Modeling the effect of biofilm formation on reverse osmosis performance: Flux, feed channel pressure drop and solute passage

The role of cell-surface interactions in bacterial initial adhesion and consequent biofilm formation on nanofiltration/reverse osmosis membranes

“…Significantly less deposition was observed on the membrane areas in front of spacer filaments. In the same way, several studies have shown experimentally and through modelling development that the stagnation zones created by the feed spacers, such as behind the spacer filament crossings on NF and RO membrane modules [122,123], enhance biofilm formation and the creation of regions of low and high liquid flow velocity [124], also called channelling. Vrouwenvelder et al [125] obtained a higher pressure drop caused by biomass accumulation with a spacer in the feed channel compared to without one, showing the importance of hydrodynamics on biofilm formation, and possibly on bacteria adhesion.…”

The effect of feed spacer geometry on membrane performance and concentration polarisation based on 3D CFD simulations

“…In the partially woven configuration, one set of filaments remains straight, with each strand being in contact with the top and bottom plane alternatingly, while the other set is woven through the gaps between the first set of filaments [30]. The middle layer configuration is built based on the 'Parallel spacer' [2,40] and 'Middle layer spacer' [31] in the literature. The filaments are arranged in a similar manner to those in the nonwoven configuration, but one set of straight filaments has a smaller diameter and pierces through the other set of filaments whose diameter is the same as the channel height.…”

“…This can be attributed to differences in boundary conditions and spacer geometries. The assumption of a constant solute rejection by the membrane, as was done in the aforementioned study [14,21] and in others [6,12,15,18,26,30,31], may need to be revisited since membrane rejection is not an intrinsic property of membranes but an output of an RO process. It would be desirable if this could be predicted as part of a CFD simulation.…”

“…Other types of spacers that have been considered in 3D CFD models include 'Parallel spacer' [2,40] and 'Middle layer spacer' [31], both consisting of thick filaments with a diameter equivalent to the channel height and thinner filaments piercing through the thick ones. However, 3D models of woven spacers with realistic spacer geometries are lacking.…”

“…As such, spacers of different types and geometries have been compared in terms of local shear stress and mass transfer coefficient [1-2, 5, 7, 9, 38-40]. A commonly used approach to account for solute permeation is to specify a constant membrane rejection combined with water flux and wall concentration [6,12,15,18,21,26,30,31]. This can be expressed as: J s = J w c m (1−R s ) where J s is the solute flux, J w the water flux (or flow velocity on the membrane wall), c m the solute concentration on the membrane wall and R s the membrane rejection.…”