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Pressure-retarded osmosis (PRO) has the potential to convert salinity gradient energy to hydraulic energy via a semi-permeable membrane. The performance of an osmotic membrane is usually evaluated using a crossflow membrane test cell with symmetric channels. However, it is not possible to test a PRO membrane under conditions of large differences in hydraulic pressure because the membrane cannot be supported in the channel inlet and outlet with a cavity. In this study, we present a newly designed PRO crossflow test cell that features asymmetric channels on both sides of the membrane. This asymmetric design prevents membrane bulge and rupture in the channel inlet and outlet even at an applied hydraulic pressure difference greater than 54.5 bar. Excellent performance of this novel PRO cell was demonstrated using two different membranes at various operating pressures. To prevent membrane deformation at open spaces between spacer strands, a feed channel spacer should be selected carefully. Accordingly, the effects of tricot feed spacers with different voidages and thicknesses on PRO performance were also investigated. [ABSTRACT FROM AUTHOR]

Abstract: Analytical solutions of the Nernst–Planck, Poisson and continuity equations for a membrane undergoing osmosis in a cross-flow system reveal that the flow of alternating ionic charge induced in the membrane during impedance measurements is actively assisted by the flow of water. The solutions yield expressions for the impedance of the membrane that characterize the dielectric, conduction and geometrical properties (Maxwell–Wagner model) as well as the actively assisted contributions that are functions of the velocity of water and the electrical field values at the membrane boundaries. The consistency of this theory with experimental results reported in literature provides a basis for interpreting in situ impedance characterizations of membrane systems, and in particular the viability of the membranes and the extent of fouling during reverse osmosis. [Copyright &y& Elsevier]

Fouling continues to be a limiting factor in membrane processes. Pulsating feed flows have the potential to decrease fouling while maintaining a constant pressure drop over the module. This study quantifies the impact of pulsating flows on particle deposition in a forward osmosis system with spacers at a mean crossflow velocity of 14 cm s−1 and permeate water flux of 18 L m−2 h−1. Carboxylate modified polystyrene beads were tracked by fluorescence microscopy in situ and real-time during the first 4 h of deposition with feed flow pulsations up to a frequency of 10 Hz, while experiments were conducted with a constant crossflow velocity as a reference. Imaging at 30 s intervals revealed deposition rates of up to 2.6 times higher at steady state, as compared to flow pulsations of 10 Hz. Consequentially, average deposition was reduced by a factor of 3.5 from steady state (109 ± 67 beads mm−2) to pulsating feed flow (31 ± 17 beads mm−2) after 4 h of deposition. The results of this research provide new fundamental insights related to the constraints and advantages of using pulsating flows for fouling mitigation in forward osmosis systems. [Display omitted] • In-situ and real-time investigation of cake layer formation in forward osmosis. • Quantification of particle deposition with fluorescence microscopy. • Pulsating flows are a promising approach for fouling mitigation in forward osmosis. • A higher pulsation frequency and amplitude leads to less deposition on the membrane. • Pulsation parameters have a significant influence on the spatial distribution of deposition. [ABSTRACT FROM AUTHOR]

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. Image 1 • 3D printed FO crossflow cell with spacer for in situ and real-time imaging. • Fluorescence microscopy for concomitant quantification of deposition and flow paths. • Time and space-resolved observation of deposition during initial cake formation. • Subtly different flow paths of beads and Bacillus subtilis at the membrane surface. • Significantly different deposition in time and space between beads and B. subtilis. [ABSTRACT FROM AUTHOR]