How cake enhanced concentration polarization can decrease the membrane resistance
Colloidal fouling (cake layer formation) and the related increase in the hydraulic resistance is, besides biofouling, one of the main causes of energetic performance loss in membrane based desalination processes. Although this loss is economically highly relevant, physicochemical mechanisms behind the fouling related increase of hydraulic resistance are still poorly understood. Relevant interactions between simultaneously occurring membrane phenomena like concentration polarization (CP), surface charge screening, and coupled fluxes through membranes are insufficiently investigated. This particularly applies to membranes with only partial salt retention like nanofiltration (NF) or in parts brackish water reverse osmosis (BWRO) membranes as the salt can intrude into and interact with the membrane material which potentially affects mass transfer across the membrane. One important reason for the knowledge gap is the lack of differentiated experimental methods by means of which contributions of specific types of resistances to the overall resistance could be explicitly determined. The derivation of such relationships is highly complex since the formation of a cake layer, with its associated fouling layer resistance (RF), also influences the resistance of the concentration polarization layer (RCP) and the resistance of the membrane (RM) by a variety of potentially unknown coupling mechanisms. Therefore, the measured fouling related increase of the overall hydraulic resistance (OHR) is not necessarily equivalent to RF but may also be due to co-existing variations of RCP and/or RM. Within the performed study a combination of dead-end and cross-flow filtration experiments was applied to achieve a differentiated quantitative assessment of the cake layer related resistance mechanisms. Unexpectedly it was found that the simultaneous determination of RM, RF and RCP resulted in an OHR smaller than the sum of the individually measured values although, according to conductivity measurements, the cake obviously increases CP and should thus also increase feed side osmotic pressure. Moreover, dead-end experiments revealed that RF of a fully consolidated colloidal cake either stays constant or increases with CP. Consequently the most likely explanation for the reduced OHR at simultaneous measurement is a significant decrease in RM when CP exceeds a certain level as a result of cake formation. However, when changes in CP were induced by performing flux variations, parallel determination of apparent retention showed that the complex salt retention mechanism of the applied NF membrane is largely independent of the membrane fouling status.