Comparison of Membrane Performance: Analysis with Fouling Models

Membrane research and engineering applications often involve optimizing flux and rejection for a particular application.  This process frequently presents the need to compare the performance of membranes having different inherent permeabilities; an important performance criterion is resistance to fouling. To identify the most appropriate (least biased) approaches to compare membrane performance, an analysis using fouling models, with an emphasis on the combined pore blockage and cake filtration model, was performed.  We illustrate how the models can be used to provide a general framework for assessing fouling and developing ways to compare membrane performance.  Comparisons at constant transmembrane pressure using the same initial flux and the same initial pressure were emphasized.  This analysis demonstrates that membrane resistance has a significant effect on flux decline - flux decline is smaller for membranes having greater resistance, especially when the feed solution and membrane-foulant interactions are similar.  This effect cannot be normalized regardless of whether flux is plotted as a function of filtration time and volume throughput.  A better approach is to compare the effects of fouling by evaluating fouling resistance as a function of volume throughput. Alternatively, comparing membranes based on total resistance provides some advantages because it can be related to permeate production and energy cost.  When comparing membranes having different resistance and rejection, the effects of fouling should be compared by evaluating fouling resistance as a function of mass accumulation on the membrane surface.  The effects of cake compressibility are also discussed.