Where, when and why? Quantifying the relation of particle deposition to crossflow velocity and permeate water flux in forward osmosis

Abstract Fouling is the main hurdle for all membrane-based systems treating water with high fouling propensity. A novel imaging approach was applied in a forward osmosis (FO) system with spacers to follow in-situ and real-time deposition of fluorescent beads as bacteria proxy on the membrane using large-field high-resolution epifluorescence microscopy. For the first time, this study quantifies the impact of crossflow velocity (1.3 or 13 cm s-1) and permeate water flux (6 or 30 L m-2 h-1) on spatiotemporal patterns of initial cake layer formation (4 h). The total amount of deposited particles on the membrane increased by 84 fold as the ratio of permeate water flux over crossflow velocity, D, was raised by 54 fold. Spatial distribution of particles was more homogenous at higher D ratio, while particle accumulation rates decreased by 50 % over 4 h. Distribution of local velocities of particle flow paths elucidated the observed spatial deposition patterns. These new quantitative results highlight that the ratio of permeate water flux over crossflow velocity impacts all aspects of particle deposition and may aid in designing new spacer geometries. We also suggest that appropriate hydrodynamic conditions may be a viable tool to postpone the onset of fouling in new and cleaned membrane modules.