Membrane scientists and engineers invest significant effort in synthesizing new membranes, often involving the optimization of surface chemistries to enhance selectivity and minimize the potential for retained species that could decrease membrane permeability (i.e., induce fouling). Furthermore, they routinely conduct comparisons of membrane performance across various applications, selecting the most suitable membrane for scaling up in a particular application When different membranes under consideration have different permeability values (or resistances,
Rm), traditional plots of flux versus time for constant pressure operation, or pressure versus volume in constant flux operation are biased, and often to not produce a valid assessment of performance. In this study, we elucidate how
Rm affects fouling kinetics across various fouling mechanisms and experimental protocols. We demonstrate that traditional plots often obscure two important performance criteria: productivity, defined as the accumulated volume throughput, and energy consumption. The practical implications of our findings include the possibility of overestimating the performance of membranes with lower productivity or higher energy consumption. Additionally, screening studies may inadvertently select lower-performing membranes under the mistaken belief of superior performance.
We have introduced two novel methods to assess membrane performance in an unbiased manner. In the first approach, we have developed a graphical approach using normalized coordinates. This approach yields linear plots with slopes solely dependent on fouling parameters, and are unbiased, i.e., independent of membrane resistance Rm. Therefore, these normalized coordinates, customized for each fouling mechanism and operational mode, effectively isolate fouling potential. In a second approach, we developed new graphical approaches to visualize the potential trade-offs between better antifouling performance but lower membrane permeability by examining either productivity (volume throughput) for constant pressure operation, or specific energy for constant flux operation. Our study establishes a comprehensive framework for evaluating membrane performance under fouling conditions, incorporating fouling, energy consumption, and volume throughput as metrics, independent of membrane resistance. This framework offers valuable guidance for process design and the development of antifouling membrane materials.
