Fluorescence enabled direct visual observation for diagnosis of ultrafiltration membrane fouling by bi‐disperse submicron particle suspensions

Whilst direct observation (DO) methodologies can describe back‐transport of supra‐micron particles, present technologies are unable to discriminate submicron particles, which are primarily responsible for membrane fouling. In this study, we therefore introduce a fluorescence enabled direct visual observation (RLF‐DVO) methodology to permit visual characterisation of submicron particle transport during cross‐flow filtration. Particle discrimination was achievable for particle diameters exceeding 0.25 µm; however, this was dependent upon particle concentration and the cross‐flow velocity employed. Nevertheless, this is considerably below the detection limit of current techniques (around 3 µm). During filtration of a binary dispersion comprised of submicron particles, deposition was observed before a change in transmembrane pressure was detected, which underpins the important role of DO for fouling diagnosis. Based on observations made during this study, recommendations are proposed that will further improve resolution. Importantly, this study demonstrates RLF‐DVO can provide real‐time description of submicron particle transport during cross‐flow filtration.