The role of bentonite addition in UF flux enhancement mechanisms for oil/water emulsion

The main problem in treating oil/water emulsion from car wash waste-water by ultrafiltration (UF) is fouling caused by oil adsorption on the membrane surface and internal pore walls. This study demonstrates that the addition of bentonite clay can reduce the adsorption layer on cellulose acetate UF membrane, resulting in a reduction of total membrane resistance (R_t). Experiments were conducted to identify and describe three possible mechanisms: (i) bulk oil emulsion concentration reduction; (ii) particle aggregation and (iii) detachment of the adsorbed gel layer by shear force. Adsorption of oil emulsion by bentonite can lead to a significant reduction of bulk oil emulsion concentration, one of the major causes of flux enhancement. Results show that contact of oil emulsion with bentonite forms larger particles resulting in flux increment. An optimum particle size of 37 μm, corresponds with a bentonite concentration of 300 mg/l and provided the highest flux. Beyond this limiting concentration, flux improvement gradually declined, possibly due to the formation of packed cake of particles on the membrane surface. The presence of bentonite in the oil emulsion promotes high shear stress which acts against the gel layer. This high shear stress, caused by bentonite particles and cross-flow velocity, reverses the adsorbed gel layer to the bulk of the liquid phase.     

Analysis of the slip effect on the permeate flux in membrane ultrafiltration

A method for predicting the mass transfer coefficient as well as the limiting permeate flux in membrane ultrafiltration has been found, based upon the boundary-layer theory which takes into account the slip velocity on the membrane surface. The theory presupposes the existence of a slip flow on a porous membrane surface, especially for the limiting permeate-flux operations. Further, the slip velocity increases with the size of the pores of the membrane, with feed velocity and also with feed concentration. The theory also showed that the permeate flux increases with the increase of the slip velocity. A considerable improvement in theoretical prediction of the permeate flux is expected if the slip flow effect is taken into consideration.     

How slug flow can enhance the ultrafiltration flux in mineral tubular membranes

The study deals with the use of a gas-liquid two-phase flow to reduce tubular mineral membrane fouling by injecting air directly into the feed stream. The injected air is supposed to create complex hydrodynamic conditions inside the ultrafiltration module which destabilize the concentration layer over the membrane surface. The experimental study was carried out by filtering suspensions (bentonite and yeast) through an ultrafiltration tubular mineral membrane. A range of transmembrane pressures and various liquid and gas flow-rates were tested. Results related to the permeate flux showed an enhancement by a factor of 3, with a slug flow-structure for the two kinds of suspension (200% of flux increase). Furthermore, the applicability of such an unsteady technique was examined with a view to reduce energy consumption.     

Suction effects on the hydrodynamics in the vicinity of an ultrafiltration plane ceramic membrane

The wall shear stress is determined at the surface of a plane ceramic ultrafiltration membrane in a tangential ultrafiltration module. At first, the shear stress is determined at the surface of a plate of Plexiglas mimicking a membrane and at the plane membrane surface without fouling particles with the aim to investigate the influence of permeation. Then, ultrafiltration of a suspension of spherical rigid particles inducing a deposit at the membrane surface is done. In order to study the influence of the foulant, two concentrations of particles were used for the experiments. The values of the mean wall shear stress and its fluctuations (turbulent intensity rate) were measured by using an electrochemical method. Twenty microelectrodes, on which an electrochemical reaction occurs, are mounted flush to the plate of Plexiglas and to the surface of the membrane to determine the maps of shear stress and turbulent intensity rate for two inlet/outlet distributors’ configurations. This work emphasized the fact that, due to permeation across the membrane, the mean velocity gradient value is enhanced and, on the contrary, its local fluctuations are clearly damped, even in the turbulent flow regime. Thus, the effect of the shape of the inlet/outlet fluid distributors is also minimized compared with the results obtained without permeation at the surface of the Plexiglas plate.     

Coagulation and ultrafiltration: Understanding of the key parameters of the hybrid process

A hybrid coagulation–ultrafiltration process has been investigated to understand membrane performance. Coagulation prior to ultrafiltration is suspected to reduce fouling by decreasing cake resistance, limiting pore blockage and increasing backwash efficiency. Coagulation followed by tangential ultrafiltration should gather the beneficial effects of particle growth and cross-flow velocity. Our study aims at determining the key parameters to improve membrane performance, by describing floc behaviour during the hollow fibre ultrafiltration process. Flocs encounter a wide range of shear stresses that are reproduced through the utilization of different coagulation reactors. Performing a Jar-test enables the formation of flocs under soft conditions, whereas Taylor-Couette reactors can create the same shear stresses occurring in the hollow fibres or in the pump. Synthetic raw water was made by adding bentonite into tap water. Five organic coagulants (cationic polyelectrolytes) and ferric chloride were selected. Floc growth was thoroughly monitored in the different reactors by laser granulometry. Coagulation–ultrafiltration experiments revealed different process performance. The effect on the permeate flux depended on the coagulant used: some coagulants have no influence on permeate flux, another enables a 20% increase in permeate flux whereas another coagulant leads to a decrease of 50%. Flocs formed with ferric chloride do not resist shear stress and consequently have no influence on permeate flux. These results show the necessity to create large flocs, but the size is not sufficient to explain membrane performance. Even if flocs show a good resistance to shear stress, a high compactness (D_f = 3) will lead to a dramatic decrease of permeate flux by increasing the mass transfer resistance of the cake. On the contrary, flocs less resistant to shear stress, then smaller and also more open have no effect on permeate flux. An optimum was quantified for large flocs, resistant enough to shear stress facilitating flow between aggregates.     

A theory of limiting flux in a stirred batch cell

Theoretical and experimental investigations of ultrafiltration (UF) of albumin solutions were conducted with a batch stirred cell. The theoretical model predicted that the limiting flux u∞ is equal to (K'_m/ø), where K'_m is the effective mass transfer coefficient and ø is the rejection coefficient. This interesting finding simply says that the limiting flux is independent of the pressure applied as was observed by many workers. Also, the limiting flux u∞ was theoretically found to increase with the stirring speed and decreases with the bulk protein concentration which is consistent with previous experimental findings. Experimental studies confirmed these predictions. Albumin ultrafiltration rate appears to be limited by the increased adsorption rate of the protein on the membrane surface and onto the pores of the membrane.     

A new generator of unsteady-state flow regime in tubular membranes as an anti-fouling technique: A hydrodynamic approach

A new generator of pulsatile flow has been developed. It consists of a rotating distributor disc judiciously perforated and placed in front of the entrance plane of a tubular membrane bundle. A laboratory-scale apparatus was built with a five membrane bundle. Two configurations were studied: upstream-disc-position (UDP) and downstream-disc-position (DDP). The main new feature is that the pulsatile flow is generated only in the membranes whereas no variation of flow or pressure occurs elsewhere in the equipment. The hydrodynamic behaviour was successfully modelled; experimental and calculated data are in good agreement. Filtration tests with an aqueous suspension of bentonite showed a close relation between the permeate flux and the pulsatile crossflow velocity. First results are encouraging: a reduction in crossflow velocity of 50% with the same power consumption per unit permeate flux as required for steady crossflow filtration.     

An annular dual-membrane continuous cation exchanger packed with ion exchange resin

Design, construction details and performance characteristics are described for an annular perfluorosulfonate (Nafion) dual tubular membrane assembly where inside of the inner membrane tube and outside of the outer membrane tube are packed with ion exchange resin beads. During operation, the solution to be ion exchanged flows in the annular space between the two membrane tubes and a regenerant solution flows countercurrent to the principal flow, through the inner tube and outside the outer tube. With a principal channel hold up volume of ∼ 125 μl and capability of operation at head pressures up to 200 psi, the device can continuously exchange 200-250 μeq of Na⁺ per minute for H⁺. While this particular device was intended for use as a “suppressor” in anion chromatography, the design concept should be of utility in other membrane-based operations.     

How slug flow can improve ultrafiltration flux in organic hollow fibres

The study deals with the use of a gas-liquid two-phase flow to reduce particle membrane fouling in organic hollow fibres by injecting air directly into the feed stream. A theoretical approach of slug flow in fibres demonstrates that the slugs created inside the fibres induce high wall shear stresses. Moreover, the membrane surface is alternately submitted to positive and negative shear stresses. This succession of stresses is expected to prevent filtered particles from settling on the membrane surface and then enhance the ultrafiltration mass transfer. Experiments were carried out with clay suspensions in hollow fibre membrane. A range of various air velocities and particle concentrations was examined and the effect of a steady gas flow was compared to that of an intermittent one. As expected, the injecting air process leads to an increase of the permeate flux by up to 110% for U_g=1 m s⁻¹ (flux multiplied by 2.1), for all the various concentrations studied. Furthermore, even at a low air velocity a significant enhancement can be achieved (+60% for U_g=0.1 m s⁻¹, flux multiplied by 1.6). An intermittent gas flow seems to be less effective than a steady one in similar experimental conditions.     

Flux enhancement in crossflow filtration using an unsteady jet

This paper deals with the influence of a new type of unsteadiness in the flow on the permeate flux in crossflow filtration. A pneumatically controlled valve generates intermittent jets from the main flow leading to the formation of large vortices moving downstream along the tubular membrane. The experimental study was carried out by filtering a bentonite suspension through an ultrafiltration mineral membrane. Flux time measurements were taken under steady and unsteady operating conditions. The unsteadiness leads to a permeate flux more than two times higher than in the usual filtration processes.     

Hollow-fibre membrane module design: comparison of different curved geometries with Dean vortices

The performance of several designs of curved membrane modules with Dean vortices was compared through experiments using a colloidal bentonite suspension and cellulose acetate hollow-fibre ultrafiltration (UF) membranes. The different module geometries were: straight, helically coiled, twisted and sinusoidal, or meander-shaped. The experiments show a remarkable increase in mass transfer in curved modules as compared to conventional straight ones. Comparisons were made for modules equipped with the same hollow fibres and the same Dean number (De) for a given Reynolds number (Re). At the same Dean number, all the curved geometries gave the same limiting permeate flux. A mass transfer correlation relating limiting UF flux with the mean wall shear stress has been obtained.     

Flux improvement by Dean vortices: ultrafiltration of colloidal suspensions and macromolecular solutions

Coiled and straight hollow-fibre modules have been built and tested; the permeate flux obtained in ultrafiltration with these two geometries is compared for two feeds: a colloidal bentonite suspension and a dextran solution. In the case of colloidal suspensions, the secondary flows induced by the coiled geometry allow fouling to be reduced and the permeate flux is multiplied by a factor of up to 2. An empirical relationship is proposed to express the limiting flux of permeate as a function of both the velocity and some geometrical parameters of the coiled modules. Analogous results are obtained during the ultrafiltration of dextran. It is also shown that under certain conditions almost no deposit was formed; the permeate flux under these conditions is three times higher for coiled modules than for straight ones. For a given energy expenditure and ultrafiltration process, the gain in permeate flux can reach a factor of 1.8.     

Electrically enhanced crossflow membrane filtration of oily waste water using the membrane as a cathode

The effect of an external electric field on the flux in crossflow membrane filtration of a model oily waste water was studied using a carbon fibre – carbon composite membrane as a cathode. Limiting fluxes for low flow rate increased significantly under the conditions studied, from 75 l/m² h without an electric field to more than 350 l/m² h using an electric field. The experimentally determined increase in the limiting flux showed good agreement with the theoretical value of 430 l/m² h calculated using a simple model. The limiting flux increase was affected by the electrophoretic mobility of the oil droplets and the applied electric field strength. When there were no cakes without an electric field due to the high flow rate, the flux increase when using an electric field under at the same conditions was minor. The critical electric field strength was determined, and experimentally obtained values were corresponded with calculated values. Decreasing the crossflow velocity above the critical electric field strength increased the flux, or had no effect, depending on the size of the particles. Permeate quality was also improved to some extent when using the electric field, and a membrane with a large pore size could be used when an electric field was applied. The main disadvantage in using the membrane as a cathode was foaming at the membrane surface causing decrease in the flux enhancement as the conductivity of the feed increased. It was not possible to restore the flux to the original value by applying an electric field after filtration of the oil emulsion without an electric field. An intermittent electric field was thus not efficient enough for keeping the flux at high level.     

Use of intermittent jets to enhance flux in crossflow filtration

This paper deals with the influence of a new flow unsteadiness on the permeate fluxes in crossflow filtration. A pneumatically controlled valve generates intermittent jets from the main flow, causing the formation of large vortices moving downstream along the tubular membrane. The main results of the numerical calculation of such flows are given. The experimental study was carried out by filtering a bentonite suspension through an ultrafiltration mineral membrane. Time evolutions of flux were achieved in steady and unsteady operating conditions. Results concerning the influence and limits of the nozzle to tube diameter ratio and the jet velocities are discussed. The applicability of such an unsteady flow is examined with a view to effects on energy consumption and possible viscosity effects.     

Gel layer formation and hollow fiber membrane filterability of polysaccharide dispersions

Gel layer formation on the membrane surface during filtration plays a significant role in membrane fouling that, in many instances, controls water production and energy consumption in the treatment of waters and wastewaters. In this study, alginate is selected as a model of the polysaccharides prevalent in wastewaters which, on membrane filtration, may form a gel on the membrane surface which subsequently limits filtrate throughput. We show that over the range of the applied pressures of 11.7–135 kPa considered here, constant pressure ultrafiltration of alginate follows the behavior of cake filtration. The material properties of the alginate are determined by the employment of the previously developed steady-state filtration approach. The consolidation of the gel layer is found to be controlled by the hydraulic flow resistance rather than the rearrangement of particles. Under these conditions, it is valid to apply the derived material properties for the quantification of both constant pressure and constant flux filtration. The gel layer formed from alginate is very compressible and far from uniform over its depth. Within the range of the applied pressures, the gel layer is very porous with a water content of more than 96% but very low Darcy permeability of less than 1 × 10⁻¹⁷ m². During hollow fiber membrane filtration, the local flux is neither uniform nor constant along the fiber length, resulting in non-uniformity of the growth rate, the average porosity and the thickness of the gel layer. The non-uniformity is most apparent at the start of filtration and then gradually diminishes as the gel layer builds up with ongoing filtration.     

Influence of inclination on gas-sparged cross-flow ultrafiltration through an inorganic tubular membrane

The effect of membrane inclination on the flux of single-phase or gas–liquid two-phase ultrafiltration in a tubular membrane has been investigated. Experimental result shows that membrane inclination has a significant enhancement on the flux of two-phase ultrafiltration operated at slug flow pattern. As the angle of inclination from the horizontal increases, the flux increases, reaches a maximum, and then decreases. The flux may be enhanced more than 1.5 when the membrane is inclined from 0 to 50°. The flux enhancement due to membrane inclination increases with increasing the gas velocity, the feed concentration, and the transmembrane pressure, while it decreases with increasing the liquid velocity. The optimal inclination angle of the membrane in a slug-flow ultrafiltration is close to 50°. An equation for determining the optimal inclination angle was also proposed in this work.     

Novel ultrafiltration membranes with the least fouling properties for the treatment of veterinary antibiotics in the pharmaceutical wastewater

Dendrimers have received more attention in all fields of research these days. In the present study, polyamidoamine (PAMAM) dendrimers were synthesized on the acrylic ultrafiltration membranes to minimize fouling as an important deficiency in the separation process. The antifouling activity of these dendrimers with different generations (G0‐3) was tested to restrict three macrolides (tylvalosin, tylosin, and tulathromycin) and two pleuromutilins (tiamulin and valnemulin) as veterinary antibiotic drugs with amine groups and positive charges at pH = 7 of the membrane surface. These compounds are risky for human consumption. Due to having several amine functional groups and branches, PAMAM dendrimers can be a great coating agent for antifouling. G3 PAMAM dendrimer‐coated membranes had the best performance (water flux: 130.7 L/m²·h, rejection of tulathromycin: 91.4%, flux recovery ratio: 86.3%). The function of this ultrafiltration process depended on pore size and also charge surface. A significant reduction for irreversible and reversible fouling was observed for this new ultrafiltration membrane (F_ir: 14.5%, F_re: 21.9%). This observation was confirmed by the power law model. Three 5‐hour cycle ultrafiltration processes were carried out for veterinary antibiotic wastewater that showed 3.18% loss of initial water flux (for the third cycle), final cleaning efficiency of 96.82%, and tylvalosin rejection of 94.1%.     

Membrane fouling in a constant-flux ultrafiltration cell

Factors affecting protein fouling of ultrafiltration membranes were investigated both in the presence and absence of a net transmembrane flux. The results obtained for the zero flux system showed that membrane-associated protein levels followed a saturation type isotherm having a maximum value of 207μg-cm⁻². In the presence of flux, membrane-associated protein showed an increase with increasing transmembrane flow. Under the conditions studied the membrane-associated protein eventually reached a flux-independent concentration which was a function of the tangential recycle flow. In all cases the limiting protein level was higher than the maximum level obtained in the absence of flux. The flow resistance of the fouling layer continued to increase after the membrane-associated protein had reached a limiting value, supporting the concept of a limited flux approach to modelling. The protein “adsorption” observed was considerable higher than could be explained by monolayer formation, and unlike Langmuir adsorption was shown to be freely reversible with changes in the hydrodynamic conditions.     

Effect of electric field during gel-layer controlled ultrafiltration of synthetic and fruit juice

Electric field enhanced ultrafiltration of pectin–sucrose mixture (synthetic juice) and mosambi (Citrus sinensis (L.) Osbeck) fruit juice using 50,000 (MWCO) polyerthersulfon membrane is studied in a cross-flow cell. Pectin, completely rejected by the membrane, forms a gel type layer over the membrane surface. Under the application of an external dc electric field across the membrane, gel-layer formation is restricted leading to an enhancement of permeate flux. During ultrafiltration of synthetic juice, application of dc electric field (800 V/m) increases the permeate flux to almost threefold compared to that with zero electric field. A theoretical model based on integral method assuming suitable concentration profile in the boundary layer is developed. The proposed model is used to predict the permeate flux in gel-layer governed electric field enhanced ultrafiltration. Predictions of the model are successfully compared with the experimental results under a wide range of operating conditions. Experiments with fruit juice also demonstrated significant increase in flux with the application of a suitable electric field.