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Nanofiltration membranes have limited ion-ion selectivity in water treatment applications, especially when separating ions with similar size and charge. To achieve greater size-based selectivity in nanofiltration, more control of pore structure is required during membrane fabrication. We demonstrate how to tailor membrane pore size and thickness using polyelectrolyte layer-by-layer assembly by alternately applying two strong polyelectrolytes, PDADMAC and PSS, to a polysulfone substrate while systematically controlling the polyelectrolyte and salt concentrations in the deposition solution. Results suggest that increasing polyelectrolyte concentration or salt concentration in the deposition solution increases polyelectrolyte multilayer thickness, but the effects on pore size may be categorized into two distinct regimes. In the first growth regime, increasing polyelectrolyte concentration in the deposition solution led to larger polymer deposition rates and smaller pore sizes. In the second growth regime, increasing polyelectrolyte concentration produced larger pore sizes. We attribute the second regime to less adsorbed polyelectrolyte on the membrane and/or less coiled polymer chains as a result of changing polyelectrolyte-salt interactions. Overall, results show that pore size modification is achievable using layer-by-layer assembly by tuning polyelectrolyte-salt interactions and can be used to study and improve size-based selectivity in membrane separation processes.

We explored the mechanisms governing the selectivity of anion- and cation-exchange membranes for the transport of four monovalent anions (i.e., fluoride, chloride, bromide, and nitrate) and four monovalent cations (i.e., sodium, potassium, cesium, and ammonium), respectively. Our ion adsorption and transport tests with mixed ion solutions reveal that an ion with larger ionic radius and lower hydration energy is more favorably adsorbed onto the ion-exchange membrane but diffuses more slowly through the polymer matrix compared to an ion with smaller ionic radius and higher hydration energy. Individual anion (as sodium salt) or cation (as chloride salt) permeation tests at different temperatures were performed to evaluate the activation behavior of ion transport through the ion-exchange membranes by calculating the energy barrier and pre-exponential factor (i.e., the ion flux when the energy barrier is negligible) for ion transport from an Arrhenius-type equation. Our results show that an ion with smaller ionic radius and higher hydration energy experiences higher energy barrier (e.g., fluoride, 10.3 kcal mol−1) and possesses higher pre-exponential factor compared to an ion with larger ionic radius and lower hydration energy (e.g., bromide, 4.6 kcal mol−1). This correlation corroborates our main hypothesis that the activation behavior observed for ion transport is a result of ion dehydration at the water-membrane interface. Our proposed ion selectivity mechanism elucidates how ion dehydration governs the extent of ion permeation into the membrane and the subsequent transport through the charged polymer matrix. Future membrane design that promotes dehydration of target ions is challenging but can result in unprecedented ion selectivity.

Polyelectrolyte multilayer membranes (PEMs) produced by the sequential, layer-by-layer deposition of polyelectrolytes on porous supports have been shown to significantly reject ions in dilute saline solutions. However, polyelectrolyte thin films are susceptible to swelling or detachment from the substrate in higher salinities and extreme pH conditions, such that their performance is highly dependent on feed water composition. In this study, we introduce tethered electrolyte active-layer membranes (TEAMs), whereby charged block copolymers are covalently grafted-from a porous support, in aim of reducing the stimuli response of layered polyelectrolyte membranes under variable solution conditions. Cellulose support layers were modified using surface-initiated atom transfer radical polymerization of neutral precursor polymers, which were then converted into charged blocks after polymerization. An ultrathin layer of a single negative or positive block with a degree of polymerization (DP) ≥ 770 exhibited ∼15–20 L m-2 h-1 bar-1 pure water permeability, 45–60% rejection of monovalent co-ions, and ∼75% rejection of divalent co-ions. In mixed feed solutions, selectivity of monovalent over divalent co-ions with single-block TEAMs fell within the range of 2–4. Single-block TEAMs slightly shrank under high salt concentration, contrary to the typical substantial swelling of PEMs. As such, this new form of polyelectrolyte membrane may provide greater stability than conventional PEMs and may have potential applications in water softening and salinity reduction of surface waters.

Membrane modification using zwitterion polymers is an excellent strategy to reduce organic fouling and initial bacterial deposition, and functionalization of the membrane surface with antibacterial material can reduce biofilm formation. In this study, we combined these two approaches to develop an ultrafiltration polyethersulfone (PES) membrane with dual antifouling and antibacterial properties. At the beginning, a zwitterion polyampholyte hydrogel was UV grafted onto PES membrane surface (p-PES membrane). Then, the hydrogel was loaded with graphene oxide (GO) nanosheets using vacuum filtration strategy (GO-p-PES membrane). Raman spectroscopy and scanning electron microscopy (SEM) confirmed the successful incorporation of the GO nanosheets into the zwitterion hydrogel, and contact angle measurements indicated that the membrane hydrophilicity was enhanced. Static adsorption and dynamic filtration experiments demonstrated that the p-PES and GO-p-PES membranes exhibited similar organic fouling propensity. Moreover, the loading of GO induced antibacterial property for the membrane as evidenced by contact killing and antibiofouling filtration experiments. Leaching of GO was very low, with over 98% of the GO remaining on the membrane surface after 7 days. Our findings highlight the potential of this GO-functionalized polyampholyte hydrogel for long-term wastewater treatment.

We fabricated polyelectrolyte multilayer (PEM) nanofiltration (NF) membranes using a layer-by-layer (LbL) method for effective removal of scale-forming divalent cations (Mg2+, Ca2+, Sr2+, and Ba2+) from feedwaters with different salinities. Two polymers with opposite charges, polycation (poly(diallyldimethylammonium chloride), PDADMAC) and polyanion (poly(sodium 4-styrenesulfonate), PSS), were sequentially deposited on a commercial polyamide NF membrane to form a PEM. Compared to pristine and PSS-terminated membranes, PDADMAC-terminated membranes demonstrated much higher rejection of divalent cations and selectivity for sodium transport over divalent cations (Na+/X2+) due to a combination of both Donnan- and size-exclusion effects. A PDADMAC-terminated membrane with 5.5 bilayers exhibited 97% rejection of Mg2+ with selectivity (Na+/Mg2+) greater than 30. We attribute the order of cation rejection (Mg2+ > Ca2+ > Sr2+ > Ba2+) to the ionic size, which governs both the hydration radius and hydration energy of the cations. The ionic strength (salinity) of the feed solution had a significant influence on both water flux and cation rejection of PEM membranes. In feed solutions with high ionic strength, abundant NaCl salt screened the charge of the polyelectrolytes and led to swelling of the multilayers, resulting in decreased selectivity (Na+/X2+) and increased water permeability. The fabricated PEM membranes can be potentially applied to the pretreatment of mild-salinity brackish waters to reduce membrane scaling in the main desalination stage.

In this study, we demonstrate nanostructured osmosis membranes that employ vapor-phase water transport to simultaneously achieve high rejection of solutes and a high permeability. The membranes consist of a hydrophobic, thermally conductive silica nanoparticle (SiNP) layer with tunable thickness supported by a hydrophilic track-etched membrane. The membrane permeability for water vapor is 1–2 orders of magnitude higher than hydrophobic microporous membranes used for osmotic distillation. This permeability is only mildly lower (~ 3 times) than the equivalent water permeability of typical forward osmosis (FO) membranes. We also demonstrate the high selectivity of the SiNP membrane via urea permeation tests, where this membrane exhibits a 2–3 orders of magnitude lower urea permeability coefficient than a thin-film composite (TFC) FO membrane. Further measurements and theoretical analysis using the dusty-gas model suggest that membranes with a smaller SiNP layer thickness are capable of having comparable water fluxes to TFC FO membranes while maintaining higher selectivity. Our work demonstrates that thin, hydrophobic nanostructured membranes composed of thermally conductive materials have a great potential to significantly extend the applications of osmosis-driven processes to treat challenging water sources.

We used layer-by-layer (LbL) self-assembly to fabricate a polyelectrolyte (PE) nanofiltration membrane for salt rejection and to immobilize trypsin on the membrane outer layer for biocatalytic activity. Poly(ethylene imine) (PEI) and poly(diallyl dimethyl ammonium chloride) (PDADMAC) were used as cationic PE while poly(acrylic acid) (PAA) and poly(styrene sulfonate) (PSS) were used as anionic PE. The impact of PE type, number of PE bilayers, and PE concentration on the rejection of inorganic salts (NaCl, MgCl2, Na2SO4, and MgSO4) and protein (bovine serum albumin, BSA) was systematically investigated. A maximum rejection of 12.7%, 45.2%, 85.5%, 94.0%, and 100% of MgCl2, NaCl, MgSO4, Na2SO4, and BSA, respectively, was obtained by the PDADMAC-PSS membrane with four bilayers. Trypsin (TRY) was immobilized on the membrane surface by electrostatic attraction or covalent bonding to produce a biocatalytic membrane and to alleviate protein fouling. Important parameters for enzymatic activity, such as immobilization time, pH, temperature, salt concentration and type, as well as the reuse number and storage time were investigated to expound the mechanism of enzyme activity in the presence of salt and BSA. BSA was used as a model protein for organic fouling experiments, and flux decline rate of the membranes was determined. Our results show that LbL-modified membranes with covalent enzyme immobilization had the lowest protein fouling rate, which we attribute to the biocatalytic activity of the immobilized trypsin.

Neutron radiography, a non-destructive imaging technique, is applied to study water and solute transport through desalination membranes. Specifically, we use neutron radiography to quantify lithium chloride draw solute concentrations across a thin-film composite membrane during forward osmosis permeation. This measurement provides direct visual confirmation of incomplete support layer wetting and reveals significant dilutive external concentration polarization of the draw solution outside of the membrane support layer. These transport-limiting phenomena have been hypothesized in previous work and are not accounted for in the standard thin-film model of forward osmosis permeation, resulting in inaccurate estimations of membrane transport properties. Our work demonstrates neutron radiography as a powerful measurement tool for studying membrane transport and emphasizes the need for direct experimental measurements to refine the forward osmosis transport model.

We conducted a comparative study to investigate the efficacies of two different types of highly hydrophilic materials (i.e., silica nanoparticles (SiNPs) and zwitterionic polymers) for antifouling surface modification of polyamide thin-film composite (TFC) membranes. Dense layers of SiNPs and zwitterionic polymer brushes were grafted on the membrane surfaces via dip-coating with aminosilane-functionalized SiNPs (i.e., SiNP-TFC membrane) and surface-initiated atom-transfer radical-polymerization of sulfobetaine methacrylate (i.e., PSBMA-TFC membrane), respectively. With the same degree of enhancement of surface hydrophilicity and identical surface roughness, the PSBMA-TFC membrane exhibited significantly higher fouling resistance than the SiNP-TFC membrane in adsorption tests of proteins and bacteria as well as in forward osmosis (FO) dynamic fouling experiments using alginate as a model organic foulant. Chemical force microscopy measurements revealed that membrane-foulant electrostatic attraction aggravates organic fouling of the SiNP-TFC membrane to a certain degree, but the primary fouling mechanism is the complexation of organic foulants with carboxylic groups on the polyamide membrane surface. We attribute the lower fouling resistance of the SiNP-TFC membrane to the high density of surface carboxylic groups that may still be accessible to foulants as well as to membrane-foulant electrostatic interaction. On the contrary, the zwitterionic polymer brushes effectively shield the surface carboxylic groups and provide steric hindrance against foulant adsorption due to significant hydration of the zwitterionic brushes.

Direct contact membrane distillation (DCMD) is a thermal desalination process that is capable of treating high salinity waters using low-grade heat. As a water treatment process, DCMD has several advantages, including the utilization of waste heat (below 100 ℃ ), perfect rejection of nonvolatile solutes, low areal footprint, and high scalability. However, the energy efficiency of DCMD is relatively low compared to other work-based and thermal desalination processes. In this study, we aim to quantify how membrane properties and process conditions affect the exergy or second-law efficiency ( η II ) of a DCMD desalination system with external heat recovery. In particular, we analyze how the membrane permeability coefficient ( B ) and thermal conduction coefficient ( K ) impact MD performance. We show that increasing the B value of a membrane by reducing its thickness, initially leads to an increase in η II before conductive heat loss through the membrane causes η II to fall. For a typical MD membrane with a porosity of 0.90 , material thermal conductivity of 0.20 W m − 1 K − 1 , and a nominal pore diameter of 0.6 μ m , we find that the optimal permeability coefficient is 1.59 × 10 − 6 kg m − 2 s − 1 Pa − 1 ( 572 kg m − 2 h − 1 bar − 1 ). This value corresponds to an optimal membrane thickness of around 95 μ m . Our analysis stresses the importance of effective heat recovery in DCMD. We show that an external heat exchanger with a minimum approach temperature of 5 ℃ reduces energy consumption by 72 % . Finally, we demonstrate that increasing the ratio B / K , rather than just the B value, is key to increasing the exergy efficiency of DCMD desalination. For example, increasing membrane porosity from 0.70 to 0.90 , which yields a 160 % increase in B / K , leads to a 42 % increase in η II from 5.3 % to 7.6 % . The advantages of reducing the bulk pressure ( P ) in the membrane pores are also explored. For a typical membrane, halving P from 1.0 bar to 0.5 ⁢ bar , results in a 21 % increase in η II from 7.0 % to 9.2 % . We conclude by identifying that the maximum exergy efficiency achievable as membrane porosity tends to unity is 10 % for a bulk membrane pressure of 1.0 bar and 12 % for a bulk membrane pressure of 0.5 bar , given perfect heat recovery.

Fully aromatic polyamide thin-film composite (TFC) membranes are the industry standard for membrane-based desalination. Due to the extensive application of TFC membranes to treat feed waters of widely varying composition, new methods are needed to modulate the water permeability, water–solute selectivity, and surface charge of the polyamide selective layer. In this study, the quenching of residual acyl-chloride groups in nascent polyamide films is performed using amine, ammonia, and alcohol solutions, including common alcohol solvents such as methanol and ethanol. Membrane transport characterization in reverse osmosis demonstrates that quenching can substantially increase water permeability. For example, quenching in ammonium hydroxide resulted in water permeability of 2.50 L m−2 h−1 bar−1, which was 2.8 times greater than that for water-quenched control membranes, paired with a small decrease in sodium chloride rejection from 99.7% to 99.3%. By using different quenching solutions, quenching resulted in a range of water permeabilities (0.15–2.50 L m−2 h−1 bar−1) and water–salt selectivities (98.4–99.7% salt rejection), with performance falling along a previously proposed permeability–selectivity trade-off. Transport behavior and surface characterization indicate that chemical reactions occur to form amides and esters during quenching. Additionally, quenching decreased the carboxyl group density from 25.5 sites/nm2 for water-quenched control membranes to 11.8 sites/nm2 for membranes quenched in ammonium hydroxide. Taken together, the results demonstrate that acyl-chloride quenching provides a novel route to alter the surface charge in addition to water permeability and water–salt selectivity. Potential ways to further optimize the method are discussed.

Osmotic power harnesses the energy of mixing between high salinity and low salinity streams to generate mechanical energy. The closed-loop osmotic heat engine (OHE) is a low-grade heat powered, membrane-based energy system that couples membrane distillation (MD), a thermally driven membrane process, with pressure retarded osmosis (PRO), an osmotically driven membrane process. The objective of this study was to evaluate the technical and economic feasibility of an OHE to generate electricity. Experimental data and previously established MD and PRO models were used to develop an OHE system model that calculates system efficiency (a ratio between the net energy output and thermal energy input), net power output, and electricity generation costs. Results show that the levelized cost of electricity generation by an OHE at the current state of the technology is $0.48 per kWh, which is not competitive with wholesale conventional U.S. grid electricity costs of $0.04/kWh [1], nor comparable to low-grade heat-powered Organic Rankine Cycle electricity generation costs ($0.08–0.13/kWh). To investigate the robustness of the OHE model, a sensitivity analysis was performed to evaluate the influence of select model inputs on electricity costs. Results indicate that improving PRO membrane power density has the highest potential benefit to reduce OHE electricity generation costs. Development of highly permeable and selective PRO membranes that are mechanically stable at increased hydraulic pressures is critical for maturation of PRO and OHE. Alternative working fluids capable of producing higher osmotic pressures and having lower reverse solute fluxes may aid in increasing OHE performance, but not substantially. Our analysis shows that substantial improvements to system operation and membrane performance could reduce electricity generation cost of large installations close to $0.10 per kWh.

Polyamide thin-film composite (TFC) membranes are the industry standard for membrane-based desalination. Negatively-charged carboxyl groups in the polyamide selective layer play an important role in membrane performance, affecting ion permeation, fouling, and scaling. As such, simple and accurate quantitation of the carboxyl group density is needed. While several methods already exist, each has important drawbacks that limit its application. In this study, we develop a simple bind-and-elute method utilizing silver ion probes, with silver quantitation performed using inductively coupled plasma mass spectrometry. First, the efficacy of the binding, wash, and elution steps is verified, most notably using ion exchange resin with known binding capacity. The method is then used to characterize the carboxyl group density and ionization behavior of six commercial polyamide TFC membranes, with total densities ranging from 7.2 to 37 sites/nm2 and ionization behaviors best described using two acid dissociation constants (pKa values). Comparison with data for polyamide layers isolated using dimethylformamide (DMF) shows a 35%–65% decrease in carboxyl density, suggesting that DMF may dissolve uncrosslinked polyamide oligomers. Lastly, the method is used to characterize the effect of two membrane surface modifications, with the results supporting an alternative physical interpretation of the ionization behavior in which a carboxyl group's pKa is dependent on whether it is located on the surface or buried within the polyamide network. The simplicity, accuracy, and accessibility of the developed method should allow for widespread usage in future studies involving membrane surface charge, as well as studies involving other charged interfaces.

We report the fabrication and characterization of a novel organic/inorganic hybrid ultrafiltration membrane with anti-fouling and self-cleaning properties. Nanoscale TiO2 clusters were grafted on the side chains of a poly(aryl ether sulfone) matrix containing trifluoromethyl and carboxyl groups (PES-F-COOH) using a silane coupling agent. Separation efficiency, fouling behavior, and self-cleaning property of the TiO2/PES-F-COOH hybrid ultrafiltration membrane were investigated by dead-end filtration experiments using a polyacrylamide foulant solution. Analysis of the membrane chemistry showed that grafting TiO2 on the side chain of the PES-F-COOH resulted in homogeneous dispersion of TiO2 clusters in the polymer matrix. The hybrid UF membrane exhibited significant self-cleaning efficiency. Specifically, water flux following polyacrylamide fouling was 53% recovered after membrane exposure to UV irradiation, which is attributable to photocatalytic degradation of the organic foulants by TiO2. We further demonstrated the anti-photocatalytic ageing property of the hybrid UF membrane, indicating resistance to decomposition of the membrane polymer matrix by photocatalytic oxidation. Our developed method can serve as a versatile platform for the development of anti-fouling and self-cleaning hybrid membranes or functional materials for a wide range of applications.

Post-treatment of electrospun nanofibers is a versatile and scalable approach for the fabrication of membranes with controlled pore size, porosity, and morphology. In this study, we demonstrate a novel solution-based approach for the fabrication of membrane distillation (MD) membranes with adjustable pore size and performance through non-solvent induced phase separation of a polymeric solution over an electrospun fiber mat. Poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) was dissolved in a blend of acetone and dimethylacetamide and used to produce a highly porous electrospun fiber mat with an average pore diameter of ~1.2 µm. Surface coating of the PVDF-HFP nanofibers with polyvinylidene fluoride (PVDF) through phase separation enabled control of the membrane pore size by filling the empty domains between the fibers. The coated fiber mats were characterized for their surface hydrophobicity, porosity, and structure. The PVDF polymeric coating layer integrated within the electrospun mat decreased the average pore diameter to 99.9%) and a water flux of 30 L m−2 h−1 in direct contact MD experiments with 40 °C temperature difference between the feed and permeate solutions. This coating procedure is compatible with current roll-to-roll membrane fabrication processes, making it a viable approach for large-scale fabrication of electrospun membranes with exceptional performance for MD applications.

Precipitation of calcium sulfate dihydrate (i.e., gypsum) on the membrane active layer negatively impacts the efficiency of reverse osmosis (RO) systems by increasing overall operation and maintenance costs. The interfacial free energy between RO membranes and scalants is expected to play a paramount role in crystal nucleation and adsorption. In this work, we modified the surface of a thin-film composite RO membrane with a zwitterionic polymer brush via atom transfer radical polymerization (ATRP) to impart superhydrophilicity for enhanced resistance to gypsum scaling. The zwitterionic polymer coating was optimized and a highly hydrophilic membrane surface displaying a water contact angle of

The performance of two ionic organic draw solutes, namely sodium acetate (NaOAc) and ethylene-diamine-tetra acetic acid disodium salt (EDTA-2Na), during osmotic membrane bioreactor (OMBR) operation was investigated in this study. Their performance was compared to that of sodium chloride (NaCl). A reverse osmosis (RO) process was integrated with OMBR to form an OMBR-RO hybrid system for draw solute recovery and clean water production. Results show that the NaOAc and EDTA-2Na draw solutes significantly reduced salinity build-up in the bioreactor in comparison with NaCl during OMBR operation. At the same osmotic pressure, these two ionic organic draw solutions produced slightly lower water flux, but considerably less reverse salt flux than NaCl. Compared to NaCl and NaOAc, EDTA-2Na resulted in significantly less fouling to the forward osmosis membrane. Regardless of the draw solutes, the OMBR-RO hybrid system could remove all 31 trace organic contaminants investigated in this study by more than 97%. Results reported here suggest that ionic organic draw solutes can be used to mitigate salinity build-up in the bioreactor during OMBR operation.

The applicability of state-of-the-art water purification membranes in harsh feed streams is limited due to their insufficient chemical robustness. Epoxide chemistry has been recently introduced to achieve pH- and chlorine-stable nanofiltration (NF). This study further investigated the influence of interfacial polymerization (IP) synthesis parameters on the resulting epoxide-based thin-film composite (TFC) membrane structure and performance. Epoxide polymerization could be initiated by N,N,N′,N′-tetramethyl-1,6-hexanediamine and NaOH. Surprisingly, neither the type of initiator, nor the initiator concentration used during IP, influenced membrane rejection of rose bengal (RB) dye (1017 g mol-1), which was constant at ~ 90%. This consistent RB rejection was primarily determined by annealing of the cross-linked polyimide support by toluene, which is the solvent used during IP. In contrast, the poly(epoxyether) top-layer determined membrane selectivity for methyl orange, a smaller dye of 327 g mol-1. The effect of solvent annealing of the membrane support by diethyl carbonate, dimethyl sulfoxide and m-xylene was also investigated and revealed that the changes induced by solvent contact are physical rather than chemical in nature. This study shows, for the first time, the substantial direct impact of the solvents used during IP to influence support properties and the resulting membrane performance. Solvent annealing can therefore be considered as a tool in membrane fabrication—during IP or as a post-treatment step—to further tune the separation performance for specific applications.

Membrane deformation under an applied hydraulic pressure, often termed compaction, is observed in almost all pressure-driven membrane processes. Most notably, compaction decreases water permeability in conventional reverse osmosis (RO) and is expected to critically hinder high-pressure reverse osmosis (HPRO) for hypersaline brine desalination. In this work, we demonstrated that compaction decreases the water permeability of commercial RO membranes from 2.0 L m−2 h−1 bar−1 at 70 bar applied hydraulic pressure to 1.3 L m−2 h−1 bar−1 at 150 bar. The morphological effects of compaction were primarily associated with changes in the support layer, where a ~60% decrease in cross-sectional thickness is observed following compaction at 150 bar hydraulic pressure. In contrast, positron annihilation lifetime spectroscopy demonstrates that the selective layer does not compact irreversibly. The mechanism that drives compaction was found to be the difference in hydraulic pressure across the interface of the selective and support layers. We further found that compaction can reduce the support layer surface porosity by up to ~95%. This decreased porosity is identified as the cause for compaction-induced water permeability decline, while the intrinsic permeability of the selective layer is not influenced by compaction. As such, we conclude that compaction of the support layer has an inextricable impact on composite membrane performance. Finally, we propose recommendations for developing compaction-resistant membranes that can maintain high water permeability, and thus good desalination performance, in high-pressure membrane applications, such as HPRO.

Dissolved silica and organic matter are major membrane foulants in desalination of brackish water and wastewater by reverse osmosis. However, the interaction between inorganic silica species and dissolved organic molecules and their combined impact on membrane fouling are poorly understood. In this study, we have used sodium silicate and sodium alginate as model inorganic and organic foulants, respectively. Membrane fouling experiments showed a cooperative effect between the dissolved silica and alginate macromolecules, which aggravated membrane fouling. The molecular interactions between sodium alginate and sodium silicate and the properties of the formed complexes were determined by X-ray diffraction and X-ray photoelectron spectroscopy. Results show that the crystallinity of the formed complexes decreased as we increased the ratio of silica in the silica-alginate mixture, indicating the formation of amorphous complexes. Full reflection Fourier transform infrared spectroscopy also confirms new functional groups following the interaction between silica and alginate. Isothermal titration calorimetry was also employed to quantitatively determine the binding parameters by measuring the heat change during the reaction. When binding occurs between silica and alginate, the measured adsorbed or released heat is used to determine the binding constants as well as the enthalpy and entropy changes. Our results suggest that the interaction between silica and alginate is spontaneously exothermic and dominated by noncovalent bonding. Our findings provide fundamental insights into the complexation between silica and alginate in reverse osmosis membrane fouling and highlight the binding mechanism between silica and alginate molecules.

We investigated the relative contributions of intra-pore diffusion (via membrane thickness) and partitioning into nanofiltration (NF) membrane pores (via membrane pore size and ion hydration energy) to the apparent energy barriers for ion transport in NF membranes. Using polyelectrolyte layer-by-layer assembly, we independently altered NF membrane thickness as well as membrane pore sizes and then determined the apparent energy barriers to bromide and fluoride transport through the fabricated membranes. Membrane thickness and pore sizes were estimated using an AFM scratch technique and the hydrodynamic pore transport model, respectively. By increasing the number of polyelectrolyte bilayers from four to ten, the polyelectrolyte film thickness increased from 28 to 77 nm, while the apparent energy barriers to bromide transport through the membranes with four, seven, and ten bilayers were negligibly affected (4.4, 3.4, and 3.9 kcal mol−1, respectively, at 1.7 bar). Instead, we found that solute flux and the apparent energy barriers to ion transport were significantly affected by both membrane pore size and ion hydration energy. Overall, our results support the traditional energy barrier based models for ion transport in membranes and the recently proposed notion that ion dehydration at the solution-membrane interface is the rate-limiting step during transport through NF membranes.

A novel osmotic dilution process using commercial liquid fertilizer for greenwall irrigation was evaluated. In this process, clean water was extracted from raw sewage by forward osmosis (FO) using a well-balanced, all-purpose commercial liquid fertilizer as draw solution. The diluted liquid fertilizer can then be used for direct sustainable greenwall irrigation. Our results show that the presence of organic matter in the liquid fertilizer draw solution did not compromise FO membrane performance. No discernible changes in water flux and key membrane transport parameters (pure water permeability coefficient, A, and salt (NaCl) permeability coefficient, B) were observed when the organic matter concentration in the draw solution was increased to 2000 mg/L. Parameters influencing the osmotic dilution process were examined in terms of reverse salt flux, liquid fertilizer concentration, cross-flow rate, and feed and liquid fertilizer draw solution temperatures. The reverse flux of phosphate was much lower compared to those of ammonium and potassium as the reverse flux of these solutes were proportionally related to their hydrated radii. Cross-flow rate had no discernible impact on either water flux or reverse nutrient transport. Water and reverse nutrient fluxes increased markedly with increasing temperature, driven by higher water and solute diffusivities. More than 80% water recovery was achieved by osmotic dilution using raw sewage feed. Water production was stable and not affected by deposition of organic matter on the membrane surface. By contrast, reverse nutrient diffusion was hindered due to enhanced steric hindrance. Results reported here have significant environmental implications. Extracting clean water from raw sewage by commercial liquid fertilizers harnesses unique FO mass transfer phenomena and balances greenwall nutrient requirement, thereby sustaining the greenwall irrigation process.

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.

A simple phase inversion technique to fabricate hydrophobic microporous films for membrane distillation (MD) is presented. Polyvinylidene fluoride (PVDF) dissolved in triethyl phosphate (TEP) was used as a casting solution with a coagulation bath comprising 2-propanol and water. The effect of processing parameters (PVDF concentration and molecular weight, and composition of coagulation bath) on membrane performance was systematically investigated. Our results demonstrate that an MD membrane with desirable properties can be achieved by carefully controlling the casting parameters. The fabricated membranes exhibited asymmetric structure with a dense top layer and highly porous bottom substrate, and showed markedly different performance (i.e., water vapor flux) depending on the MD membrane orientation. Our results suggest that membrane surface porosity as well as the structure of the membrane are the key factors determining MD membrane performance.

In this study, we analyze the effects of membrane properties, namely water permeability, solute permeability, and structural parameter, on the overall performance of an FO membrane module to extract water from simulated seawater (0.6 M NaCl). By considering the thermodynamic limit of operation, we demonstrate that the maximum achievable water recovery is practically independent of membrane properties, and higher maximum water recovery is achievable with counter-current compared to co-current mode. Analysis of the module-scale model indicates that reducing the support layer structural parameter offers substantial reductions in the membrane area required to achieve a specified water recovery. For example, a 25% reduction of the structural parameter of a state-of-the-art thin-film composite (TFC) membrane (from 400 to 300 μm) yields a sizable 20% reduction in membrane area. In contrast, quintupling the water permeability coefficient (from 2.0 to 10.0 L m−2 h−1 bar−1) of a modern TFC membrane generates only a modest 10% saving in membrane area. In addition, because of the permeability-selectivity trade-off that governs current polymeric membranes, doubling the water permeability coefficient would cause crippling ~7-fold increases in forward and reverse solute permeation. This quantitative study models the potential performance of a module-scale FO desalination process and firmly highlights the need to prioritize the reduction of support layer mass transport resistances over water permeability increases in membrane development.

Facile and effective strategies are needed to modify forward osmosis (FO) membranes for improved resistance to organic fouling. Fouling resistant FO membranes will advance the commercial implementation of FO for treating feed waters with high fouling potential, such as wastewater and brines. We report a membrane modification technique for post-fabrication grafting of a poly(ethylene glycol) (PEG) block copolymer to the surface of commercial thin-film composite (TFC) FO membranes via an amide coupling reaction. The PEG concentration for membrane modification is optimized based on increased membrane hydrophilicity and reduced water permeability that result from increasing PEG concentrations during modification. Modified membranes exhibit improved resistance to organic fouling compared to unmodified control membranes when exposed to an aggressive synthetic wastewater mixture. The fouling resistance is achieved despite the non-uniform grafting of PEG, which is attributed to the limited accessibility of carboxylic group binding sites on the membrane surface. The fouling resistance of membranes modified using this post-fabrication technique compares favorably to TFC-FO membranes modified using other procedures. The modification technique we report in this work has the advantages of being relatively inexpensive, easy to implement, and applicable to commercial membranes.

We evaluated the performance of trimethylamine–carbon dioxide (TMA–CO 2 ) as a potential thermolytic draw solution for engineered osmosis. Water flux and reverse solute flux with TMA–CO 2 draw solution were measured in forward osmosis (FO) and pressure retarded osmosis (PRO) modes using thin-film composite (TFC) and cellulose triacetate (CTA) FO membranes. Water flux with the TMA–CO 2 draw solution was comparable to that obtained with the more common ammonia–carbon dioxide (NH 3 –CO 2 ) thermolytic draw solution at similar (1 M) concentration. Using a TFC–FO membrane, the water fluxes produced by 1 M TMA–CO 2 and NH 3 –CO 2 draw solutions with a DI water feed were, respectively, 33.4 and 35.6 L m −2 h −1 in PRO mode and 14.5 and 15.2 L m −2 h −1 in FO mode. Reverse draw permeation of TMA–CO 2 was relatively low compared to NH 3 –CO 2 , ranging from 0.1 to 0.2 mol m −2 h −1 in all experiments, due to the larger molecular size of TMA. Thermal separation and recovery efficiency for TMA–CO 2 was compared to NH 3 –CO 2 by modeling low-temperature vacuum distillation utilizing low-grade heat sources. We also discuss possible challenges in the use TMA–CO 2 , including potential adverse impact on human health and environments.

Forward osmosis (FO) is an emerging technology that has received much global interest due to its potential applications in wastewater reclamation and seawater desalination. One of the major challenges to overcome is the detrimental effects of concentration polarization (CP), which reduce the effective osmotic pressure driving force and thus decrease productivity of the FO process. In this study, pressure assisted osmosis (PAO) was investigated as a method to increase the effective driving force and water flux by combining an osmotic pressure driving force with an additional hydraulic pressure. Experiments were carried out to examine the efficiency of the PAO process using a bench-scale setup specially designed to prevent membrane deformation under the applied hydraulic pressure. Results showed that PAO water flux increased with increasing the applied hydraulic pressure in FO mode (i.e., active layer facing the feed solution). The measured water fluxes were in good agreement with predictions based on a model developed to describe the water flux in PAO operation. However, the PAO water flux was lower than model predictions in PRO mode (i.e., active layer facing the draw solution). This observation is attributed to the spacer ‘shadow effect’ and the resulting reduction in the effective membrane area by the spacers. The results also showed that reverse solute flux decreased with increasing the applied hydraulic pressure in both FO and PRO modes. Although applying hydraulic pressure to FO increases energy consumption, the higher water flux in PAO reduces the number of membrane modules for the FO process. In addition, control of the driving force is easier in PAO than FO, leading to flexibility in system design and operation. Based on these results, a possible combination of FO and RO system with PAO was proposed for allowing higher energy efficiency in seawater desalination.

This study systematically investigates the organic fouling behavior of a superhydrophilic polyvinylidene fluoride (PVDF) ultrafiltration membrane functionalized via post-fabrication tethering of surface-tailored silica nanoparticles to poly(methacrylic acid)-grafted PVDF membrane surface. Sodium alginate (SA), Suwannee River natural organic matter (SRNOM), and bovine serum albumin (BSA) were used as model organic foulants to investigate the antifouling behavior of the superhydrophilic membrane with combined-fouling (mixture of foulants) and individual-fouling (single foulant) tests. A membrane bioreactor (MBR) plant supernatant was also used to verify the organic antifouling property of the superhydrophilic membrane under realistic conditions. Foulant size distributions and foulant–membrane interfacial forces were measured to interpret the observed membrane fouling behavior. Molecular weight cutoff measurements confirmed that membrane functionalization did not adversely affect the intrinsic membrane selectivity. Both filtration tests with the synthetic foulant-mixture solution (containing SA, SRNOM, and BSA) and MBR plant supernatant demonstrated the reliability and durability of the antifouling property of the superhydrophilic membrane. The conspicuous reduction in foulant–membrane interfacial forces for the functionalized membrane further verified the antifouling properties of the superhydrophilic membrane, suggesting great potential for applications in wastewater treatment.

In this study, we systematically investigated the propensity and reversibility of combined organic–colloidal fouling in forward osmosis (FO) under various solution chemistries (pH and calcium ion concentrations) and applied hydraulic pressure on the feed side. Alginate, silica colloids, and their mixture (i.e., combined organic–colloidal) were used as model foulants. Our findings demonstrate that combined organic–colloidal foulants caused more rapid flux decline than the individual foulants due to the synergistic effect of alginate and silica colloids. As a result, much lower flux recovery was achieved by physical cleaning induced by increasing the cross-flow rate, in contrast to single foulants of which the fouling layer was easily removed under all solution conditions. Interestingly, less flux decline was observed at neutral pH for combined fouling, while acidic conditions were favorable for alginate fouling and basic solutions caused more silica fouling, thereby providing clear evidence for the combined fouling effect. It was also found that calcium ions enhanced water flux decline and induced the formation of less reversible combined organic–colloidal fouling layers. Lastly, the role of applied hydraulic pressure on the feed side in FO was examined to elucidate the mechanism of fouling layer formation, fouling reversibility, and water flux recovery. Higher fouling propensity and lower fouling reversibility of combined organic–colloidal fouling were observed in the presence of applied hydraulic pressure on the feed side. This observation suggests that the lower fouling propensity and greater fouling reversibility in FO compared to reverse osmosis (RO), are attributable to unpressurized operating conditions in FO.

Direct contact membrane distillation (DCMD) can desalinate saline waters using low-grade heat and is thus economically attractive when low-temperature thermal energy is readily available. Coupling DCMD with a heat exchanger (HX) can significantly enhance the energy efficiency of the process by recovering the latent heat accumulated in the permeate (distillate) stream. This study evaluates the mass recovery rate (i.e., fraction of feed water recovered), γ, and the specific heat duty (i.e., energy input per unit mass of product water), β, of DCMD desalination using low-grade heat coupled with HX. Mass and heat transfer in DCMD and HX were modeled at the module scale and thermodynamic analysis of the system was performed. The relative flow rate (between the permeate and feed streams), α, was found to be a critical operation parameter to optimize process performance, regardless of the mass and heat transfer kinetics. Both numerical evaluation and analytical analysis reveal a critical relative flow rate, α⁎, that demarcates DCMD operation between a permeate limiting regime (when α α⁎), when mass transfer kinetics are not limiting. Similarly, we identified mass-limited and temperature-limited heat recovery regimes in the HX that are dependent on α. Our analysis shows that the highest γ and lowest β achievable are solely determined by the thermodynamic properties of the system and always occur at the critical relative flow rate, α⁎. For example, the thermodynamic limits for γ and β are 6.4% and 27.6 kJ kg−1, respectively, for seawater desalination by single-pass DCMD at 60 °C with HX. However, in practical operation, as the DCMD membrane area and permeability cannot be infinitely large, the process is in a mass-transfer-limiting-regime and performance departs from the thermodynamic limits. Lastly, we demonstrate that heat transfer across a thermally-conductive DCMD membrane further reduces the recovery rate and energy efficiency of the process. The findings from this study have important implications for optimization of the DCMD process and for serving as criteria to assess process performance.

Forward osmosis (FO) has gained significant research interest due to the wide range of potential applications in desalination and wastewater reuse. However, the FO process being concentration (osmosis) driven has its own intrinsic limitations. Net transfer of water across the membrane occurs until the point of osmotic equilibrium between the draw solution (DS) and the feed solution (FS). Without external intervention, it is impossible to dilute the DS beyond the point of osmotic equilibrium. In this study, the concept of osmotic equilibrium in the FO process is introduced by simulating conditions in a plate-and-frame FO membrane module using established mass transport models. The simulations evaluated the influence of various operating parameters on process performance, assessed in terms of water flux, feed recovery rate and the final concentration of the diluted DS. The counter-current crossflow mode of operation has been observed to be advantageous because it can achieve higher module average water flux, higher feed water recovery rates and higher DS final dilution. Based on the osmotic equilibrium concept and mass balance analysis, a modified equation for the water extraction capacity of a draw solute has been proposed. This study underscores the need for process optimisation for large-scale FO operations.

Biofouling remains a major challenge for membrane processes. Several methods may curtail biofouling, including membrane surface modification with hydrophilic and antimicrobial polymers and nanomaterials. Although many of these modifications rely on compounds that leach from the membrane surface, the release of these compounds is not always characterized, understood, or controlled. Here, we adapt a technology used for drug delivery — controlled release — and apply it to the membrane biofouling problem. Capsules for controlled release can be designed using a number of polymers, and the loading and release rate of the capsules depends on a number of tunable variables. We have encapsulated two antimicrobial compounds, cinnamaldehyde and kanamycin, in biodegradable poly(lactic-co-glycolic acid), PLGA, particles. These capsules were then bound to the surface of thin-film composite polyamide membranes, targeting biofouling where it is most problematic. Cinnamaldehyde released from the capsules for ~2 days. Kanamycin, encapsulated in larger PLGA particles, showed continuous release over the 80 h period tested. Biofilm formation on modified membranes was assessed using model bacteria (Escherichia coli), and significant reductions in biofilm were observed on membranes modified with kanamycin capsules, indicating that sufficient kanamycin was released to curtail bacterial growth and biofilm development. This proof of concept study demonstrates that the controlled-release platform can be used to encapsulate a variety of compounds to slow membrane biofouling.

Despite significant advances in forward osmosis (FO) membrane development, organic fouling, due to adsorption of organic molecules prevalent in natural waters and wastewater effluents, remains a major technical problem, decreasing process performance and membrane life. We report the fabrication, characterization, and testing of thin film composite (TFC) FO membranes functionalized with poly(ethylene glycol) (PEG) for improved fouling resistance. The membranes comprise a microporous polysulfone support over which a polyamide selective layer is synthesized by interfacial polymerization. To facilitate PEG grafting, a second interfacial reaction is carried out between ethylenediamine and acyl chloride groups on the nascent polyamide layer, yielding a selective layer rich in primary amine groups. The resulting amine-rich active layer is functionalized with PEG diglycidyl ether, whose epoxide groups readily react with primary amines. Surface characterization by ATR-FTIR spectroscopy and zeta-potential measurements shows the presence of PEG and amine groups on the membrane surface, and contact angle measurements demonstrate that the PEG-functionalized active layer is more hydrophilic than that of control polyamide membrane surfaces (whose active layer does not contain a grafted PEG layer). We report the results of dynamic fouling experiments using alginate as model organic foulant, showing that membrane fouling is significantly reduced in the case of PEGylated membranes, due to the presence of a PEG barrier that hinders foulant adsorption. Finally, we use AFM adhesion force measurements to demonstrate that the interaction energy between a carboxylated colloidal probe and the PEGylated membrane is significantly weakened as compared to the control polyamide surface. The approach of amine enrichment of the membrane active layer that is reported in this work holds promise as a versatile platform on which to study different anti-fouling modifications.

A hybrid system that combines forward osmosis with a reverse osmosis seawater desalination process could reduce both energy requirements and environmental impacts by osmotic dilution of the seawater and concentrated brine with an impaired low salinity stream, such as treated wastewater effluent. In this study, we investigate the membrane fouling behavior in forward osmosis under conditions simulating the osmotic dilution process and the use of hydrodynamic methods without the use of cleaning chemicals, to control membrane fouling. Fouling runs with seawater or SWRO brine draw solution and deionized (DI) water feed solution showed insignificant water flux decline, which implies negligible effect of particulate and organic matter in the seawater/brine on fouling of the FO membrane support layer. Fouling of the membrane active layer was evaluated by using an enriched synthetic wastewater effluent containing a mixture of inorganic and organic foulants, focusing on the impact of permeate drag force on fouling layer formation. Our results demonstrate that higher permeate water flux causes an increase in concentration build-up of foulants at the membrane surface, thereby forming a dense inorganic/organic combined fouling layer during FO fouling runs. We also examined three hydrodynamic methods for minimizing FO membrane fouling in the osmotic dilution process: (1) applying shear force on the membrane surface by increasing the cross-flow velocity, (2) using a feed-channel spacer to induce turbulence, and (3) employing pulsed flow to remove foulants from the membrane surface. Our results show that these hydrodynamic methods substantially reduce fouling and flux decline rate.

The effects of feed and draw solution temperature and transmembrane temperature difference on the rejection of 12 trace organic contaminants (TrOCs) by two forward osmosis (FO) membranes were investigated. The membrane structure parameter (S) and the reverse salt (NaCl) flux selectivity (RSFS) were constant over the temperature range of 20–40 °C, suggesting that within this range, the solution temperature did not significantly influence the membrane polymeric structure. Draw solution properties, including diffusivity, viscosity, and osmotic pressure, played an important role in water and reverse salt (NaCl) flux behaviour and TrOC rejection. Pure water and salt (NaCl) permeability coefficients of the two forward osmosis membranes increased as both the feed and draw solution temperatures increased from 20 to 40 °C due to an increase in solute diffusivity and a decrease in water viscosity. Rejection of charged TrOCs was higher than that of neutral TrOCs and their rejection was insensitive to temperature variation. On the other hand, rejection of neutral TrOCs decreased significantly when the feed and draw solution temperatures were 40 and 20 °C, respectively, due to the increase in their diffusivity at an elevated temperature. By contrast, rejection of neutral TrOCs increased when the feed and draw solution temperatures were 20 and 40 °C, respectively. The reverse salt (NaCl) flux increased due to an increase in the draw solute diffusivity. In addition, at a higher draw solution temperature, the dilution effect induced by higher water flux and the hindrance effect enhanced by a higher reverse salt (NaCl) flux led to the increase in the rejection of neutral TrOCs.

The physical and chemical factors affecting boron solute flux behavior and membrane transport mechanisms in forward osmosis (FO) have been systematically investigated. Boron solute flux behavior in FO was further compared with that in reverse osmosis (RO) by employing identical plate-and-frame cells and membranes under the same filtration conditions. The influence of draw solution pH, draw solution type, and membrane orientation on boron solute flux was examined for FO, and the effects of water flux, cross-flow velocity, feed water boron concentration, and solution pH on boron solute flux were examined for both FO and RO. Results show that reverse salt diffusion, a unique feature of FO, is a key mechanism governing boron solute flux in FO. Boron solute flux through the FO membrane was inversely proportional to the degree of reverse salt diffusion by draw solution. The higher boron rejection observed in FO compared to RO is also attributed to reverse salt diffusion in FO. It is also shown that membrane orientation in FO plays an important role, affecting boron solute flux due to different degrees of internal concentration polarization. In both FO and RO, boron solute flux increased with increasing water flux. However, the influence of water flux on boron solute flux was less significant in FO than RO. Furthermore, boron solute flux decreased with increasing feed water pH due to the conversion of the neutral boric acid to borate anions. The findings provide new insight into the mechanisms and factors controlling boron solute transport in FO. © 2012 Elsevier B.V.

Seawater desalination for agricultural irrigation will be an important contributor to satisfying growing water demands in water scarce regions. Irrigated agriculture for food production drives global water demands, which are expected to increase while available supplies are further diminished. Implementation of reverse osmosis, the current leading technology for seawater desalination, has been limited in part because of high costs and energy consumption. Because of stringent boron and chloride standards for agricultural irrigation water, desalination for agriculture is more energy intensive than desalination for potable use, and additional post-treatment, such as a second pass reverse osmosis process, is required. In this perspective, we introduce the concept of an integrated forward osmosis and reverse osmosis process for seawater desalination. Process modeling results indicate that the integrated process can achieve boron and chloride water quality requirements for agricultural irrigation while consuming less energy than a conventional two-pass reverse osmosis process. The challenges to further development of an integrated forward and reverse osmosis desalination process and its potential benefits beyond energy savings are discussed.

Understanding the simultaneous forward water flux and reverse draw solute flux in osmotically driven membrane processes is essential to the development of these emerging technologies. In this work, we investigate the reverse fluxes of three neutral draw solutes-urea, ethylene glycol, and glucose-across an asymmetric forward osmosis membrane. Our experiments reveal that for the rapidly permeating draw solutes (urea and ethylene glycol), an additional resistance to mass transfer develops due to external concentration polarization on the feed side of the membrane. A model to predict the reverse flux for these highly permeable solutes is derived and then validated using independently determined transport parameters. The experimentally measured water fluxes generated by these solutes are consistently lower than those predicted by theories for calculating the water flux in osmotically driven membrane processes-even when the effects of external concentration polarization are taken into account. These results are indicative of a coupling between the forward water flux and reverse solute flux, and a reflection coefficient is introduced to account for the solute–solvent coupling. Interestingly, our experiments demonstrate that solute–solvent coupling does not significantly impact the reverse flux of draw solute. The effect of this solute–solvent coupling on the reverse flux selectivity (i.e., the ratio of the forward water flux to the reverse solute flux) is demonstrated.

a b s t r a c t Colloidal fouling behavior in forward osmosis (FO) was investigated, focusing on the role of reverse salt diffusion. Two suspensions of silica nanoparticles, with average particle diameters of 24 and 139 nm, were used as model colloidal foulants. To verify the effect of reverse salt diffusion on the colloidal fouling behavior, NaCl and LaCl3 were employed as draw solutions because they exhibit different reverse diffusion rates. Our results suggest that in colloidal fouling of FO, salts diffuse from the draw to the feed solution and accumulate within the colloidal fouling layer that forms on the membrane surface. The accumulated salts result in a marked acceleration of cake-enhanced osmotic pressure (CEOP), which reduces the net osmotic driving force for permeate water flux. Fouling was not observed with the small, 24-nm particles because of the lack of substantial cake formation, but was notable for the 139-nm particles and for a feed containing a mixture of the 24 and 139 nm particles. Our findings further indicate that colloidal fouling is enhanced under solution conditions (ionic strength and pH) within the colloidal cake layer that promote aggregation or destabilization of the silica particles. Colloidal fouling reversibility was also examined by varying the cross-flow velocity during the FO fouling runs. We showed that in the absence of colloidal particle destabilization/aggregation, the permeate flux during colloidal fouling in FO recovered almost completely when the cross-flow velocity was increased from 8.5 to 25.6 cm/s. Our results suggest that reverse salt diffusion in FO is a key mechanism that controls colloidal fouling behavior as well as fouling reversibility. Therefore, minimization of reverse salt diffusion through the selection of proper draw solutes and optimization of FO membrane selectivity are important for minimizing colloidal fouling as well as enhancing FO operation efficiency.

The effect of different modes of cleaning of RO membranes fouled by wastewater treatment plant effluent has been investigated. Characterization of the wastewater effluent revealed the presence of foulants containing carboxylic and phenolic functional groups as well as calcium ions. The chemical cleaning agents, sodium hydroxide (NaOH), ethylenediaminetetraacetic acid (EDTA), sodium dodecyl sulfate (SDS), and sodium chloride (NaCl), were selected as models for alkaline solutions, metal chelating agents, surfactants, and salt cleaning, respectively. Specifically, we examined the impact of a sequence or a combination of two cleaning agents compared to the use of single cleaning agents. Increased cleaning efficiency was demonstrated when two cleaning agents were applied in a certain order and mixture. In particular, it was shown that addition of NaOH can enhance the overall cleaning performance when introduced with other chemical agents, due to its ability to loosen the fouling layer. Cleaning efficiency as high as 94% was obtained by simply increasing the pH of an NaCl cleaning solution, compared to 65% in the case of the individual salt solution with no pH adjustment. On the other hand, combining chemical cleaning agents was not advantageous in some cases, possibly because of the competing cleaning mechanisms of some of the agents. The most and the least effective cleaning modes were highlighted, suggesting a rationale for the design of chemical cleaning of RO membranes fouled by wastewater effluent. This study demonstrates that careful selection of cleaning agents and the steps through which those agents are applied allows the regeneration of high water productivity after fouling while minimizing both the cleaning time and the amount of chemicals.

The surface characteristics of reverse osmosis membranes and their relation to membrane fouling are systematically investigated by measuring membrane dynamic hysteresis based on the Wilhelmy plate method. Dynamic hysteresis represents the difference between the forces applied to a membrane surface when it is advanced into and withdrawn from a liquid or solution. Our results demonstrate that the chemical surface heterogeneity of various RO membranes could be quantified by measuring their dynamic hysteresis. The chemical heterogeneity was mostly related to the distribution of surface charge rather than average zeta potential. There was a remarkable correlation between the chemical surface heterogeneity and membrane dynamic hysteresis. It was clearly shown that dynamic hysteresis varied substantially with respect to the solution chemistry of test solutions. The dynamic hysteresis of RO membranes measured in the presence of organic foulants was further related to the flux-decline rate determined from bench-scale fouling experiments. It was found that higher flux-decline rate was obtained for RO membranes with larger dynamic hysteresis. Based on the results in this study, it is demonstrated that dynamic hysteresis measurements can be a promising tool for characterizing membrane surfaces as well as assessing membrane fouling.

Fouling behaviors during forward osmosis (FO) and reverse osmosis (RO) are compared. Alginate, humic acid, and bovine serum albumin (BSA) are used as model organic foulants, and two suspensions of silica colloids of different sizes are chosen as model particulate foulants. To allow meaningful comparison of fouling behavior, identical hydrodynamic operating conditions (i.e., initial permeate flux and cross-flow velocity) and feed water chemistries (i.e., pH, ionic strength, and calcium concentration) are employed during FO and RO fouling runs. The observed flux-decline behavior in FO changed dramatically with the type of organic foulant, size of colloidal foulant, and the type of the draw solution employed to generate the osmotic driving force. Based on these experimental data and the systematic comparisons of fouling behaviors of FO and RO, we provide new insights into the mechanisms governing FO fouling. In FO, reverse diffusion of salt from the draw solution to the feed side exacerbates the cake-enhanced osmotic pressure within the fouling layer. The elevated osmotic pressure near the membrane surface on the feed side leads to a substantial drop in the net osmotic driving force and, thus, significant decline of permeate flux. Our results further suggest that the structure (i.e., thickness and compactness) of the fouling layers of FO and RO is quite different. By varying the cross-flow velocity during the organic fouling runs, we were able to examine the fouling reversibility in FO and RO. The permeate flux during organic fouling in FO recovered almost completely with increasing cross-flow velocity, while no noticeable change was observed for the RO system. Our results suggest that organic fouling in FO could be controlled effectively by optimizing the hydrodynamics in the feed stream without employing chemical cleaning.

The recently resurgent forward osmosis (FO) membrane process has the potential to become a sustainable alternative to conventional membrane processes. However, the fouling and cleaning behavior of FO membranes remains largely unknown. There is a need to fully understand the fouling phenomena in FO in order to take advantage of this emerging technology. In this study, we used alginate as a model organic foulant to examine FO membrane fouling and cleaning behavior with the ultimate goal of determining the underlying FO fouling/cleaning mechanisms. Results showed that alginate fouling in FO is almost fully reversible, with more than 98% recovery of permeate water flux possible after a simple water rinse without any chemical cleaning reagents. We also studied the role of applied hydraulic pressure in membrane fouling and cleaning by performing fouling tests in FO (without hydraulic pressure) and RO (with hydraulic pressure) modes. Flux recovery in the FO mode was much higher than that in the RO mode under similar cleaning conditions, although the rate of membrane flux decline was similar in the two modes. The fouling reversibility of FO was attributed to the less compact organic fouling layer formed in FO mode due to the lack of hydraulic pressure. Our results suggest that operating in FO mode may offer an unprecedented advantage in reducing or even eliminating the need for chemical cleaning. AFM force measurements were used to elucidate the impact of membrane materials (cellulose acetate versus polyamide) on membrane fouling and cleaning behavior. Adhesion force data revealed that a small percentage of relatively adhesive sites on the membrane surface play an important role in increasing membrane fouling potential and decreasing cleaning efficiency. This finding implies that using average adhesion force to predict membrane fouling potential is inadequate. Extensive long-range adhesion forces are observed for the polyamide membrane in the presence of alginate and calcium ions. The long-range interactions are attributed to calcium bridging of alginate molecules between the AFM probe and the adhesive sites on the polyamide membrane surface.

Concentrating sugar solutions is a common process used in the production of many food products for either dewatering a high value product or concentrating waste streams prior to disposal. Thermal and pressure-driven dewatering methods are widely used, but they are prohibitively energy intensive and hence, expensive. Osmotically driven membrane processes, like forward osmosis, may be a viable and sustainable alternative to these current technologies. Using NaCl as a surrogate draw solution, this investigation shows that forward osmosis processes can lead to sucrose concentration factors that far exceed current pressure-driven membrane technologies, such as reverse osmosis. For instance, a concentration factor of 5.7 was achieved by forward osmosis with a starting sucrose concentration of 0.29 M, compared to reported concentration factors of up to 2.5 with reverse osmosis. Water fluxes were found to be lower than those commonly obtained in reverse osmosis, which is a consequence of the significantly higher concentration factors in conjunction with internal concentration polarization. The latter is a common problem in forward osmosis processes that utilize current generation anisotropic polymeric membranes. Further advances in forward osmosis membrane technology would yield higher water fluxes and concentration factors.

The growing attention to forward osmosis (FO) membrane processes from various disciplines raises the demand for systematic research on FO membrane fouling. This study investigates the role of various physical and chemical interactions, such as intermolecular adhesion forces, calcium binding, initial permeate flux, and membrane orientation, in organic fouling of forward osmosis membranes. Alginate, bovine serum albumin (BSA), and Aldrich humic acid (AHA) were chosen as model organic foulants. Atomic force microscopy (AFM) was used to quantify the intermolecular adhesion forces between the foulant and the clean or fouled membrane in order to better understand the fouling mechanisms. A strong correlation between organic fouling and intermolecular adhesion was observed, indicating that foulant–foulant interaction plays an important role in determining the rate and extent of organic fouling. The fouling data showed that FO fouling is governed by the coupled influence of chemical and hydrodynamic interactions. Calcium binding, permeation drag, and hydrodynamic shear force are the major factors governing the development of a fouling layer on the membrane surface. However, the dominating factors controlling membrane fouling vary from foulant to foulant. With stronger intermolecular adhesion forces, hydrodynamic conditions for favorable foulant deposition leading to cake formation are more readily attained. Before a compact cake layer is formed, the fouling rate is affected by both the intermolecular adhesion forces and hydrodynamic conditions. However, once the cake layer forms, all three foulants have very similar flux decline rates, and further changes in hydrodynamic conditions do not influence fouling behavior.