Low-pressure membrane filtration of secondary effluent in water reuse: Pre-treatment for fouling reduction

Fouling in the low-pressure membrane filtration of secondary effluent for water reuse can be severe due to the complex nature of the components in the water. Pre-filtration, coagulation and anion exchange resin were investigated as pre-treatments for reducing fouling of microfiltration (MF) and ultrafiltration (UF) membranes in the treatment of activated sludge-lagoon effluent. The key fouling components were determined using several analytical techniques to detect differences in the organic components between the feed and permeate.Pre-filtration (1.5 μm) enhanced the permeate flux for MF by removing particulates, but had little effect for UF. Marked flux improvement was obtained by coagulation pre-treatment at 5 mg L⁻¹ Al³⁺ with internal membrane fouling being substantially alleviated. Anion exchange resin removed >50% of effluent organic matter but did not improve the flux or reduce irreversible membrane fouling. These results, together with detailed organic compositional analyses, showed that the very high-molecular weight organic materials (40–70 kDa) comprised of hydrophilic components such as soluble microbial products, and protein-like extracellular matter were the major cause of membrane fouling.     

Influence of Extracellular Polymeric Substances on Membrane Fouling and Cleaning in a Submerged Membrane Bioreactor

There is little information available on the correlation between the concentration of extracellular polymeric substances (EPS) and membrane fouling as well as cleaning efficiency. In this study, two lab-scale flat submerged membrane bioreactors (SMBRs) at sludge retention times (SRTs) of 25 and 250 days were operated at a constant permeate flux (12.5 l m⁻² h⁻¹). Samples of activated sludge were tested to quantify the concentration of extractable EPS using cation exchange resin. Batch filtration tests were also performed to determine the specific cake resistances and the flux recoveries. The extractable EPS and protein concentrations were relatively low at the prolonged SRT, leading to cake layers easily removable by the physical manual cleaning or the de-ionized water backwashing and the chemical cleaning with sodium hypochlorite methods. The extent of flux recoveries (both in SMBRs and batch filtration tests) and macroscopic as well as microscopic images indicated that the chemical cleaning could enhance the effectiveness of cleaning. The membrane fouling and cleaning mechanisms were also discussed.__________From Kolloidnyi Zhurnal, Vol. 67, No. 3, 2005, pp. 392–397.Original English Text Copyright © 2005 by Chackrit Nuengjamnong, Ji Hyang Kweon, Jinwoo Cho, Kyu-Hong Ahn, Chongrak Polprasert.This article was submitted by the authors in English.     

Free-solvent model shows osmotic pressure is the dominant factor in limiting flux during protein ultrafiltration

In protein ultrafiltration (UF), the limiting flux phenomenon has been generally considered a consequence of the presence of membrane fouling or the perceived formation of a cake/gel layer that develops at high operating pressures. Subsequently, numerous theoretical models on gel/cake physics have been made to address how these factors can result in limiting flux. In a paradigm shift, the present article reestablishes the significance of osmotic pressure by examining its contribution to limiting flux in the framework of the recently developed free solvent osmotic pressure model. The resulting free-solvent-based flux model (FSB) uses the Kedem–Katchalsky model, film theory and the free solvent representation for osmotic pressure in its development. Single protein tangential-flow diafiltration experiments (30 kDa MWCO CRC membranes) were also conducted using ovalbumin (OVA, 45 kDa), bovine serum albumin (BSA, 69 kDa), and immuno-gamma globulin (IgG, 155 kDa) in moderate NaCl buffered solutions at pH 4.5, 5.4, 7 and 7.4. The membrane was preconditioned to minimize membrane fouling development during the experimental procedure. The pressure was randomly selected and flux and sieving were determined. The experimental results clearly demonstrated that the limiting flux phenomenon is not dominated by membrane fouling and the FSB model theoretically illustrates that osmotic pressure is the primary factor in limiting flux during UF. The FSB model provides excellent agreement with the experimental results while producing realistic protein wall concentrations. In addition, the pH dependence of the limiting flux is shown to correlate to the pH dependency of the specific protein diffusion coefficient.     

Comparison between filtrations at fixed transmembrane pressure and fixed permeate flux: application to a membrane bioreactor used for wastewater treatment

Membrane bioreactors for wastewater treatment must operate for long periods without chemical cleaning. This paper investigates the critical flux concept introduced by Field et al. as a means for achieving this goal. Experiments were conducted on a membrane bioreactor containing 600 l of activated sludge, equipped with a 0.25 m² ceramic membrane and located in Compiegne wastewater treatment plant. Hydraulic retention time was set at 24 h and sludge retention time at 60 days, so that suspended solids concentration stabilises at 10 g/l. We conducted two series of tests: at fixed transmembrane pressure (TMP) and at fixed permeate flux, set by a volumetric pump on the permeate. In both cases, velocity was varied from 1 to 5 m/s. In fixed flux tests, the flux was increased by 10 l/h m² increments and the TMP was observed to rise moderately first and then stabilise in about 15 min until a critical value of the flux is reached. Above this critical flux, the TMP rises rapidly and does not stabilise, as in dead-end filtration. The critical flux was found to increase approximately linearly with velocity, reaching about 115 l/h m² at 4 m/s. These data were reproducible at various dates between 30 and 120 days of continuous operation of the bioreactor and permit to know at which flux a membrane bioreactor must be operated. Comparison of constant pressure and constant flux tests under same conditions showed that the critical flux is almost identical to the limiting or pressure independent flux obtained in constant pressure. More generally, constant flux procedure below the critical flux avoids overfouling of the membrane in the initial stage and is more advantageous for membrane bioreactor operation.     

Xylose recovery by nanofiltration from different hemicellulose hydrolyzate feeds

Xylose is an intermediate product in xylitol production. Nanofiltration could simplify and enhance this separation step conventionally done by chromatographic methods. Here different hemicellulose hydrolyzate feeds were nanofiltered to recover xylose into the permeate.Two different batches of hemicellulose hydrolyzate were prepared: the hydrolyzate as such and modified with crystalline xylose addition. Both feed solutions were diluted to a total dry solids (TDS) content of approximately 21 wt.% and the xylose contents were 48.7% and 59.1% of the TDS_f (total dry solids in feed). The filtration experiments were made at 40, 50 and 60 °C in total reflux mode for approximately 30 min at each pressure of 20, 25, 30, 35 and 40 bar. In addition, a 20-h filtration was made at 50 °C and 30 bar. A DDS LabStak M20-filter was used and it was equipped with Desal-5 DK, Desal-5 DL and NF270 membranes.In short-term filtrations, the nanofiltered permeate of the original hydrolyzate had 78–82% xylose of the TDS_p (total dry solids in permeate) and the modified hydrolyzate 86–88% xylose of the TDS_p. Thus, considerable xylose purification was obtained. The addition of crystalline xylose into the hemicellulose hydrolyzate gave a notable increase in permeate fluxes. The 20-h filtration showed fouling and compaction effects as a flux decrease of approximately 10–25% was detected in the retention integrity test. According to the results, xylose purification from hemicellulose hydrolyzate could be enhanced by nanofiltration.     

Tubular ultrafiltration flux prediction for oil-in-water emulsions: analysis of series resistances

Using the resistance-in-series (RIS) approach to permeate flux modeling, a general relationship between permeate flux, transmembrane pressure, cross-flow velocity, and feed kinematic viscosity was developed for the tubular ultrafiltration (UF) of synthetic oil-in-water emulsions. The fouling layer resistance, R_f, was 63% of the total membrane resistance, R_m′; however, concentration polarization was the predominant factor controlling resistance in the tubular UF system. An explicit form of the resistance index, Φ, was postulated based on the observed interactions between Φ, cross-flow velocity and feed kinematic viscosity and the RIS model was modified to further describe the interactions between permeate flux and operational parameters. The modified model adequately predicted flux–pressure data over the range of experimental variables examined in this study. Additionally, a set point operating pressure was determined as a function of cross-flow velocity and feed viscosity to achieve a balance between polarization and total membrane resistance.     

Adsorption of bovine serum albumin on fused silica: Elucidation of protein–protein interactions by single-molecule fluorescence microscopy

The adsorption of bovine serum albumin (BSA) on fused silica at pH 4.7 was studied at the single molecules level by total-internal-reflection fluorescence microscopy. This pH value was the isoelectric point of BSA. At low [BSA] of 20 pM, protein molecules adsorbed as monomers. At intermediate [BSA] of 500 pM, protein molecules adsorbed as clusters of about five monomers on average. Both monomers and clusters had adsorption rate coefficients of the order 10⁻⁷ m s⁻¹ and desorption rate coefficients of about 2 × 10⁻² s⁻¹. The respective steady-state coverage was about 10× higher than that at neutral pH, presumably because of the more favorable BSA–silica electrostatics. At pH 4.7 and with [BSA] higher than 100 nM, adsorption begot further adsorption to produce nonlinear isotherms. The coverage at 1 μM BSA was 2.5× that of the linearly extrapolated coverage. This suggests that at pH 4.7, solute–adsorbate affinity was the dominant factor that explains the enhanced adsorption observed in ensemble measurements.     

Effective arsenic removal using polyacrylonitrile-based ultrafiltration (UF) membrane

Applicability of polyacrylonitrile (PAN)-based negatively charged ultrafiltration (UF) membrane for effective arsenic removal has been demonstrated, to our knowledge, for the first time. The hydrolysis of PAN-based UF membrane surface by NaOH leading to the formation of carboxylate (COO⁻) groups and reduction in initial pore size rendered As-V rejection capability by Donnan exclusion principle. A lowering in pore size was indicated by the reduction in water flux and elevation in rejection of protein and polyethylene glycol (PEG). NaOH treatment leading to formation of carboxylate group on the membrane surface was indicated by FTIR-ATR, while contact angle measurement indicated increased hydrophilicity. This treatment rendered membrane surface smoothening as confirmed by SEM and AFM analyses. The molecular weight cut off after the NaOH treatment was found to be 6 kDa. The rejection of pentavalent arsenic (As-V) by these surface modified membranes was studied with different feed concentration, cross-flow velocity, pressure, temperature and pH. Experiments with 50 ppb As-V in feed showed that arsenic rejection was close to 100% and remained constant up to 6 h. Feed sample concentration of 1000 ppb and 50 ppm of As-V showed >95% rejection at pH 7 and room temperature, but for 1000 ppm feed concentration, the rejection was 40–65%. For concentrations ≤50 ppm of arsenic in the feed, the rejection coefficient was not dependent on cross-flow velocity or transmembrane pressure. The rejection for 1000 ppm concentration of As-V varied from 40 to 65% with variation in the cross-flow velocity and transmembrane pressure as the concentration polarization was important.     

Modeling of the permeate flux decline during MF and UF cross-flow filtration of soy sauce lees

Cross-flow ultrafiltration and microfiltration have been used to recover refined soy sauce from soy sauce lees for over 25 years. The precise mechanism which dominated the permeate flux during batch cross-flow filtration has not been clarified. In the present study, we proposed a modified analytical method incorporated with the concept of deadend filtration to determine the initial flux of cross-flow filtration and carried out the permeate recycle and batch cross-flow filtration experiments using soy sauce lees. We used UF and MF flat membrane (0.006 m² polysulfone) module under different transmembrane pressures (TMP) and cross-flow velocities. The modified analysis provided an accurate prediction of permeate flux during the filtration of soy sauce lees, because this model can consider the change in J₀ at initial stage of filtration which was caused by the pore constriction and plugging inside membrane, and these changes may not proceed when the cake was formed on the membrane surface. Mean specific resistance of the cake increased with TMP due to the compaction of the cake and decreased with cross-flow velocity due to the change of deposited particle size, but less depended on the membrane in the present study. These results indicate that the value of J₀ determined by modified method was relevant to exclude the effects of the initial membrane fouling by pore constriction due to protein adsorption and plugging with small particles. The modified analytical method for the cake filtration developed in the present study was considered to be capable of selecting an appropriate operating conditions for many cross-flow filtration systems with UF, MF membranes.     

Unsteady state flux response: a method to determine the nature of the solute and gel layer in membrane filtration

The unsteady-state permeate flux response to a step change in transmembrane pressure is shown to result in unique flux–pressure profiles for the three types of solutes common in membrane ultrafiltration (UF): (a) solutes which exert an osmotic pressure but do not form a ‘gel’; (b) solutes which do not exert an osmotic pressure but form a ‘gel’ and (c) solutes which exert an osmotic pressure and also form a ‘gel’. It is also shown that for stirred cell UF, changes in the bulk feed solution properties (concentration, volume) are negligible on the time scale needed to attain a stable permeate flux. Unsteady-state permeate flux measurements could therefore be made at short filtration times so that the results would not be masked by changes in bulk properties.     

Pulse radiolysis study of ion-species effects on the solvated electron in alkylammonium ionic liquids

The spectra and kinetic behavior of solvated electrons (e_sol⁻) in alkyl ammonium ionic liquids (ILs), i.e. N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)imide (DEMMA-TFSI), N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium tetrafluoroborate (DEMMA-BF₄), N,N,N-trimethyl-N-propylammonium bis(trifluoromethanesulfonyl)imide (TMPA-TFSI), N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide (PP13-TFSI), N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide (P13-TFSI), and N-methyl-N-butylpyrrolidinium bis(trifluoromethanesulfonyl)imide (P14-TFSI) were investigated by the pulse radiolysis method. The e_sol⁻ in each of the ammonium ILs has an absorption peak at 1100 nm, with molar absorption coefficients of 1.5–2.3×10⁴ dm³ mol⁻¹ cm⁻¹. The e_sol⁻ decayed by first order with a rate constant of 1.4–6.4×10⁶ s⁻¹. The reaction rate constant of the solvated electron with pyrene (Py) was 1.5–3.5×10⁸ dm³ mol⁻¹ s⁻¹ in the various ILs. These values were about one order of magnitude higher than the diffusion-controlled limits calculated from measured viscosities. The radiolytic yields (G-value) of the e_sol⁻ were 0.8–1.7×10⁻⁷ mol J⁻¹. The formation rate constant of e_sol⁻ in DEMMA-TFSI was 3.9×10¹⁰ s⁻¹. The dry electron (e_dry⁻) in DEMMA-TFSI reacts with Py with a rate constant of 7.9×10¹¹ dm³ mol⁻¹ s⁻¹, three orders of magnitude higher than that of the e_sol⁻ reactions. The G-value of the e_sol⁻ in the picosecond time region is 1.2×10⁻⁷ mol J⁻¹. The capture of e_dry⁻ by scavengers was found to be very fast in ILs.     

Reactions of CO2 with H, H2 and deuterated analogues

Combining a temperature variable 22-pole ion trap with a cold effusive beam of neutrals, rate coefficients k(T) have been measured for reactions of CO₂⁺ ions with H, H₂ and deuterated analogues. The neutral beam which is cooled in an accommodator to T_ACC, penetrates the trapped ion cloud with a well-characterized velocity distribution. The temperature of the ions, T_22PT, has been set to values between 15 and 300 K. Thermalization is accelerated by using helium buffer gas. For reference, some experiments have been performed with thermal target gas. For this purpose hydrogen is leaked directly into the box surrounding the trap. While collisions of CO₂⁺ with H₂ lead exclusively to the protonated product HCO₂⁺, collisions with H atoms form mainly HCO⁺. The electron transfer channel H⁺ + CO₂ could not be detected (<20%). Equivalent studies have been performed for deuterium. The rate coefficients for reactions with atoms are rather small. Within our relative errors of less than 15%, they do not depend on the temperature of the CO₂⁺ ions nor on the velocity of the atoms (k(T) lays between 4.5 and 4.7 × 10⁻¹⁰ cm³ s⁻¹ with H as target, and 2.2 × 10⁻¹⁰ cm³ s⁻¹ with D). For collisions with molecules, the reactivity increases significantly with falling temperature, reaching the Langevin values at 15 K. These results are reported as k = α (T/300 K)^β with α = 9.5 × 10⁻¹⁰ cm³ s⁻¹ and β = −0.15 for H₂ and α = 4.9 × 10⁻¹⁰ cm³ s⁻¹ and β = −0.30 for D₂.     

Ultrafiltration of soybean oil/hexane extract by porous ceramic membranes

This study investigated the ultrafiltration of soybean oil/hexane extract (miscella) using porous ceramic membrane. The evaporation energy can be saved in the soybean oil production by pre-separating a portion of hexane through the ceramic membrane. Raw soybean oil/hexane extract with 33 wt% of oil was used without pretreatment. A cross-flow ultrafiltration was performed using an anodisc membrane with a pore diameter of 0.02 μm and thickness of ∼1 μm. The concentrations of oil/hexane mixture were measured by UV adsorption at a wavelength of 458 nm. The separation mechanism was suggested to be the hindrance diffusion of soybean oil. Agitation in the feed side significantly increased the rejection of soybean oil. A small stage cut could also yield a higher rejection. Above observations were attributed to the reduction of concentration polarization by increasing the shear rate and small permeate flux, respectively. The optimum separation was achieved under the conditions of 4 kg/cm² transmembrane pressure, 0.04 stage cut and 120 rpm agitation speed. The concentration of soybean oil decreased from 33 wt% of feed to 27 wt% in permeate, that is, near 20% rejection. A gel-layer polarization model was proposed to estimate the gel concentration and thickness. The gel concentration was found 43–53 wt%. Agitating feed side reduced gel thickness, thus enhanced the rejection and permeate flux.     

Palladized aluminum electrode covered by Prussian blue film as an effective transducer for electrocatalytic oxidation and hydrodynamic amperometry of N-acetyl-cysteine and glutathione

The mediated oxidation of N-acetyl cysteine (NAC) and glutathione (GL) at the palladized aluminum electrode modified by Prussian blue film (PB/Pd–Al) is described. The catalytic activity of PB/Pd–Al was explored in terms of Fe^III[Fe^III(CN)₆]/Fe^III[Fe^II(CN)₆]¹⁻ system by taking advantage of the metallic palladium layer inserted between PB film and Al, as an electron-transfer bridge. The best mediated oxidation of NAC and GL on the PB/Pd–Al electrode was achieved in 0.5 M KNO₃ + 0.2 M potassium acetate of pH 2. The mechanism and kinetics of the catalytic oxidation reactions of the both compounds were monitored by cyclic voltammetry and chronoamperometry. The charge transfer-rate limiting step as well as overall oxidation reaction of NAC or GL is found to be a one-electron abstraction. The values of transfer coefficients α, catalytic rate constant k and diffusion coefficient D are 0.5, 3.2 × 10² M⁻¹ s⁻¹ and 2.45 × 10⁻⁵ cm² s⁻¹ for NAC and 0.5, 2.1 × 10² M⁻¹ s⁻¹ and 3.7 × 10⁻⁵ cm² s⁻¹ for GL, respectively. The modifying layers on the Pd–Al substrate have reproducible behavior and a high level of stability in the electrolyte solutions. The modified electrode is exploited for hydrodynamic amperometry of NAC and GL. The amperometric calibration graph is linear in concentration ranges 2 × 10⁻⁶–40 × 10⁻⁶ for NAC and 5 × 10⁻⁷–18 × 10⁻⁶ M for GL and the detection limits are 5.4 × 10⁻⁷ and 4.6 × 10⁻⁷ M, respectively.     

Resistance-in-series analysis in cross-flow ultrafiltration of fermentation broths of Bacillus subtilis culture

Quantitative analysis of various resistances that lead to flux decline during cross-flow ultrafiltration (UF) of the fermentation broth of Bacillus subtilis ATCC (American Type Culture Collection) 21332 culture was studied. Polyethersulfone membrane with a molecular weight cut-off (MWCO) of 100 kDa was used. Prior to cross-flow UF, the broth was treated by acid precipitation (pH 4.0) and centrifugation, and the precipitate was re-dissolved in NaOH solution. Experiments were performed at a feed pH of 7.0, a feed surfactin concentration of 1.48 g L⁻¹, and a cross-flow velocity of 0.32 m s⁻¹ but at different transmembrane pressures (ΔP, 20–100 kPa). The resistance-in-series model was used to analyze the flux behavior, which involves the resistances of membrane itself and cake as well as those due to adsorption and solute concentration polarization. It was shown that the resistance due to solute concentration polarization and of membrane dominated under the conditions examined. The resistances due to cake formation and solute adsorption were comparable, and their sum contributed below 20% of the overall resistance.     

A non-equilibrium thermodynamics approach to model mass and heat transport for water pervaporation through a zeolite membrane

Pervaporation through zeolite membranes involves local heat effects and combined heat and mass transport. The current state-of-the-art Maxwell–Stefan (M–S) models do not take these effects into account. In this study, transport equations for the coupled heat and mass transport through a zeolite membrane are derived from the framework of non-equilibrium thermodynamics (NET). Moreover, the assumption of equilibrium between the adjacent bulk phases at the feed and permeate sides of the zeolite layer is abandoned in favor of local equilibrium. The equations have been used to model pervaporation of water through a 2 m thick NaA type zeolite membrane, deposited on an asymmetric -alumina support, at a feed temperature of 348 K. Assuming a flux of 10 kg m⁻² h⁻¹(0.15 mol m⁻² s⁻¹), the transport through the zeolite layer, as well as the liquid feed side boundary layer and the support layers is modeled. The activity, fugacity, and temperature profiles are calculated with and without taking coupling effects and surfaces into account. The profiles show distinct differences between the two cases. Including the surface effects leads to discontinuities in the activity and temperature at the membrane interfaces. A significantly higher temperature drop of 1.3 K is calculated across the zeolite, compared to 0.4 K when surface and coupling effects are not accounted for. The calculated decrease in temperature over the zeolite layer is dominated by the surfaces. This could indicate that temperature polarization is, to a large extent, a surface effect. The heat flux induces an extra driving force for mass transport, reducing the activity difference over the membrane. A positive jump in activity is observed at the interfaces, revealing the mass transport across the interfaces is governed by the coupling with the heat flux. The support layers contribute significantly to the total mass transport resistance.     

Improved enzyme retention from an electropolymerized polypyrrole-alginate matrix in the development of biosensors

A novel pyrrole-alginate was synthesized providing a gel by Ca²⁺ cross-linking. The subsequent in situ electrochemical polymerization of the linked pyrrole groups at 0.93 V led to the formation of a composite polypyrrole–gel matrix exhibiting a greater enzyme retention as well as increased alginate stability towards the destructive effect of phosphate anions unlike the natural alginate gel. The presence of the electropolymerized chains was clearly indicated by the decrease of the permeability when compared to natural alginate gel, namely 2.7 × 10⁻² and 3.65 × 10⁻¹ cm s⁻¹ respectively, using as an electroactive permeate, hydroquinone. Moreover, the analytical performance of glucose oxidase-based composite alginate for the determination of glucose was determined.     

Flux enhancement of stagnant sandwich compared to supported liquid membrane systems in the removal of Gemfibrozil from waters

Removal of the drug Gemfibrozil (GEM), as a target molecule, from aqueous media by using a carrier mediated transport in supported liquid membrane (SLM) and Stagnant Sandwich LM (SSwLM) systems has been investigated. Optimal chemical conditions to use in the transport tests were determined by means of solubility and liquid–liquid extraction tests. The results showed that the best LM phase to realize stable LM systems was tributylphosphate (TBP) 30% (v/v) in n-decane. Transport tests by using the “traditional” SLM system showed an average flux J_AV(0–CTT) of 0.421 mmol h⁻¹ m⁻² and a system stability of 1410 min. Three different microfiltration membranes, GH-Polypro, FP-Vericel and Supor 200, made of polypropylene, polyvinylidene fluoride and polyethersulphone polymers, respectively, were used to assemble the SSwLM. Contact angle and adsorption measurements evidenced hydrophilic/lypophilic character of the supports. The best results in terms of average flux (0.873 mmol h⁻¹ m⁻²), permeability coefficient (21.88 L h⁻¹ m⁻²) and stability (7170 min ≈120 h) were obtained by using the SSwLM made with the Supor 200 support. The overall results showed that the SSwLM made with this type of support achieves both high flux and high stability compared to the SLM. Thus SSwLMs seems very interesting to employ transport in LM for removing molecular species (e.g. drugs) from aqueous solutions.     

Free radical scavenging and radioprotective effects of carnosine and anserine

Two histidine-containing natural dipeptides, carnosine and anserine (β-alanyl-1-methyl-l-histidine), have been examined for their antioxidant and radioprotective abilities. Pulse radiolysis studies indicated the antioxidative properties of carnosine and anserine aqueous solutions at different pH. The rate constants for the reaction OH radical with carnosine at neutral pH were determined to be 5.3×10⁹ M⁻¹ s⁻¹ at 300 nm, and 4.1×10⁹ M⁻¹ s⁻¹ at 400 nm, respectively. Carnosine and anserine also protected plasmid pUC18 DNA from X-ray radiation-induced strand breaks as evidenced from the studies by agarose gel electrophoresis. Carnosine showed higher protective efficiency under the experimental conditions. Our data demonstrated that carnosine and anserine may play an important role in the maintenance of the antioxidant system.     

Concentration and purification of malate ion from a beverage industry waste water using electrodialysis with homopolar membranes

Recovery of l-malate from a by-product of fermentation industry was investigated in order to decrease its polluting content as well as to ensure a better valorisation. Environmental-friendly process could consist in a first homopolar electrodialysis step to purify and concentrate malate and a subsequent electrodialysis step involving bipolar membranes in order to recover malic acid. The feasibility of the first step was assessed. Purification experiments on a two-compartment pilot device operated batch-wise with different homopolar membranes showed that the non-ionised impurities (sugars, alcohols) were retained better than 96% in the diluate stream and that the colour of the purified stream changed from dark brown to very pale yellow. More than 84% malate could be recovered with current efficiencies better than 90%. An additional purification effect was observed with the retention of a part of citrate, calcium and magnesium ions. The best flux conditions observed (316 g h⁻¹ m⁻²) corresponded to the Neosepta CMX-Sb/AXE 01 arrangement. Moreover, membrane arrangement using a monoselective cation-exchange membrane (Neosepta CMX-S) proved very efficient in removing divalent cations from the purified stream. Concentration was undertaken through a series of ten consecutive batches, allowing a malate concentration of 130 g L⁻¹ (i.e. approximately 2 equiv. L⁻¹) to be reached which complies with criteria for a subsequent bipolar electrodialysis step. According to these results, it was shown that a malate concentration as high as 300 g L⁻¹ could be expected in industrial operating conditions.