirculation rate in the module cylinder is very low (< 1 cm/s). At the given operational conditions, with a membrane module consisting of 54 hollow fibers, a critical flux of 15 L/(m 2  h) has earlier been determined [1]. The permeability, membrane- and fouling resistances and the UV absorbance (260 nm) of bulk supernatant (to measure the levels of extracellular polymeric substances (EPS) from the yeast cells) at supra- and sub-critical flux are determined. A vibrating membrane bioreactor - operated at supra- and sub-critical flux Søren Prip Beier, Gunnar Jonsson CAPEC, Department of Chemical Engineering, Technical University of Denmark, DK-2800 Lyngby, Denmark 1. Introduction 4. Discussion and Conclusion References: [1] S.P. Beier, M. Guerra, A. Garde, G. Jonsson, Dynamic Microfiltration with a Vibrating Hollow Fiber Membrane Module; Filtration of Yeast Suspensions, Journal of Membrane Science 281 (2006) 281-287. [2] S.P. Beier, G. Jonsson, Separation of Enzymes and Yeast Cells with a Vibrating Hollow Fiber Membrane Module, Separation and Purification Technology 53 (2007) 111-118. Submerged suction pressure driven membrane units are of expanding interest. Such set-ups, better known as membrane bioreactors (MBR’s), are often seen in relation to waste water treatment, but applications in relation to biomass and artificial particles can be found. The drop in permeability at supra- and sub-critical flux supports the statement of the critical flux being an interval flux level rather than a specific value (Figure 2). Fouling does occur even at sub-critical flux. However, the difference is obvious as the fouling resistance around twice as large after supra-critical MF (Figure 2). 2. Methodology Figure 2: Bulk supernatant UV absorbance (260 nm) and fouling resistance for constant flux filtration at supra-critical flux (30 L/(m2  h)) and sub-critical flux (14 L/(m2  h)) for 5 hours. Feed dry yeast content = 4 g/l. Figure 2: Permeabilities and resistances for i) clean module, ii) for constant flux filtration at sub-critical flux (14 L/(m2  h)) for 5 hours and iii) for constant flux filtration at supra-critical flux (30 L/(m2  h)) for 5 hours. Feed dry yeast content = 4 g/l. 3. Results Thus, fouling occur even below the critical flux, but a clear differences in fouling resistances and EPS levels are observed at supra- and sub-critical flux. Therefore, operation at sub-critical flux seem to be a sensible compromise between controllable EPS level and acceptable increase in fouling resistance on one hand and acceptable flux level on the other hand. The relative control of fouling reduces the expenses associated with cleaning of the membrane, just to mention one of the benefits. We present results from micro- filtration (MF) experiments with a vibrating MBR, at which fouling is reduced by vibrations. The system, also know as a dynamic microfiltration set-up, has earlier been tested in MF of yeast cell suspensions [1] and in separation of yeast cells from macromolecules [2]. The critical flux has earlier been determined by experiments and the aim of this work is to evaluate the critical flux concept by cond- ucting constant flux experiments above (supra-critical) and below (sub-critical) an experimentally determined critical flux. Operation at sub-critical flux is desirable in order to reduce the expenses related to membrane cleaning, but improved trans- mission of macromolecules [2] and low pumping costs is also an additional benefit. Figure 1: Vibrating MBR set-up. Variable frequency (0-30 Hz) and variable amplitude (0 – 1.375 mm). Very low feed recirculation rate (< 1 cm/s) in the module cylinder) and trans-membrane pressure (< 100 mbar). Operated at constant flux. 3. Results There is a clear difference in the levels and in the way the fouling resistances increase at supra- and sub- critical flux (Figure 3). The UV absorbance of bulk supernatant indicates differences in the washing- out mechanism of EPS from the yeast cells. The level of EPS continually increases at supra-critical flux whereas it is almost constant at sub-critical flux (Figure 3).