Constant Flux Operation Research Articles

Evaluation of fouling deposition, fouling reversibility and energy consumption of submerged hollow fiber membrane systems with periodic backwash

The filtration behavior and energy consumption of submerged hollow fiber membranes were investigated in a dead-end mode under constant flux operation with periodic backwash. Various operating parameters such as filtration flux, feed concentration, backwash duration, backwash strength, and the aid of air scouring during backwash were investigated to optimize the filtration performance and energy consumption. Baker's yeast was used as the model foulant. The transmembrane pressure (TMP) profiles obtained with this model foulant indicated the existence of two regions: a low fouling region followed by a high fouling region. In the low fouling region, periodic backwash could effectively remove the fouling layer. The percentage of reversible fouling by backwash was significantly lower in the second region which led to a rapid increase in TMP. The Taguchi experimental design method was used to determine the critical parameters, the effect of individual parameters and the optimum conditions. Since the transition into the second region of filtration is not favorable, the fouling rate in the first 10 cycles, the net permeate volume and specific energy consumption at the end of the first region were chosen as the design method responses. The results showed that filtration flux and feed concentration have significant effects on the membrane fouling rate. In order to maximize the net permeate volume, filtration flux, backwash duration and backwash strength were found to be the most important parameters, which must be carefully selected. To minimize the specific energy consumption, filtration flux, backwash duration and aeration rate during backwash were the most important factors.

Flux dependency of particulate/colloidal fouling in seawater reverse osmosis systems

Abstract Fouling is the main operational problem in seawater reverse osmosis systems (SWRO). Particulate fouling is traditionally measured through the silt density index (SDI) and through the modified fouling index (MFI). In recent years, ultrafiltration membranes were used successfully at constant flux – MFI-UF – to measure particulate/colloidal fouling potential and tested in sea water applications. Furthermore, constant flux operation allows predicting the rate of fouling in RO systems. The objectives of this study are: (1) to measure the flux effect in MFI-UF with different membranes (100, 30 and 10 kDa) for raw seawater and pre-treated water before reverse osmosis in three different locations; (2) to study the particulate and colloidal fouling potential of seawater in reverse osmosis systems; (3) to project the increase in pressure due to cake resistance in reverse osmosis systems. In this research, flat ultrafiltration membranes (100, 50, 30 and 10 kDa) are used in a constant flux filtration mode to test and compare real seawaters from various locations (North and Mediterranean Sea) and from various full scale facilities including different pre-treatments (i.e., ultrafiltration and coagulation + dual media filtration). The operated fluxes range from 350 down to values close to real RO operation, 15l (m2 h)−1. After each filtration test, the MFI-UF is calculated to assess the particulate fouling potential. The obtained results showed that: (1) the articulate and colloidal fouling potential is directly proportional to the applied flux during filtration. This proportionality is related to the compression of the cake deposit occurring at high flux values; (2) the higher the flux, the higher the required pressure, the less porous the cake and therefore the higher the specific cake resistance; (3) particulate and colloidal fouling potential of seawater is site specific and is influenced by pre-treatment.

Colloidal and bacterial fouling during constant flux microfiltration: Comparison of classical blocking laws with a unified model combining pore blocking and EPS secretion

The instantaneous transmembrane pressure needs to be continuously increased to compensate for foulant accumulation during constant flux microfiltration. Herein, we compare predictions of a unified mathematical model and conventional blocking laws with laboratory data obtained during constant flux operation. Foulants employed included single species cultures of bacteria and coagulated natural colloids, both of which are known to form compressible cakes. The first principles model unifies fouling arising from pore blocking by individual cells, cake formation, as well as bacterial secretion of exopolymers. It also incorporates non-uniform spatial deposition patterns that have been observed during unstirred dead-end filtration. Mechanistically, these heterogeneities arise either from non-uniform membrane surface porosity or stochastic initial binding of foulants. Previous studies have shown that the initial patchy or uneven deposit morphology is magnified over longer time-scales during bacterial filtration by differential extracellular polymeric substances (EPS) secretion through quorum sensing. However, in this study, we are primarily interested in the averaged behavior (mainly flux and pressure). By spatially averaging the microscale variables we are able to compare with classical blocking law models. We show that blocking laws and the unified model both accurately model fouling under our experimental conditions. The unified model provides mechanistic insights into (bio)colloid deposition and associated fouling during constant flux microfiltration particularly since it is obtained excellent predictive agreement with experimental data using parameters taken exclusively from our recent study of constant pressure filtration.

A New Flat-Sheet Submerged Membrane Photoreactor (FSMPR) for the Removal of Humic Acid in Water

The study focuses on the degradation humic acid (HA) by a hybrid system combining a flat-sheet submerged membrane module and a photocatalysis reactor (FSMPR). The influences of UV irradiation intensity, titanium dioxide (TiO2) and initial humic acid concentration, pH, H2O2 dosage were investigated in batch kinetic tests. and the relevant dynamic analysis was carried out. The transmembrane pressure (TMP) with time under the range of constant flux operation (20-40L/m2h) does not increase obviously. It was also found that aeration backflush availably reduced TMP of the FSMPR. Therefore, the FSMPR can be potentially applied in drinking water treatment.

Analysis of full-scale membrane filtration processes using econophysics and econometrics

Abstract In this paper, econophysics and econometric approaches were employed to investigate statistical and dynamic properties of membrane performance, respectively, using filtration data provided by Woongjin Chemical Co., Ltd., Korea. For the econophysics approach, the filtration index tree was built to comprehensively visualize the closeness between filtration variables, analogous to the hierarchical index tree for the dynamics of stock prices. The logarithmic changes of the filtration variables follow the normal distribution, which implies that the filtration variables themselves are governed by the log-normal distribution due to periodic cleaning and/or seasonal variations. In the econometrics approach, the autoregressive model verified the periodicity of the pressure profile due to cleaning events and natural environmental changes. As developed in this study, the semi-loglinear and autocorrelation models captured pressure growth rates of 1.4% and 0.3 kg/cm 2 per week, respectively. Combined and complementary roles of econophysics and econometrics provided new methodological insights to statistically analyze and further forecast membrane filtration performance in terms of the pressure growth rate in the constant flux operation.

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

This article describes a predictive model to account for the performance of a reverse osmosis (RO) system under the influence of colloidal fouling (using silica particles as model colloids) in terms of transmembrane pressure increase (δTMP) in constant flux operation or flux drop (δJ v) in constant pressure operation. The predictive model considers the implications of both the critical flux (J crit) and cake enhanced osmotic pressure (CEOP) phenomenon and is based on the membrane retention, concentration polarization, mass transfer, resistance in series and osmotic pressure model, cake resistance, and critical flux concept.

Subcritical fouling behaviour modelling of membrane bioreactors for municipal wastewater treatment: The prediction of the time to reach critical operating condition

After the advent of critical flux concept, studies on constant subcritical flux operation of membrane bioreactors (MBRs) gained interest, presumably, because of preferred least fouling operational strategy; however, such a chosen operational condition has demonstrated the continuous slow and irreversible fouling followed by a sudden rise in transmembrane pressure (TMP). Such peculiar fouling behaviour has been explained to be evolved due to extracellular polymeric substances (EPS) originated through the course of biological processes inherent to MBRs. A subcritical flux fouling model based on the role of two forms of EPS, viz. bound and free, has been presented here. The model, studied for MBR treating municipal wastewater, could successfully predict the time at which a sudden change in the TMP–time profile occurs; the predicted time of 200 h by the model has shown good agreement with the observed time of approximately 190 h. Although the fouling problem is one of the major bottlenecks for the MBR technology to become widely applicable as an efficient municipal wastewater treatment technology, from the operational point of view, prediction of the time after which severe fouling occurs (a sudden jump in the TMP–time profile) during subcritical operation could be an important step. Such a prediction would help in developing an efficient and well regulated cleaning protocol for better management of MBR treatment plant operations.

Protein Fouling of Virus Filtration Membranes: Effects of Membrane Orientation and Operating Conditions

The capacity of virus filters used in the purification of therapeutic proteins is determined by the rate and extent of membrane fouling. Current virus filtration membranes have a complex multilayer structure that can be used with either the skin-side up or with the skin-side facing away from the feed, but there is currently no quantitative understanding of the effects of membrane orientation or operating conditions on the filtration performance. Experiments were performed using Millipore's Viresolve 180 membrane under both constant pressure and constant flux operation with sulfhydryl-modified BSA used as a model protein. The capacity with the skin-side up was greater during operation with constant flux and at low transmembrane pressures, with the flux decline or pressure rise due primarily to osmotic pressure effects. In contrast, data obtained with the skin-side down showed a slower, steady increase in total resistance with the cumulative filtrate volume, with minimal contribution from osmotic pressure. The capacity with the skin-side down was significantly greater than that with the skin-side up, reflecting the different fouling mechanisms in the different membrane orientations. These results provide important insights for the design and operation of virus filtration membranes.

Factors affecting the membrane performance in submerged membrane bioreactors

Submerged membrane bioreactors (SMBRs) operate at constant imposed flux usually with the membranes in direct contact with the mixed liquor. Membrane fouling is evident as a rise in the required transmembrane pressure (TMP). In this paper we describe the rise in TMP as a three stage process. Stage 1 occurs in a period of a few hours and involves abrupt TMP rise due to ‘conditioning’, presumably by pore blockage and closure. Stage 2 is a prolonged period of slow TMP rise, which we ascribe to accumulation of extracellular polymeric substances (EPS) and other products of bioactivity, either deposited from the bulk liquor or produced in biofilms on the membrane surface. Polysaccharides are predominant in the supernatant EPS, and appear to be major membrane foulants deposited by the filtration process. Stage 3 is a sudden rise in TMP, which rapidly leads to inoperability of the membranes. This stage could have several causes, which we summarize, and which are all driven by the self-accelerating nature of fouling under constant flux operation. In order to understand the complex nature of fouling in submerged MBRs we have developed the MBR ‘roadmap’ that shows the relationship between the many operational and design characteristics of the MBR and the generic ‘fouling factors’, i.e. the nature of the feed (to the membrane), the membrane properties, and the hydrodynamic environment. We also present a MBR ‘fouling mechanism map’ which depicts the three stages of fouling and the multiple fouling mechanisms that could occur. Experimental observations are provided in support of many of the proposed fouling mechanisms.

Blocking laws analysis of dead-end constant flux microfiltration of compressible cakes

New blocking law models for dead-end constant flux microfiltration of colloids forming cakes that compressed in a linear and power law manner were derived. Constant pressure and constant flux experiments were performed using bacteria, colloidal silica, and treated natural waters to validate these new models and quantitatively verify blocking law predictions on the role of cake compressibility in microfilter fouling. Statistically invariant values of cake specific resistance and compressibility were obtained for constant flux and constant pressure operation for each feed suspension. This suggests that colloids formed cakes whose hydraulic resistance is dominated by a morphology that did not depend on their mode of deposition, confirming that the cake permeability was determined by the instantaneous pressure. Additionally, an inverse relationship between extracellular polymeric substances (EPS) secreted by bacteria and hydrodynamic flux restoration procedures was obtained demonstrating the importance of linking EPS to backwashing frequency when bacteria are present in the feed water.

Discussion on: “Observer Scheme for State and Parameter Estimation in Asynchronous Motors with Application to Speed Control”

The paper by Ţiclea and Besancon is an interesting contribution to the area of state and parameter estimation for both speed sensorless operation and insensitivity to parameter variation of an induction motor drive. It treats the problem of simultaneous state and parameter estimation in induction motor. It is to be noted that simultaneous estimation of speed and rotor resistance was, for a long time, considered impossible by many researchers in this area. This is because under constant flux operation in rotor flux oriented control, the rotor model is reduced to one equation containing both speed and rotor resistance. So, a change in either rotor speed or rotor resistance has an equal effect on the electrical behaviour of the motor. The aim of this discussion is to highlight the essential points about the proposed observer that can help the reader to better appreciate the significance of the paper. The authors present a method of simultaneous estimation based on an exact Kalman-like observer. The use of such an observer is made possible by an immersion of the nonlinear model of the machine into a model which is affine with respect to variables to be estimated. The choice of this observer is due to the drawbacks of the extended Kalman filter, usually used for this kind of problem [2], such as uncertain global, low accuracy at small speeds and the required computational time. The theoretical background of the machine modeling in order to use an exact Kalmanlike observer is interesting and the simulation results are satisfactory for state and parameter estimation and speed control. The method developed is weakened by the large amount of computations, when all variables of state and parameter are to be estimated as Ţiclea and Besancon have mentioned in their conclusion and it can be seen in the state vector size of the developed machine model. Also, the load torque is considered as a constant parameter which is not always the case, and this reduces the application area of the study. These disadvantages will not reduce the interest generated by this work. The machine model takes into account the coupling between the electrical and mechanical modes, and is expressed in a reference frame fixed to stator ( , ), and made under an affine representation of the form: X 1⁄4 Aðu, y, ÞXþ Bu Y 1⁄4 CX ð1Þ

Unbalanced magnetic pull induced by arbitrary eccentric motion of cage rotor in transient operation. Part 1: Analytical model

Eccentric rotor motion of an electric motor induces an unbalanced magnetic pull. This eccentricity force may couple the electromagnetic system with the flexural vibration modes of the shaft. This paper presents an analytic model for this eccentricity force allowing an arbitrary rotor motion and transient operation. The general equations were simplified for the constant flux and steady-state operation. These simplified equations correspond well to the results presented earlier.

Performance of crossflow microfiltration during constant transmembrane pressure and constant flux operations

Abstract Crossflow microfiltration of lactalbumin suspensions was studied to gain a better understanding of fouling mechanisms. The performance under constant flux mode and constant transmembrane pressure (TMP) mode was compared. Critical flux J cr values, below which no membrane fouling occurs, were determined at different crossflow velocities. J cr for all crossflow velocity levels studied were very close to the corresponding steady-state flux J ss value measured under constant TMP mode. Under similar operating conditions (same feed side pressure, crossflow velocity and total membrane fouling), the process performance was different under the two modes of operation. At all the crossflow velocities studied, the internal fouling was greater under constant flux mode. This was attributed to the smaller particle size deposition and hence greater pore penetration under the lower flux force in low constant flux operation. For the same reason, smaller particles were present in the cakes deposited under constant flux mode compared with the cakes from the corresponding constant TMP mode runs. The two modes were combined in a sequence to further study the process performance. Constant flux followed by constant TMP operation fouled the membrane both internally and externally very severely. However, when constant TMP operation was followed by low constant flux operation, the surface fouling could be partly reversed. This suggests the possibility of improving the process performance by intermittently switching to low constant flux from the normal constant TMP mode during the operation.

The effects of electrolyte concentration and pH on protein aggregation and deposition: critical flux and constant flux membrane filtration

The performance of membranes used in the separation of solution mixtures containing proteins can be substantially limited by aggregation and deposition inside membrane pores and on their surfaces. In this study flux, stepping and constant flux operation have been used in conjunction with varying electrolyte concentrations and pHs in the microfiltration of aqueous solutions of bovine serum albumin. Experiments were performed to investigate the hydrodynamic and solution conditions during the incipient fouling stage which lead up to eventual cake formation. Results from flux stepping showed that an increase in wall concentration coincides with the onset of an apparent critical flux. This is followed by a time lag before an increase in observed rejection is exhibited. Sub-critical constant flux operation showed that there exists an aggregation and deposition time lag after which the membrane suddenly experiences a rapid increase in hydraulic resistance due to protein aggregates blocking a majority of membrane pores. These incipient fouling conditions were shown to be dependent on pH and ionic strength and were concluded to be the product of a balance between electrostatic, solubility and hydrophobic effects which were manifested in protein–protein and protein–membrane interactions.

A new coiled hollow-fiber module design for enhanced microfiltration performance in biotechnology

The microfiltration performance of a novel membrane module design with helically wound hollow fibers is compared with that obtained with a standard commercial-type crossflow module containing linear hollow fibers. Cell suspensions (yeast, E. coli, and mammalian cell cultures) commonly clarified in the biotechnology industry are used for this comparison. The effect of variables such as transmembrane pressure, particle suspension concentration, and feed flow rate on membrane performance is evaluated. Normalized permeation fluxes versus flow rate or Dean number behave according to a heat transfer correlation obtained with centrifugal instabilities of the Taylor type. The microfiltration performance of this new module design, which uses secondary flows in helical tubes, is significantly better than an equivalent current commercial crossflow module when filtering suspensions relevant to the biotechnology industry. Flux and capacity improvements of up to 3.2-fold (constant transmembrane pressure operation) and 3.9-fold (constant flux operation), respectively, were obtained with the helical module over those for the linear module.

Optimal efficiency analysis of induction motors fed by variable-voltage and variable-frequency source

The authors discuss the efficiency analysis and experimental data for an induction motor fed by a variable-voltage and variable-frequency (VVVF) source. Nonideal factors (core saturation, source harmonics, and skin effect) affecting the efficiency are included in the analysis to yield practical results from computer simulation. Based on the simulated results, an experimental system, composed of a DC link power converter with a VVVF feature, a dynamometer and a PC/AT, was developed to evaluate the efficiencies of the induction motor from low to rated speed and torque. Both analysis and experimental results indicate that the efficiency with VVVF is superior to that of constant flux operation. Experimental results showed that 10-15% improvement in the efficiency of a 2 hp induction motor at 0.4 per unit load using VVVF can be achieved as compared to the constant flux operation.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Control System Design of a Current Inverter Induction Motor Drive

An analytical design is presented of the control system of a slip-controlled current inverter induction motor drive. First of all, the drive dynamics are modeled in a control-oriented perspective. Afterwards, the transfer function between the rectifier output voltage and the inverter current is thoroughly analyzed and the results used to design the current controller. Two function generators are then synthetized and incorporated in the speed loop so as to obtain constant rotor flux operation and satisfactory torque dynamics. Lastly, the speed controller is designed for a fast and accurate response. All the parameters of the control system are given in terms of performance specifications and drive data. A computer implementation of the drive system is carried out and performance tests are shown.

Performance of a Thermoelectric Converter under Constant Heat Flux Operation

The performance characteristics of a conventional thermoelectric generator operating under constant heat flux conditions have been computed. Expressions for power output, optimum efficiency, and optimum ratio of internal-to-load resistance show that optimization requires operating conditions appreciably different from those for the operation of thermoelectric generators having constant hot and cold junction temperatures. Performance curves are presented for the optimum ratio of internal-to-load resistance when either the internal resistance or the load resistance is under the control of the designer.