Fouling propensity of forward osmosis: investigation of the slower flux decline phenomenon

Abstract

Forward Osmosis (FO) is a membrane process that uses the natural osmotic pressure of a concentrated draw solution to extract pure water from a feed stream. The attraction of the FO process is that it uses dense membranes, while operating at ambient pressure. This means that the FO process could potentially produce high quality water with lower energy consumption, as compared to the other desalination or reclamation processes. As FO does not entail the use of hydraulic pressure, FO has been hypothesized to have lower fouling propensity than pressure driven membrane processes. Membrane fouling has significant impact on the operational sustainability and economics of the process. This study examines the possible contributing factors to the slower flux decline observed in FO experiments based on a combined experimental and modelling approach. It was found that these factors could include low water fluxes, use of hydrophilic and smooth membranes, and the effect of internal concentration polarisation that is inherent of FO. It was also found that the transmission of draw solutes from the draw solution into the feed can have significant effect on FO performance.

Unpacking compaction: Effect of hydraulic pressure on alginate fouling

“…Fouling layers created in FO and RO have also been imaged using confocal laser scanning microscopy (CLSM) to show that both alginate cakes [15] and biofilms [22] are thinner and more uniform in RO than in FO. Although this has been considered to be evidence for foulant compaction by high pressure, the ICP self-compensation effect [17] contributes to the larger foulant thickness in FO [19]. Furthermore, no justification has yet been given for why pressure should lead to a more uniform foulant layer.…”

“…The slower flux decline of FO at a given initial flux has been explained by the internal concentration polarization (ICP) self-compensation effect [17,18,19], which is unrelated to the system pressure. However, the lower effectiveness of cleaning fouled RO membranes is typically attributed to the high hydraulic pressure of the feed.…”

“…Lay et al [17] find the idea of compaction by high hydraulic pressure "contradictory to the well established critical flux concept," [24] which implies that, "regardless of the type of driving force, the effect of membrane fouling should be comparable under similar flux and operational conditions." However, Xie et al [15] show through modeling that high pressure could cause compaction of compressible foulant cakes, i.e., those with Poisson's ratios less than 0. contradictions, but they suggest that foulant compaction by high feed hydraulic pressure could still occur because hydraulic pressure and drag "work simultaneously and therefore reinforce each other" [15].…”

“…Diffusion of NaCl from the draw solution to the feed, which can significantly affect cake layer ion concentration [7,17], may lead to differences in gel properties between FO and RO tests with alginate fouling and a draw solution of NaCl due to the ion exchange 29 reaction between sodium and calcium ions in alginate gels [43]. LeRoux et al [44] measured compressive and shear moduli of alginate gels in calcium chloride solutions with and without sodium chloride and found that compressive and shear moduli decrease by 63% and 84%, respectively, due to the addition of 0.15 M sodium chloride.…”

Influence of Functionalized Multiwall Carbon Nano-Tube on the Structure and Performance of Cellulose Di-Acetate Based Forward Osmotic Membrane for Desalination Process

“…Compared to Reverse Osmosis (RO), FO has many potential advantages: (1) it has high rejection for a wide range of contaminants, and less membrane fouling [1][2][3][4]; (2) it is a spontaneous low pressure process that needs lower external energy. As a low pressure membrane separation process, it is expected to lead to energy-efficient separations in many areas including seawater desalination [5,6], food processing [7][8][9], waste water treatment [10][11][12], electric power production [13][14][15], pharmaceutical industry [16,17] and so on.…”