A critical flux to avoid biofouling of spiral wound nanofiltration and reverse osmosis membranes: Fact or fiction?

The relation between biofouling and membrane flux in spiral wound nanofiltration and reverse osmosis membranes in drinking water stations with extensive pretreatment such as ultrafiltration has been studied. The flux – water volume flowing through the membrane per unit area and time – is not influencing the development of membrane biofouling. Irrespective whether a flux was applied or not, the feed spacer channel pressure drop and biofilm concentration increased in reverse osmosis and nanofiltration membranes in a monitor, test rigs, a pilot scale and a full-scale installation. Identical behavior with respect to biofouling and feed channel pressure drop development was observed in membrane elements in the same position in a nanofiltration installation operated with and without flux. Calculation of the ratio of diffusive and convective flux showed that the diffusive flux is considerably larger than the convective flux, supporting the observations that the convective flux due to permeate production is playing an insignificant role in biofouling. Since fouling occurred irrespective of the actual flux, the critical flux concept stating that “below a critical flux no fouling occurs” is not a suitable approach to control biofouling of spiral wound reverse osmosis and nanofiltration membranes.     

Nuclear magnetic resonance microscopy studies of membrane biofouling

There is a substantial need for novel measurement techniques that enable non-invasive spatially resolved observation of biofouling in nanofiltration (NF) and reverse osmosis (RO) membrane modules. Such measurements will enhance our understanding of the key design and operational parameters influencing biofilm fouling. In this study we demonstrate the first application of nuclear magnetic resonance microscopy (NMR) to a spiral wound reverse osmosis (RO) membrane module. The presented NMR protocols allow the extraction of the evolution with biofouling of (i) the spatial biofilm distribution in the membrane module, (ii) the spatially resolved velocity field and (iii) displacement propagators, which are distributions of molecular displacement of a passive tracer (in our case, water) in the membrane. From these measurements, the effective membrane surface area is quantified. Despite the opaque nature of membrane design, NMR microscopy is shown to be able to provide a non-invasive quantitative measurement of RO membrane biofouling and its impact on hydrodynamics and mass transport. Minimal biofilm growth is observed to have a substantial impact on flow field homogeneity.     

Biofouling in spiral wound membrane systems: Three-dimensional CFD model based evaluation of experimental data

A three-dimensional (3D) computational model describing fluid dynamics and biofouling of feed channels of spiral wound reverse osmosis and nanofiltration membrane systems was developed based on results from practice and experimental studies. In the model simulations the same feed spacer geometry as applied in practice and the experimental studies was used. The 3D mathematical model showed the same trends for (i) feed channel pressure drop, (ii) biomass accumulation, (iii) velocity distribution profile, resulting in regions of low and high liquid flow velocity also named channeling. The numerical model predicted a dominant biomass growth on the feed spacer, consistent with direct in situ observations on biofouling of spiral wound membrane modules and monitors using Magnetic Resonance Imaging (MRI). The model confirms experimental results that feed spacer fouling is more important than membrane fouling. The paper shows that mathematical modeling techniques have evolved to a stage that they can be used hand-in-hand with experiments to understand the processes involved in membrane fouling.     

Effect of process parameters on transmembrane flux during direct osmosis

Direct osmosis is a non-thermal membrane process employed for the concentration of fruit juices at ambient temperature and atmospheric pressure, thereby maintaining the organoleptic and nutritional properties of fruit juices. In the present study, concentration of pineapple juice by direct osmosis was explored. Aqueous solution of sucrose (0–40%, w/w)–sodium chloride (0–26%, w/w) combination was investigated as an alternative osmotic agent. The sucrose–sodium chloride combination can overcome the drawback of sucrose (low flux) and sodium chloride (salt migration) as osmotic agents during direct osmosis process. The effect of the hydrodynamic conditions in the module and feed temperature (25–45 °C) on transmembrane flux was evaluated. For a range of hydrodynamic conditions studied, it was observed that transmembrane flux increases with Reynolds number. The increase in feed temperature resulted in an increase in transmembrane flux. The pineapple juice was concentrated upto a total soluble solids content of 60 °Brix at ambient temperature. The effect of direct osmosis process on physico-chemical characteristics of pineapple juice was also studied. The ascorbic acid content was well preserved in the pineapple juice concentrate by direct osmosis process.     

Periodic air/water cleaning for control of biofouling in spiral wound membrane elements

The main problem during the operation of nanofiltration or reverse osmosis membrane plants is fouling of feed spacers in membrane elements due to biofouling and particulate fouling. In order to control biofouling and particulate fouling in membrane elements, both daily air/water cleaning (AWC) and daily copper sulphate dosing (CSD) were investigated and compared to a reference without daily cleaning. A pilot study was carried out for 110 days with three parallel spiral wound membrane elements; AWC, CSD and the reference which were fed by tap water enriched with a biodegradable compound (100 μg acetate-C/L). The CSD element, which combined daily copper sulphate dosing and sporadically air/water cleaning, performed best with an increase in pressure drop of 18% and a biomass concentration of 8000 pg ATP/cm² within 110 days. This was followed by the AWC element with a pressure increase of 37% and biomass concentration of 20,000 pg ATP/cm² within 110 days. The reference element showed a pressure increase of 120% within 21 days. The presented approach is considered very successful in controlling particulate fouling and biofouling, especially when air/water cleaning is combined with copper sulphate dosing.     

Chitosan‐modified graphene oxide as a modifier for improving the structure and performance of forward osmosis membranes

Chitosan (CS) with good hydrophilicity and charged property was used to modify graphene oxide (GO), the obtained GO‐CS was used as a novel modifier to fabricate thin film composite forward osmosis (FO) membranes. The results revealed that the amino groups on CS reacted with carboxyl groups on GO, and the lamellar structure of the GO nanosheets was peeled off by CS, resulting in the reducing of their thicknesses. The GO‐CS improved the hydrophilicity of polyethersulfone (PES) substrate, and their contact angles decreased to 64° with the addition of GO‐CS in the substrate. GO‐CS also increased the porosity of the substrate and surface roughness of FO membrane, thereby optimizing the water flux and reverse salt flux of FO membrane. The average water flux of the FO membrane reached the optimal flux of 21.34 L/(m² h) when GO‐CS addition was 0.5 wt%, and further addition of GO‐CS to the substrate would decrease the water flux of FO membrane, and the reverse salt flux also decreased to the lowest value of 2.26 g/(m² h). However, the salt rejection of the membrane increased from 91.4% to 95.1% when GO‐CS addition increased from 0.5 to 1.0 wt% under FO mode using 1 mol/L sodium chloride (NaCl) solution as draw solution (DS). In addition, high osmotic pressure favored water permeation, and at the same concentration of DS, magnesium chloride (MgCl₂) exhibited better properties than NaCl. These results all suggested that GO‐CS was a good modifier to fabricate FO membrane, and MgCl₂ was a good DS candidate.     

Osmotic pressure control in response to a specific ion signal at physiological temperature using a molecular recognition ion gating membrane

A molecular recognition gating ion membrane was prepared by graft copolymerization of N-isopropylacrylamide and benzo[18]crown-6-acrylamide onto the pore surface of porous polyethylene film. This membrane captured Ba2+ with its crown ether receptors and generated osmotic pressure in response to Ba2+ autonomously and reversibly. However, the membrane never generated osmotic pressure in response to Ca2+. In addition, the concentration gradient of both the ion and other solute such as dextran could be used as the driving force; using a dextran concentration gradient, we can control the critical concentration and the duration time of the osmosis response.     

Effect of colloids on salt transport in crossflow nanofiltration

The role of colloid deposition on the performance of a salt-rejecting NF membrane was evaluated by modeling salt transport using a two-layer transport model, which quantified the relative contributions of advection and diffusion in the cake and the membrane layers, and the effects of flux on the membrane sieving coefficient. The model was able to accurately describe how the measured permeate concentration, rejection, osmotic pressure, and flux decline varied with time. The two-layer model confirmed that the Peclet number in the cake layer was about an order of magnitude higher than that in the membrane layer, leading to significant concentration polarization at the membrane surface, as shown by others. However, the cake layer also increased overall resistance, which resulted in flux decline during constant pressure operation. Flux decline caused an increase in the actual sieving coefficient, leading to higher solute flux, lower observed rejection, and thus lower the bulk concentration. These coupled phenomena tended to mitigate the increase in concentration polarization caused by the cake. Therefore, as predicted by the model and verified by experiment, the osmotic pressure does not increase monotonically as the cake grows, and in fact can decrease when the cake layer is thick and the flux decline is significant. In our experimental system, the pressure drop across the cake layer, which was proportional to the cake thickness, was significant under the conditions studied. The effects of cake-enhanced osmotic pressure analyzed here are lower than those observed in previous studies, possibly because the transport model employed explicitly accounts for the effect of flux decline due to cake growth on the membrane sieving coefficient, and possibly because we used a somewhat different methodology to estimate cake porosity.     

Effect of porous support fabric on osmosis through a Loeb-Sourirajan type asymmetric membrane

Commercially available asymmetric membranes of the Loeb-Sourirajan (L-S) type comprise a support fabric, bonded to the porous substructure. The influence of this fabric on osmotic permeation flux was examined, mostly with a Toray CA-3000 membrane from which, with care, it was possible to remove the support fabric. In osmosis experiments with 12% MgCl₂ solution on one side (either side) and 6% solution on the other, the permeation flux (J₁) was of the order of 0.01 and 0.06 m³/m² d with and without fabric, respectively. These results could be generalized by considering the resistivity to solute diffusion in the non-skin part of the membrane. This resistivity term averaged 104 and 17 d/m for membranes with and without fabric, respectively, and in further tests without fabric, it was between 15 and 25 d/m over a wide range of MgCl₂ concentrations. Four other L-S membranes, all with support fabric, were tested in osmosis experiments. Their resistivity values were similar to or higher than those of the Toray membrane with fabric, but, with one of the four, the results were affected by switching the location of the high and low concentration solutions. It was concluded that existing commercially available L-S membranes are not appropriate for large-scale osmosis applications because their support fabric decreases permeation flux excessively.     

Separate and Concentrate Lactic Acid Using Combination of Nanofiltration and Reverse Osmosis Membranes

The processes of lactic acid production include two key stages, which are (a) fermentation and (b) product recovery. In this study, free cell of Bifidobacterium longum was used to produce lactic acid from cheese whey. The produced lactic acid was then separated and purified from the fermentation broth using combination of nanofiltration and reverse osmosis membranes. Nanofiltration membrane with a molecular weight cutoff of 100–400 Da was used to separate lactic acid from lactose and cells in the cheese whey fermentation broth in the first step. The obtained permeate from the above nanofiltration is mainly composed of lactic acid and water, which was then concentrated with a reverse osmosis membrane in the second step. Among the tested nanofiltration membranes, HL membrane from GE Osmonics has the highest lactose retention (97 ± 1%). In the reverse osmosis process, the ADF membrane could retain 100% of lactic acid to obtain permeate with water only. The effect of membrane and pressure on permeate flux and retention of lactose/lactic acid was also reported in this paper.     

Molecular weight spectra of ultrafilter rejection. II. Measurements on highly rejecting filters

A mixture of maltodextrins (10,000-80,000 daltons) has been used to follow the response of rejection by membranes to molecular weight. Two ultrafilters and a reverse osmosis membrane having rejection values greater than 0.999 for the high molecular weight fractions were tested. The procedure described permits determination of sieving values (1-R) as low as 1 x 10^?5. The results indicate that only the reverse osmosis membrane was sufficiently retentive to assure the absence of significant levels of endotoxins in the product streams.     

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.     

Power generation with pressure retarded osmosis: An experimental and theoretical investigation

Pressure retarded osmosis (PRO) was investigated as a viable source of renewable energy. In PRO, water from a low salinity feed solution permeates through a membrane into a pressurized, high salinity draw solution; power is obtained by depressurizing the permeate through a hydroturbine. A PRO model was developed to predict water flux and power density under specific experimental conditions. The model relies on experimental determination of the membrane water permeability coefficient (A), the membrane salt permeability coefficient (B), and the solute resistivity (K). A and B were determined under reverse osmosis conditions, while K was determined under forward osmosis (FO) conditions. The model was tested using experimental results from a bench-scale PRO system. Previous investigations of PRO were unable to verify model predictions due to the lack of suitable membranes and membrane modules. In this investigation, the use of a custom-made laboratory-scale membrane module enabled the collection of experimental PRO data. Results obtained with a flat-sheet cellulose triacetate (CTA) FO membrane and NaCl feed and draw solutions closely matched model predictions. Maximum power densities of 2.7 and 5.1 W/m² were observed for 35 and 60 g/L NaCl draw solutions, respectively, at 970 kPa of hydraulic pressure. Power density was substantially reduced due to internal concentration polarization in the asymmetric CTA membranes and, to a lesser degree, to salt passage. External concentration polarization was found to exhibit a relatively small effect on reducing the osmotic pressure driving force. Using the predictive PRO model, optimal membrane characteristics and module configuration can be determined in order to design a system specifically tailored for PRO processes.     

Flux decline during nanofiltration of naturally-occurring dissolved organic matter: effects of osmotic pressure,membrane permeability,and cake formation

Nanofiltration of naturally-occurring dissolved organic matter (NOM) by an aromatic polyamide membrane was measured in a crossflow bench-scale test cell and modeled using a semi-empirical osmotic pressure/cake formation model. Our objective was to examine flux decline due to NOM fouling while explicitly accounting for flux decline due to osmotic effects and changes in membrane permeability. This approach allowed quantification of the effect of ionic composition on specific NOM cake resistance, and yielded insight into flux decline due to enhanced NaCl rejection by the NOM deposit. In the absence of NOM, increasing NaCl concentration reduced salt rejection and decreased membrane permeability. Flux decline was modeled by accounting for changes in osmotic pressure with time, and by employing an effective permeability. The addition of calcium significantly reduced rejection of sodium and feed conductivity, and thus mitigated flux decline. Increasing pH from 4 (near membrane pI) to 10 increased the effective permeability but also increased NaCl rejection, which resulted in greater flux decline. The presence of NOM caused greater flux decline resulting from a combination of NOM cake resistance and increased rejection of NaCl by negatively charged NOM functional groups. Increasing NaCl concentration had little effect on the mass of NOM deposited, but significantly increased the specific resistance of the NOM cake. The effect of ionic strength on specific resistance correlated with a reduction in NOM size, estimated by separate UF permeation experiments and size exclusion chromatography analysis of UF permeate. Therefore, increased specific cake resistance is consistent with a more compact, less porous cake. Flux decline by NOM solutions showed a maximum at pH 7, where salt rejection was also a maximum. Binding of calcium reduced the ability of NOM to enhance NaCl rejection, and likely increased NOM cake resistance. Flux decline caused by NOM fouling in the presence of calcium was only significantly different than that caused by NOM in a solution of NaCl at the same ionic strength when the calcium concentration corresponded to saturation of NOM binding sites.     

Direct osmotic concentration of tomato juice in tubular membrane – module configuration. II. The effect of using clarified tomato juice on the process performance

A study was carried out by using a new tubular direct osmosis module, constructed by PCI UK and equipped with a novel AFC99 membrane 400 μm thick, to investigate the effect of clarifying tomato juice on the rate of direct osmotic concentration. Under virtually the same operating conditions five respective clarifications of juice were tried including full, unclarified, tomato juice. The process performance was calculated in terms of permeation flux. In all the experimental runs the osmotic medium was sodium chloride brine (initial concentration approx. 23% NaCl). A remarkable increase of permeation flux (over 100%) was observed shifting from unclarified to the clarified tomato juice. Clarification was carried out by passing the juice through 35 μm mesh and also by using membranes of molecular weight cut-off 200 000, 100 000 and 25 000 Daltons (Da), respectively, in order to obtain the juice ultrafiltrates. It is also worth mentioning that the flux value obtained with 25 000 Daltons (Da) ultrafiltrate appeared to be considerably higher than values reported in experiments carried out by other researchers, where unclarified juice was used, despite the disadvantage of a far thicker membrane being used in the present investigation. This specific finding discloses great potential in using a combined low temperature and pressure ultrafiltration and direct osmosis process to produce tomato concentrates.     

The osmotic pressure of concentrated protein and lipoprotein solutions and its significance to ultrafiltration

Side-by-side measurements were made of osmotic pressure and ultrafiltration flux from a stirred cell for separate saline solutions of the proteins, bovine serum albumin, bovine fibrinogen, human low density lipoprotein and for polyethylene oxide .in distilled water. Albumin osmotic pressures were large enough to conclude that concentration polarization limits ultrafiltrate flux mostly by decreasing the driving forceΔP -Δπ. For the other large macrosolutes, concentrated-solution osmotic pressures were so small that polarization probably limits flux at the usual levels of applied pressure by adding a hydrodynamic resistance (gel layer) in series with the membrane resistance.     

Experimental study of desalination using direct contact membrane distillation: a new approach to flux enhancement

New membrane distillation configurations and a new membrane module were investigated to improve water desalination. The performances of three hydrophobic microporous membranes were evaluated under vacuum enhanced direct contact membrane distillation (DCMD) with a turbulent flow regime and with a feed water temperature of only 40 °C. The new configurations provide reduced temperature polarization effects due to better mixing and increased mass transport of water due to higher permeability through the membrane and due to a total pressure gradient across the membrane. Comparison with previously reported results in the literature reveals that mass transport of water vapors is substantially improved with the new approach. The performance of the new configuration was investigated with both NaCl and synthetic sea salt feed solutions. Salt rejection was greater than 99.9% in almost all cases. Salt concentrations in the feed stream had only a minor effect on water flux. The economic aspects of the enhanced DCMD process are briefly discussed and comparisons are made with the reverse osmosis (RO) process for desalination.     

Preparation,structure characteristics and separation properties of thin-film composite polyamide-urethane seawater reverse osmosis membrane

A novel thin-film composite (TFC) seawater reverse osmosis membrane was developed by the interfacial polymerization of 5-chloroformyloxyisophthaloyl chloride (CFIC) and metaphenylenediamine (MPD) on the polysulphone supporting membrane. The performance of the TFC membrane was optimized by studying the preparation parameters, which included the reaction time, pH of the aqueous-MPD solution, monomer CFIC concentration, additive isopropyl alcohol content in aqueous solution, curing temperature and time. The reverse osmosis performance of the resulting membrane was evaluated through permeation experiment with synthetic seawater, and the structure of the novel membrane was characterized by using SEM, AFM and XPS. Furthermore, the separation properties of the TFC membrane were tested by examining the reverse osmosis performances of various conditions, the boron rejection performance and the long-term stability. The results show that the desired TFC seawater reverse osmosis membrane has a typical salt rejection of 99.4% and a flux of about 35 L/m² h for a feed aqueous solution containing 3.5 wt.% NaCl at 5.5 MPa, and an attractive boron rejection of more than 92% at natural pH of 7–8; that the novel seawater reverse osmosis membrane appears to comprise a thicker, smoother and less cross-linking film structure. Additionally, the TFC membrane exhibits good long-term stability.