Permeate Flux Decline in Cross-Flow Microfiltration at Constant Pressure

Abstract:
In membrane filtration processes, the layer of deposited matter formed on the surface of the membrane is leading to a significant increase in filtration resistance. In dead-end mode, the resulting permeate flux decline is compensated by (i) an increasing driving force (i.e. an increasing transmembrane pressure, TMP) or (ii) by backwashing sequences conducted to remove the reversible cake layer from the membrane. Alternatively, a tangential flow in the feed channel is applied to generate shear at the surface of the membrane. This tangential flow can be realized by (i) using a force pump within the feed line (cross-flow filtration) or (ii) by injecting air bubbles into the stationary suspension to induce flow (submerged filtration). In medium to large scale municipal wastewater treatment plants, mainly submerged membrane filtration units are used to separate the suspended microorganisms from the biologically cleaned water. However, cross-flow test cells are frequently used for fundamental cake layer formation observations since the respective shear forces are easier to predict compared to the bubble-induced shear forces in submerged mode. Hence, numerous scientific studies in this field are conducted in cross-flow mode while the results are transferred to submerged membrane bioreactor (sMBR) applications. The influence of a variety of operating parameters on the permeate flux evolution in membrane filtration processes has been extensively investigated by a number of research groups. Within previous publications, it was reported that, after a rapid permeate flux decline due to an initial pore blocking in the beginning of the filtration process, a (reversible) cake layer is formed on the surface of the membrane, indicated by a nearly constant permeate flux. Furthermore, it was found that with an increasing cross-flow velocity and by that higher shear rates within the boundary layer in the vicinity of the membrane, the thickness of the cake layer is decreasing and the long-term permeate flux is increasing. Experiments using silica respectively latex suspensions as a filtration medium have consistently shown that an increase in particle concentration is resulting in a decreasing permeate flux. Besides this, it was determined that smaller particles are leading to higher specific filtration resistances within the cake layer, which is in turn reducing the long-term permeate flux. Interestingly, opposed trends were observed with respect to the impact of TMP on the long-term permeate flux. While Hong et al. reported that the long-term permeate flux is decreasing with an increasing TMP, it is independent from feed pressure variations according to Wang and Song. However, also Hong et al. observed a less significant impact of TMP after reaching the permeate flux plateau. The overall objective of this study is to estimate the impact of operating parameters (feed pressure, average cross-flow velocity) and characteristics of the model suspension (solids concentration, particle size distribution, dynamic viscosity, type of continuous liquid) on the permeate flux decline. For this purpose, a single hollow-fibre microfiltration membrane was used. A cross-flow test cell was operated at constant TMP while monitoring the permeate flux over time. Subsequently, the observed trends were compared with published results.

Keywords:
membrane bioreactor (MBR), model suspension, reversible cake layer formation and removal, shear stress

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