NMR imaging of local cumulative permeate flux and local cake growth in submerged microfiltration processes

Abstract

Microfiltration processes are frequently used for rejecting dispersed solid matter from biological suspensions such as activated sludge. Previous studies in the field of these so-called membrane bioreactors (MBRs) have shown that the cake formation is self-accelerating for a submerged filtration at imposed flux. This means that the local permeation is reduced where the local cake layer is relatively thick, which is compensated by an enhanced filtration in regions with thinner cakes. Hence, a permeate flux distribution and a transient non-uniform cake growth are occurring along the membrane and were investigated by nuclear magnetic resonance (NMR) imaging in the framework of this study. The investigations have shown that the local cumulative permeate flux increases linearly with a decrease in vertical distance from the point of permeate extraction. This results in thicker cake layers in the vicinity of the permeate line. 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 levels off for longer aeration cycles. Nevertheless, 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.