Role of pressure in organic fouling in forward osmosis and reverse osmosis

Abstract

Fundamental understanding of membrane fouling in osmosis-driven membrane processes is important for further deployment of this emerging technology in desalination and wastewater reuse. In this study, we investigated the role of pressure in organic fouling and reversibility in forward osmosis (FO) and reverse osmosis (RO) using alginate as a model organic foulant. Varying contributions of pressure (i.e., osmotic versus hydraulic) to the overall driving force were realized in forward osmosis (FO), pressure-assisted FO (PFO), and reverse osmosis (RO) experiments, while the same total driving force for water permeation was applied. Confocal laser scanning microscopy was used to examine alginate fouling layer structure in the hydrated state, which informed two key parameters: fouling layer thickness and foulant volume. We observed that the resulting fouling layer became increasingly more compact in the order of FO, PFO, and RO experiments. Fouling layer reversibility followed the same trend, with the highest and lowest reversibility observed for the FO and RO fouling experiments, respectively. Possible mechanisms for fouling layer compaction in RO were discussed, including permeate drag force and foulant compressibility, as opposed to FO where only permeate drag force applies. Our findings suggest that pressure mechanistically alters the membrane fouling layer structure and fouling reversibility, leading to higher fouling reversibility in FO, where the driving force is osmotic pressure, than RO, where the driving force is hydraulic pressure. ABSTRACTFundamental understanding of membrane fouling in osmosis-driven membrane processes is important for further deployment of this emerging technology in desalination and wastewater reuse. In this study, we investigated the role of pressure in organic fouling and reversibility in forward osmosis (FO) and reverse osmosis (RO) using alginate as a model organic foulant.Varying contributions of pressure (i.e., osmotic versus hydraulic) to the overall driving force were realized in forward osmosis (FO), pressure-assisted FO (PFO), and reverse osmosis (RO) experiments, while the same total driving force for water permeation was applied. Confocal laser scanning microscopy (CLSM) was used to examine alginate fouling layer structure in the hydrated state, which informed two key parameters: fouling layer thickness and foulant volume.We observed that the resulting fouling layer became increasingly more compact in the order of FO, PFO, and RO experiments. Fouling layer reversibility followed the same trend, with the highest and lowest reversibility observed for the FO and RO fouling experiments, respectively.Possible mechanisms for fouling layer compaction in RO were discussed, including permeate drag force and foulant compressibility, as opposed to FO where only permeate drag force applies. Our findings suggest that pressure mechanistically alters the membrane fouling layer structure and fouling reversibility, leading to higher fouling reversibility in FO, where the driving fo...

Quantifying osmotic membrane fouling to enable comparisons across diverse processes

“…6 shows no effect of pressure on foulant accumulation rate, even between pressures of 36 and 64.5 barg. This result contrasts with the findings of Xie et al [7] on the effect of pressure in forward osmosis, in which confocal laser scanning microscopy was used to show that thicker foulant layers develop in FO at atmospheric pressure than in RO 2.0% NaCl, 36 barg 3.1% NaCl, 64.5 barg 3.7% NaCl, 64.5 barg Figure 6: Comparison of calculated foulant accumulation in RO tests with feeds of different salinity on membrane coupons of varying permeability at different pressures, showing strong similarity. The cake structural parameter was calculated using the model presented here for the same tests that exhibited differences in flux decline in Fig.…”

“…Xie et al [7] found a similar result using confocal laser scanning microscopy on alginate gel layers harvested from fouled membranes: when operated under similar conditions with the same initial flux, thicker cake layers developed in FO than in RO; however, that study attributed the thinner layer in RO to compaction by hydraulic pressure. Future work should address whether the disparity in foulant accumulation is solely driven by the difference in flux decline rates of FO and RO systems, or whether other differences contribute as well.…”

“…Many fouling studies are concerned with comparisons, such as the relative fouling propensity of FO vs. RO [5], pressurized vs. unpressurized FO [6,7], or new membrane coatings vs. commercial membranes [8]. Although flux decline comparisons often keep initial flux constant because of the dependence of fouling rate on flux [9], differences in membrane properties and solution composition can lead to differences in flux decline between experiments even if the foulant layers are identical in size and structure.…”

Silica scaling in forward osmosis: From solution to membrane interface

“…Water flux was continuously monitored throughout the fouling 120 experiments by a data logger. A baseline experiment (i.e., feed without silica foulant) was also 121 carried out to correct the flux decline due to the continuous concentration of the feed solution 122 and dilution of the draw solution, as described in our previous publication (Xie et al 2015b). The 123 feed solution was continuously sampled to quantify the evolution of silica polymerisation.…”

Shielding membrane surface carboxyl groups by covalent-binding graphene oxide to improve anti-fouling property and the simultaneous promotion of flux

“…The cross-flow velocity during this stage was 0.11 m/s. The same TMP of 0.45 MPa was chosen to exclude the impact of TMP on membrane fouling (Xie et al, 2015). Alginate was used as a model macromolecule for the fouling experiment.…”