Constant Flux Operation Research Articles

Investigation of direct Coagulation-Flocculation-Ultrafiltration (CFU) at lab-scale constant pressure and flux operation of copper removal

Limited heavy metal concentrations in drinking water are harmful. The size-exclusion separation process was found to be a technology for removing heavy metals and organic substances. Although promising, a single ultrafiltration process is insufficient for the separation of heavy metals. Therefore, an additional process is required. The combination of coagulation flocculation followed by ultrafiltration was investigated. In this study, water matrix was used to simulate the worst-case scenario by adding 2 mg of copper to the surface water. For the filtration process, a comparison of single ultrafiltration with and without pretreatment using aluminum sulfate was investigated. Filtration was performed in a lab-scale experiment employing a polyethersulfone (PES) membrane with an average pore size of 30 nm operated at constant flux 120 L/m²⋅h and constant pressure of 0.7 bar. Furthermore, TDS retention, copper concentration, and turbidity were observed. Higher Cu removal was found at filtration under constant flux compared to constant pressure (81% and 66%, respectively. In the case of treated water with coagulation (optimum coagulation of 30 mg/L), higher removal of Cu was observed at constant flux operation compared to constant pressure, with 73% and 89% removal, respectively. Additional coagulation resulted in less membrane fouling during the filtration experiment, which explained the better performance almost double that of single ultrafiltration.

Temporal resistance fluctuations during the initial filtration period of colloidal matter filtration

Membrane filtration processes with flux reversal for backwashing are employed to manage fouling during colloidal fluid treatment. This flux reversal accompanies filtration cycles where the total resistance increases over time. However, little is known about the initial phase of filtration. We focus on early filtration phase and present a microfluidic filtration system that allows precise observation of filtration resistances under constant flux or constant pressure operation for soft matter dead-end filtration with subsequent percolation of pure solvent. We identified temporal hydraulic resistance fluctuations during the start-up phase, which significantly differed from the steady-state condition. We focus on the experimental filtration of soft core–shell poly(N-isopropylacrylamide)-co-acrylic-acid microgels and observed a distinct peak in filter cake resistance with a subsequent reduction down to 50% of the initial peak resistance. To comprehend the phenomenon, we complement the experiments with temporal and spatial confocal microscopy studies linking cake compression and cake density gradients to its resistance. By comparing filtration, dialysis, and centrifugation as different microgel purification methods, we discover microgel shell degradation leading to detached polymer chains permeating in the downstream which causes the observed resistance reduction. This result demonstrates the permeation-induced degradation of macromolecular colloids during membrane filtration.

Development of a new modeling framework to describe sterile filtration of mRNA-Lipid nanoparticles

Sterile filtration is one of the critical steps in the production of lipid nanoparticle (LNP)-based biotherapeutics. However, LNP fouling can limit the overall capacity of the sterilizing-grade filter. Effective design and control of this unit operation enables a robust manufacturing process. The objective of this study was to examine the sterile filtration of mRNA-LNP through the dual-layer Sartopore 2 XLG membrane during both constant flux and constant transmembrane pressure (TMP) filtration experiments. The complete pore blockage model effectively described the fouling behavior at constant TMP, with the rate of pore blockage decreasing with increasing TMP. However, a novel modification of the complete pore blockage model was needed to describe the fouling behavior during constant flux operation, with the rate of pore blockage found to be a function of both the instantaneous TMP and the TMP gradient. This new model successfully describes the TMP profiles during constant flux operation at multiple fluxes and the flux profiles during constant TMP operation at multiple TMPs, all using the same model parameters. These findings establish a foundational framework that mathematically describes the fouling behavior of mRNA-LNP and can be used to design and optimize sterile filtration processes for this class of biotherapeutic.

Productivity and energy consumption as performance criteria to overcome bias caused by membrane resistance

Membrane scientists and engineers routinely compare the performance of membranes for different applications. Such comparisons are required to optimize surface chemistries and to evaluate and select the best membrane to scale-up industrial applications. In most cases, the potential of the feed to foul membrane surfaces is also critical. When different membranes under consideration have different resistances Rm, such comparisons must be done with care. Here we elucidate the impact of Rm on fouling kinetics for a range of fouling mechanisms and experimental protocols. We show that traditional plots of flux versus time for constant pressure operation, or pressure versus volume in constant flux operation, obscure two important performance criteria: accumulated volume over a defined time period (i.e., productivity) and energy consumption; practical implications include the risk that higher performance may be attributed to membranes that actually have lower productivity and/or that consume more energy, and that lower performing membranes may be inadvertently chosen from screening studies as offering superior performance. To facilitate membrane comparison with different Rm, we have developed a graphical approach using normalized coordinates. This approach yields linear plots having slopes that are a function of fouling parameters only, and are independent of membrane resistance Rm. Therefore, such normalization coordinates, presented for each fouling mechanism and each operation mode, can be employed to isolate fouling potential. In addition, we developed a new graphical approach employing contour plots of iso-volume or iso-energy lines to visualize the potential trade-offs between antifouling performance, membrane permeability, and either productivity in the case of constant pressure operation, or specific energy in the case of constant flux operation. This study provides a framework for judging the performance of different membranes under fouling conditions, considering fouling, energy consumption and productivity as metrics, independent of membrane resistance. This framework will help guide process design and the design and preparation of antifouling membranes.

Evaluation of Organic and Inorganic Foulant Interaction Using Modified Fouling Models in Constant Flux Dead-End Operation with Microfiltration Membranes.

The goal of this study was to elucidate the interaction of complex feed solutions under modified membrane fouling models for constant flux operation. The polyvinylidene fluoride membrane (PVDF) was tested for three types of solutions containing inorganic foulants (Al, Mn, and Fe), organic foulants, and suspended solids at 0.5 mM Ca2+ ionic strength. The membrane's performance was evaluated by measuring the increase in transmembrane pressure (TMP) during two different filtration scenarios: continuous filtration lasting 1 h and cyclic filtration lasting 12 min, with 3 min backwashing cycles included. Statistical analysis (linear regression results (R2), p-value) was used to verify the fouling model propagation along with the determination of the contributing constant of each fouling model. An increasing TMP percentage of 164-302%, 155-300%, and 208-378% for S1 (HA + Ca2+), S2 (inorganics + kaolin + Ca2+), and S3 (HA + inorganics + kaolin + Ca2+) was recorded for 1 h filtration, respectively. Furthermore, a five percent increase in irreversible resistance was noted for the S3 solution due to the strong adsorption potential of foulants for the PVDF membrane caused by the electrostatic and hydration forces of foulants. In addition to that, the participation equation elucidated the contribution of the fouling model and confirmed that complete blocking and cake layer contribution were dominant for the S1 and S3 solutions, while standard blocking was dominant for the S2 solution with a high significance ratio. Moreover, R2 and cyclic filtration analysis also confirmed the propagation of these fouling models. The statistical confirmation and regression results analysis of the modified model gave comparative results and satisfied the filtration mechanism and can be used for the constant flux dead filtration analysis of water treatment.

Protein fouling during constant-flux virus filtration: Mechanisms and modeling.

As biomanufacturers consider the transition from batch to continuous processing, it will be necessary to re-examine the design and operating conditions for many downstream processes. For example, the integration of virus removal filtration in continuous biomanufacturing will likely require operation at low and constant filtrate flux instead of the high (constant) transmembrane pressures (TMPs) currently employed in traditional batch processing. The objective of this study was to examine the effect of low operating filtrate flux (5-100 L/m2 /h) on protein fouling during normal flow filtration of human serum Immunoglobulin G (hIgG) through the Viresolve® Pro membrane, including a direct comparison of the fouling behavior during constant-flux and constant-pressure operation. The filter capacity, defined as the volumetric throughput of hIgG solution at which the TMP increased to 30 psi, showed a distinct minimum at intermediate filtrate flux (around 20-30 L/m2 /h). The fouling data were well-described using a previously-developed mechanistic model based on sequential pore blockage and cake filtration, suitably modified for operation at constant flux. Simple analytical expressions for the pressure profiles were developed in the limits of very low and high filtrate flux, enabling rapid estimation of the filter performance and capacity. The model calculations highlight the importance of both the pressure-dependent rate of pore blockage and the compressibility of the protein cake to the fouling behavior. These results provide important insights into the overall impact of constant-flux operation on the protein fouling behavior and filter capacity during virus removal filtration using the Viresolve® Pro membrane.

Effect of operation mode on membrane fouling from traditional Chinese medicine water extracts

Membrane technology is a viable alternative to many conventional methods currently used to process traditional Chinese medicine (TCM). However, membrane fouling is a significant obstacle to the application of membrane technology in TCM. In this study, the ultrafiltration (UF) membrane fouling from TCM water extracts under constant transmembrane pressure (TMP) and constant flux operation was studied in connection with critical and threshold flux determinations. Meanwhile, the physicochemical foulant–membrane interactions were analyzed using the extended Derjaguin, Landau, Verwey, and Overbeek (XDLVO) theory. At constant TMP operation, the high pressure resulted in a rapid drop in permeate flux at the start of the filtration period, but the steady-state fluxes did not increase significantly with an increase in TMP. At constant flux operation, the TMP increased somewhat when the permeate flux was below the threshold flux. Fouling was severe above the threshold flux, and the TMP rapidly increased. By displaying fouling resistance as a function of permeate volume, the constant flux operation was compared to a constant TMP operation beginning at the same flux imposed during the constant flux operation. Below the threshold flux, fouling resistance was similar for constant TMP and constant flux operations. However, above the threshold flux, fouling resistance under constant flux operation was much greater than fouling resistance under constant TMP operation. The results of physicochemical interactions showed that there are attractive interactions between hydrophobic membrane and foulants in the TCM water extracts. These interactions should be considered a key factor leading to serious membrane fouling.

Design and implementation of constant flux controller for VSI assisted SEIG feeding induction motor pump

In small scale, stand-alone, wind-power generation employing self excited induction generator (SEIG), water pumping using an induction motor is a typical application. In this paper, a simple voltage regulation scheme for the constant flux operation of the inverter assisted SEIG feeding three-phase induction motor pump is presented. The behaviour of DC-link voltage, frequency, amplitude modulation index (ma) and the shaft speed for load and speed perturbations are discussed in detail. Also, the overall active and reactive power flow for constant flux operation at the point of common coupling (PCC) is analysed. The proposed work presents a simple and reliable controller for SEIG-based stand-alone system for frequency-dependent loads using only a DC-link voltage sensor. Detailed system simulations are performed using Matlab/Simulink and the results of a laboratory prototype are presented to validate the theoretical analysis and practical operability of the proposed system.

Membrane aging effects on water recovery during full-scale potable reuse: Mathematical optimization of backwashing frequency for constant-flux microfiltration

One tool in efforts to tackle the ever growing problem of water scarcity is municipal wastewater reclamation to produce drinking water. Microfiltration (MF) is a central technology for potable reuse because it is highly effective in removing pathogenic protozoa, bacteria, and other colloids and for reverse osmosis pretreatment. However, as microfiltered materials accumulate at the membrane surface, its productivity is reduced requiring periodic removal of foulants. A mathematical model of MF is described in the context of hollow fiber filtration that focused on optimizing constant flux operation with backwashing. Design curves were also proposed for determining backwash timing. The model analysis is evaluated against real-world MF fouling for membranes that range in age from a few weeks to three years, observed at the world’s largest water reuse facility operated by the Orange County Water District. The presented model compares well with the full-scale operational data, and model parameters accurately capture variations in fouling kinetics with membrane age, providing clues to changes in optimal regeneration timing and frequency as membrane performance declines over long time scales.

Neuro-fuzzy based estimation of rotor flux for Electric Vehicle operating under partial loading

The primary objective of this work is to optimize the induction motor rotor flux so that maximum efficiency is attained in the facets of parameter and load variations. The conventional approaches based on loss model are sensitive to modelling accuracy and parameter variations. The problem is further aggravated due to nonlinear motor parameters in different speed regions. Therefore, this work introduces an adaptive neuro-fuzzy inference system-based rotor flux estimator for electric vehicle. The proposed estimator is an amalgamation of fuzzy inference system and artificial neural network, in which fuzzy inference system is designed using artificial neural network. The training data for neuro-fuzzy estimator is generated offline by acquiring rotor flux for different values of torque. The conventional fuzzy logic and differential calculation methods are also developed for comparative analysis. The efficacy of developed system is established by analyzing it under varying load conditions. It is revealed from the results that suggested methodology provides an improved efficiency i.e. 94.51% in comparison to 82.68% for constant flux operation.

Quorum quenching in anaerobic membrane bioreactor for fouling control

Quorum quenching (QQ) is an effective method to control membrane biofouling in aerobic membrane bioreactors (AeMBRs). However, it is not clear if QQ is feasible in an anaerobic membrane bioreactor (AnMBR). In this study, Microbacterium. sp that has QQ capability was embedded in alginate beads, known as QQ beads (QQB), and applied in a lab-scale AnMBR to investigate their potential in fouling control. With the addition of QQB, the operating period of AnMBR-QQB reactor was prolonged by about 8–10 times at constant flux operation before reaching the pre-set maximum transmembrane pressure (TMP). The concentration of Acyl-homoserine lactones (AHLs) in the bulk liquid was significantly higher during the ‘TMP jump’ period compared to QQB and control phases, while AHLs in the membrane foulants were remarkably lower in QQB phase compared to control phase. Furthermore, a much lower level of soluble microbial production (SMP) was observed in QQB phases. Extracellular polymeric substance (EPS), protein in particular, was reduced by 39.73–80.58% in the cake layer of the membrane from QQB phases. Significant changes of organic functional groups were observed in cake layer from QQB membrane as compared with that from control membrane. At the end of operation, bio-polymer (BP), building blocks (BB) and low molecular weight (LMW) organic matters increased in the foulant from control phases but such increase was not observed in QQB phase. After long-term operation, revival of QQB is required due to the declined activity for AHLs degradation.

Entropy generation minimisation for heat exchangers with heat leakage

ABSTRACT A model of a heat transfer process in heat exchangers with heat leakage is established, and the optimal temperature relationship between hot- and cold-fluids for the minimum entropy generation is derived by applying optimal control theory. The optimisation results are also compared with those for the two conventional strategies of constant heat flux and constant hot fluid temperature operation. The results show that the heat leakage has some effects on the minimum entropy generation of the process and the corresponding fluid temperature configurations. Considering the three strategies of constant hot fluid temperature, constant heat flux, and the minimum entropy generation, one sees that the paths that generate more entropy generation benefit from necessitating small installed capacities. Hence, the eventual selection of economically optimal strategies will depend on the relative costs of avoided entropy generation versus installed capacity.

Ultrafiltration with intermittent relaxation using colloidal silica and humic acid as model foulants

Crossflow microfiltration and ultrafiltration are widely used in membrane bioreactors for wastewater treatment and applications in the food, biotechnology and process industries. Membrane fouling is an endemic problem in these operations that can be mitigated by backwashing and intermittent relaxation of the pressure or flux. Intermittent relaxation offers advantages relative to backwashing since it does not alter the membrane morphology or damage susceptible membranes, provides a more uniform removal of the foulant cake layer, and is potentially more energy-efficient. This paper reports a study of intermittent relaxation to mitigate fouling under both constant flux and constant pressure operation using a regenerated cellulose flat sheet ultrafiltration membrane (MWCO 30 kDa) and two model foulants: colloidal silica that is completely rejected and highly compressible and metastable; and, humic acid that is partially rejected and thereby can cause internal pore fouling and is relatively incompressible. Intermittent relaxation more effectively mitigated humic acid fouling owing to the compressibility and metastability of colloidal silica. An optimum relaxation frequency exists owing to the counterbalancing effects between increasing the permeation flux and reducing the time for permeation, which is determined for both constant flux and constant pressure operation. Design heuristics are advanced for the operation of intermittent relaxation.

Techniques for efficiency improvement in PWM motor drives

In this paper a comprehensive efficiency analysis on an inverter-fed induction motor drive is proposed. The drive under study integrates both switching losses minimization by D-PWM (Discontinuous Pulse Width Modulation) for the voltage source inverter and an original version of an ELMT (Electric Loss Minimization Technique) for the motor. The two techniques are implemented, either separately or contemporary in a unique control platform in order to allow the contribution of both the chosen PWM and ELMT strategies on the power total losses to be assessed and benchmarked. Results have been compared with those achieved by the use of the classic space-vector PWM in constant rated flux operation; from this comparison the improvement of the efficiency of the whole electrical drive with the proposed techniques is demonstrated. A complete dynamic model of the drive system has been developed, which permits the drive behaviour in terms of efficiency/power losses to be evaluated in numerical simulation, properly separating each loss term. Such a complete dynamic model has been validated. Experimental results are given as well.

Enhancement of the Antifouling Properties and Filtration Performance of Poly(ethersulfone) Ultrafiltration Membranes by Incorporation of Nanoporous Titania Nanoparticles

Nanoporous titania nanoparticles (NTNs) were synthesized and used as an additive at a low concentration of 0.1–1 wt % in the fabrication of poly(ethersulfone) (PES) ultrafiltration membranes via non-solvent-induced phase separation. The structure and properties of nanoparticles were characterized using nitrogen sorption, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The NTNs have a size distribution with a particle size of mainly <100 nm and have a Brunauer, Emmett, and Teller surface area of ∼100 m2 g–1. The modified membranes were fabricated and investigated in terms of their pure water flux, solute rejection, and fouling resistance. The water permeability and molecular weight cutoffs (MWCOs) of membranes were determined under constant-pressure filtration in dead-end mode at 100 kPa. The membrane fouling resistance was characterized under constant flux operation using bovine serum albumin as a model foulant. The membranes were characterized in terms of morphology, porosity, pore size distribution, energy-dispersive X-ray spectroscopy, contact angle goniometry, surface free energy, and viscosity of the dope solution. Overall, the modified membrane showed increased wettability and reduced surface free energy and pore size. The modified UF membrane with 0.5 wt % NTN loading exhibited improved fouling resistance (fouling rate of 0.58 kPa/min compared to a rate of 0.70 kPa/min for the control membrane) with ∼80% water flux recovery. The same membrane showed an ∼20% increase in water flux, an improvement in MWCO, and a narrower pore size distribution.

Enhanced performance of submerged hollow fibre microfiltration by fluidized granular activated carbon

The effect of fluidized granular activated carbon (GAC) on membrane filtration performance was investigated using a bench-scale upflow hollow fibre membrane filtration setup under constant flux operation. The membrane fouling tendencies were compared among five different experimental parameters, namely, GAC size, GAC packing amount, hollow fibre spacing, filtration/idle duration, and fluidization/idle duration. The results indicate that larger-sized GAC particles, higher packing amounts and a ratio of hollow fibre spacing to fluidized particle size of approximately 3–5 are most beneficial for fouling control. Unexpectedly, the intermittent filtration (under continuous fluidization) could not further alleviate membrane fouling compared to continuous filtration, possibly due to inefficient interaction of fluidized GAC particles with membrane surface in absence of permeate driving force. To lower energy consumption, the optimization of intermittent fluidization (under continuous filtration) was performed. Results indicate that the nature of the cake layer formed during non-fluidization period determined the membrane fouling development rather than the fluidization time span. Finally, by comparing the membrane permeability, sodium alginate rejection rate, and membrane properties before and after GAC abrasion, the GAC scouring on membrane integrity was negligible.

Application of dispersed and immobilized hydrolases for membrane fouling mitigation in anaerobic membrane bioreactors

Enhancing the hydrolysis of microbial macromolecules by supplementing exogenous hydrolases may improve membrane performance via structural disruptions of fouling layers and alterations to sludge characteristics. This was investigated by short batch filtration (<1h) and 30-day extended filtration experiments using laboratory-scale anaerobic membrane bioreactors. Crude hydrolases were either dispersed directly into the reactor, or immobilized onto microfiltration membranes. Under constant flux operation, dispersed enzymes consistently moderated increases in transmembrane pressures (TMPs) compared to the control setup. Immobilized hydrolases appeared effective in the short filtration test, but in the extended experiment, the pseudo-stable TMP was not significantly lower compared to the control TMP. With dispersed enzymes, the average TMP was almost 30% lower than the control value. This was associated with a 33% reduction in the protein content of the bulk extracellular polymeric substances, and a 45% reduction in the membrane cake density. Immobilized enzymes limited cake formation to a similar extent through hydrolysis at the base of the cake, but this was negated by the increase in gel resistance attributed to the hydrophobic attraction between the immobilization layer and proteinaceous hydrolysis products. Even as the dispersed hydrolases exhibited greater effectiveness under the conditions studied, there is scope for further enhancement in both approaches.

In situ investigation of combined organic and colloidal fouling for nanofiltration membrane using ultrasonic time domain reflectometry

Ultrasonic time domain reflectometry (UTDR) was used to monitor the deposition of combined organic–colloidal fouling on a nanofiltration membrane. The fouling experiments were performed with different feed solutions: a mixture of 1000mg/L silica and 1000mg/L NaCl, a mixture of 1000mg/L silica and 250mg/L BSA, and 250mg/L BSA alone, respectively. Results showed that the rate of flux decline obtained in the constant-pressure experiment with the mixture of silica+BSA was greater than that with the mixture of silica+NaCl and BSA alone. The acoustic measurements indicated that the fouling layer obtained from the combined organic–colloidal fouling was denser than that obtained from the colloidal fouling layer in the presence of NaCl. Furthermore, the mixed foulants rapidly deposited on the membrane surface in the early fouling phase, and then reached a plateau in the later fouling phase under a constant pressure operation, whereas the mixed foulants gradually deposited on the membrane surface as the fouling progressed under a constant flux operation. The main reason for the rapid foulant deposition during the initial fouling stage under constant pressure operation was because the initial flux was above the critical flux. The off-line AFM analysis and zeta potential measurements corroborated the ultrasonic measurements.

Understanding and modeling alternating tangential flow filtration for perfusion cell culture.

Alternating tangential flow (ATF) filtration has been used with success in the Biopharmaceutical industry as a lower shear technology for cell retention with perfusion cultures. The ATF system is different than tangential flow filtration; however, in that reverse flow is used once per cycle as a means to minimize fouling. Few studies have been reported in the literature that evaluates ATF and how key system variables affect the rate at which ATF filters foul. In this study, an experimental setup was devised that allowed for determination of the time it took for fouling to occur for given mammalian (PER.C6) cell culture cell densities and viabilities as permeate flow rate and antifoam concentration was varied. The experimental results indicate, in accordance with D'Arcy's law, that the average resistance to permeate flow (across a cycle of operation) increases as biological material deposits on the membrane. Scanning electron microscope images of the post-run filtration surface indicated that both cells and antifoam micelles deposit on the membrane. A unique mathematical model, based on the assumption that fouling was due to pore blockage from the cells and micelles in combination, was devised that allowed for estimation of sticking factors for the cells and the micelles on the membrane. This model was then used to accurately predict the increase in transmembane pressure during constant flux operation for an ATF cartridge used for perfusion cell culture.

Effect of addition of two-dimensional ZIF-L nanoflakes on the properties of polyethersulfone ultrafiltration membrane

A new two-dimensional zeolitic imidazolate framework with leaf-shaped morphology (ZIF-L) was incorporated into polyethersulfone (PES) ultrafiltration membranes to investigate how the ZIF nanoflakes affect functional membrane properties. The membranes were characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), atomic force microscopy (AFM), and contact angle goniometry. The water permeability and molecular weight cut-offs (MWCO) of membranes were also determined under constant pressure filtration. Membrane fouling resistance was characterized under constant flux operation using bovine serum albumin (BSA) as foulant. The modified UF membrane with 0.5% ZIF-L loading showed around 75% increase in water flux without greatly affecting the MWCO. Also, the same membrane showed almost twice the fouling resistance improvement against BSA with more than 80% water flux recovery. The improvement was due to the combined effect of the lower zeta potential of the modified membrane, increased hydrophilicity and reduced surface roughness, which made the attachment of BSA protein on the membrane surface more difficult. These results demonstrate that the addition of 2-dimensional ZIF-L nanoflakes is effective for improving polymer membrane fouling resistance and water flux.