Thin-film Composite Forward Osmosis Membranes Research Articles

Probing the key foulants and membrane fouling under increasing salinity in anaerobic osmotic membrane bioreactors for low-strength wastewater treatment

Anaerobic osmotic membrane bioreactors (AnOMBRs) have attracted much attention in recent years due to their ability to produce high-quality effluent with low energy demand. Here, the performance of a thin film composite (TFC) forward osmosis (FO) membrane in an AnOMBR system is evaluated, and the key membrane foulants are examined by analyzing the characteristics of the mixed liquor and the fouling layer. As the experiment progressed, the water flux declined due to membrane fouling and salt accumulation in the bioreactor, and reached approximately 3.5 LMH by the end of each operating cycle where membrane cleaning was adopted. The conductivity increased from 3.3 mS/cm to 23.6 mS/cm over the 28-day treatment. The contaminant rejection rates of total organic carbon, total phosphorous and ammonia-nitrogen were more than 93%, 98% and 60%, respectively. Results also suggested that extracellular polymeric substances (EPS) was the major contributor to membrane fouling, and the increase of EPS in the mixed liquor with salinity was primarily composed of polysaccharides. The comprehensive characterization of the fouling layer via X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy and confocal laser scanning microcopy (CLSM) revealed the coexistence of biofouling and organic fouling. The CLSM images further demonstrated the prevalence of β polysaccharides, proteins and microorganisms on the fouled TFC FO membrane surface. In addition, the flow cytometric analysis indicated that the foulant sample was “bacterial viable”, the large number of bacteria in suspension (96.61% live cells) maybe corresponded to the high EPS content in the bulk mixed liquor.

Electrospun nanofiber supports with bio-inspired modification enabled high-performance forward osmosis composite membranes

Electrospun nanofiber mats (ENMs) are desired candidates as supports for composite forward osmosis (FO) membranes owning to the high porosity and low tortuosity of pores. However, it remains challenge to construct defect-free thin selective layer on ENMs controllably to achieve simultaneous high permeability and selectivity. In order to meet the demands, a novel thin film composite (TFC) FO membrane was fabricated via interfacial polymerization on a polydopamine (PDA) modified ENM substrate (PDA-HPENM), which was hot-pressed (HPENMs) prior to the PDA deposition to improve the mechanical strength and reduce the mat thickness. The internal concentration polarization (ICP) effects and the transmembrane resistance in FO membranes are reduced significantly attributed to the unique pore structure and good hydrophilicity of the PDA-HPENMs, resulting in doubled water flux compared with that of the TFC membranes using HPENM substrates. Moreover, the optimized TFC FO membranes exhibit relatively high rejection (~100%) to heavy metal ions and antibiotics, holding great potential for wastewater treatment.

Improved performance of thin-film composite forward osmosis membrane with click modified polysulfone substrate

In this study, polysulfone-graft-poly(2-dimethylaminoethyl methacrylate) (PSf-g-PDMA) as a pH-responsive graft copolymer was synthesized for the first time via a combination of click chemistry and atom transfer radical polymerization (ATRP). This graft copolymer was blended with polysulfone during the phase inversion. The so formed substrate was used for the fabrication of a forward osmosis (FO) membrane via interfacial polymerization. The influence of the copolymer concentration on hydrophilicity, morphology, porosity, FO performance, and TFC layer characteristics were thoroughly investigated using diverse analysis. To prove pH-sensitivity of the PSF-g-PDMA blended substrate, the pure water permeability (PWP) of substrate containing 10.0 wt% of PSf-g-PDMA copolymer (G10) was measured at different pH values (3.0–12.0). As the pH of the feed solution was changed from 3.0 to 12.0, the PWP increased from 221.3 to 299.4 LMH/bar. In addition, the performance of PSF-g-PDMA based TFC membrane in FO process was studied. Compared to the pure PSf based TFC membrane, the PSf-g-PDMA modified TFC-FO membranes demonstrated increased water flux without adversely affecting their selectivity. In addition, the TFC-G10 membrane exhibited a reversible pH-dependence water flux. The pH-reversibility was investigated using draw solution with different pH values (3.0 and 10.0) where acidic draw solutions show higher water fluxes than basic draw solutions.

Polydopamine nanoparticles modified nanofiber supported thin film composite membrane with enhanced adhesion strength for forward osmosis

To minimize the internal polarization concentration (ICP) in FO process and improve the structural stability of thin film composite (TFC) forward osmosis (FO) membrane, a reactable hydrophilic nanofiber substrate was prepared by vacuum filtrating an interlayer composed of polydopamine nanoparticles (PDA NPs) onto an electrospun polyacrylonitrile nanofiber substrate. Then a TFC FO membrane with dense selective layer, low structure parameter, high water permeability and low salt permeability was prepared by the interfacial polymerization thereon. According to the T-peel test, the resulting TFC FO membrane showed greatly improved adhesion strength between selective layer and substrate due to the chemical bonding and entanglement between polyamides and the PDA NPs, and the strong adhesion between PDA NPs and the nanofibers. The ICP in FO process for the resulting membranes were minimized due to the application of the nanofiber substrates with low structure parameter (in the range of 290 ± 36 μm–354 ± 20 μm). These FO membranes performed well in the heavy metal ions contained water treatment, showed 1–2 times higher water flux (29.2 ± 0.3 L/m2h) compared with commercial TFC membrane (HTI-TFC) and high heavy metal ions rejection (99.5% ± 0.4%, 99.1% ± 0.5%, 98.1% ± 0.7% for Cr3+, Cu2+ and Ni2+, respectively) when using 2.0 M NaCl as draw solution. The findings provide the pathway for the design of nanofiber based FO membrane with improved structure stability and high performance by interfacial polymerization.

Improving water flux and salt rejection by a tradeoff between hydrophilicity and hydrophobicity of sublayer in TFC FO membrane

This paper presents a novel approach using sulfonated polyethersulfone/zeolitic imidazolate framework (SPES/ZIF-8) porous nanocomposite substrate to enhance the salt rejection and water flux of forward osmosis (FO) membrane. The effect of SPES/ZIF-8 substrate on FO membrane properties containing morphologies, hydrophilicity, water permeability and salt rejection were investigated and completely discussed. Based on the obtained results, the selectivity and permeability of thin-film nanocomposite (TFN) FO membranes can be improved simultaneously through adjusting the both hydrophilic and hydrophobic characteristics. But these two results, is achieved in two different processes and with reverse impacts. After the sulfonation process and using the more hydrophilic polymer (SPES) instead of the neat PES, the porosity and consequently the water flux was increased. Because of more hydrophilicity of SPES which reduces the cross-linking degree of the corresponding polyamide (PA) layer, the salt rejection was reduced. In contrast, after the incorporation of ZIF-8 into the SPES substrate, the salt rejection was increased due to the hydrophobic nature of these nanoparticles (NPs). In terms of both salt rejection and water flux, the sulfonated TFN membrane with 0.5% ZIF-8 NPs (TFN-S0.5 membrane) showed the improved results comparing with the bare Thin-film composite (TFC) membrane. Furthermore, the best result for the ratio of reverse salt flux to water flux (Js/Jw) as the selectivity parameter, was about 0.16 g/L using 10 mM and 2 M NaCl solutions as the feed solution (FS) and the draw solution (DS), respectively, in the FO experiment. Also, the structural parameter (S) which is believed to have a strong relationship with the internal concentration polarization (ICP), was decreased to 0.39 nm as the best result, after modification of FO membrane.

Fabrication and modification of forward osmosis membranes by using graphene oxide for dye rejection and sludge concentration

In this study, the preparation procedures for the polysulfone (PSf) substrate and polyamide (PA) selective layers were systematically investigated to determine their effects on the separation performance of thin-film composite (TFC) forward osmosis (FO) membranes in terms of the permeate flux (Jw) and reverse solute flux (Js). Furthermore, the PA active layer was modified by adding different proportions of an emerging material, graphene oxide (GO), to increase Jw and decrease Js. The experimental results indicated that special attention should be paid to the preparation of the PSf casting solution, which required thorough degassing, sealing, and humidity and temperature control. The optimum casting height was discovered to be 175 μm. For PA layer formation, the same amount of polymer solutions (resulting thickness of 78.5 μm) on the top surface of the PSf substrate (on the side facing the water during phase inversion) resulted in the highest FO performance. GO modification of the PA layer at the dosage of 0.0175 wt% considerably enhanced Jw to 14 L m−2 h−1 and reduced Js to 0.23 mol m−2 h−1. However, higher GO dosage (0.02 wt%) led to lower membrane performance due to aggregation of GO nanoparticles, as confirmed using scanning electron microscopy. Next, the prepared membranes were applied to dye rejection and sludge concentration for water recovery. The virgin and modified FO membranes both exhibited high rejection efficiency (≥96.0 %) for dyes commonly used in the textile industry. The 0.0175 %-GO-modified FO membrane exhibited a higher concentration factor (1.67) and greater water recovery (40.0 %) than the virgin membrane (1.45 and 31.2 %, respectively). Therefore, the application of FO for water recovery is economic and environmentally friendly in terms of saving the transportation cost of sludge disposal while recovering water for reuse in wastewater treatment plants.

New insights into the interaction between surface-charged membranes and positively-charged draw solutes in the forward osmosis process

In this study, a flat-sheet polyamide thin film composite (TFC) membrane with different surface charges has been designed as a forward osmosis (FO) membrane while a positively-charged organic chitooligosaccharide (COS) applied as a draw solution (DS). The flat-sheet TFC FO membrane consists of three layers (from top to bottom): the polyamide selective layer, electrospun cellulose triacetate (CTA) nanofibers and mechanically robust polyacrylonitrile (PAN) nanofibers coated with dopamine (DPA)/polyethyleneimine (PEI). The DPA/PEI modification on the PAN nanofibers has proven to be efficient and resulted in a more positively-charged surface with improved hydrophilicity. Meanwhile, the polyamide layer formed by the interfacial polymerization (IP) method was negatively charged on top of the FO membrane. The interaction between surface-charged membranes and positively-charged draw solutes in the FO process has been investigated comprehensively. The result showed that hydrophilicity and electrostatic repulsion could alleviate the fouling generated by organic DS, while hydrophobicity and electrostatic attraction could magnify the fouling. Finally, the positively-charged DS was recycled by the positively-charged PEI NF membranes with a reasonably high special water flux and salt rejections. Therefore, this study shows that the combination of the surface-charged membranes and positively-charged COS DS has the desirable water flux, little reverse salt flux (RSF) and fouling effects. In conclusion, the developed membrane is providing a new way to design practical FO processes.

Desalination performance of thin-film composite forward osmosis membranes based on different carbon nanomaterials

The incorporation of nanomaterials in thin film composite (TFC) has provided a new way to increase the permeability of the forward osmosis membrane during the desalination of water. In this research graphene oxide (GO), reduced graphene oxide (rGO) and carbon nanotube (MWCNT) was prepared and introduced into a polyamide layer to form TFC membrane via interfacial polymerization reaction. The properties of the membranes were characterized by using ATR- FTIR, SEM and water contact angle measurements. The obtained results illustrated that a TFC-FO membrane doped with GO (0.7wt%) has a high water flux (27.15 L m-2h-1) while membrane doped with rGO (0.5wt%) exhibits water flux (24.05 L m-2h-1), and the membrane doped with MWCNT (0.5wt%) has water flux (21.67 L m-2h-1) compared with pure membrane with water flux of (10.24 L m-2h-1).

Performance Improvement and Biofouling Mitigation in Osmotic Microbial Fuel Cells via In Situ Formation of Silver Nanoparticles on Forward Osmosis Membrane.

An osmotic microbial fuel cell (OsMFC) using a forward osmosis (FO) membrane to replace the proton exchange membrane in a typical MFC achieves superior electricity production and better effluent water quality during municipal wastewater treatment. However, inevitable FO membrane fouling, especially biofouling, has a significantly adverse impact on water flux and thus hinders the stable operation of the OsMFC. Here, we proposed a method for biofouling mitigation of the FO membrane and further improvement in current generation of the OsMFC by applying a silver nanoparticle (AgNP) modified FO membrane. The characteristic tests revealed that the AgNP modified thin film composite (TFC) polyamide FO membrane showed advanced hydrophilicity, more negative zeta potential and better antibacterial property. The biofouling of the FO membrane in OsMFC was effectively alleviated by using the AgNP modified membrane. This phenomenon could be attributed to the changes of TFC–FO membrane properties and the antibacterial property of AgNPs on the membrane surface. An increased hydrophilicity and a more negative zeta potential of the modified membrane enhanced the repulsion between foulants and membrane surface. In addition, AgNPs directly disturbed the functions of microorganisms deposited on the membrane surface. Owing to the biofouling mitigation of the AgNP modified membrane, the water flux and electricity generation of OsMFC were correspondingly improved.

Surface modification of thin-film composite forward osmosis membranes with polyvinyl alcohol–graphene oxide composite hydrogels for antifouling properties

In this study, the polyamide (PA) layers of commercial thin-film composite (TFC) forward osmosis (FO) membranes were coated with glutaraldehyde cross-linked polyvinyl alcohol (PVA) hydrogel comprising of graphene oxide (GO) at various loadings to enhance their fouling resistance. The optimal GO concentration of 0.02 wt% in hydrogel solution was confirmed from the FO membrane performance, and its influence on membrane antifouling properties was studied. The properties of the modified membranes, such as surface morphology, surface charge and wettability, were also investigated. PVA/GO coating was observed to increase the smoothness and hydrophilicity of the membrane surface. The foulant resistances of the pristine, PVA-coated and PVA/GO-coated membranes were also reported. PVA hydrogel-coated TFC membrane with a GO loading of 0.02 wt% showed a 55% reduction in specific reverse solute flux, only a marginal reduction in the water flux, and the best antifouling property with a 58% higher flux recovery than the pristine TFC membrane. The significant improvement in the selectivity of the modified membranes meant that the hydrogel coating could be used to seal PA defects. The biocidal GO flakes in PVA hydrogel coating also improved the biofouling resistance of the modified membranes, which could be attributed to their morphologies and superior surface properties.

Improved antifouling and antibacterial properties of forward osmosis membranes through surface modification with zwitterions and silver-based metal organic frameworks

This study investigates the effect of surface functionalization of a thin-film composite forward osmosis membrane with zwitterions and silver-based metal organic frameworks (Ag-MOFs) to improve the antifouling, anti-biofouling, and antimicrobial activity of the membrane. Two types of zwitterions, namely, 3-bromopropionic acid and 1,3-propane sultone, are chemically bonded, with and without incorporation of Ag-MOFs, over the surface of a polyamide membrane. Spectroscopy measurements indicate successful grafting of the modifying agents on the membrane surface. Contact angle measurements demonstrate a notable improvement in surface wettability upon functionalization. The performance of the membranes is evaluated in terms of water and salt fluxes in forward osmosis filtrations. The transport data show demonstrably increased water flux of around 300% compared to pristine membranes, with similar or slightly reduced salt reverse flux. The antifouling and anti-biofouling properties of the modified membranes are evaluated using sodium alginate and E. coli, respectively. These experiments reveal that functionalized membranes exhibit significant antifouling and anti-biofouling behavior, with high resilience against flux decline.

Effect of a novel hydrophilic double-skinned support layer on improving anti-fouling performance of thin-film composite forward osmosis membrane

A thin-film composite (TFC) membrane with hydrophilic double-skinned support layer was developed to improve anti-fouling performance. The double-skinned separate casting technology was applied to obtain the double-skinned support layer and the polyacrylonitrile (PAN) concentration in the bottom casting solution was adjusted. Then, the support layer was carboxylated by immersing in a 1.5 M NaOH solution for 2 h. Finally, TFC membrane with hydrophilic double-skinned support layer was obtained by the interfacial polymerization. When the PAN concentration in the bottom casting solution was 10 wt%. TFC membrane with hydrophilic double-skinned support layer (TFC-10 %) showed the best performance. The water flux of TFC-10 % reached 17.6 LMH and the reverse salt flux was 5.6 gMH. Under the double interception effect of the double-skinned structure and the steric hindrance of the hydration layer on the support layer, good anti-fouling property of TFC-10 % was observed under bovine serum albumin (BSA) foulant. Compared to the traditional TFC membrane, the normalized flux of TFC-10 % increased by 80 %. The anti-fouling performance was also analyzed by microaction mechanisms using AFM and QCM-D. It was found that the adhesion force between TFC-10 % and BSA was small, therefore fewer pollutants were adsorbed on TFC-10 %, and its adsorption layer was loose and easy-cleaning.

Influences of Combined Organic Fouling and Inorganic Scaling on Flux and Fouling Behaviors in Forward Osmosis

This study investigated the influence of combined organic fouling and inorganic scaling on the flux and fouling behaviors of thin-film composite (TFC) forward osmosis (FO) membranes. Two organic macromolecules, namely, bovine serum albumin (BSA) and sodium alginate (SA), and gypsum (GS), as an inorganic scaling agent, were selected as model foulants. It was found that GS scaling alone caused the most severe flux decline. When a mixture of organic and inorganic foulants was employed, the flux decline was retarded, compared with when the filtration was performed with only the inorganic scaling agent (GS). The early onset of the conditioning layer formation, which was due to the organics, was probably the underlying mechanism for this inhibitory phenomenon, which had suppressed the deposition and growth of the GS crystals. Although the combined fouling resulted in less flux decline, compared with GS scaling alone, the concoction of SA and GS resulted in more fouling and flux decline, compared with the mixture of BSA and GS. This was because of the carboxyl acidity of the alginate, which attracted calcium ions and formed an intermolecular bridge.

Exploring the use of cheap natural raw materials to reduce the internal concentration polarization in thin-film composite forward osmosis membranes

Internal concentration polarization (ICP) is a significant problem in Forward osmosis (FO) membranes, which reduces the water flux. In order to mitigate the ICP phenomenon, rice bran (RB) and wood sawdust (WSD) particles were selected as natural green pore formers and incorporated into the polyethersulfone (PES) matrix to fabricate mixed matrix membranes (MMMs). Fabricated MMMs were used as the porous support layer (SL) to make thin-film composite (TFC) FO membranes. Firstly, the water uptake experiment was performed to evaluate the water adsorption capacity of the RB and WSD particles. Furthermore, all samples were characterized by FTIR, FESEM, AFM, XPS, DLS, static contact angle (CA), and tensile strength. Also, performance tests in reverse osmosis (RO) and the FO units were performed to evaluate the fabricated membranes. The results showed that the use of RB and WSD particles dramatically reduced the structural parameter in all MMMs, resulting in lower ICP effects and high water flux. Due to the softer structure, smaller size, and more water uptake, the RB-based TFC membranes recorded better results. The TFC-RB-5 (with 5% of RB in the SL) was the best membrane with a water flux of about 65.71 L/m2.h for Caspian seawater desalination, while the FO water flux for DI water as the feed solution (FS) was 83.65 L/m2.h. The present study showed the membranes made in this study are competitive with the existing FO membranes and very cost-effective for broad applications.

Effect of the support layer morphological structure on the performance of forward osmosis hollow fiber membranes

A thin-film polyamide layer was synthesized on three different hollow-fiber support layers, each of which possess different morphological structures (i.e., sponge-like structures (SM-1), finger-like macrovoid close to the outer surface (SM-2), and middle-partitioned finger-like structures (SM-3)). In this work, we found that forward osmosis (FO) performance of thin-film composite (TFC) membranes could be significantly affected by their sub-layer morphology. During FO tests, SM-1 showed the highest water permeability of 0.742 LMH/bar followed by SM-3 and SM-2. In addition, SM-1 exhibited the lowest solute permeability of 0.039 LMH, followed by SM-2 and SM-3. To more systematically evaluate the effect of the support layer structures on the performance of TFC FO membranes, the structural parameters (S) of the membranes were estimated using the dusty gas model (DGM) prior to interfacial polymerization on the support layers. The S values of the support layers obtained from the DGM method are in accord with the FO membrane performance. Although the conventional S parameter determination method showed relatively similar values with the DGM method, the overall trend corresponds, confirming that the smaller S value is related to a smaller internal concentration polarization decrease. Moreover, this study clearly demonstrates that the support layer needs to be considered alongside the active layer of TFC FO membranes, and that a sponge-like support layer can be a better option for high-performance FO TFC membranes.

Characterization and control of membrane fouling during dewatering of activated sludge using a thin film composite forward osmosis membrane

This study investigated the feasibility of applying a thin film composite (TFC) forward osmosis (FO) membrane in the dewatering of activated sludge (AS). Membrane fouling was investigated and controlled to enhance the system’s performance. Investigations showed that the TFC FO membrane provided a water flux that was 120 % higher and a concentration factor that was three times higher compared to a cellulose tri-acetate (CTA) membrane. The foulant layer on the TFC membrane surface was mostly irreversible when 1.44 mg-C/cm2 and 0.13 mg-C/cm2 dissolved organic carbon (DOC) were extracted in sodium hydroxide (NaOH) and deionized (DI) water, respectively. The results of principle component analysis (PCA) revealed that among the operating conditions, the amount of aromatic organic compounds (indicated by UV254 values) followed by their hydrophilicity (specific ultraviolet absorbance (SUVA) indices) were the dominant factors controlling the different fouling potentials. SUVA value indices ranged from 0.4 to 0.6 L/m-mg DOC, illustrating that hydrophilic compounds were more responsible for membrane fouling than hydrophobic components. These results implied that aromatic and hydrophilic substances, in particular protein and polysaccharides were key components of the fouling layers, which need to be considered to enable a reduction of membrane fouling. We thus employed several novel fouling control methods, in which the combination of mono-chloramine pre-treatment and membrane cleaning by NaOH resulted in the recovery up to 86 % of the water from raw AS.

Feasibility of the highly-permselective forward osmosis membrane process for the post-treatment of the anaerobic fluidized bed bioreactor effluent

A laboratory-made high-performance thin-film composite (TFC) forward osmosis (FO) membrane supported by a polyacrylonitrile (PAN) support was firstly employed to treat the effluent produced by an anaerobic fluidized bed bioreactor (AFBR). Synthetic wastewater was prepared to operate the AFBR, which produced the effluent for the FO process as post-treatment. The performance of the FO membrane was characterized via the measurement of the permeate flux, reverse salt (NaCl) flux and the rejection efficiency of ammonium nitrogen (NH4-N) mostly present in the AFBR effluent having low removal characteristics. An ultrathin and highly-crosslinked polyamide selective layer combined with the PAN support having a low structural parameter of the prepared PAN-supported TFC (PAN-TFC) membrane resulted in higher rejection to both nitrogen and salt compared to a commercial FO membrane. Importantly, the rejection of NH4-N higher than 70% was achieved by the PAN-TFC FO membrane. At 0.5 M NaCl draw solution, the PAN-TFC membrane exhibited the nitrogen flux of 0.92 ± 0.01 g m−2 h−1 and the permeate flux of 25 L m−2 h−1. The results demonstrate that the FO membrane can be one of the promising ways to effectively control the removal of contaminants present in AFBR effluent such as NH4-N as a post-treatment.

Tailoring the Polyamide Active Layer of Thin-Film Composite Forward Osmosis Membranes with Combined Cosolvents during Interfacial Polymerization

In this study, it was explored the method of using combined co-solvents to tailor the polyamide (PA) active layer during interfacial polymerization (IP) process for improving the performance of thi...

Insight into organic fouling behavior in polyamide thin-film composite forward osmosis membrane: Critical flux and its impact on the economics of water reclamation

A strategy of using critical fluxes to control organic fouling of polyamide thin film composite (PA-TFC) forward osmosis (FO) membranes during wastewater reclamation was developed for FO mode. This work was a comprehensive investigation with various organic foulants covering complex mixtures as well as single foulants. The foulants were alginate (ALG), Humic acid (HA), and Bovine Serum Albumin (BSA) and the study covered different concentration (40; 80; 120; 160 mg/L). Our results indicated that there was a single value of critical flux, 35 LMH for 160 mg/L and single foulant. However the presence of mixed foulants i.e., ALG + BSA, ALG + HA, HA + BSA, at an overall foulant concentration of 160 mg/L gave rise to foulant-foulant-membrane interactions that caused a significant decrease in critical flux values to 25–30 LMH. Using these results as a guide, long-term tests in which there was no fouling or negligible fouling were successfully implemented. Operating below critical flux maintains a sustainable operation with the characteristic of full reversibility, which is vital if chemical cleaning is to be minimized. Characterization of fouling around critical values was made through physico-chemical analyses including SEM, EEM, aggregate size, zeta potential, and FTIR. It was found that FO fouling became irreversible when operated at a flux ≥35 LMH for single foulants and fluxes of 25 LMH and 30 LMH for ALG + BSA and HA + BSA foulants respectively, being a foulant concentration of 160 mg/L; such conditions are favorable for the formation of the cohesive and compact cake layer. Economic assessments based on specific energy consumption facilitated the production of guidelines for practical design and operation.

The role of polyvinyl butyral additive in forming desirable pore structure for thin film composite forward osmosis membrane

A thin-film composite forward osmosis (TFC-FO) membrane was developed by combining a new substrate consisting of cellulose acetate butyrate (CAB) and polyvinyl butyral (PVB) with the conventional polyamide active layer. The incorporation of the PVB additive (0–4 wt%) was to improve the hydrophilicity and alter the pore formation mechanism of the substrate to reduce the internal concentration polarization (ICP) effect in FO. The results showed that promising structural parameters of the CAB/PVB substrate such as A value (1.08 L m−2 h−1 bar−1) and S value (363.5 μm) could be obtained for the M2 substrate with 2 wt% PVB, due to the combined effect of finger-like pores and high porosity with high pore connectivity. The evaluation of the TFC-FO membranes showed that the water flux of the TFC-M2 was up to 27.5 L m−2 h−1 with 1 M NaCl draw solution in active-layer facing draw solution (AL-DS) mode, which was 40% and 130% flux enhancement compared to the reported commercial CTA and pure CAB control TFC membrane, respectively, with lower specific salt flux Js/Jw (0.35 g/L). The fouling experiments using model feed containing BSA foulant showed that the TFC-M2 was almost unaffected (98%) by the dilutive ICP effect in the active-layer facing feed solution (AL-FS) mode; while it also exhibited the highest flux recovery of 88% in the AL-DS mode. Overall, the total mass transfer resistance Rt of the TFC-M2 was the lowest compared to all as-prepared TFC-FO membranes, indicating the key role of the PVB additive in tuning the CAB substrate structure for designing high performance FO membranes.