Osmotic backwash mechanism of reverse osmosis membranes

A new osmotic backwash (BW) model for reverse osmosis (RO) membranes was developed for conditions of no applied pressures across the membrane. This analytical model has one adjustable parameter representing the coefficient of a linearized convection term in the general convection–diffusion equation. An experimental RO/BW system was used for 12 data sets to verify the proposed BW model and illustrate its predictability. Results show deviations of the model from the data within a range of 5–15%. The described dilution mechanism of the feed concentration polarization (CP) layer is based on RO originated concentrated layer detachment from the membrane surface followed by its gradual dilution.The understanding gained in this research may be applied to automatic RO/BW cleaning cycles. A dominant RO parameter of the BW process is the RO initial driving force—the concentration difference across the membrane. Other RO process parameters – applied pressure and feed flow rate – have lesser effects. Both theoretical and experimental methods provide quantitative relationships between RO and BW variables that enable an understanding and control of the BW process.     

Development of robust organosilica membranes for reverse osmosis

Hybrid organically bridged silica membranes have attracted considerable attention because of their high performances in a variety of applications. Development of robust reverse osmosis (RO) membranes to withstand aggressive operating conditions is still a major challenge. Here, a new type of microporous organosilica membrane has been developed and applied in reverse osmosis. Sol-gel derived organosilica RO membranes reject isopropanol with rejection higher than 95%, demonstrating superior molecular sieving ability for neutral solutes of low molecular weight. Due to the introduction of an inherently stable hybrid network structure, the membrane withstands higher temperatures in comparison with commercial polyamide RO membranes, and is resistant to water to at least 90 °C with no obvious changes in filtration performance. Furthermore, both an accelerated chlorine-resistance test and Fourier transform infrared analysis confirm excellent chlorine stability in this material, which demonstrates promise for a new generation of chlorine-resistant RO membrane materials.     

Structures of cellulose acetate membranes for reverse osmosis

Conclusions Studies under the scanning electron microscope have shown that the cellulose acetate membranes used for reverse osmosis are high-molecular-weight condensation structures of the cellular type resulting from the dropwise separation of a new liquid phase under diffusional enrichment of the polymer solution by water, the solvent. The pore diameter, and the total pore volume, both diminish on approaching the membrane surface; the diffuse character of the active layer traces back to the concentration distribution resulting from vaporization of acetone, the volatile component, from the acetone- formamide cellulose acetate solution.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 1, pp. 111–115, January, 1977.     

Produced water treatment by nanofiltration and reverse osmosis membranes

Produced water, water that is co-produced during oil and gas manufacturing, represents the largest source of oily wastewaters. Given high oil and gas prices, oil and gas production from non-conventional sources such as tar sands, oil shale and coal bed methane will continue to expand resulting in large quantities of impaired produced water. Treatment of this produced water could improve the economic viability of these oil and gas fields and lead to a new source of water for beneficial use.Two nanofiltration and one low-pressure reverse osmosis membrane have been tested using three produced waters from Colorado, USA. The membranes were analyzed before and after produced water filtration using field emission scanning electron microscopy (FESEM), attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS). In addition, membrane–water contact angles have been measured. XPS data indicate adsorption of organic and inorganic species during filtration. FESEM and ATR-FTIR data support theses findings. Water contact angles indicate the effect of membrane hydrophilicity on fouling. Our results highlight the value of using multiple surface characterization methods with different depths of penetration in order to determine membrane fouling. Depending on the quality of the produced water and the water quality requirements for the beneficial uses being considered, nanofiltration may be a viable process for produced water treatment.     

Removal of organics from produced water by reverse osmosis using MFI-type zeolite membranes

Separation of an organics/water mixture was carried out by reverse osmosis using an α-alumina-supported MFI-type zeolite membrane. The organic rejection performance is strongly dependent on the ionic species and dynamic size of dissolved organics. The membrane showed high rejection efficiency for electrolytes such as pentanoic acid. An organic rejection of 96.5% with a water flux of 0.33 kg m⁻² h⁻¹ was obtained for 100 ppm pentanoic acid solution at an operation pressure of 2.76 MPa. For non-electrolyte organics, separation efficiency is governed by the molecular dynamic size; the organics with larger molecular dynamic size show higher separation efficiency. The zeolite membrane gives an organic rejection of 99.5% and 17% for 100 ppm toluene and 100 ppm ethanol, respectively, with a water flux of 0.03 kg m⁻² h⁻¹, 0.31 kg m⁻² h⁻¹ at an operation pressure of 2.76 MPa. It was observed that organic rejection and water flux were affected by the organic concentration. As pentanoic acid concentration increased from 100 ppm to 500 ppm, both organic rejection and water flux decreased slightly.     

Adhesion of waste water bacteria to reverse osmosis membranes

A stirred cell was used to study initial adhesion of three sewage bacteria belonging to the genus Pseudomonas to the three reverse osmosis (RO) membranes BW30, PVD and CAB2, and the nanofiltration membrane NF45. Membranes were immersed in suspensions containing 10⁸ bacteria/ml for 10 min. All three strains were capable of rapidly colonising the four membranes, but to different extents. It was found that bacteria would sometimes aggregate upon adhering to particular RO membranes. The effects of solution ionic strength and pH, and conditioning of membranes (by prior exposure to filtrates of treated and untreated sewage) on the number of adherent bacteria were investigated. Minimal bacterial attachment occurred in a very low ionic strength milieu (deionised water). Salt concentrations corresponding to waste water and to twice that concentration resulted in significantly higher but statistically similar numbers of attached microbes. Adhesion of the three isolates was not affected by pH in the range of 4–8. The number of bacteria attaching to the membranes could be increased or reduced by conditioning films of sewage origin, conditioning films could also trigger or inhibit aggregation of adherent cells. Some surface properties of the membranes (roughness, hydrophobicity) and bacterial cells (electrophoretic mobility, functional groups by affinity chromatography) were also investigated.     

Surface modification of commercial composite polyamide reverse osmosis membranes

Radical grafting of two monomers, methacrylic acid and polyethylene glycolmethacrylate, onto commercial composite polyamide reverse osmosis membranes was performed. A redox system was used for initiation, and grafting was performed in an aqueous medium at room temperature. Surface grafting was characterized by ATR-FTIR, ESCA and streaming potential measurements. It was found that the membranes were surface modified without damage to their transport properties.     

Ion-containing reverse osmosis membranes obtained by radiation grafting method

Cationic membranes obtained by radiation grafting of acqueous acrylic acid onto low density polyethylene films followed by alkaline treatment to confer ionic character in the graft chains, were tested for reverse osmosis desalination of saline water. Selected physical properties of such membranes were investigated. The grafted membranes prossess good mechanical and electrical properties. Water uptake for the alkali-treated membrane was much higher than that of the alkali-untreated one. The effect of operation time, degree of grafting, applied pressure and feed concentration on the water flux and salt rejection for the grafted membranes was investigated. Such cationic membranes showed good durability, thermal and chemical stability, acceptable water flux and salt rejection which may make them acceptable for practical use in reverse osmosis desalination of sea water.     

Membrane potential across reverse osmosis membranes under pressure gradient

Membrane potential measurement has been widely used for the characterization of ionic membranes such as ion-exchange membranes without solvent permeability. However, there have been few studies on membrane potentials across pressure-driven processes such as reverse osmosis (RO) membranes with solvent permeability. In the present study, the membrane potential across RO membranes in NaCl and MgCl2 under the pressure gradient, DeltaP=0-0.3 MPa, was measured. The experimental results were analyzed by the theoretical model based on the Donnan equilibrium and the extended Nernst-Planck flux equation considering the pressure effect. The theoretical values agreed well with the experimental ones. This indicates that membrane potential is useful for characterizing the effective charge density of the active layer of RO membranes under pressure gradient.     

PEG-coated reverse osmosis membranes: Desalination properties and fouling resistance

This study focuses on the use of surface-coated reverse osmosis (RO) membranes to reduce membrane fouling in produced water purification. A series of crosslinked PEG-based hydrogels were synthesized using poly(ethylene glycol) diacrylate as the crosslinker and poly(ethylene glycol) acrylate, 2-hydroxyethyl acrylate, or acrylic acid as comonomers. The hydrogels were highly water permeable, with water permeabilities ranging from 10.0 to 17.8 (L μm)/(m² h bar). The hydrogels were applied to a commercial RO membrane (AG brackish water RO membrane from GE Water and Process Technologies). The water flux of coated membranes and a series-resistance model were used to estimate coating thickness; the coatings were approximately 2 μm thick. NaCl rejection for both uncoated and coated membranes was 99.0% or greater, and coating the membranes appeared to increase salt rejection, in contrast to predictions from the series-resistance model. Zeta potential measurements showed a small reduction in the negative charge of coated membranes relative to uncoated RO membranes. Model oil/water emulsions were used to probe membrane fouling. Emulsions were prepared with either a cationic or an anionic surfactant. Surfactant charge played a significant role in membrane fouling even in the absence of oil. A cationic surfactant, dodecyltrimethyl ammonium bromide (DTAB), caused a strong decline in water flux while an anionic surfactant, sodium dodecyl sulfate (SDS), resulted in little or no flux decline. In the presence of DTAB, the AG RO membrane water flux immediately dropped to 30% of its initial value, but in the presence of SDS, its water flux gradually decreased to 74% of its initial value after 24 h. DTAB-fouled membranes had lower salt rejection than membranes not exposed to DTAB. In contrast, SDS-fouled membranes had higher salt rejection than membranes not exposed to SDS, with rejection values increasing, in some cases, from 99.0 to 99.8% or higher. In both surfactant tests, coated membranes exhibited less flux decline than uncoated AG RO membranes. Additionally, coated membranes experienced little fouling in the presence of an oil/water emulsion prepared from DTAB and n-decane. For example, after 24 h the water flux of the AG RO membrane fell to 26% of its initial value, while the water flux of a PEGDA-coated AG RO membrane was 73% of its initial value.     

Sodium cyanide separation and parameter estimation for reverse osmosis thin film composite polyamide membrane

Sodium cyanide separation data are obtained from synthetic binary aqueous solutions using a commercial thin film composite polyamide reverse osmosis membrane. The separation data are analyzed with the help of two membrane transport models, i.e., combined film theory–solution–diffusion (CFSD) model and combined film theory–Spiegler–Kedem (CFSK) model. The membrane transport parameters and mass transfer coefficients are estimated, simultaneously, from both the models, using a graphical method in case of CFSD model, and a nonlinear parameter estimation method for CFSK model. The new phenomena that there exists a maximum in rejection when it is plotted against product flux, which was observed previously using the same membrane for the phenol–water system, is observed in the present case too. This behavior is explained using the above said models. Though both the models predict the membrane performance reasonably, the values of CFSK model are more accurate and the mass transfer coefficients estimated from the CFSK model are comparable with the literature values.     

Gas and liquid permeation properties of modified interfacial composite reverse osmosis membranes

Gas permeation tests using nitrogen, oxygen, hydrogen, helium and carbon dioxide were performed to assess how membrane modification procedures affect the separating layer morphology of thin-film composite reverse osmosis membranes. Gas selectivity data provided evidence for the presence of nanoscale separating layer defects in dry samples of six commercial membrane types. These defects were eliminated when the membrane surface was coated with a polyether–polyamide block copolymer (PEBAX 1657), as indicated by a 25-fold decrease in gas permeance and at least a 2-fold increase in most selectivity values. Treatment with n-butanol followed by drying reduced water flux and gas flux by 30% and 75%, respectively, suggesting that using n-butanol as a solvent for applying coatings negatively affects membrane performance. The results of this study demonstrate that gas permeation measurements can be used to detect morphological features that impact gas and water membrane flux.     

Observations on the structure of first generation polybenzimidazole reverse osmosis membranes

The structure-property relationships of polybenzimidazole (PBI) reverse osmosis (RO) membranes are important in process optimization. The membrane transport properties are influenced by the morphology which, in turn, is established by the process variables. Microscopical observation of the in situ porous textures of experimental membranes required development of special sample preparation methods. The preparation involves gradual replacement of water, and infiltration by an embedding resin, which permits thin sectioning for microscopical observation without collapse of the structure. p]Optical microscopy observations using the newly developed method provide an excellent overview useful for screening process variables. Scanning electron microscopy (SEM) was used to obtain useful three-dimensional fracture views of samples, although the membrane structure is somewhat collapsed during specimen preparation. Pore size and distribution, particularly in the region of the dense surface layer critical for effective separation properties (the active layer), were analyzed using transmission electron microscopy (TEM). Membrane surfaces were characterized using SEM and high resolution scanning electron imaging (SEI). Preliminary study revealed major morphological variation as flux levels varied. A study of high-flux as-cast and annealed first generation membranes revealed finger-like macrovoids below the top 500 nm layer. The top 20 nm has no resolvable pores (resolution limit about 10 nm, limited by sample thickness), whereas the bulk membrane exhibits pores about 100 nm across. The annealing process appears to shrink the membrane and reduce the pores to half the as-cast size.     

Microstructure and permeability of “dense” cellulose acetate reverse osmosis membranes

Measurements of water permeability and salt rejection have been made on a series of dense cellulose acetate reverse osmosis membranes prepared from different types of homopolymers and blends, from different solvents, and from different casting solution formulations. Each of the membranes was characterized by means of differential scanning calorimetry measurements. Both the thermal properties and the reverse osmosis performance of the dense films were affected by the composition of the casting solution and by subsequent heat treatment. Development of some degree of microcrystallinity appeared favorable to reverse osmosis performance, possibly because of the restraints it imposed on the amorphous phase. A 50 : 50 blend of cellulose triacetate and CA-398-3 was found to give better reverse osmosis performance than was obtained from films prepared from either homopolymer.     

Modeling permeation of volatile organic molecules through reverse osmosis spiral-wound membranes

Reverse osmosis is an interesting process to eliminate organic solutes from distillery condensates before recycling them into the fermentation step. However, organic solutes transport phenomena through reverse osmosis membranes are specific. Rejection and sorption of five compounds were studied on a brackish water membrane. Acetic acid and 2,3-butanediol were not sorbed on the membrane while furfural and 2-phenylethanol presented strong sorption following the Langmuir pattern. These sorption effects coupled with solute molecular weight (MW) led to low rejections of acetic acid and furfural (30–60%) and high rejections of 2,3-butanediol and 2-phenylethanol (80–98%). With intermediate sorption and MW, butyric acid showed rejections between 70 and 80%. A modified solution-diffusion model was developed to take into account the sorption pattern and predict the concentration profile along the membrane on the retentate and permeate sides. Equilibrium properties were determined experimentally while transport properties were identified with data obtained from a synthetic condensate. This model was validated for various operating conditions with the synthetic and the industrial condensates. It was then used to simulate the influence of the recovery rate on the retentate and permeate concentrations. It showed the behavior differences between solutes with a linear sorption and solutes with a saturating sorption.     

Concentration profiles retention—flux curves for composite membranes in reverse osmosis

The influence of a transition layer between the skin porous sublayer in asymmetric membranes was investigated, using a numerical integration of the Spiegler—Kedem equation. It is shown that the transition layer will only interfere if the resistance to solute diffusion within this layer is of the same order of magnitude as in the skin layer.Experiments with multi-layer membranes were performed by placing a highly selective asymmetric membrane on top of a membrane of lower selectivity, and vice versa. The resulting retention-flux curves can be simulated by a three-layer model: σ $\text{P}$/Δx for the two skin layers were determined from reverse osmosis data on the individual membranes whereas the values for the interjacent porous layer were correlated with the porosity and thickness of that layer.     

Synthesis of organically bridged trialkoxysilanes bearing acetoxymethyl groups and applications to reverse osmosis membranes

New bridged trialkoxysilanes bearing acetoxymethyl groups were synthesized by double hydrosilylation of 1,6‐diacetoxy‐2,4‐butadiyne, using two equivalents of triethoxysilane and a metal catalyst. With a Ru catalyst, the reaction proceeded via anti‐addition to provide BTES‐Ac‐a as a single isomer, while a similar reaction with a Pt or Rh catalyst provided an isomeric mixture of syn ‐adducts BTES‐Ac‐b. Reverse osmosis (RO) silica membranes were prepared by the sol–gel process with BTES‐Ac‐a and BTES‐Ac‐b and the membranes were examined with respect to water desalination using a 2000 ppm NaCl aqueous solution. NaCl rejection of the membranes increased to reach 96% at the early stage of the RO experiments. However, the rejection decreased gradually to 85% after 70 and 200 h for BTES‐Ac‐a and BTES‐Ac‐b, respectively, due to hydrolytic decomposition of the silica network during the experiments. In contrast, a membrane prepared from copolymerization of BTES‐Ac‐a with ethane‐bridged bistrialkoxysilane (BTES‐E1) showed improved stability towards hydrolysis with stable NaCl rejection of 96% with higher water permeance (3.5 × 10⁻¹³ m³ m⁻² s⁻¹ Pa⁻¹) than that of a membrane prepared by homopolymerization of BTES‐E1 (2.7 × 10⁻¹⁴ m³ m⁻² s⁻¹ Pa⁻¹) reported previously.