Particle counting and tracking: Zooming on deposition and flow paths during initial stages of cake formation in forward osmosis with spacers

Abstract

Deposition and flow paths of particles (live and inert) often control the first stages of “cake” (fouling) formation in membrane systems. Developments in crossflow cell design such as three-dimensional (3D) printing and state of the art imaging techniques enable real-time and in situ tracking of initial cake formation. In this work, high-resolution large-field fluorescent microscopy was used to concomitantly study and compare the deposition and flow paths of inert beads and Bacillus subtilis as biofilm forming bacteria. The membrane surface was imaged continuously (30 s intervals) during the initial stages of cake formation (< 4 h) in a 3D printed forward osmosis (FO) cell containing diamond-shaped spacers. Deposition patterns were analyzed using particle detection and counting for quantitative comparison. Flow paths and velocities (< 17 μm s−1) of beads were similar to those of B. subtilis, thus providing a reliable approximation for bacteria passing through the feed channel. However, spatiotemporal deposition of beads and bacteria were markedly different: Final bacteria deposition was 20 times lower than that of beads when normalized to the initial foulant concentration in the feed solution, although beads had a linear deposition increase, while bacteria deposition rose exponentially. Additionally, deposition of beads was homogeneously distributed within the spacer element compared to bacteria, which were mostly captured around the spacer filaments obstructing the flow. Our results provide a novel approach to quantify cake formation in membrane systems as well as new insights on the impact of inert and living particles on the deposition patterns and flow paths during the first stages of cake formation. These insights could be applicable to design new membrane surfaces and spacer shapes that minimize and delay fouling development.