Non-Invasive Observation of Permeate Flux Distribution and Local Cake Growth in Submerged Microfiltration Processes via Nuclear Magnetic Resonance (NMR) Imaging

Abstract:
Microfiltration processes are frequently used for rejecting dispersed solid matter from biological suspensions such as activated sludge. The overall filtration performance is mostly characterised by monitoring the transmembrane pressure (TMP) rise at constant permeate flux due to the formation of a deposition layer on the membrane surface. Previous studies in the field of these so-called membrane bioreactor (MBR) processes have shown that the cake formation is self-accelerating for a submerged filtration at imposed permeate flux [1]. This means that the local permeation is reduced where the local cake layer is relatively thick, which is compensated by an enhanced local permeation in regions with thinner cakes. Hence, a permeate flux distribution along the membrane as well as transient and non-uniform cake growth patterns are expected to significantly affect the overall filtration performance. In the framework of this study, the steady-state permeate flux distribution and dynamics of cake formation were investigated by applying nuclear magnetic resonance (NMR) imaging. This non-invasive technique is based on an excitation and relaxation of protons in a specimen under the influence of an external magnetic field [2]. For these lab-scale experiments, a setup equipped with a single hollow-fibre membrane for a submerged outside-in filtration of water (with or without colloidal silica particles) was operated at constant permeate flux. The steady-state permeate flow velocity was measured for a pure water filtration at different vertical positions, thus allowing an estimation of the local cumulative permeate flux along the membrane. The transient cake growth was determined during a filtration of silica suspensions in a series of consecutive NMR measurements. The latter approach is an extension of the methodology suggested by Airey et al. [3] and permits a step-wise estimation of the increase in local cake layer thickness with time by a subsequent image processing. The investigations have shown that the local cumulative permeate flux increases linearly with an increase in average permeate flux or a decrease in vertical distance from the point of permeate extraction. In addition to that, the cake layer on the membrane surface was highly heterogeneous, with thicker cakes formed next to the point of permeate extraction. The cake growth rate was found to increase as the setpoint permeate flux increases or the solids concentration increases. The efficiency of cake removal due to air bubbling was higher in case of higher aeration pressures and longer aeration sequences. The impact of aeration pressure on the cleaning efficiency was more pronounced for shorter aeration cycles, whereas the benefit in cake removal levels off for longer aeration cycles. An increase in duration of aeration in the range of lower aeration pressures was consistently followed by a reduction in local cake layer thickness. This suggests that critical aeration conditions might exist.