Revisiting the distinct concepts of critical flux and threshold flux

Classically the fouling of membranes has been categorised into four modes. Apart from the pore filling mode, the other three seek to model surface fouling be it the foundational layer on the membrane itself or the cake layer that may or may not be built up upon the initial layer. The assumptions are rigorously and critically re-examined. With more realistic assumptions, a new expression is derived for the foundational layer for both constant flux and constant pressure operation. Herein the virtues of assessing the evolution of overall resistance with time is emphasised and the usefulness of examining the evolution of resistance with respect to volume collected is illustrated with data from Membrane Bioreactors. Threshold flux is best defined with respect to rates of fouling and is the flux for a particular operation at which or below which a low and near constant rate of fouling occurs, but above which the rate of fouling increases markedly. When quoting a value for threshold flux, the length of operating is highly relevant. The definition for critical flux (strong form) remains as the flux at which the transmembrane pressure starts to deviate from the pure water line, i.e. the total resistance is identical to R m , the resistance of the clean membrane. For the weak form, there is the assumption that following very modest rapid fouling on start-up, the flux–TMP relationship remains linear until the critical flux (weak form) is exceeded. • Untangling a tangled web: critical fluxes and threshold flux revisited. • Criteria for specifying threshold flux clarified. • Introduction of the hindrance model for the foundational layer. • Re-evaluation of complete pore blocking mode of fouling: constant flux operation. • Re-evaluation of complete pore blocking mode of fouling: constant pressure operation.