Critical and sustainable fluxes: Theory, experiments and applications

Abstract

Over the last 10 years, numerous membrane filtration data have been viewed through the lens of the concept of critical flux. This concept, used in a number of different ways often without explicit redefinition, is here clarified both theoretically and from an experimental viewpoint. A link is made with the concept of sustainable flux and an approach given for the determination of the apparent sustainable flux. To retain the utility of the critical flux concepts, it is emphasised firstly that the strong form and weak form of the critical flux, J cs and J cw, must be evaluated via a check on whether or not the overall resistance has remained invariant. Secondly, the critical flux for irreversibility, J ci, has a sound theoretical basis; it represents the shift from repulsive interaction (dispersed matter—polarised layer) to attractive interaction (condensed matter—deposit). The various methods for measuring critical flux and the influence of membrane and suspension properties on critical flux are reviewed. Dispersive forces that are the key to the existence of a critical flux are discussed. The concept of a critical concentration for phase transition is also introduced. For theoreticians and experimentalists, this and the clarified concept of a small set of critical fluxes will continue to provide a valuable framework. But in addition, and especially for membrane users dealing with industrial process streams, the concept of a sustainable flux (which has evolved from critical flux thinking) is of a great utility. Above the sustainable flux (dependent on hydrodynamics, feed conditions and process time), the rate of fouling is economically and environmentally unsustainable.