Determination Of Critical Flux Research Articles

Investigation of fouling in perfusion cell culture processes using alternating tangential flow filtration

Microfiltration membranes are increasingly used to retain the cells inside bioreactors while continuous harvest of the desired biopharmaceutical occurs in perfusion processes. One method of microfiltration is Alternating Tangential Flow (ATF) filtration, which involves moving the cultivation broth tangentially back and forth across the membrane, which results in a backwash effect that reduces fouling. In this study, fouling was investigated with asymmetric polysulfone hollow fibers operated in ATF mode attached to a bioreactor producing a recombinant protein in Chinese Hamster Ovary (CHO) cells. Fouling was assessed through different approaches, including determination of critical flux using an improved flux-step method. Fouling was studied through measurements of Transmembrane Pressure (TMP), protein transmission, membrane pore size and staining of the membrane after operation to visualize the distribution of biological fouling inside the membrane. For critical flux determination, fluxes of up to 69 LMH were used without exceeding the critical flux. No sign of fouling was observed for the short-term (<3.5 h) critical flux experiment. However, during prolonged operation at 8.3 LMH the TMP jumped to 0.9–0.95 bar, indicating fouling. At this state, the protein transmission remains at the same high level (>88 %).

The effects of pristine and silanized nanodiamond on the performance of polysulfone membranes for wastewater treatment by MBR system

In this study, silanized nanodiamond (SND) was synthesized and used as a nano additive to develop the polysulfone (PSf) nanocomposite membrane in order to investigate the effects of the functionalization of nanodiamond (ND) nanoparticles on the structure and performance of PSf membranes for wastewater treatment process. ND was functionalized by silanization method using vinyltrimethoxysilane (VTS) and the results of Fourier transform infrared spectroscopy (FTIR) analysis confirmed the presence of silane groups on the surface of NDs. Afterward, neat and nanocomposite PSf membranes containing 1.0 wt.% ND and 1.0 wt.% SND were prepared via phase inversion method. Prepared membranes were then characterized by field emission scanning electron microscopy (FESEM), energy-dispersive x-ray spectroscopy (EDX), pure water flux (PWF), contact angle and mechanical strength analyses. FESEM images showed that the number of pores on the surface of membranes increased at the presence of ND and SND nanoparticles but the average pore size decreased. PSf/SND nanocomposite membrane had the maximum PWF of 112 Lm−2 h-1 and the lowest contact angle value of 76.44°. However, the presence of ND and SND nanoparticles had no significant effect on mechanical strength of the membranes. Moreover, the EDX analysis showed that the SNDs were dispersed uniformly throughout the PSf matrix. After determination of critical flux, the performance of prepared membranes was investigated in a membrane bioreactor (MBR) system with pharmaceutical wastewater feed for 360 min operation. The obtained results revealed that PSf/SND nanocomposite membrane had the lowest irreversible fouling ratio (IFR) and the highest flux recovery ratio (FRR).

Critical flux-based membrane fouling control of forward osmosis: Behavior, sustainability, and reversibility

Membrane fouling is closely related to the concept of critical flux. Therefore, a fouling control strategy for forward osmosis (FO) membranes that is based on the critical flux is necessary. This study systematically investigated the critical flux behavior of FO membranes (CTA and PA-TFC) in the short-term using a stepping method (draw solution (DS) concentration stepping). In addition, to test the reliability of this method, long-term experiments were conducted to evaluate the influences of operational critical flux on the fouling behavior (sustainable operation and fouling reversibility/irreversibility), thereby determining the critical flux for reversibility. Our results showed that the DS concentration stepping could be applied for critical flux determination in FO. Both membranes exhibited higher critical flux values for alginate fouling compared to other single foulants such as colloidal silica or gypsum. The values were 15.9 LMH for a cellulose triacetate membrane (CTA) and 20.5 LMH for the polyamide thin-film composite (PA-TFC). Whilst these values should be adequate in FO applications they were determined for single foulants. The presence of multispecies of foulants caused a significant decline in the critical flux values. This study found 5.4 LMH for the CTA membrane and 8.3 LMH for the PA-TFC membrane for the combined foulants of alginate + gypsum. This indicates that the critical flux behavior in FO was dependent on the foulant type and membrane type. Importantly, the high restoration of water flux was achieved with the PA-TFC membrane at an operation either close to critical flux (92–98%) or below critical flux (98–100%) (i.e., with negligible irreversible fouling). The critical fluxes for reversibility obtained in this study will aid the efficient operation of practical FO processes.

Preparation, characterisation and critical flux determination of graphene oxide blended polysulfone (PSf) membranes in an MBR system

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.

Colloidal Fouling of Nanofiltration Membranes: Development of a Standard Operating Procedure

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.

Recover energy from domestic wastewater using anaerobic membrane bioreactor: Operating parameters optimization and energy balance analysis

Anaerobic membrane bioreactor (AnMBR) is a promising process to recover energy and water resource from domestic wastewater; however, its energy-efficiency needs substantial improvement for real applications. In this study, based on the methanogenic activity batch tests and critical flux determination, an optimization protocol for AnMBRs is reported and a flux-centered energy balance analysis is conducted. The results demonstrate that organic loading rate by sludge (OLRsludge) should be controlled within 0.43–0.90 kgCOD/(kgVSS·d), and the corresponding sludge retention time (SRT) should be in the range of 50 d to infinity. Energy balance analysis shows that the AnMBR systems at the temperature of 35 °C and 25 °C could achieve net energy recovery. For realizing energy-neutral operation, the corresponding fluxes should range from 8.3 to 9.5 L/(m2·h) at 35 °C and 6.0 to 6.7 L/(m2·h) at 25 °C, respectively. In the process design and operation, a relatively short hydraulic retention time (HRT) and an SRT close to 50 d should be considered in order to achieve an energy-efficient AnMBR performance.

Dynamic filtration – A novel approach for critical flux determination using different textiles

Dynamic filtration which is characterized by high flux rates and low filter resistance is an attractive alternative to conventional microfiltration for membrane bioreactor (MBR) operation. Therefore the cake layer formation has been subject to intensive studies and modeling. In the case of dynamic MBRs (DMBRs) the understanding of the underlying principles of cake formation is of vital importance for further process optimization. The combination of purposely designed experimental set-ups and mathematical models are particularly useful to look into these complex processes. The main disadvantage using the constant pressure method is that stable situation occurs not straight away. Therefore an exact determination of filter resistance or critical flux is not possible. For the first time a modified set-up based on constant flux method to overcome the described problem is described for the first time. Outcomes, comparing the constant pressure and the new approach showed that the experimental design plays an essential role on cake formation in dynamic membrane systems as well as on the results which are obtained. The novel approach allows a better simulation of the critical flux model and is more suitable for the evaluation of the filtration behavior of mesh filters due to far better realistic simulation. Results gained concerning flux and trans-filter pressure behaviors in the large scale experiments could be verified in laboratory using this filtration set-up.This short communication is intended to be a thought-provoking impulse to discuss the mentioned implications and to offer suggestions for experimental approaches that allow improved insight into the cake formation process.

CRITICAL FLUX DETERMINATION IN ULTRAFILTRATION OF WASTEWATER FROM A FOOD INDUSTRY BY OPTIMIZATION METHOD

Two ultrafiltration membranes with different geometries (spiral polymeric and tubular ceramic) but similar cutoffs were used to treat wastewater from a food industry. Hydrodynamic conditions were optimized by statistical methods as a strategy to get more accurate values of the critical parameters and then to produce higher water flux and minimization of membrane fouling. The validation of the optimization method was obtained by experimental critical flux determination at critical parameters. Membrane fluxes revealed significant differences during filtration. The polymeric membrane showed an optimal flux of 45.60 Lh−1 m−2 at 3.21 bar while operating at a stable time of 11.61 h, whereas optimal flux of the ceramic membrane was 32.43 Lh−1 m−2 at 3.98 bar for 16.03 h. Experimental critical flux values were only slightly lower than optimal fluxes for both membranes, showing the validity of the statistics models applied. Negligible osmotic pressure was found on the two membranes at critical flux parameters, indicating irreversible fouling for both cases. The polymeric membrane revealed strong fouling behavior and the ceramic membrane showed a weak form; the flux decline occurred first in the polymeric membrane, whereas the ceramic membrane exhibited high stability during the filtration operations. A high degree of purification of wastewater was obtained by this membrane at critical flux conditions.

Determination of fouling-related critical flux in self-forming dynamic membrane bioreactors: Interference of membrane compressibility

A new measurement protocol was proposed for appropriate determination of the fouling-related critical flux of highly compressible self-forming dynamic membranes (SFDMs) developed on coarse-pore supporting materials. Intermittent relaxation periods and parallel clean water filtration tests were incorporated into the common transmembrane pressure (TMP) step method to minimize the interference of SFDM compressibility on fouling-related critical flux determination and to better understand the filtration characteristic of SFDMs. The average fouling-related critical flux of SFDMs determined by the new measurement protocol turned out to be significantly higher than that obtained with the common measurement protocol. This is attributed partly to the exclusion of the flux reduction associated with SFDM compression by conducting the parallel clean water filtration tests, and partly to the minimization of the effect of “continuous” compression on the structure and fouling propensity of SFDMs by integrating the intermittent relaxation periods. The intermittent relaxation periods also prevented the occurrence of irreversible fouling and steady compression of SFDMs, embodied by the relatively smaller and faster disappeared hysteresis observed in the new measurement protocol. As one way to control SFDM compression, intermittent effluent production mode appears to be a potentially promising strategy for further optimization of SFDMBR operation. Stepwise increment of relaxation time and proper selection of higher initial TMP turned out to be the options recommended for reducing the time consumption of the new measurement protocol.

Various operating conditions affecting the performance of aerobic digestion coupled with membrane filtration

An aerobic digestion coupled with membrane filtration (ADMF) system was investigated to determine the influence of various operating conditions to the efficiency of sludge thickening. Three different parameters were studied: the effect of hydraulic retention time (HRT), aeration and addition of coagulant. The effect of varying HRT was noticeable with the decrease in viscosity with increasing HRT. Transmembrane pressure, which indicates the extent of membrane fouling, increased with the decrease in HRT. Effect of aeration on the rate of increase in mixed liquor suspended solid concentration was observed to be insignificant, indicating that the aeration rate affected the filtration performance rather than the sludge reduction efficiency. Study on the effect of coagulant was evaluated by comparing two kinds of coagulants, organic and inorganic, at different concentrations. Dead-end filtration experiment and determination of critical flux were conducted to investigate the permeate performance according to coagulant addition. Higher value of critical flux was obtained with the addition of inorganic coagulant, while the total resistance was decreased with increasing coagulant concentration. Results show that while the addition of inorganic coagulant enhanced the ADMF system by improving membrane permeability, it is recommended that the system be operated at high HRT and aeration rate.

Avaliação da remoção de matéria orgânica carbonácea e nitrogenada de águas residuárias em biorreator de membranas

O presente trabalho objetivou a avaliação da remoção de matéria orgânica carbonácea e nitrogenada, bem como a determinação do fluxo crítico, em biorreator de membranas, com zona pré-anóxica, tratando águas residuárias industriais da produção de aminoácidos. O reator foi operado sob carga orgânica volumétrica de 1,91 kg.DQO.m-3.d-1 e 0,18 kg.NTK.m-3.d-1; a recirculação do reator aeróbio para o reator anóxico foi de quatro vezes a vazão afluente. O reator apresentou médias de remoção de DQO, NTK e NT de 97, 98 e 92%, respectivamente. O sistema de ultrafiltração foi testado em vários fluxos entre 25 e 37 L.m-2.h-1 e determinou-se o fluxo crítico de 28 L.m-2.h-1 quando operado com 11,4 g.L-1 de SST e 35 dias de tempo de retenção celular. Os resultados mostraram que houve viabilidade técnica no uso de biorreator de membranas para remoção de matéria orgânica de águas residuárias industriais da produção de aminoácidos.

Fouling behavior of microstructured hollow fibers in cross-flow filtrations: Critical flux determination and direct visual observation of particle deposition

Abstract The fouling behavior of microstructured hollow fiber membranes was investigated in cross-flow filtrations of colloidal silica and yeast. In addition to the as-fabricated microstructured fibers, twisted fibers made by twisting the microstructured fibers around their own axes were tested and compared to round fibers. In silica filtrations, the three different fibers showed similar behavior and increasing Reynolds number increased the critical fluxes significantly. In yeast filtrations, the twisted fiber performed similar to the round fiber and better than the structured fiber. Among the three fibers, during yeast filtrations the critical flux for irreversibility was highest for the twisted fiber. The Reynolds number had little effect on the critical fluxes for particle deposition, which was attributed to the strong adsorption of yeast particles on the membrane. On the other hand, the critical fluxes for irreversibility increased with increasing Reynolds number for all three fibers. Direct visual observation of yeast particles on the surface of the three different hollow fibers revealed that for the structured and twisted fibers, the initial deposition rate on the fins is much lower than that in the grooves. This is attributed to the shear-induced migration of the yeast particles from areas of high shear (fins) to those of low shear (grooves). Furthermore, on the fins of the twisted fiber the deposition rate was lower than that on the fins of the structured fiber. This observation, together with the observed high critical fluxes for the twisted fiber led to the conclusion that the twisting induces a secondary flow in the liquid. This secondary flow is effective in depolarizing the buildup of micron-sized yeast particles since the diffusion of these particles is strongly effected by gradients in shear rate. On the other hand, for the silica colloids which are much smaller, shear-induced diffusion is not significant and twisting does not have an improving effect on filtration.

Fouling Behavior of Microstructured Hollow Fiber Membranes in Dead-End Filtrations: Critical Flux Determination and NMR Imaging of Particle Deposition

The fouling behavior of microstructured hollow fibers was investigated in constant flux filtrations of colloidal silica and sodium alginate. It was observed that the fouling resistance increases faster with structured fibers than with round fibers. Reversibility of structured fibers' fouling was similar during silica filtrations and better in sodium alginate filtrations when compared with round fibers. The deposition of two different silica sols on the membranes was observed by NMR imaging. The sols had different particle size and solution ionic strength and showed different deposition behaviors. For the smaller particle-sized sol in deionized solution (Ludox-TMA), there was more deposition within the grooves of the structured fibers and much less on the fins. For the alkali-stabilized sol Bindzil 9950, which had larger particles, the deposition was homogeneous across the surface of the structured fiber, and the thickness of the deposit was similar to that on the round fiber. This difference between the deposition behavior of the two sols is explained by differences in the back diffusion, which creates concentration polarization layers with different resistances. The Ludox sol formed a thick polarization layer with very low resistance. The Bindzil sol formed a slightly thinner polarization layer; however, its resistance was much higher, of similar magnitude as the intrinsic membrane resistance. This high resistance of the polarization layer during the Bindzil sol filtration is considered to lead to quick flow regulation toward equalizing the resistance along the fiber surface. The Ludox particles were trapped at the bottom of the grooves as a result of reduced back diffusion. The fouling behavior in sodium alginate filtrations was explained by considering the size-dependent deposition within the broad alginate size distribution. The better reversibility of fouling in the structured fibers is thought to be the result of a looser deposit within the grooves, which is more easily removed than a compressed deposit on the round fibers.

Assessments of critical flux in a pilot-scale membrane bioreactor

In this study, the influence of various parameters such as determination methods and step height and length of incremental flux on critical flux values were investigated. Experiments were carried out on a pilot-scale membrane bioreactor (MBR) treated municipal wastewater. Three of the five critical flux determination methods, such as flux linearity, 90% permeability and flux cycling conducted in this study, indicated a decline in critical flux values as the step height of incremental flux increased. However, the hysteresis method and the two-third (2/3) flux limitation method showed an increase and independence of critical flux to the step height of incremental flux, respectively. On the other hand, the step length of incremental flux had no obvious effect on critical flux values evaluated by all critical flux determination methods. Like critical flux, sustainable flux has negative relationship with the increase of step height but no influence of step length was found in this study.

Determination of critical flux by mass balance technique combined with direct observation image analysis

An improved method combining mass balance and image analysis was developed in this study to determine the critical flux during crossflow microfiltration. The proposed method gave a better representation of the physical deposition processes on the membrane surface, and the results obtained were more accurate than the previous methods based on the observation technique alone. In addition, by studying the fractional deposition of the membrane surface when the operating flux is above the critical flux, it was found that not all the particles convected toward the membrane would eventually deposit onto the membrane surface when the operating flux was above the critical flux. The fractional deposition also varied under different operating conditions of the crossflow microfiltration.

Enhancing filterability of flat-sheet membrane by addition of cationic polymer for sludge thickening system

The efficiency of coagulation-coupled membrane filtration as an alternative process for sludge thickening was evaluated using various parameters that could influence the thickened sludge, such as mixed liquor suspended solids (MLSS) concentration, viscosity, critical flux and soluble microbial products. Dead-end filtration experiment and determination of critical flux were conducted to investigate the change in permeation across the membrane after addition of coagulant. PVDF flat-sheet membrane with 0.08 mm pore size was used in the experiment. Two separate submerged systems were run in parallel, one with coagulant and the other served as the control. A significant difference between the transmembrane pressures (TMPs) of the two systems was observed. It was found that an increase in soluble microbial products (SMPs) concentration decreases filtration flux, which in turn increases TMP. PSD analysis confirmed that larger flocs were formed after addition of coagulant. Confocal laser scanning microscopy (CLSM...

Critical flux determination by flux‐stepping

AbstractIn membrane filtration related scientific literature, often step‐by‐step determined critical fluxes are reported. Using a dynamic microfiltration device, it is shown that critical fluxes determined from two different flux‐stepping methods are dependent upon operational parameters such as step length, step height, and flux start level. Filtrating 8 kg/m3 yeast cell suspensions by a vibrating 0.45 × 10−6 m pore size microfiltration hollow fiber module, critical fluxes from 5.6 × 10−6 to 1.2 × 10−5 m/s have been measured using various step lengths from 300 to 1200 seconds. Thus, such values are more or less useless in itself as critical flux predictors, and constant flux verification experiments have to be conducted to check if the determined critical fluxes can predict sustainable flux regimes. However, it is shown that using the step‐by‐step predicted critical fluxes as start guesses, in our case, in constant flux verification experiments for 5 and 1/2 hours, a sustainable flux was identifiable. © 2009 American Institute of Chemical Engineers AIChE J, 2010

Characterisation of polysaccharide fouling of an ultrafiltration membrane using model solutions

Polysaccharides (PSs) have been identified to cause long term mechanically irreversible fouling in membrane bioreactors, although findings are still contradictory, partly due to the fact that generally lump concentrations are measured. To elucidate contributing factors to the varying fouling propensity of PSs, the influence of the characteristics of the PSs on membrane fouling was studied by comparing the filterability of different solutions of model PSs using the Berlin filtration method (BFM), which uses a novel air-sparged in situ filtration test cell. Critical fluxmeasurements were performed with solutions of eight different substances: xanthan gum, alginate, carrageenan, starch, carboxymethylcellulose (CMC), pectin, agarose and heparin, in which pH and conductivity were adjusted at 7.5 and 800 μS/cm, respectively. The protocol used for the critical flux determination included relaxation and, in some cases, irreversible fouling could be detected. No relationship was found between critical flux charact...

Implications of critical flux and cake enhanced osmotic pressure (CEOP) on colloidal fouling in reverse osmosis: Experimental observations

This paper describes the determination of critical flux and the cake enhanced osmotic pressure (CEOP) effect of colloidal silica fouling in reverse osmosis (RO) using a sodium chloride tracer response technique. The tracer technique gave the transient values of concentration polarization, CP, and cake resistance, R f. From these values, the rate of accumulation of deposit, d m f/d t, could be readily determined. The critical flux, J crit, was defined as the flux at which the rate of deposition of colloidal silica on the membrane, became zero, i.e. d m f/d t = 0. It was found that the critical flux concept was applicable to colloidal silica fouling in the RO process. The critical flux was strongly influenced by the crossflow velocity, such that J crit ∼ v 0.4 . In addition, the critical Peclet number, ( J v/ k m) crit, which is a measure of the convective to diffusive forces, to trigger silica fouling was approximately 0.52. Furthermore, it was observed that the amount of particles convected to and finally deposited on the membrane surface, known as the fractional deposition constant, Φ, was strongly influenced by the crossflow velocity. At low crossflow (<0.1 m s −1, shear rate <430 s −1) Φ = 1.0, but at 0.22 m s −1 (shear rate = 940 s −1) Φ dropped to about 0.1. The CEOP effect, as deduced from the transient CP data, was found to be more severe at high flux and low crossflow velocity conditions, due to formation of a thicker cake layer and flux-induced polarization. Even for the small (20 nm) colloids used in this study, the CEOP effect can readily exceed the R f effect due to fouling.

On-line cake-layer management by trans-membrane pressure steady state assessment in Anaerobic Membrane Bioreactors for wastewater treatment

Membrane bioreactors have been increasingly applied for wastewater treatment during the last two decades. High energy requirements and membrane capital costs remains as their main drawback. A new strategy of operation is presented based on a continuous critical flux determination, preventing excessive cake-layer accumulation on the membrane surface. Reactor operation is divided in cycles of 500 s filtration followed by a short back-flush of 15 s. If cake-layer formation is detected during continuous operation, a decrease in flux or an increase in cross flow velocity is performed. The proposed approach keeps reactor operation oscillating around the critical flux, minimizing reactor maintenance and maximizing performance. An easy to operate statistical steady state determination tool for the trans-membrane pressure was used to detect cake-layer formation. The developed control approach was tested on two Anaerobic MBRs equipped with submerged membranes. Despite the existence of very different critical fluxes and cake-layer formation characteristics, proposed approach was able to keep pressure increase during filtration cycles below 20 mbar. The developed approach is an efficient tool for on-line control of cake-layer formation over the membranes, changing cross flow velocities by manipulating gas sparging in submerged MBRs.