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Key operating variables to predict the necessary scour air flowrate in full-scale Membrane Bioreactor (MBR) systems are identified, aiming to optimize energy consumption while avoiding the limiting condition (i.e., rapid increasing total resistance). The resulting metric, referred to here as the K value, was derived by balancing hydrodynamic conditions between the particle deposit rate imposed by permeate flux normalized by fouling condition and its removal by shear stress induced from air scouring. The metric includes air scouring flow, permeate flow, Mixed Liquor Suspended Solids (MLSS) concentration, Mixed Liquor (ML) viscosity, membrane packing density, and total resistance. Long-term (year-long) data from two full-scale MBR plants were analyzed. The value of K corresponding to limiting operational operation and referred to as the limiting K value, KLim, is estimated by detecting the occurrence of threshold limiting flux from the data stream and calculating the resulting value for K. Then, using KLim, the minimum required specific air demand per permeate (SADp,Crit) is calculated, indicating a potential reduction of over half the air scouring energy in typical operational conditions. The results from this data driven analysis suggest the feasibility of employing KLim to predict the adequate scour air flowrate in terms of dynamically varying operational conditions. This approach will lead to the development of energy-efficient algorithms, significantly reducing scour air energy consumption in the full-scale MBR system.

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The low-fouling propensity of commercially available polyethersulfone (PES) membranes was studied after modification of the membrane surface via coating with polymerizable bicontinuous microemulsion (PBM) materials. The PBM coating was polymerized within 1 min using ultraviolet (UV) light exposure. It was detected on the PES membrane surface via attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy. The PBM coating led to an average 10% increase in the hydrophilicity of the PES membrane surface and an increase in total organic content (TOC) removal by more than 15%. Flux-step tests were conducted with model foulant comprising 100 mg L-1 humic acid (HA) solution to detect the onset of critical fouling, characterized by a rapid and substantial increase in TMP, and to compare the fouling propensity of commercially available PES membranes with PBM-coated membranes. The critical flux was found to be about 40% higher for PBM spray-coated membrane and 20% lower for PBM casting-coated membrane than the commercial PES membrane. This demonstrates the performance advantages of the thin PBM layer spray-coated on PES membrane compared to the thick casting-coated PBM layer. The study showcases the potential of PBM spray-coated membranes over commercial PES membranes for use in membrane bioreactors (MBR) for wastewater treatment systems with reduced maintenance over longer operation periods.

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In pursuit of sustainability, we explored replacing conventional dissolved air floatation (DAF) in poultry processing wastewater (PPW) treatment with a precisely tuned 0.02 µm stainless-steel ultrafiltration (SSUF) membrane. SSUF is a robust, homogenously porous membrane with strong chemical resistance, ease of cleaning, and exceptional resistance to organic fouling. Unlike polymeric membranes, it can be regenerated multiple times, making it a cost-effective choice due to its compatibility with harsh chemical cleaning. The PPW used for the study was untreated wastewater from all processing units and post-initial screening. Our study revealed the SSUF membrane's exceptional efficiency at eliminating contaminants. It achieved an impressive removal rate of up to 99.9% for total suspended solids (TSS), oil, grease, E. coli, and coliform. Additionally, it displayed a notable reduction in chemical oxygen demand (COD), biochemical oxygen demand (BOD), and total Kjeldahl nitrogen (TKN), up to 90%, 76%, and 76%, respectively. Our investigation further emphasized the SSUF membrane's ability in pathogen removal, affirming its capacity to effectively eradicate up to 99.99% of E. coli and coliform. The measured critical flux of the membrane was 48 Lm-2h-1 at 38 kPa pressure and 1.90 m/s cross-flow velocity. In summary, our study highlights the considerable potential of the SSUF membrane. Its robust performance treating PPW offers a promising avenue for reducing its environmental impact and advocating for sustainable wastewater management practices.

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A long-term membrane resistance model (LMR) was established to determine the sustainable critical flux, which developed and simulated polymer film fouling successfully in a lab-scale membrane bioreactor (MBR) in this study. The total polymer film fouling resistance in the model was decomposed into the individual components of pore fouling resistance, sludge cake accumulation and cake layer compression resistance. The model effectively simulated the fouling phenomenon in the MBR at different fluxes. Considering the influence of temperature, the model was calibrated by temperature coefficient τ, and a good result was achieved to simulate the polymer film fouling at 25 and 15 °C. The relationship between flux and operation time was simulated and discussed through the model. The results indicated that there was an exponential correlation between flux and operation time, and the exponential curve could be divided into two parts. By fitting the two parts to two straight lines, respectively, the intersection of the two straight lines was regarded as the sustainable critical flux value. The sustainable critical flux obtained in this study was just 67% of the critical flux. The model in this study was proven to be in good agreement with the measurements under different fluxes and different temperatures. In addition, the sustainable critical flux was first proposed and calculated in this study, and it was shown that the model could be used to predict the sustainable operation time and sustainable critical flux, which provide more practical information for designing MBRs. This study is applicable to polymer films used in a wide variety of applications, and it is helpful for maintaining the long-term stable operation of polymer film modules and improving the efficiency of polymer film modules.

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In membrane technology for water/wastewater treatment, the concepts of critical flux (JC) and limiting flux (JL) suggest the existence of a threshold flux below which no fouling occurs. However, their important roles on stable flux duration have not been sufficiently understood. This work adopts a collision-attachment approach to clarify the relationship of JC, JL to metastable (i.e., short-term stable) and long-term stable fluxes based on their dependence on initial flux (J0), foulant-clean-membrane energy barrier (Ef-m), and foulant-fouled-membrane energy barrier (Ef-f). When J0 is below JL, water flux remains stable over a long time even for the case of J0 over JC, thanks to the strongly repulsive Ef-f. At J0 > JL and J0 > JC, the water flux is unstable at the beginning of filtration, and the flux ultimately decreases to JL as the long-term stable flux. Under the condition of JL < J0 ≤ JC, an initial metastable flux appears owing to the high Ef-m, with longer metastable period observed at lower J0 and for more hydrophilic/charged membrane or colloids. Nevertheless, rapid flux decline occurs subsequently due to the energy barrier shifting to weak Ef-f, and the water flux eventually degenerates to JL in long-term fouling duration. Our results provide significant guidelines for fouling control strategies with respect to membrane design, feedwater pretreatment, and operational optimization.

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Recent approval of several viral-vector-based therapeutics has led to renewed interest in the development of more efficient bioprocessing strategies for gene therapy products. Single-Pass Tangential Flow Filtration (SPTFF) can potentially provide inline concentration and final formulation of viral vectors with enhanced product quality due. In this study, SPTFF performance was evaluated using a suspension of 100 nm nanoparticles that mimics a typical lentivirus system. Data were obtained with flat-sheet cassettes having 300 kDa nominal molecular weight cutoff, either in full recirculation or single-pass mode. Flux-stepping experiments identified two critical fluxes, one based on boundary-layer particle accumulation (Jbl) and one based on membrane fouling (Jfoul). The critical fluxes were well-described using a modified concentration polarization model that captures the observed dependence on feed flow rate and feed concentration. Long-duration filtration experiments were conducted under stable SPTFF conditions, with the results suggesting that sustainable performance could potentially be achieved for as much as 6 weeks of continuous operation. These results provide important insights into the potential application of SPTFF for the concentration of viral vectors in the downstream processing of gene therapy agents.

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In this study, the behavior of permeate flux decline due to scale precipitation of calcium sulfate on reverse osmosis membranes was investigated. The proposed scaling-based flux model is able to explain that permeate fluxes attributed to three mechanisms of scale precipitation-cake formation, surface blockage, and mixed crystallization-converge to the same newly defined scaling-based critical flux. In addition, a scaling index is defined, which determines whether scale precipitates on the membrane. The experimental results were analyzed based on this index. The mass-transfer coefficients of flat membrane cells used in the experiments were measured and, although the coefficients differed, they could be summarized in the same form as the Leveque equation. Considering the results of the scale precipitation experiments, where the operating conditions of pressure, solute concentration, temperature, and Reynolds number were varied, the convergent values of the permeate fluxes are explained by the scaling-based critical fluxes and the scale precipitation zones by the scaling indexes.

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Membrane fouling remains one of the most critical drawbacks in membrane filtration processes. Although the effect of various operating parameters-such as flow velocity, concentration, and foulant size-are well-studied, the impact of particle shape is not well understood. To bridge this gap, this study investigated the effect of polystyrene particle sphericity (sphere, peanut and pear) on external membrane fouling, along with the effect of particle charge (unmodified, carboxylated, and aminated). The results indicate that the non-spherical particles produce higher critical fluxes than the spherical particles (i.e., respectively 24% and 13% higher for peanut and pear), which is caused by the looser packing in the cake due to the varied particle orientations. Although higher crossflow velocities diminished the differences in the critical flux values among the particles of different surface charges, the differences among the particle shapes remained distinct. In dead-end filtration, non-spherical particles also produced lower flux declines. The shear-induced diffusion model predicts all five particle types well. The Derjaguin-Landau-Verwey-Overbeek (DLVO) and extended DLVO (XDLVO) models were used to quantify the interaction energies, and the latter agreed with the relative critical flux trends of all of the PS particles. As for the flux decline trends, both the DLVO and XDLVO results are in good agreement.

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Fouling tends to cause a significant increase in hydraulic resistance, decreased permeate flux, or increased transmembrane pressure (TMP) when a process is operated under constant TMP or constant flux conditions. To control membrane fouling and maintain sustainable operation, the concept of critical flux has been discussed by several researchers. Various fouling mechanisms, such as macromolecule adsorption, pore plugging, or cake build-up, as well as hydrodynamic conditions, for example aeration, can take place at the membrane surface. This study aimed to investigate the effects of mixed liquor suspended solid (MLSS) concentration and air bubble flow rate (ABFR) on the critical flux and fouling behavior, when treating refinery-produced wastewater. To determine the critical flux values, the experimental flux-steps were the following: (1) the filtration began with a 30 min step duration at a low flux (10 to 20 L/m2h); (2) at the end of this step (after 30 min), the permeate flux was increased, (3) this step was repeated until the TMP did not remain constant at the constant permeate flux, (4) the critical flux was then achieved. A critical flux model with an R2 of 0.9 was, therefore, derived, which indicates that the particle properties were regulated by the suspended solids. The increase of MLSS concentration, from 3 mg/L to 4.5 mg/L, resulted in a decrease of the permeate flux by 18%. Moreover, an increase in ABFR, from 1.2 mL/min to 2.4 mL/min, increased the permeate flux, but this decreased with a greater flow rate of aeration. To assess the stability and reversibility of fouling during critical flux (Jc) determination using a mixed matrix membrane, flux-step methods were utilized. A step height of 14.3 L/m2h and 30 min duration were arbitrarily chosen. The flux increased to 32.5 L/m2h with a slight increase of trans membrane pressure (TMP), while the rate of increase became significant at a higher flux of 143.6 L/m2h, due to fouling. Overall, this study proved that the response of MLSS concentration and aeration affected the membrane performance, based on the critical flux and fouling behavior.

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Membrane fouling occurs when the operating flux exceeds a certain point (i.e., critical flux). Critical flux has therefore been widely adopted to determine the initial operating flux in membrane bioreactor (MBR) processes. The flux steeping method currently used to measure the critical flux is time-consuming and uneconomical. This study was conducted to develop a novel approach for the evaluation of critical flux. Given that particle fouling is dominant during the initial fouling stage, we hypothesized that particle properties may be closely related to critical flux. A critical flux prediction model with an R2 of 0.9 was therefore derived, which indicates that particle properties regulate critical flux. The results imply that most of the fouling potential during the early stages of operation is caused by SS, and that the formation of cakes that comprise large particles is the dominant fouling mechanism. The new method proposed in this study reduced the measurement cost and time to evaluate critical flux by 3.5-and 8 times, respectively, compared to the flux-stepping method. In terms of practical application, the applicability of the model equation was identified by system reliability analysis, which indicates that the system failure increases significantly as the standard deviation of the variables increases. This study demonstrated that the prediction of critical flux and system reliability can be achieved through particle characteristic measurement. A similar approach is expected to be employed in real MBR plants as an economical and convenient fouling control strategy to solve problems involving resource shortages.

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Counting rate is an important factor for CdZnTe photon counting detectors as high-flux devices. Until recently, there has been a lack of knowledge on the relationship between X-ray photocurrent response and the photon counting performance of CdZnTe detectors. In this paper, the performance of linear array 1 × 16-pixel CdZnTe photon counting detectors operated under different applied biases is investigated. The relation between experimental critical flux and applied bias show an approximate quadratic dependence, which agrees well the theoretical prediction. The underlying relationship among X-ray photocurrents, carrier transport properties, and photon counting performance was obtained by analyzing X-ray current-voltage and time current curves. The typical X-ray photocurrent curve can be divided into three regions, which may be explained by the photoconductive gain mechanism and electric field distortion characteristics. To keep CdZnTe photon counting detectors working in a "non-polarized state", the applied bias should be set on the left side of the "valley region" (high bias direction) in the X-ray I-V curves. This provides an effective measurement for determining the proper working bias of CdZnTe detectors and screening photon counting detector crystals.

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Minimising the usage of potable water in industrial and cleaning processes is essential to conserve fresh water. Recycling treated wastewater will help to do so. However, high quality treated wastewater is required for reuse and recycling. This study evaluated the performance of an enhanced membrane bioreactor (eMBR) in treating car wash wastewater for the purpose of reuse. The eMBR consisted of an anaerobic tank, an anoxic tank, an aerobic membrane bioreactor (AMBR) and a UV disinfection unit. The effects of hydraulic retention time of the eMBR on the treated water quality parameters and operating parameters were evaluated. The eMBR produced high quality recyclable water (0.5-10.2 mg/L of COD, 0.18-0.83 NTU of turbidity, 0 org. of E. Coli/100 mL) meeting Class A recycle water standards. Decrease in the mixed liquor suspended solids concentration in the AMBR (from 294 to 117 mg/L) reduced the fouling of the membrane which increased the permeate flux (from 5.9 to 6.7 L/m2h). This is unique to the eMBR system used in this study. However, when the flux exceeded the critical flux, the trans-membrane pressure increased significantly.

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Whey, produced in large quantities during cheese production, is a rapidly fermentable high strength wastewater characterized by a high biodegradability and low alkalinity. In this study, a lab-scale cross-flow anaerobic membrane bioreactor was used to address the commonly experienced difficulties such as unstable reactor performance and unexpected biomass losses when treating whey wastewater with conventional anaerobic reactors. The anaerobic membrane bioreactor provided a stable treatment performance, i.e. more than 90% chemical oxygen demand removal, and moderate membrane fluxes between 8 and 11 L m-2 h-1 could be obtained, applying a low cross-flow velocity of about 0.5 m s-1. Short term critical flux tests revealed that higher fluxes up to 36 L m-2 h-1 are possible at elevated cross-flow velocities and/or reduced mixed liquor suspended solids concentrations. Sludge filterability indicated by capillary suction time and specific resistance to filtration deteriorated throughout the study. Chemical cleaning efficiency gradually decreased, indicating irreversible membrane fouling during long term operation.

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Haemophilus influenzae type b (Hib) is a Gram-negative bacterium, responsible for serious infections, such as pneumonia, sepsis and meningitis, especially in children younger than 5 years old and immunocompromised individuals. The main virulence factor of Hib is its polyribosyl-ribitol-phosphate capsular exopolysaccharide (PRP), which is used in the formulation of Hib vaccines, conjugated to a carrier protein. The production of the conjugated vaccine is complex and costly, particularly the PRP purification process, that comprises several steps of ethanol precipitation and centrifugation, and uses organic solvents and detergents. Targeting a more affordable vaccine that meets the public health systems demands, the Process Development Laboratory of Butantan Institute has been developing a simpler, economical and easily scalable purification process based on tangential flow filtration. In this process, one major bottleneck is the broth clarification step by microfiltration, which results in low PRP recovery and high variability, probably due to membrane fouling. Regarding this matter, a small scale study was previously performed, in order to identify the best operational conditions of relevant process variables, namely transmembrane pressure (TMP), feed flow, and permeate flow. The aim of this work was to scale up the broth clarification step using the defined conditions for these parameters and to perform a critical flux analysis, which is a very useful tool in tangential flow filtration optimization. Although PRP recovery was relatively smaller in the higher scale (68.5% against 97.5%), the permeate flow and TMP profiles were very similar. Analysis of PRP recovery profile indicated that extending dialysis might improve PRP yield. Critical flux of around 35 L.h-1 .m-2 was determined experimentally, which explains the high PRP recovery when a limit permeate flux of 25 L.h-1 .m-2 was employed in the cell clarification step. In conclusion, the process designed for broth clarification is quite promising in terms of product yield and scalability.

Haemophilus influenzae do tipo b (Hib) é uma bactéria Gram-negativa, responsável por infecções graves, como pneumonia, sepse e meningite, especialmente em crianças menores de 5 anos de idade e indivíduos imunocomprometidos. Seu principal fator de virulência é o exopolissacarídeo capsular de poliribosil-ribitol- fosfato (PRP), que é utilizado na formulação de vacinas contra Hib, conjugado a uma proteína carreadora. A produção da vacina conjugada é complexa e onerosa, principalmente o processo de purificação do PRP, caracterizado por várias etapas de precipitação com etanol, centrifugação e uso de solventes orgânicos e detergentes. Buscando uma vacina mais acessível, que possa atender à demanda de sistemas públicos de saúde, o Laboratório de Desenvolvimento de Processos vem desenvolvendo nos últimos anos um método de purificação mais simples, econômico e facilmente escalonável, baseado em filtração tangencial. Nesse processo, um dos gargalos identificados é a etapa de separação celular por microfiltração, que apresentava baixa recuperação de PRP e alta variabilidade dos resultados, provavelmente relacionados ao fenômeno de fouling. Um estudo em pequena escala foi realizado previamente para identificar as melhores condições de variáveis operacionais relevantes - pressão transmembrana (TMP), fluxo de entrada e fluxo do filtrado. O presente trabalho teve como objetivos escalonar o processo, mantendo essas condições, e realizar uma análise do fluxo crítico do filtrado, ferramenta bastante útil para a otimização da filtração tangencial. Embora a recuperação de PRP tenha sido relativamente menor no processo escalonado (68,5% contra 97,5%), os perfis de fluxo do filtrado e de TMP se mostraram muito semelhantes, e as análises da tendência de recuperação mostram que um aumento no número de diavolumes utilizado pode contornar essa questão. O valor de fluxo crítico determinado experimentalmente foi próximo de 35 L.h-1 .m-2 , o que corrobora os bons resultados obtidos com o uso de fluxo limite de 25 L.h-1 .m-2 para o filtrado no processo de separação celular. Os resultados obtidos mostram que o desenho de processo estabelecido para a clarificação de PRP é bastante promissor em termos de rendimento de PRP e escalabilidade do processo.

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Membrane biofouling, due to Soluble Microbial Products (SMP) and Extracellular Polymeric Substances (EPS) deposition, results in reduction of the performance of Membrane Bioreactors (MBRs). However, recently, a new method of biofouling control has been developed, utilizing the interference of the bacterial inter- and intra-species' communication. Bacteria use Quorum Sensing (QS) to regulate the production of SMP and EPS. Therefore, disruption of Quorum Sensing (Quorum Quenching: QQ), by enzymes or microorganisms, may be a simple mean to control membrane biofouling. In the present study, a novel QQ-bacterium, namely Lactobacillus sp. SBR04MA, was isolated from municipal wastewater sludge and its ability to mitigate biofouling was evaluated by monitoring the changes in critical flux and transmembrane pressure, along with the production of EPS and SMP, in a lab-scale MBR system treating synthetic wastewater. Lactobacillus sp. SBR04MA showed great potential for biofouling control, which was evidenced by the ∼3-fold increase in critical flux (8.3 → 24.25 L/m2/h), as well as by reduction of the SMP and EPS production, which was lower during the QQ-period when compared against the control period. Furthermore, the addition of the QQ-strain did not affect the COD removal rate. Results suggested that Lactobacillus sp. SBR04MA represents a novel and promising strain for biofouling mitigation and enhancement of MBRs performance.

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Microfiltration membranes having different blends of graphene-oxide (GO) (0-1 wt%) and Polysulfone (PSf) (15-20 wt%) were prepared using the classical non-solvent induced phase inversion process. The prepared membranes were characterised for their structural morphology, surface properties, mechanical strength, porosity and pure water flux. Based on the initial characterisation results, four membranes (15 wt% PSf, 15 wt% PSf + 0.25 wt% GO, 15 wt% PSf + 1 wt% GO and 20 wt% PSf + 1 wt% GO) were chosen for critical flux study, that was conducted using flux-step method in a lab scale MBR system. In order to study the application potential of GO blended membranes, the critical flux of each membrane was evaluated in two operational modes i.e., continuous and intermittent modes with backwash. The membranes with maximal GO concentration (15 wt% PSf + 1 wt% GO and 20 wt% PSf + 1 wt% GO) showed higher critical flux (16.5, 12.8 L/m2h and 19, 15 L/m2h for continuous and intermittent mode, respectively). It was observed that the operational modes did not have a significant effect on the critical flux of the membranes with low GO concentration (15 wt% PSf and 15 wt% PSf + 0.25 wt% GO), indicating a minimal of 1 wt% GO was required for an observable effect that favoured intermittent mode of operation. Through these results, ideal operating condition was arrived (i.e., flux maintained at 6.4 L/m2h operated under intermittent mode) and the membranes 15 wt% PSf and 15 wt% PSf + 1 wt% GO were studied for their long-term operation. The positive effect of GO on filtration time, cleaning frequency and against fouling was demonstrated through long term TMP profile of the membranes, indicating the suitability of GO blended membrane for real time wastewater treatment.

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Filtration flux is one of the key factors in regulating the performance of membrane bio-reactors (MBRs) for wastewater treatment. In this study, we explore the effectiveness of a mechanical sieve unit for effective flux enhancement through retardation of the fouling effect in a modified MBR system (SiMBR). In brief, the coarse sieve unit having 100 µm and 50 µm permits small size microorganism flocs to adjust the biomass concentration from the suspended basin to the membrane basin. As a result, the reduced biofouling effect due to the lowered biomass concentration from 7800 mg/L to 2400 mg/L, enables higher flux through the membrane. Biomass rejection rate of the sieve is identified to be the crucial design parameter for the flux enhancement through the incorporation of numerical simulations and operating critical-flux measurement in a batch reactor. Then, the sieve unit is prepared for 10 L lab-scale continuous SiMBR based on the correlation between sieve pore size and biomass rejection characteristics. During continuous operation of lab-scale SiMBR, biomass concentration is maintained with a higher biomass concentration in the aerobic basin (7400 mg/L) than that in the membrane basin (2400 mg/L). In addition, the SiMBR operations are conducted using three different commercial hollow fiber membranes to compare the permeability to that of conventional MBR operations. For all cases, the modified MBR having a sieve unit clearly results in enhanced permeability. These results successfully validate that SiMBR can effectively improve flux through direct reduction of biomass concentration.

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There is renewed interest in the possibility of using precipitation for initial capture of high value therapeutic proteins as part of an integrated continuous downstream process. These precipitates can be continuously washed using tangential flow filtration, with long term operation achieved by operating the membrane modules below the critical filtrate flux for fouling. Our hypothesis was that the critical flux for the precipitated protein would be a function of the properties of the precipitate as determined by the precipitation conditions. We evaluated the critical flux using a flux-stepping procedure for model protein precipitates (bovine serum albumin) generated using a combination of a crosslinking agent (zinc chloride) and an excluded volume precipitant (polyethylene glycol [PEG]). The critical flux varied with shear rate to approximately the 1/3 power, consistent with predictions of the classical polarization model. The critical flux increased significantly with increasing zinc chloride concentration, going from 60 L/m2 /h for a 2 mM ZnCl2 solution to 200 L/m2 /h for an 8 mM ZnCl2 solution. In contrast, the critical flux achieved a maximum value at an intermediate PEG concentration. Independent measurements of the effective size and viscosity of the protein precipitates were used to obtain additional understanding of the effects of ZnCl2 and PEG on the precipitation and the critical flux. These results provide important insights into the development of effective tangential flow filtration systems for processing large quantities of precipitated protein as would be required for large scale continuous protein purification by precipitation. © 2017 American Institute of Chemical Engineers Biotechnol. Prog., 33:1561-1567, 2017.

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In the last decades, membrane processes have gained a significant share of the market for wastewater purification. Although the product (i.e., purified water) is not of high added value, these processes are feasible both technically and from an economic point of view, provided the flux is relatively high and that membrane fouling is strongly inhibited. By controlling membrane fouling, the membrane may work for years without service, thus dramatically reducing operating costs and the need for membrane substitution. There is tension between operating at high permeate fluxes, which enhances fouling but reduces capital costs, and operating at lower fluxes which increases capital costs. Operating batch membrane processes leads to increased difficulties, since the feed fed to the membrane changes as a function of the recovery value. This paper is concerned with the operation of such a process. Membrane process designers should therefore avoid membrane fouling by operating membranes away from the permeate flux point where severe fouling is triggered. The design and operation of membrane purification plants is a difficult task, and the precision to properly describe the evolution of the fouling phenomenon as a function of the operating conditions is a key to success. Many reported works have reported on the control of fouling by operating below the boundary flux. On the other hand, only a few works have successfully sought to exploit super-boundary operating conditions; most super-boundary operations are reported to have led to process failures. In this work, both sub- and super-boundary operating conditions for a batch nanofiltration membrane process used for olive mill wastewater treatment were investigated. A model to identify a priori the point of transition from a sub-boundary to a super-boundary operation during a batch operation was developed, and this will provide membrane designers with a helpful tool to carefully avoid process failures.

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Fouling of nanofiltration (NF) membranes is the most significant obstacle to the development of a sustainable and energy-efficient NF process. Colloidal fouling and performance decline in NF processes is complex due to the combination of cake formation and salt concentration polarization effects, which are influenced by the properties of the colloids and the membrane, the operating conditions of the test, and the solution chemistry. Although numerous studies have been conducted to investigate the influence of these parameters on the performance of the NF process, the importance of membrane preconditioning (e.g., compaction and equilibrating with salt water), as well as the determination of key parameters (e.g., critical flux and trans-membrane osmotic pressure) before the fouling experiment have not been reported in detail. The aim of this paper is to present a standard experimental and data analysis protocol for NF colloidal fouling experiments. The developed methodology covers preparation and characterization of water samples and colloidal particles, pre-test membrane compaction and critical flux determination, measurement of experimental data during the fouling test, and the analysis of that data to determine the relative importance of various fouling mechanisms. The standard protocol is illustrated with data from a series of flat sheet, bench-scale experiments.