Controlled flux behaviour of membrane processes

Controlling ultrafiltration (UF) and microfiltration (MF) membrane fluxes at or around a region where fouling is minimal can provide an interesting and economic operating regime. Selectivity may be enhanced and cleaning may be easier. For a given flux it is sometimes possible to filter a product suspension at the same trans-membrane pressure (TMP) as for pure water (PWP), but this can require a lot of energy input to maintain cross-flow or high shear in other ways if high fluxes are required. The critical flux is the flux above which one starts to observe fouling. By operating at lower cross-flow velocities and just above the critical flux, and thus, with lower TMPs, periodic cleaning can be effected by temporarily stopping permeation. A change in feed rate demands a change in flux which is obtained by temporarily increasing energy inputs. Controlled flux improves macromolecular fractionation. As flux increases the rejection of high molecular weight materials decreases whilst that of lower molecular weight materials decreases. This paper discusses the causes of fouling and the use controlled flux operation to mitigate its effects.     

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

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

Impact of ultrafiltration operating conditions on membrane irreversible fouling

The main limitation of the ultrafiltration (UF) process identified in drinking water treatment is membrane fouling. Although adsorption of natural organic matter (NOM) is known to cause irreversible fouling, operating conditions also impact the degree of irreversible fouling. This study examined the impact of several operating parameters on fouling including flux, concentrate velocity in hollow fibers, backwash frequency, and transmembrane pressure. A hydrophilic cellulose derivative membrane and a hydrophobic acrylic polymer membrane were used to conduct these tests. Pilot testing showed that when short-term reversible fouling was limited during a filtration cycle by increasing the concentrate velocity, reducing the flux, and increasing the backwash frequency, the evolution of the membrane toward irreversible fouling could be controlled. It appeared that operating parameters should be adjusted to maintain the increase of transmembrane pressure below a certain limit, determined to be approximately 0.85 to 1.0 bar for the tested UF membrane, in order to minimize the rate of irreversible fouling. This threshold for transmembrane pressure was confirmed empirically by compiling data from over 36 pilot studies. Other testing results demonstrated that hydraulic backwash effectiveness decreased as the transmembrane pressure applied in the previous filtration cycle increased. Backwash efficiency in terms of membrane flux recovery after hydraulic backwash was reduced by 50% when the transmembrane pressure was increased from 0.4 bar to 1.4 bar.     

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

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

Fouling of a ceramic microfiltration membrane by corn starch hydrolysate

The effect of operating parameters on fouling of a ceramic microfiltration membrane by corn starch hydrolysate of 95 dextrose equivalence was studied. Transmembrane pressures above 100 kPa had little or no effect on flux. Cross-flow velocity had a significant beneficial effect. The rate of flux decline was reduced significantly when the feed was adjusted from its natural pH of 4.2 to 10. However, this resulted in a dark brown clarified syrup (permeate). Scanning electron microscopy showed extensive fouling layers on the alumina surface with conventionally processed dextrose solutions and the least fouling layer with corn starch hydrolysate adjusted to pH 10. Maximum steady state flux for unconcentrated hydrolysate at its natural pH was 178 LMH obtained at low transmembrane pressures (103 kPa, 15 psi) and high cross-flow velocities (5 m s⁻¹). Adjustment of the pH to 10 can increase the flux by 40%.     

Filtration method characterizing the reversibility of colloidal fouling layers at a membrane surface: analysis through critical flux and osmotic pressure

A filtration procedure was developed to measure the reversibility of fouling during cross-flow filtration based on the square wave of applied pressure. The principle of this method, the apparatus required, and the associated mathematical relationships are detailed. This method allows for differentiating the reversible accumulation of matter on, and the irreversible fouling of, a membrane surface. Distinguishing these two forms of attachment to a membrane surface provides a means by which the critical flux may be determined. To validate this method, experiments were performed with a latex suspension at different degrees of destabilization (obtained by the addition of salt to the suspension) and at different cross-flow velocities. The dependence of the critical flux on these conditions is discussed and analysed through the osmotic pressure of the colloidal dispersion.     

Ultrafiltration of oil-in-water emulsion by using ceramic membrane: Taguchi experimental design approach

In this study, a Taguchi experimental design methodology was used to determine the importance of process parameters influencing the ultrafiltration (UF) of oil-in-water emulsions. Four parameters including pH (5–11), oil concentration (φ) (0.5–3% (v/v)), temperature (T) (25–45°C) and trans-membrane pressure (TMP) (1–5 bar) were studied at three levels. The highest flux was used as optimization criterion. In order to reduce the number of experiments, a Taguchi method was applied. Analysis of variance (ANOVA) was used to determine the most significant parameters affecting the optimization criterion. Filtration experiments were performed in a cross-flow operation at a total recycle condition in a laboratory-scale plant. The ceramic UF membrane with a pore size of 50 nm was employed in a tubular module with an active area of 0,418 m². We used water-soluble cutting oil mixed with water as a model oil-in-water emulsion. During the experiment, the drop size and zeta potential distributions were evaluated. The optimum conditions for UF providing the highest flux were found at TMP = 5 bar, pH = 7, and φ = 0.5 v/v%. The pH of emulsion had the highest impact on COD retention. The results of this study could be used as a guideline for operating UF systems with ceramic membranes at optimal conditions.     

Fouling in air sparged submerged hollow fiber membranes at sub- and super-critical flux conditions

The overall objective of the present study was to characterize the effect of operating a submerged air sparged membrane system over a wide range of operating conditions (i.e. sub- and super-critical flux conditions) on the extent and mechanisms of membrane fouling during drinking water treatment. The sub- and super-critical flux conditions considered were generated by varying the operating permeate flux, the bulk cross-flow velocity, the flow characteristics (i.e. with or without air sparging) and the configuration of the membrane modules.     

Performance of partially permeable microfiltration membranes under low fouling conditions

Controlling the onset of fouling and concentration polarization is critical in many membrane operations, particularly in the bioseparation area. By using stepping and constant flux experiments, the fouling threshold or `incipient fouling' region was studied for various microfiltration membranes, pH's, and bulk concentrations using bovine serum albumin. Experiments were conducted to try to decouple effects such are porosity and pore size on incipient fouling by using a combination of tracked etched and polyvinylidene difluoride membranes. Changes in protein transmission and wall concentrations near the fouling threshold were also compared across these membranes. While porosity determined the fouling rate after the exceeding the fouling threshold, pore size appear to be an dominant factor in determining level of the fouling threshold itself. The effect of pH also supports the hypothesis that the rejections are initially dominated by membrane–solute interactions but are subsequently modified by protein adsorption to the surface as the wall concentration increases. Repulsive forces between membrane and solute allow greater rejection (greater wall concentration) to be maintained without fouling but did not increase the critical flux substantially. Attractive electrostatic forces allow greater passage of solute (lower wall concentration), but the protein adsorption soon dominated and the onset of fouling occurred much more quickly. Using a conventional concentration polarization model, analysis of the results indicates that the onset of fouling is occurring at a relatively low wall concentrations.     

Critical flux concept for microfiltration fouling

Several constant-flux filtration experiments for yeast cell suspensions, yeast cell debris, and dodecane-water emulsion were performed at various operating conditions in both flat-sheet and tubular-membrane systems. The aim of the paper is two-fold. Firstly the relationship between constant-flux behaviour and membrane fouling is discussed. In some cases constant-flux filtration was realized at a constant transmembrane pressure which was below a critical value. In general constant-flux filtration was obtained with moderately increasing transmembrane pressure, and this approach is shown to have some advantages over normal constant-pressure filtration because it clearly provides for the possibility of avoiding over-fouling and so reduces the severity of fouling. Secondly, the concept of critical flux is introduced. Whilst it has long been recognised that low-pressure microfiltration is much more effective than high-pressure microfiltration, the emphasis in this work is upon the possible existence of a critical flux and the desirability of starting filtration operations at a low flux. The critical-flux hypothesis is that on start-up there exists a flux below which a decline of flux with time does not occur. Equations which may enable identification of the appropriate flux level are included.     

Dimensional analysis of steady state flux for microfiltration and ultrafiltration membranes

Dimensional analysis of the mass, length and time shows that the steady state flux observed for microfiltration or ultrafiltration through inorganic composite membrane can be expressed using two dimensionless numbers. The shear stress number N_S compares the shear stress against the membrane wall to the driving pressure, while the resistance number N_f compares the convective cross-flow transport to the drived transport through a layer, whose resistance is the sum of all the resistances induced by the different processes which limit the mass transport. Experimental data obtained in ultrafiltration of hydrocarbon emulsions and microfiltration of methanogenic bacteria suspensions and secondary treated wastewater were recalculated in terms of these dimensionless groups. Straight lines were plotted whose slope depends solely on the suspension and the membrane and not on the solute concentration. A negative slope and a positive intersection with the N_S axis means that a cake layer or a polarization layer can be completely eliminated at a critical cross-flow velocity; this was the case for an inorganic particles suspension and for the methanogenic suspension. A straight line of negative slope followed by a plateau means that an irreversible fouling is superimposed to the reversible phenomenon; this was observed for a secondary treated wastewater. A positive slope means that fouling predominates; this was observed with hydrocarbon emulsions.     

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

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

Productivity enhancement in a cross-flow ultrafiltration membrane system through automated de-clogging operations

A membrane system only has a limited operational lifetime, whereby it becomes so severely fouled that continued operation must be stopped. In the cross-flow configuration of membrane filtration of wastewater, both increased cross-flow velocities and decreased operational transmembrane pressures can be used to decrease membrane fouling and extend the life cycle of the membrane separation process. The study found that an optimised usage of two de-clogging techniques, with a 1 h production period followed by a 1 min relaxation period and then a 1 min high cross-flow rate period, resulted in a net productivity increase of 14.8%.

The study involved a detailed investigation into the utilization of two automated cleaning techniques to reduce fouling problems encountered when cross-flow membrane systems are operated with high permeate flux rates. The two cleaning techniques studied were periodic membrane relaxation and a periodic high rate cross-flow. During both the relaxation and high rate cross-flow periods, permeate production was stopped. This results in an operational loss in productivity. When each cleaning technique was operated individually, there was a net productivity decrease of 0.7%, due to the 3.2% operational loss due to cleaning technique being implemented.

The system was developed using a Programmable Logic Controller (PLC) and a Supervisory Control and Data Acquisition (SCADA) system to accurately control and monitor the process.     

Analysis of humic acid fouling during microfiltration using a pore blockage–cake filtration model

Fouling by natural organic matter, such as humic substances, is a major factor limiting the use of microfiltration for water purification. The objective of this study was to develop a fundamental understanding of the underlying mechanisms governing humic acid fouling during microfiltration using a combined pore blockage–cake filtration model. Data were obtained over a range of humic acid concentrations, transmembrane pressures, and stirring speeds. The initial flux decline was due to pore blockage caused by the deposition of large humic acid aggregates on the membrane surface, with a humic acid deposit developing over those regions of the membrane that have first been blocked by an aggregate. The rate of cake growth approaches zero at a finite filtrate flux, similar to the critical flux concept developed for colloidal filtration. The data were in good agreement with model calculations, with the parameter values providing important insights into the mechanisms governing humic acid fouling during microfiltration. In addition, the basic approach provides a framework that can be used to analyze humic acid fouling under different conditions.     

Effect of operating conditions on separation performance of reactive dye solution with membrane process

Membrane process has increasingly developed as a reliable and effective means of improving product yield and reducing manufacturing costs in the reactive dye industry. In order to improve a product's quality, ultrafiltration (UF) membrane has been applied to perform Reactive Brilliant Blue KN-R desalting and concentration. The performance of this membrane's separation process was evaluated under different operating conditions, through which the influence of operating pressure, temperature, cross-flow velocity, pH, concentration of feed and operating time on permeate flux, rejection of Reactive Brilliant Blue KN-R and sodium sulfate were studied.     

Critical flux measurement for model colloids

The conventional operating membrane of a laboratory membrane filtration process is to apply controlled transmembrane pressures to the retentate side of the membrane, with the permeate side open ended. Often the minimum transmembrane pressure available is sufficient to cause membrane fouling in a given system. A membrane rig has been built which monitors transmembrane pressure in increments of 0.001 bar and by pumping permeate at a specified rate controls the flux to be constant. The technique used allows sensitive detection of trace fouling. Under a variety of low flux conditions fouling was not observed and it was found to be useful to produce an experimentally related definition of two types of critical flux. In the first definition a `strong form' of critical flux exists if the flux of a suspension is identical to the flux of clean water at the same transmembrane pressure. In the second definition a `weak form' of the critical flux exists if the relationship between transmembrane pressure and flux is linear, but the slope of the line differs from that for clean water. This paper describes how the use of this operating mode led to the successful experimental measurements of critical fluxes for two colloidal silica suspensions, BSA solution and a baker's yeast suspension with a 50k MWCO membrane. These measurements could not be made successfully in constant-pressure mode. The paper also reports experimental evidence in support of a `strong form' of the critical flux for the filtration of X30 silica suspension. Finally, we report the effect of membrane pore size on critical flux measurements for the three types of feed fluids.     

Whey protein fouling of large pore-size ceramic microfiltration membranes at small cross-flow velocity

Microfiltration of whey protein solutions by tubular ceramic membranes, under constant cross-flow and trans-membrane pressure, with periodic backwashing, is investigated using a fully instrumented pilot unit. Relatively large nominal membrane pore size (0.8 μm) insures very high protein transmission, which is desirable in applications such as microbial load reduction. In the first of a sequence of three filtration-backwashing cycles, irreversible and reversible fouling are identified, over the tested pressure range of 5–17.5 psi. Early in the first cycle, especially at the higher pressures, a pore constriction/blocking mechanism appears to be responsible for the irreversible fouling. In the other two cycles only the reversible fouling is significant, possibly due to some kind of protein layer formation on the membrane surface. The permeate flux level tends to increase by increasing trans-membrane pressure up to a near-optimum value of 10 psi, beyond which pressure has a negative effect. This interesting trend is attributed to the interplay of cross-flow velocity, which tends to reduce fouling by promoting re-suspension and breakage of colloidal protein agglomerates, with the trans-membrane pressure (and related flux) which leads to protein layer formation on the membrane and may impose compressive stresses, thereby increasing its resistance to permeation.     

Influence of operating conditions on performance of ceramic membrane used for water treatment

The removal of natural organic matter (NOM) is a critical aspect of potable water treatment because NOM compounds are precursors of harmful disinfection by-products, hence should be removed from water intended for human consumption. Ultrafiltration using ceramic membranes can be a suitable process for removal of natural substances. Previously reported experiments were dedicated to evaluating the suitability of ultrafiltration through ceramic membrane for water treatment with a focus on the separation of natural organic matter. The effects of the membrane operating time and linear flow velocity on transport and separation properties were also examined. The experiments, using a 7-channel 300 kDa MWCO ceramic membrane, were carried out with model solutions and surface water at trans-membrane pressure of 0.2–0.5 MPa. The results revealed that a loose UF ceramic membrane can successfully eliminate natural organic matter from water. The permeability of the membrane was strongly affected by the composition of the feed stream, i.e. the permeate flux decreased with an increase in the NOM concentration. The permeate flux also decreased over the period of the operation, while this parameter did not influence the effectiveness of separation, i.e. the removal of NOM. It was observed that the increased cross-flow velocity resulted in the decrease in the membrane-fouling intensity and slightly improved the retention of contaminants.     

Fouling of reverse osmosis membranes by biopolymers in wastewater secondary effluent: Role of membrane surface properties and initial permeate flux

Reverse osmosis (RO) is being increasingly used in treatment of domestic wastewater secondary effluent for potable and non-potable reuse. Among other solutes, dissolved biopolymers, i.e., proteins and polysaccharides, can lead to severe fouling of RO membranes. In this study, the roles of RO membrane surface properties in membrane fouling by two model biopolymers, bovine serum albumin (BSA) and sodium alginate, were investigated. Three commercial RO membranes with different surface properties were tested in a laboratory-scale cross-flow RO system. Membrane surface properties considered include surface roughness, zeta potential, and hydrophobicity. Experimental results revealed that membrane surface roughness had the greatest effect on fouling by the biopolymers tested. Accordingly, modified membranes with smoother surfaces showed significantly lower fouling rates. When Ca²⁺ was present, alginate fouled RO membranes much faster than BSA. Considerable synergistic effect was observed when both BSA and alginate were present. The larger foulant particle sizes measured in the co-existence of BSA and alginate indicate formation of BSA-alginate aggregates, which resulted in greater fouling rates. Faster initial flux decline was observed at higher initial permeate flux even when the flux was measured against accumulative permeate volume, indicating a negative impact of higher operating pressure.