Direct Microscopic Observation of Forward Osmosis Membrane Fouling

Abstract

This study describes the application of a noninvasive direct microscopic observation method for characterizing fouling of a forward osmosis (FO) membrane. The effect of the draw solution concentration, membrane orientation, and feed spacer on FO fouling was systematically investigated in a cross-flow setup using latex particles as model foulant in the feedwater. Higher draw solution (DS) concentrations (and thus increased flux levels) resulted in dramatic increase in the surface coverage by latex particles, suggesting that the critical flux concept might be applicable even for the osmotically driven FO process. Under identical draw solution concentrations, the active-layer-facing-the-feed-solution orientation (AL-FS) experienced significantly less fouling compared to the alternative orientation. This may be explained by the lower water flux in AL-FS, which is consistent with the critical flux concept. The use of a feed spacer not only dramatically enhanced the initial flux of the FO membrane, but also significantly improved the flux stability during FO fouling. Despite such beneficial effects of using the feed spacer, a significant amount of particle accumulation was found near the spacer filament, suggesting further opportunities for improved spacer design. To the best of the authors' knowledge, this is the first direct microscopic observation study on FO fouling.

Quantifying osmotic membrane fouling to enable comparisons across diverse processes

“…14, is within the range that would be expected for this membrane: using the the ratio τ S / S = S S /δ S = 7.125 (based on measured membrane thickness), Eq. 15 gives an estimate of 6.5 µm for the product of pore diameter and inhomogeneity factor in the support layer, which is somewhat smaller than but comparable in order of magnitude to the pore sizes seen in micrographs [20] of the same type of FO membrane used here.…”

“…As described in Appendix A.2.1, the value of dispersivity fit from our foulant-free FO flux measurements across a wide range of feed and draw concentrations was α = 1.65 × 10 −4 m, which corresponds to σd p ≈ 6.5 µm for the product of support layer pore diameter and inhomogeneity factor. SEM and optical micrographs of the same CTA FO membrane used in the present experiments show support layer pore diameters on the order of 10 µm [20], so we would expect to see some enhancement of diffusion.…”

“…As in all trials, alginate concentration is short compared to the time over which the initial flux is calculated (15 minutes). In the present experiments, the foulant layer thickness (on the order of tens or hundreds of micrometers) is much greater than the roughness of the membrane (order 100 nm in RO [40] or less in FO [20]) or the pore size of alginate (order 10 nm). Additionally, different membrane functional groups that affect initial alginate adsorption rate in RO have been…”

Tailoring Membrane Surface Charges: A Novel Study on Electrostatic Interactions during Membrane Fouling

“…Therefore, a uniform fouling reagent with tunable surface charge is essential to describe the role of electrostatic interaction while membrane fouling, e.g., polystyrene (PS) beads. As already described by several groups, PS beads can be used as model fouling reagent [31][32][33][34][35][36][37][38][39][40][41][42][43][44]. Nevertheless, an investigation regarding the fouling of differently charged PS beads is missing so far.…”

In situ visualization of organic fouling and cleaning mechanisms in reverse osmosis and forward osmosis

“…Thompson et al [6] visualized combined biofouling and scaling in RO at pressures up to 25 bar to show that biofouling enhances scaling due to biofilm-enhanced concentration polarization. Microscopic observation of fouling has also been conducted in FO with latex particulates [7], showing that the rapid particle deposition that occurs beyond the critical flux occurs only between the embedded mesh filaments.…”