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There is some controversy in the literature on the effects of combined fouling of organics and colloids on water flux. There are reports of more flux decline for combined fouling than that of fouling by individual foulants, which has been attributed to synergistic effects. However, some authors observed lower flux decline in combined fouling. In addition, some authors have reported on increased flux declines by calcium addition, whereas others have shown improved fluxes after calcium addition. The discrepancies observed in the literature are systematically investigated in more detail in this manuscript. The organic–colloid concentration ratio was varied in order to explain discrepancies in previously observed effects in combined fouling. Further, the probable contributing factor of colloid type and calcium concentration in combined fouling was also examined. Organic, colloidal and combined fouling was found to be aggravated in the presence of Ca 2+ due to organic–Ca 2+ complexation and the reduction in foulant–membrane repulsive interactions, but this only occurred up to certain Ca 2+ -concentrations. Above these concentrations, the opposite effect was observed. In addition, membrane–foulant as well as foulant–foulant affinity interactions were determined and related to initial and later fouling rates. A novel type of “sequential” fouling experiments were carried out for the first time, and yielded information on how colloids and organics interact when both are present in fouling solutions. It was concluded from these experiments that hindered back diffusion of organics by a layer of colloids was the main factor responsible for flux decline in combined fouling. [ABSTRACT FROM AUTHOR]

Patterned microfiltration membranes were fabricated via Phase Separation MicroFabrication (PSMF) using vapor-induced phase separation to avoid skin formation on the nonpatterned surface. A starting solution of 10% PES, 60% PEG400, 5% water and 25% NMP yielded symmetric membranes with similar pore size all throughout the cross section when 10 or 15 min of vapor exposure was used before further coagulation in water. On the other hand, 5 min of vapor exposure resulted in what may be called inverse asymmetric membranes, which had smaller pores on the patterned (mold) side and macrovoids throughout the cross-section. The fidelity in replicating the pattern was higher when the vapor exposure was shorter and surface area enhancements of 103% and 52% were obtained when vapor exposure times were 5 and 10 min, respectively. The phase inversion rates of the polymer solutions in liquid water, humid air and humid air followed by liquid water were examined by measuring the light transmission through the solutions in time. It was observed that water in the starting solution increased the overall phase inversion rate and decreased the delay time before phase separation, in consistence with the membrane morphologies obtained with the solutions with and without water under different coagulation conditions. The patterned and nonpatterned membranes were tested in constant flux crossflow filtration of yeast suspensions at different fluxes and it was seen that the fouling rate of nonpatterned membranes was higher than patterned membranes. However, the major contribution to the improved fouling behavior appears to be the increased surface area since when the fluxes were normalized by the patterned (actual) surface area for foulant deposition, the difference between the fouling rate of patterned and nonpatterned membranes was not significant. [ABSTRACT FROM AUTHOR]

Pressure-retarded osmosis (PRO) has the potential to convert salinity gradient energy to hydraulic energy via a semi-permeable membrane. The performance of an osmotic membrane is usually evaluated using a crossflow membrane test cell with symmetric channels. However, it is not possible to test a PRO membrane under conditions of large differences in hydraulic pressure because the membrane cannot be supported in the channel inlet and outlet with a cavity. In this study, we present a newly designed PRO crossflow test cell that features asymmetric channels on both sides of the membrane. This asymmetric design prevents membrane bulge and rupture in the channel inlet and outlet even at an applied hydraulic pressure difference greater than 54.5 bar. Excellent performance of this novel PRO cell was demonstrated using two different membranes at various operating pressures. To prevent membrane deformation at open spaces between spacer strands, a feed channel spacer should be selected carefully. Accordingly, the effects of tricot feed spacers with different voidages and thicknesses on PRO performance were also investigated. [ABSTRACT FROM AUTHOR]

Previous work has suggested that membrane patterning is a promising approach to fouling mitigation. In this systematic study, we describe performance metrics for the cross-flow filtration of colloidal suspensions through ultrafiltration membranes with lithographically patterned surfaces. The effects of particle size, cross-flow velocity, pattern size, and flow configuration on membrane performance are specifically considered. Using an unpatterned membrane as a reference, results show that the presence of surface patterns increases the critical flux associated with filtration of colloidal feed solutions. For the patterned membranes, the critical flux increases with particle size, cross-flow velocity, the angle between the feed flow and the pattern lines, and the pattern height. These experimental findings can be correlated with a back-transport mechanism such as shear-induced diffusion, which ultimately increases the threshold permeation flux associated with the onset of particle deposition. [ABSTRACT FROM AUTHOR]

Abstract: Fouling, more precisely biofouling, remains one of the main obstacles for a wider application of membrane technology. One of the most significant problems limiting membrane fouling control is the inadequate understanding of the basic mechanisms governing adsorption/deposition of foulants in and on membranes. A detailed knowledge with regard to fouling characteristics and development would be clearly advanced if a reliable and repeatable, non-invasive study tool would be available. For this purpose, a direct visual observation system, containing multiple separated cross-flow filtration units, coupled with either an optical or a confocal laser scanning microscope was developed. This approach allows the online, non-destructive monitoring of the fouling process while filtering and the high number of parallel filtrations makes it possible to either analyze different membranes, to apply statistical evaluations, run different feed solutions or to sample and perform membrane autopsies at different time points. This technique allows for the investigation of microbial ecology with respect to microbe–membrane attachment, site-specific associations of micro-organisms, deposition of foulants on the membrane surface and spatial organization of microbial communities on membrane surfaces through the use of fluorescent microscopy techniques. This manuscript illustrates the reproducibility of the developed research tool with respect to membrane flux decline and fouling characterization. Two membrane types, a polyvinylidene fluoride microfiltration (under filtering conditions) and a polyamide reverse osmosis membrane (in non-permeating mode), with respectively a bacterial foulant, Pseudomonas aeruginosa and tap water, were chosen as model systems. [Copyright &y& Elsevier]

Abstract: The cross-flow microfiltration applied to wine filtration has become a legitimate alternative to conventional filtration processes. However, membrane fouling which affects the operating costs and the plant maintenance, limits the widespread application of this technique. The aim of this review is to provide a better understanding of the development of the cross-flow microfiltration in wine industry, as well as the complexity of wine composition and its consequences on membrane fouling. This review covers also the impact of the operating conditions and the membrane characteristics on fouling mechanisms. Strategies to limit fouling as well as the latest innovations and commercial proposal are discussed in this paper. [Copyright &y& Elsevier]

Abstract: In the dairy industry, whey protein is concentrated by means of cross-flow ultrafiltration (UF). In cross-flow UF, it is considered that the surface part of the fouling layer can be removed by the effect of the shear stress exerted by the feed flow. As a result, the flux decline is suppressed and a steady state flux eventuates. However, the means to express the relationship between the feed flow and the permeate process has not been come to definite conclusion. In this study, the compressive yield stress model for the permeation through a fouling layer on a membrane in dead-end UF was applied to cross-flow UF considering the removal rate of the fouling layer. The time course of permeate flux estimated based on the model. In the estimation, the concentration at the top of the fouling layer must be specified and the concentration distribution in the layer should be calculated. The rheology of a whey protein concentrate (WPC) suspension, which is used as a model fluid, was investigated. The concentration at the top of the fouling layer was determined based on the relation between the shear stress exerted by the feed flow and the rheology investigated. The validity of the model was discussed by comparing the results of estimation by the model and experiments. [Copyright &y& Elsevier]

Abstract: Electric field-assisted cross-flow ultrafiltration of mosambi (Citrus sinensis (L.) Osbeck) juice has been carried out using a 50kDa molecular weight cut-off membrane. Electric field is applied in both constant as well as pulse mode. The effect of process parameters, e.g., electric field, pulse ratio, cross-flow velocity and transmembrane pressure on the permeate flux and energy consumption are studied. Application of dc electric field has resulted in significant improvement of permeate flux. Application of pulsed electric field is found to be more beneficial in terms of energy consumption, and equally effective in terms of flux augmentation as compared to constant field. An already developed resistance-in-series model based on gel layer theory is used incorporating the effect of constant as well as pulsed electric field and numerically solved to quantify the flux decline and gel layer deposition. The predictions from the numerical solution of the model are successfully compared with experimental results under various operating conditions. [Copyright &y& Elsevier]

Abstract: The purpose of the study was to investigate the change in membrane resistance in relation to operating pressure for clean RO and NF membranes during filtration of a bioreactor effluent. Experiments were conducted using a SEPA CFII membrane element cell and effluent from a bioreactor with high-ionic content. Flux–pressure profiles of two RO and one NF membranes were analyzed and compared with those for filtration of deionized water. The flux–pressure profile increased linearly with increasing pressure for all three membranes during filtration of deionized water. However, during filtration of the bioreactor effluent, the rate of increase in flux decreased after the pressure reached a critical level with both RO membranes. The changes in flux and membrane resistance were analyzed in relation to the operating pressure which was correlated to transmembrane pressure using logarithmic and second order polynomial correlations. Experimental results showed that a second order polynomial function described the flux–pressure profile more adequately than the linear correlation. The use of the flux ratio (i.e., flux with bioreactor effluent to flux with deionized water) allowed a basis for comparison of change in membrane resistance with transmembrane pressure for different membranes. The critical pressure which resulted in the minimum membrane resistance was determined by the change in the membrane resistance expressed in dimensionless form as the ratio of the observed membrane resistance with bioreactor effluent and deionized water. [Copyright &y& Elsevier]