Cellulose Triacetate Forward Osmosis Membranes Research Articles

Higher boron rejection with a new TFC forward osmosis membrane

Due to the stringent limits for boron in drinking and irrigation water, water treatment facilities have to incur additional treatment to remove boron down to a safe concentration. Forward osmosis (FO) is a membrane technology that may reduce the energy required to remove boron present in seawater. In direct FO desalination hybrid systems, fresh water is recovered from seawater using a recoverable draw solution, FO membranes are expected to show high boron rejection. This study focuses on determining the boron rejection capabilities of a new generation thin-film composite (TFC) FO membrane compared to a first generation cellulose triacetate (CTA) FO membrane. The effects of water permeate flux, membrane structure, draw solute charge, and reverse solute flux on boron rejection were determined. For TFC and CTA FO membranes, experiments showed that when similar operating conditions are applied (e.g. membrane type and draw solute type) boron rejection decreases with increase in permeate flux. Reverse draw solute flux and membrane fouling have no significant impact on boron rejection. Compared to the first generation CTA FO membrane operated at the same conditions, the TFC FO membrane showed a 40% higher boron rejection capability and a 20% higher water flux. This demonstrates the potential for boron removal for new generation TFC FO membranes.

Impacts of different draw solutions on a novel anaerobic forward osmosis membrane bioreactor (AnFOMBR).

Two anaerobic forward osmosis (FO) membrane bioreactors (AnFOMBRs), Rchloride and Rsulfate, were operated for 100 days using NaCl and Na2SO4 as the draw solution, respectively. The operating conditions were identical for both systems, with a solids retention time of 30 d, hydraulic retention time of 8 h and using cellulose triacetate FO membrane. High rejection performance of FO membranes resulted in salinity accumulation in the bioreactors. Rchloride and Rsulfate reached a stable conductivity of about 35 and 11 mS/cm, respectively, at the end of the experimental run. Hypersalinity of Rchloride undesirably impacted biological growth; mixed liquor volatile suspended solids in Rchloride was much lower at 376 mg/L, whereas that of Rsulfate was 1,170 mg/L. Organic removals were excellent due to reduced organic loadings at low fluxes and thus, Rsulfate and Rchloride achieved secondary total organic carbon (TOC) removal efficiencies of at least 75%. Both AnFOMBRs started with an initial flux of 5 LMH. Flux for Rchloride stabilized at 0.25 LMH, while Rsulfate at 0.96 LMH. The high salinities of both reactors negatively impacted methanogenic growth. Application of the fluorescence in-situ hybridization (FISH) technique confirmed the ousting of methanogens by sulfate reducing bacteria from the anaerobic consortium. Sparsely located methanogens were detected in Rchloride but none were detected in Rsulfate.

Assessing the major factors affecting the performances of forward osmosis and its implications on the desalination process

This study evaluates the influence of some of the major factors affecting the performances of forward osmosis (FO) desalination and assessed their potential implications on the overall process. The major factors assessed include membrane properties, draw solution (DS) properties, feed solution (FS) properties and the operating conditions. The influence of the membrane properties was evaluated using three types of membranes and in doing so we have also introduced one newly synthesized proprietary thin film composite FO (TFC-FO) membrane. The performances of TFC-FO membrane in terms of water flux and reverse solute flux were significantly higher than the commercial cellulose triacetate FO membrane and TFC reverse osmosis membrane in FO process. Although adequate osmotic pressure of DS is desirable for FO process, the influence of DS osmotic pressure was less significant at higher DS osmotic pressure and therefore selecting an optimum initial osmotic pressure is essential for FO process to minimize pumping energy. A critical DS concentration has been hypothesized to minimize the implications of DS concentrations on the capital and operational cost of the FO desalination plant. Total dissolved solids (TDS) of the FS play a significant role in the performance of FO process however the influence of feed TDS was less significant for feed higher than 20,000mg/L indicating that FO has a promising potential for use with high TDS feed water. Although, water flux decreased, the reverse solute flux (RSF) and specific RSF also decreased slightly at higher feed TDS. For operating parameters, the influence of crossflow velocity and the crossflow direction was also investigated.

Cellulose triacetate forward osmosis membranes: preparation and characterization

Cellulose triacetate (CTA) forward osmosis membranes were prepared by immersion precipitation. Following a standard recipe for the preparation of reverse osmosis membranes, the concentrations of polymer, additives (lactic acid, methanol), and solvent composition in the casting solution were investigated with respect to membrane permeation and salt rejection. The membrane preparation parameters including the evaporation time and the coagulation bath temperature were investigated as well. A CTA membrane with a quasi double-skinned morphology was obtained. The CTA FO membrane showed a water flux higher than 10 kg/m2 h using 2M glucose water solution as the draw solution and 0. 5M NaCl as feed. A NaCl rejection of 95% was obtained.

Novel hydrophilic nylon 6,6 microfiltration membrane supported thin film composite membranes for engineered osmosis

Previous investigations of engineered osmosis (EO) concluded that hydrophobic support layers of thin film composite membrane causes severe internal concentration polarization due to incomplete wetting. Incomplete wetting reduces the effective porosity of the support, inhibiting mass transport and thus water flux. In this study, novel thin film composite membranes were developed which consist of a poly(piperazinamide) or polyamide selective layer formed by interfacial polymerization on top of a nylon 6,6 microfiltration membrane support. This intrinsically hydrophilic support was used to increase the “wetted porosity” and to mitigate internal concentration polarization. Reverse osmosis tests showed that the permselectivity of our best poly(piperazinamide) and polyamide thin film composite membranes approached those of a commercial nanofiltration and a commercial reverse osmosis membrane, respectively. The osmotic flux performance of the new polyamide thin film composite membrane showed matched water flux, 10 fold lower salt flux and 8–28 fold lower specific salt flux than the standard commercial cellulose triacetate forward osmosis membrane from Hydration Technology Innovations™. The relatively good performance in osmotic flux tests of our thin film composite membranes was directly related to the high permselectivity of the selective layers coupled with the hydrophilicity of the nylon 6,6 support. These results suggest that these nylon 6,6 supported thin film composite membranes may enable applications like forward osmosis or pressure retarded osmosis.

Organic fouling of thin-film composite polyamide and cellulose triacetate forward osmosis membranes by oppositely charged macromolecules

Fouling of cellulose triacetate (CTA) and thin-film composite (TFC) forward osmosis (FO) membranes by organic macromolecules were studied using oppositely charged lysozyme (LYS) and alginate (ALG) as model foulants. Flux performance and foulant deposition on membranes were systematically investigated for a submerged membrane system. When an initial flux of 25 L/m2h was applied, both flux reduction and foulant mass deposition were severe for feed water containing the mixture of LYS and ALG (e.g., 50% LYS and 50% ALG at a total foulant concentration of 100 mg/L). In comparison, fouling was much milder for feed water containing either LYS or ALG alone. Compared to the CTA FO membrane, the TFC FO membrane showed greater fouling propensity under mild FO fouling conditions due to its much rougher surface. Nevertheless, under severe FO fouling conditions, fouling was dominated by foulant-deposited-foulant interaction and membrane surface properties played a less important role. Furthermore, when the feed water contained both LYS and ALG in sufficient amount, the deposited cake layer foulant composition (i.e., the LYS/ALG mass ratio) was not strongly affected by membrane types (CTA versus TFC) nor testing modes (pressure-driven NF mode versus osmosis-driven FO mode). In contrast, solution chemistry such as pH and calcium concentration had remarkable effect on the cake layer composition due to their effects on foulant–foulant interaction.

Reverse Permeation of Weak Electrolyte Draw Solutes in Forward Osmosis

The successful development of forward osmosis relies on the identification of draw solutes that can be easily regenerated, produce high water fluxes, and minimize leakage into the feed solution. One promising draw solute currently under investigation, ammonia–carbon dioxide, is a weak electrolyte. Therefore, in this work, we report a fundamental study on the reverse permeation of a model weak electrolyte draw solute, propanoic acid/propanoate ion, through a commercial cellulose triacetate forward osmosis membrane. Reverse solute flux and permeate water flux were monitored as the draw solution pH was varied from pH 4–7. Draw solutions with pH < 4.87, the pKa of propanoic acid, exhibited significantly higher reverse draw solute fluxes. However, pH had little effect on the water flux generated by the draw solutions. A mathematical model for the solute and water fluxes, which accounts for the pH-dependent equilibrium of the propanoic acid dissociation reaction, was developed and compared with experimental dat...

Forward osmosis desalination using polymer hydrogels as a draw agent: Influence of draw agent, feed solution and membrane on process performance

We have previously reported the use of hydrogel particles as the draw agent for forward osmosis desalination. In the present work, the effects of draw agent, feed concentration and membrane on the process performance were systematically examined. Our results showed that the incorporation of carbon filler particles in polymer hydrogels led to enhanced swelling ratios of the draw agents and thus higher water fluxes in the FO process. The composite polymer hydrogel particles of sizes ranging from 100 μm to 200 μm as draw agents induced greater water fluxes in FO desalination as compared with those with larger particle sizes (500–700 μm). Similar to other types of draw solutes, as the salt concentration in the feed increased, the water flux created by the polymer hydrogel draw agent decreased; the use of a cellulose triacetate forward osmosis membrane resulted in higher water flux compared with the use of a polyamide composite reverse osmosis membrane.

Using macromolecules as osmotically active compounds in osmosis followed by filtration (OF) system

Finding a suitable osmostically active solute is the most important problem in forward osmosis (FO). Even though there are a number of osmotically active compounds that exist, the major problem occurs during the separation of product water from the solute. Osmotically active macromolecules (polyethylene glycol [PEG] and humic acid [HA]) were investigated in this research as possible draw solutes for FO. Cellulose triacetate FO membranes (Hydration Technology Innovations, LLC) and several ultrafiltration and nanofiltration membranes were used in osmosis and filtration steps of the system, respectively. Molecular weights (MW) of PEG were selected as 2 k, 10 k, and 20 kDa for 400 and 600 g/L concentrations. HA solutions were prepared in concentrations ranging from 200 to 800 g/L. Increased MW resulted in higher water permeation when PEGs were used. The relationship between the reflection coefficient and the viscosity was investigated for PEG/water separation by membrane filtration. The combined effect of the osmotic pressure and the viscosity of the PEG solutions was found to be greater than the effect of the reflection coefficient on the permeability.

REMOVED: Organic Fouling of Thin-film Composite and Cellulose Triacetate Forward Osmosis Membranes by Oppositely Charged Macromolecules

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The forward osmosis membrane bioreactor: A low fouling alternative to MBR processes

A novel osmotic membrane bioreactor (OsMBR) is presented. The system utilizes a submerged forward osmosis (FO) membrane module inside a bioreactor. Through osmosis, water is transported from the mixed liquor across a semi-permeable membrane, and into a draw solution (DS) with a higher osmotic pressure. To produce potable water, the diluted DS is treated in a reverse osmosis (RO) unit; the by-product is a reconcentrated DS for reuse in the FO process. Preliminary results from experiments conducted with a flat-sheet cellulose triacetate FO membrane demonstrated high sustainable flux and relatively low reverse transport of solutes from the DS into the mixed liquor. Membrane fouling was controlled with osmotic backwashing. The FO membrane was found to reject 98% of organic carbon and 90% of ammonium-nitrogen; the OsMBR process (bioreactor and FO membrane) was found to remove greater than 99% of organic carbon and 98% of ammonium-nitrogen, respectively; suggesting a better compatibility of the OsMBR with downstream RO systems than conventional membrane bioreactors.