Unraveling the role of NaCl on microfiltration fouling: Insights from In situ analysis of dynamic interfacial behaviors

Resolving the interfacial behaviors plays a pivotal role in elucidating the formation of membrane fouling. However, quantifying the short-term rapid changes in these processes remains challenging, particularly for complex solutions like food waste digestate. In this study, a poly (vinylidene fluoride) (PVDF) membrane was employed to filter food waste digestate; the dynamic membrane-foulant and foulant-foulant interactions during the initial filtration were in situ examined using optical coherence tomography (OCT) to ascertain the effects of varying concentrations of sodium chloride (NaCl). Results indicate that NaCl could alter the solution structure by disrupting the hydration bonds. The hydration repulsion caused by Na+ ions gave rise to a slower deposition of foulants, thereby delaying the transition from membrane-foulant to foulant-foulant interactions; the hydrationrepulsion effect was intensified by increasing the salt concentration. On the other hand, when foulant-foulant interactions began to dominate, compression of the electric double layer could result from the elevated ionic strength and thereby contribute to a denser cake layer. Moreover, the hydrated layer could also be an effective inhibitor to the internal fouling that plagues the membrane cleaning. These mechanistic insights would be in favor of designing more efficient fouling-control strategies for membrane-based treatment of high salinity wastewater.