NF Process Research Articles

Nanofiltration process for separating Cr(III) from acid solutions: Experimental and modelling analysis

This study aims to analyse the nanofiltration process at laboratorial scale, for separating Cr(III) from acid solutions. Thus, several experiments were tested and the results interpreted with a suitable mathematical model. The rejection of trivalent chromium salts under acidic conditions using the nanofiltration membrane Desal 5 DK was investigated. The experimental data were described by a Donnan steric partitioning pore and dielectric exclusion model which was based on the extended Nernst–Planck equation and incorporates also the concentration polarization effect in the membrane/feed solution interface. Such approach enabled to estimate the charge membrane density which varies from 1.5 × 10 − 3 to 0.1 mol m − 3 , and the pore dielectric constant nearly 50. The predictions obtained with the mathematical model are in good agreement with the experimental data. The parametric sensitivity analysis showed that Cr(III) concentration profile across the membrane is particularly sensitive to initial concentrations of chromium and chloride. The pore dielectric constant also has a significant effect on the observed rejections of counter-ions (Cr(III) and H +) and co-ion (Cl −). The model used may be helpful to optimize NF process applied to the treatment of metal salt solutions under acidic conditions.

Flux decline study for textile wastewater treatment by membrane processes

Membrane methods, including MF, UF and NF, belong to high-efficiency processes which can be competitive with traditional methods of water treatment. However, their efficiency can be affected by some parameters such as rejection and permeate flux. In this work, three membrane processes (MF, UF and NF) were used for the treatment of textile effluent sample and factors influencing the flux decline of the used membranes were investigated. MF and UF processes were studied in order to be investigated as pretreatment for the NF process in the case of textile effluent treatment. Linearized forms of cake formation equations were used according to Wiesner and Aptel equations and the various filtration resistances were determined using the resistance-in-series model. Experiments demonstrated that more than 90% of colour, turbidity and TDS were removed from the treated water accompanied with a reduction of COD. Experimental results concluded that concentration polarization was the principal cause of flux decline for UF membrane. On the other hand, fouling was observed to make an important contribution to flux decline for NF membrane. We concluded also that rapid cake formation occurred during initial filtration and caused an initial sharp decrease in permeate flux for MF and UF processes. In the case of NF process, the cake formation took place progressively and increased in the second state of filtration.

Removal of organic dyes in the hybrid photocatalysis/nanofiltration system

AbstractThe removal of azo dyes acid red 18 (AR18), direct green 99 (DG99) and acid yellow 36 (AY36) in hybrid photocatalysis/nanofiltration (NF) system was investigated. The photocatalytic reactions were conducted in the flow reactor with immobilized photocatalyst bed. A commercially available titanium dioxide (Aeroxide® P25, Degussa, Germany) was used as a photocatalyst. The solution after the photocatalytic reaction was applied as feed for NF process. The changes of various parameters, including concentration of dyes, pH and conductivity of the solution, total organic carbon (TOC) and total dissolved solids (TDS) content were analyzed during the process.It was found that azo dyes AR18, DG99 and AY36 solutions could be successfully decolorized in the quartz labyrinth flow reactor with immobilized photocatalyst bed. The rate of color removal can range as follows: AR18 > DG99 > AY36. Copyright © 2009 Curtin University of Technology and John Wiley & Sons, Ltd.

Application of nanofiltration process in 5′-GMP production

In this study, an NF membrane named S-372 (200 MWCO and the negative charge) was successfully applied in 5′-GMP production. With NF process, 5′-GMP was concentrated from 20.9 g·L −1 to 130.6 g·L −1 and NaCl was desalted from 19.7 g·L −1 to 1.00 g·L −1. The pure water flux could be completely recovered by the alkaline detergent solution with pH 9.0. These results proved that NF process with S-372 could replace vacuum concentration and ethanol crystallization in the conventional process, having advantages of low energy consumption and being environment friendly.

Wastewater treatment and water reuse of food processing industries. Part II: Techno-economic study of a membrane separation technique

To reuse pretreated water effluent of the El-Nile Company for the food industry in Egypt, NF and RO membrane separation techniques were investigated. Two sets of experiments were carried out by using laboratory separation unit with maximum pressure of 7 bars and pilot plant membrane separation unit with maximum pressure of 14 bars. Based on the analyses of collected samples, a hydration model was used to determine the combined salts in the wastewater. The combined salts were used in the preparation of a synthetic solution simulated to the wastewater of El Nile Company. The experiments were divided into two parts: the first part was concerned with the application of membrane separation experiments to synthetic solutions and the second part was concerned with the pretreatment and membrane separation experiments of the industrial wastewater. The first part was carried out by using NF followed by RO membrane separation technology. Best performance for NF process is observed in separation of divalent ions and the less in separation of monovalent ions, NF membrane succeeded in ~30% removal of divalent and trivalent ions, while RO membrane proved separation of 99% of sulphate ions, 96% of iron, 93% of bicarbonate, ~90% of sodium, magnesium and sulphide ions, 86% of potassium, 73% of phosphate and 25% of calcium ions. The second part was concerned with industrial wastewater, where oil and grease were completely removed by floatation, 11% of TSS and 12% of COD were removed by clarification, while coagulation by using 2 mg/l of anionic polymer removed 93% and 30% of TSS and COD respectively. The pretreated wastewater was allowed to pass through NF membrane at 14 bars followed by RO membrane at pressure of 14 bars. The separation efficiencies of monovalent, divalent and trivalent ions were nearly 2-3% less than that in case of synthetic solutions, while COD and BOD were completely removed. The water resulted from NF may be used in industrial processes and low pressure boilers while the water resulted from RO membrane may be used for high pressure boilers. Based on the previous results, preliminary techno-economic evaluation for 1200 m 3/day treatment plant was performed. The designed plant consists of: holding tank, oil separator, flash mixer, coagulation unit, clarification and settling unit, MAF unit, MIF unit, UF unit, NF unit and RO unit. The fixed capital cost was 1.46 million Egyptian pounds. The cost of treatment of 1 m 3 wastewater was 1.3 LE.

Viral safety of C1-inhibitor NF

We studied the efficacy of virus reduction by three process steps (polyethylene glycol 4000 (PEG) precipitation, pasteurization, and 15 nm virus filtration) in the manufacturing of C1-inhibitor NF. The potential prion removing capacity in this process was estimated based on data from the literature. Virus studies were performed using hepatitis A virus (HAV) and human immunodeficiency virus (HIV) as relevant viruses and bovine viral diarrhea virus (BVDV), canine parvovirus (CPV) and pseudorabies virus (PRV) as model viruses, respectively. In the PEG precipitation step, an average reduction in infectious titer of 4.5 log 10 was obtained for all five viruses tested. Pasteurization resulted in reduction of infectious virus of >6 log 10 for BVDV, HIV, and PRV; for HAV the reduction factor was limited to 2.8 log 10 and for CPV it was zero. Virus filtration (15 nm) reduced the infectious titer of all viruses by more than 4.5 log 10. The overall virus reducing capacity was >16 log 10 for the LE viruses. For the NLE viruses CPV and HAV, the overall virus reducing capacities were >8.7 and >10.5 log 10, respectively. Based on literature and theoretical assumptions, the prion reducing capacity of the C1-inhibitor NF process was estimated to be >9 log 10.

NF process for the recovery of caustic soda and concentration of disodium terephthalate from alkaline wastewater from polyester fabrics

The recovery of caustic soda and concentration of disodium terephthalate (DST) from alkali waste of polyester fabrics were investigated using SelRo (MPT-34) nanofiltration membranes with membrane area of 0.024 m 2 and 5.2 m 2, respectively. Flux behaviors were observed with respect to filtration time, volumetric concentration factor (VCF), operating pressure, temperature, and cleaning. The mass balance of caustic soda, DST, and ethylene glycol (EG) were estimated according to different values of VCFs. As the VCF increased, more amounts of disodium terephthalate and ethylene glycol permeated while almost the same concentration of sodium hydroxide was permeable. The NF operation end with a concentrated disodium terephthalate solution that needed a reduced amount of acid for precipitation and a permeate of caustic soda solution containing EG and DST. Concentrations of EG and DST that govern reusability of permeate in the caustic treatment of polyester fabrics were measured to be dependent of VCF. The most suitable VCF in terms of maintaining relatively good membrane productivity and high rejection of DST was approximately 4.5. At a VCF of 4.5, the rejections of DST and EG were 86% and 42%, while NaOH recovery was 84% with fluxes between 12 to 36 LMH.

Water demineralisation by NF/MD integrated processes

The production of demineralised water from drinking water by nanofiltration/membrane distillation system (NF/MD) has been investigated. The precipitation of CaCO 3 on the membrane surface and a rapid flux decline was observed when tap water was used directly as a feed. The problem of scaling was eliminated by the acidification of the feed to pH = 4 and stable module efficiency was observed. The performance of the MD module was significantly improved when tap water was subjected to softening in NF process. The precipitation of the silica compounds during the operation of MD process with the NF permeate was found. These compounds caused clogging of the capillary membrane inlets, resulting in a gradual decline of the module efficiency. The application of filtration net assembled at the module inlet prevented blocking of the capillary membrane inlets. The MD process can be run without a flux decline, associated with the scaling, when the pH of NF permeate was adjusted at a level of 4. A partial wettability of the membrane resulting in a flux decline by 30% was observed after 1100 h of process duration. The quality of obtained distillate was stable and practically independent of the feed concentration. The produced distillate has the electrical conductivity in the range of 1.5–2.5 μS cm −1 and contained 0.5 ppm of inorganic carbon and below 1 ppm TOC.

A comprehensive review of nanofiltration membranes:Treatment, pretreatment, modelling, and atomic force microscopy

Nanofiltration membranes (NF) have applications in several areas. One of the main applications has been in watertreatment for drinking water production as well as wastewater treatment. NF can either be used to treat all kinds of water including ground, surface, and wastewater or used as a pretreatment for desalination. The introduction of NF as a pretreatment is considered a breakthrough for the desalination process. NF membranes have been shown to be able to remove turbidity, microorganisms and hardness, as well as a fraction of the dissolved salts. This results in a significantly lower operating pressure and thus provides a much more energy-efficient process. Similar to other membrane processes, a major problem in NF membrane applications is fouling. Several studies have investigated the mechanisms of fouling in NF membranes and suggested methods to minimize and control the fouling of NF membranes. For NF membrane characterizations and process prediction, modeling of NF processes and the use of atomic force microscopy (AFM) are very important. The ability to predict the performance of NF processes will lead to a lower number of experiments, saving of time and money, and help to understand the separation mechanisms during NF. A comprehensive review of NF in water treatments is presented including a review of the applications of NF in treating water as well as in the pretreatment process for desalination; the mechanism as well as minimization of NF membrane fouling problems; and theories for modelling and transport of salt, charged and noncharged organic compounds in NF membranes. The review will also address the application of AFM in studying the morphology of membrane surfaces as part of the NF membrane characterization.

Effect of feed pH on permeate pH and ion rejection under acidic conditions in NF process

This study explored the influence of feed pH on permeate pH and rejections of ions under acidic conditions in NF process. A Desal-5 NF membrane was tested. Town water, 10 mM NaCl and 10 mM NaNO 3 in deionized water were used as raw feed. Feed pH in the range of 1–7 was investigated. A critical feed pH was observed for each solution. Above the critical pH value NF permeate pH was lower than feed pH, but below the critical pH value permeate pH was higher than feed pH. The critical pH value for town water, NaCl and NaNO 3 solutions were 4.25, 4.50 and 4.55, respectively. These are close to the isoelectric point (4.0) of the NF membrane used. An explanation of these observed results, that have not been reported so far, is offered. The experimental results showed that rejection of NaCl in town water was lower than that in pure NaCl solution due to higher rejection of bivalent ions in town water. For NaNO 3 solution at a feed pH below 1.6, a lower pH of permeate than feed was observed again due to positively charged groups on the membrane being shielded by anions at high salt concentration and significant reduction of the electrostatic repulsion between protons and the positively charged membrane surface. Ion rejection first passed through a minimum and then a maximum with a decrease in feed pH. Moreover, operating pressure and feed velocity in NF process did not appear to influence the relationship between feed pH and permeate pH. In addition, it was observed that rejection of NaCl in town water was lower than in pure NaCl solution due to the influence of co-existing divalent ions on reducing the rejections of monovalent ions. It was concluded that feed pH significantly influenced the electrostatic repulsion between the charged membrane surface and ions and therefore, permeate pH and ion rejection in NF process. A maximum in ion transmission corresponded to a peak in membrane flux and vice versa. Change in osmotic pressure does not satisfactorily explain the change in flux with pH.

Seawater desalination and salt production in a hybrid membrane-thermal process

Several hybrid systems, which are developed now, namely NF-MSF, NF-RO-MSF, NF-RO-MD and others offer promising performance. An integrated system for complete usage of components in seawater is also being considered. In all the aforementioned systems NF as water pre-treatment method is applied and a considerable decrease in scale forming components is achieved. Higher recovery may be then reached when desalting in comparison to traditionally treated water. Very compact NF membranes are applied which results in a very high rejection of divalent ions and relatively high rejection of sodium chloride. A simultaneous decrease of NaCl concentration in NF process is then obtained which causes an increase in the RO recovery since the osmotic pressure value of NF permeate is diminished. IfNF-MSF (or NF followed by any thermal method) system is considered a NaCl concentration decrease in the pre-treatment is however not essential for the NF step. The salt concentration influence on thermal method performance is very weak in comparison to RO. Furthermore the highly concentrated MSF brine might be used as a possible by-product, e.g. for obtaining salt. A dual-purpose desalination-salt production system may be then suggested. Two arrangements are presented, namely: UF-NF-MSF-crystallisation and UF-NF-RO-MSF-crystallisation systems. The high rate of water recovery is accompanied by salt obtainment in these systems. If $30/ton of salt is assumed the water cost is equal to $0.71/m 3 in UF-NF-MSF-crystallisation system and $0.43/m 3 in UF-NF-RO-MSF-crystallisation system. The above data demonstrate that these water costs are competitive compared to those of potable water produced in thermal or RO seawater plants.

Field Test Results of a Nanofiltration Application for Separating almost Saturated Brine Solutions of the Vacuum Salt Production

To produce vacuum evaporated salt from a salt feedstock in high purity a high amount of mother liquors has to be purged which causes a loss of salt and represents a big environmental problem. A solution is to be seen in a selective recycling of sulphate ions from the mother liquors back to the brine treatment. Laboratory tests have shown that a NF process would offer a new way in the salt winning technology if this technique can be successfully applied within the salt production. To investigate the feasibility and the usefulness pilot tests were carried out at the Austrian Salt Works in Ebensee. The investigation was conducted over a period of 9 months with an automated 25 m 2 membrane separation plant in which data on the membrane performance and stability were collected. The field tests have proven the membrane process to be a satisfactory and technically feasible application.

Direct measurement, by intracavity laser spectroscopy, of the population difference for the b—X transition in the NF radical

An intracavity laser spectroscopy method was employed in recording for the first time the absorption spectrum of the NF radical in the vicinity of the b—X transition. Experiments showed that the method could be used successfully in direct measurements of the population difference. When the duration of a dye laser pulse was 1.3 μs, the minimum recorded population difference was 3.5×1015 cm-3. The rate constant for the NF(X) +NF(X) process was determined: it was (5.5 ± 1.5)×10-12 cm3 s-1.