Direct Observation Through The Membrane Research Articles

Improved in-situ characterization for the scaling-induced wetting in membrane distillation: Unraveling the role of crystalline morphology

Despite being recognized as a promising technique for treating high salinity water, membrane distillation (MD) has been plagued by the scaling of sparingly soluble salts. The growth of crystals can not only create additional resistance to evaporating water at the feed-membrane interface, but also alter the hydrophobic network to bridge the feed and distillate (i.e., result in the phenomenon of wetting). When recognizing the uncertain behaviors of calcium sulfate (CaSO4) scaling in MD, this study was motivated to ascertain whether the crystal-membrane interactions could be dependent on the variation in crystalline morphology. In particular, optical coherence tomography (OCT) was employed to characterize the scaling-induced wetting via a direct-observation-through-the-membrane (DOTM) mode, which mitigated the effects of developing an external scaling layer on resolving the crystal-membrane interactions. The improved in-situ characterization suggests that the crystalline morphology of CaSO4 could be effectively regulated by varying the stoichiometry of crystallizing ions; the richness of calcium in the aqueous environment for crystallization would be in favor of weakening the crystal-membrane interactions. The stoichiometry-dependent growth of CaSO4 crystals can be exploited to develop an effective strategy for preventing the hydrophobic network from being wetted or irreversibly damaged.

Critical flux of colloidal foulant in microfiltration: Effect of organic solvent

As membrane technology becomes more popular for separation and purification in the aqueous phase, the advantages have increasingly attracted attention for organic solvent filtration applications. However, although the knowledge base on the inevitable membrane fouling in water is rich by now, the understanding for organic solvents is limited. Accordingly, this study was targeted at providing insights on the fouling behavior of the same colloidal foulant (namely, silica) and microfiltration membrane (namely, Anopore) in four different solvents (namely, DI water, ethanol, hexane and formamide). The direct observation through the membrane (DOTM) technique was used to characterize critical flux (Jcrit), which affirmed the distinctly different values in the four solvents at various Reynolds numbers. The shear-induced diffusion model predicted well for water and formamide only, indicating the model has to be enhanced to account for solvent effects. To quantify the interfacial interactions, the Derjaguin-Landau-Verwey-Overbeek (DLVO) and extended (XDLVO) models were used. Both models agreed in that silica-membrane and silica-silica interactions were the most attractive in hexane, which underlies the immediate membrane fouling and extensive clumping, respectively. For the other three solvents, the relative DLVO interaction energies agreed with the relative Jcrit values at fixed Re, indicating the greater relevance of the DLVO (rather than XDLVO) model and Re (rather than crossflow velocity). Because the greatest silica-silica repulsion in ethanol was mis-predicted by both DLVO and XDLVO models, the solvation film effect was additionally harnessed to provide an explanation. The understanding gained from this study on membrane fouling in organic solvents are expected to be beneficial in the design and operation of such emerging membrane filtration systems.

In-situ characterization of cake layer fouling during crossflow microfiltration of oil-in-water emulsion

In view of membrane-filtration being a promising means for treating the large volumes of oily wastewater, much efforts have been dedicated to understanding membrane fouling by oil emulsions. However, the understanding on cake fouling, which is the governing form of fouling throughout filtration, by oil emulsions remain poorly understood. To bridge this gap, this study focused on characterizing the evolution of the oil-emulsion cake layer characteristics on hollow fiber (HF) membranes in real time using the direct observation through the membrane (DOTM) technique. The more well-studied latex particles were studied alongside to provide a benchmark for comparison. Despite the same feed concentration and crossflow velocity used, the transmembrane pressure (TMP) and cake layer resistance magnitudes were greater for oil emulsion than for latex particles at the same permeate flux. DOTM images revealed that this was linked to the following behaviors: (i) oil tended to deposit as a moving cake layer, whereas latex as scattered deposits; and (ii) oil gave a thinner yet denser cake layer. Moreover, it was observed that the cake thickness and porosity fluctuated significantly with respect to permeate volume for the oil emulsions, because the moving cake layer gave greater variations of cake thicknesses along the membrane. Finally, the interfacial interaction energy predicted by the XDLVO model gave higher oil-membrane and oil-oil affinities compared to that of latex, which underlie respectively the more extensive fouling and denser cake exhibited by oil. The unique characteristics of the cake layer of oil emulsion vis-à-vis the well-studied particulate foulant (i.e., latex) shown here provides new perspectives on the monitoring and control of oil fouling.

Critical flux and fouling mechanism in cross flow microfiltration of oil emulsion: Effect of viscosity and bidispersity

Membrane-based filtration is promising for applications in the food industry, for example, for processing dairy products, but membrane fouling restricts the efficiency of the process. The deposition of the main constituents in skim milk of casein, lactose and milk fat globules onto the membrane due to the permeate drag is inevitable, which necessitates a mechanistic study of the fouling phenomena. The Direct Observation through the Membrane (DOTM) technique was used to understand the effects of viscosity and bidispersity on the microfiltration of oil emulsions. Deposits at the feed-membrane interface clearly slowed down as the feed viscosity increased. The higher drag to the crossflow as a result of the higher viscosity caused more oil droplets to foul the membrane. However, although both casein and lactose increased the feed and permeate viscosity, the presence of lactose did not alter the fouling phenomena of a moving cake layer of oil droplets much, but the presence of casein caused the oil droplets to deposit instead as clusters. For casein, other than the viscosity effect, it also resulted in a bidisperse suspension, with the smaller particulates promoting membrane fouling. Agreement between the experimental data and the shear-induced diffusion model was good at the lower cross-flow velocities (CFVs) but poorer at higher CFVs, which indicate that the effect of viscosity appeared negligible at the lower CFVs but more dominant at the higher CFVs. The DLVO model agreed with the Jcrit and TMP results. This study sheds light on the membrane fouling phenomena for viscous feeds, and are expected to be useful particularly for food applications.

Understanding membrane fouling by oil-in-water emulsion via experiments and molecular dynamics simulations

Membrane-based filtration is promising for the treatment of oily wastewater with stable micron-sized oil droplets, but is unfortunately limited by the inevitable membrane fouling phenomenon. To advance the understanding on membrane fouling by oil emulsion, this study made use of the direct observation through the membrane (DOTM) technique to experimentally visualize the evolution of fouling and determine critical flux, and also molecular dynamics simulations to unveil the interfacial interactions and behaviors underlying the different fouling behaviors. Three oil emulsion types with similar mean droplet sizes were studied, namely, one without surfactant, one stabilized by sodium dodecyl sulfate (SDS; a negatively charged surfactant) and one stabilized by dodecyltrimethylammonium bromide (DTAB; a positively charged surfactant). DOTM results indicate that the critical flux was the highest in the absence of surfactant and lowest for the DTAB-stabilized ones, while simulation results indicate that the interaction energies are clearly different among the different oil emulsion types. Both hence affirm that the presence of surfactant and different surfactant type distinctly changes the fouling tendencies. The key conclusions on the different fouling tendencies among the three oil emulsion types are summarized as follows. Firstly, the highest critical flux in the absence of any surfactant is linked to greatest oil-water interaction and least oil-membrane interaction, both of which cause the oil molecule to prefer being in the bulk aqueous feed. The radial distribution function (RDF) further substantiates this. Secondly, the higher critical flux exhibited by the SDS-stabilized oil emulsion compared to the DTAB-stabilized ones could be attributed to the negative charge of the former. Simulations affirm the comparatively greater oil-membrane repulsion through the RDF profile, lower oil-membrane interaction and higher oil-water interaction of the SDS-stabilized oil emulsion. Thirdly, despite surfactants generally have a stabilizing effect on oil emulsions, DOTM images show that coalescence was most extensive for the DTAB-stabilized oil emulsion, because of the greatest oil-membrane attraction and least oil-oil repulsion. Coupling both experiments and simulations, this study enhanced the mechanistic understanding on the effect of surfactant on membrane fouling by emulsions of the same oil.

Striping phenomenon during cross-flow microfiltration of oil-in-water emulsions

Abstract The striping phenomenon in membrane filtration, whereby the foulants deposit as regular streaks rather than a more uniform layer, was first observed more than 30 years ago (Jonsson, 1987), but the understanding has remained limited to a few subsequent studies (Henriksen and Hassager, 1993; Larsen, 1991; Li et al., 2016; Tanudjaja et al., 2017; Tarabara et al., 2002). In view of the potential practical implications of the stripes in terms of membrane fouling and fouling mitigation, this study was targeted at an in-depth characterization of the stripes. The direct observation through the membrane (DOTM) technique was employed to observe the conditions when the stripes formed by oil emulsions stabilized by three Tween surfactants at a lower cross-flow velocity, and higher oil concentrations and permeate fluxes. The results indicate that (i) other than hydrodynamic factors (e.g., permeate drag, tangential shear), foulant-membrane and foulant-foulant interactions played a role in stripe formation or disappearance, as evident in the formation of stripes only by the oil emulsion stabilized by the Tween surfactants and the effect of pH; (ii) stripes made up of oil droplets appeared to be easier to remove than the more uniform layer of oil droplets, as evident in the shorter time taken for the former to detach; and (iii) among the three Tween surfactants, the striping characteristics investigated were largely similar, except for the time taken for the deposits to detach. These are expected to have implications for fouling control and mitigation.

Assessment of oil fouling by oil-membrane interaction energy analysis

Membrane-filtration is promising for treating the voluminous oily wastewater, especially when the oil emulsions are smaller than 20 µm. However, studies on the inevitable membrane fouling phenomenon by oil are rather scarce. In particular, a question that remained to be addressed was whether the DLVO or XDLVO model provides better predictions of the oil-membrane interfacial interactions and thereby the extent of fouling. Accordingly, this study investigated four oil types (namely, hexadecane, soybean oil, fish oil and crude oil) that were stabilized by the same non-ionic surfactant (namely, Tween 20) and had similar mean droplet diameters. The direct observation through the membrane (DOTM) technique was used to quantify the critical flux of the different emulsions, and both the XDLVO and DLVO models were used to quantify the foulant-membrane and foulant-foulant interactions. DOTM results indicated that the critical flux values were similar for all oils under the conditions tested, except crude oil. Although the XDLVO model appears to be more comprehensive than the DLVO in terms of accounting for the additional Lewis acid-base polar (AB) interaction that is acknowledged to be important in membrane-filtration, results indicate that the dominance of the AB component drowns out the other interactions like that of electrostatics (EL) in this case, which impedes accurate prediction of the different fouling tendencies by the different oil types.

Characteristics of non-spherical fluidized media in a fluidized bed–membrane reactor: Effect of particle sphericity on critical flux

Abstract The fluidization of millimeter-sized media to mitigate the membrane fouling by micron-sized particles has received increasing attention, but a question that remained unanswered is the effect of the sphericity of the fluidized media on the energy-efficiency of fouling mitigation, which motivated the current study. Four millimeter-sized fluidized media types, of which three had similar equivalent volume diameters, were investigated, with micron-sized polystyrene particles serving as the particulate foulant. The direct observation through the membrane (DOTM) technique was used to determine the local critical flux, while a high-speed video camera was used to capture the local fluidization hydrodynamics (namely, particle velocity, particle rotation velocity, particle contact area and particle momentum). The results indicate that (i) although smaller media mitigated fouling better among higher-sphericity media, the smallest, low-sphericity media here performed similarly or worse; (ii) lower-sphericity media gave lower particle rotation, greater contact area with the membrane, and lower particle momentum; (iii) in contrast to higher-sphericity media, the extent of local fouling mitigation decreased as the particle velocities increased and the local critical flux was not the most strongly correlated with particle momentum; (iv) lower-sphericity media generally gave lower overall critical flux for the same hydrodynamic characteristic; (v) particle sphericity is a negligible factor in the energy-efficiency of fouling mitigation by similar-sized fluidized media, but, between low-sphericity media, the larger media was more energy-efficient. The findings here highlight the non-negligible influence of the particle sphericity of the fluidized media, and are expected to be valuable in the optimization of the properties of the fluidized media towards more energy-efficient membrane fouling mitigation.

Ultrafiltration of saline oil-in-water emulsions stabilized by an anionic surfactant: Effect of surfactant concentration and divalent counterions

The paper presents results of a direct visualization study of membrane fouling by hexadecane-in-water emulsions (0.1% v/v) stabilized by sodium dodecyl sulfate (SDS; 0.1mM or 3mM) in the presence of divalent counterions (Mg2+; 0mM, 6.7mM, or 42.6mM). Direct Observation Through the Membrane (DOTM) tests were performed using an ultrafiltration membrane (Anopore; dpore =0.02µm) and low wettability by hexadecane (contact angle, θ, in 3mM aqueous solution of SDS is ~164°). Three emulsions employed in DOTM tests had different values of interfacial tension and surface charge and exhibited distinctly different behaviors of oil droplets on the membrane surface. In addition to decreasing the solubility of SDS, MgSO4 had two opposing effects on emulsion stability wherein both interfacial tension and ζ-potential of oil droplets decreased; the overall effect of MgSO4 was a more facile droplet coalescence that was further promoted by permeate flux and concentration polarization of oil. The dominant fouling mechanism was cake filtration with multilayer and sub-monolayer coverages observed for different conditions. Because of the relative oleophobicity of the membrane, the attached oil did not form contiguous oil films. Under conditions of extensive coalescence (high MgSO4), oil droplets reached a critical size and were then removed by the crossflow resulting in minimal membrane fouling.

Effect of bubble characteristics on critical flux in the microfiltration of particulate foulants

Fouling control by bubbling is widely implemented in membrane bioreactors (MBRs), but a mechanistic understanding of the relationship between the bubble characteristics and critical flux is missing. This study is targeted at bridging the gap. A microfiltration setup employing bubbling as a means of fouling mitigation was set up. The direct observation through the membrane (DOTM) technique was used to determine the critical flux of a feed containing micron-sized polystyrene particles, while a high-speed video camera was used to characterize the bubble behavior. The results indicate that (i) the local critical flux (Jc,local) values increased with height along the membrane and with gas flow rate, but was relatively invariant with liquid flow rate; (ii) with respect to height, the mean bubble velocity and mean bubble area per bubble increased, while the number of bubbles per unit membrane area decreased; (iii) bubble momentum and mean area per bubble had the most positive correlation with Jc,local,; and (iv) the overall critical flux (Jc,overall, which is the average of the local critical fluxes) increased linearly with respect to the ratio of gas to total flow rates, and steeply then more gradually with respect to power required. Finally, a comparison between the two two-phase flow systems employed for fouling mitigation (namely, bubbling and fluidized granular activated carbon (GAC)) was carried out.

Effect of cross-flow velocity, oil concentration and salinity on the critical flux of an oil-in-water emulsion in microfiltration

Microfiltration is an attractive means for treating oily wastewater, especially when the size of the oil droplets are micrometer-sized since the conventional techniques become deficient. A systematic study on the critical flux of oil-in-water emulsion, which behaves differently from other colloidal foulants with regards to deformation, coalescence and splitting, has not been carried out to date. This was the goal of the current study, which employed the Direct Observation Through the Membrane (DOTM) technique to characterize the critical flux of oil-in-water emulsions of various concentrations, and at various cross-flow velocities (CFV) and salt concentrations. Five observations can be highlighted here. Firstly, the oil droplets with a mean droplet diameter of approximately 5µm exhibited critical fluxes equal to or greater than latex particles of 10µm. This is likely due to the twin effects of membrane oleophobicity promoting back-transport of the oil foulant from the membrane and the presence of a droplet size distribution with larger drops that can enhance the shear-induced diffusion of the average drops. Secondly, the critical flux values did not agree with the model that is valid for the size range the mean droplet diameter falls in, but instead agreed with the model adapted for smaller particulate foulants. Thirdly, the increase in the critical flux with CFV was more significant for the lower oil concentration. Fourthly, a striping phenomenon was observed at higher oil concentrations and lower CFV values. Striping was not observed for latex particles. Fifthly, the critical flux decreased with salt concentration. These findings highlight the unique fouling behavior of oil-in-water emulsions in microfiltration.

Flow-field mitigation of membrane fouling (FMMF) via manipulation of the convective flow in cross-flow membrane applications

Membrane fouling by particulates is largely driven by permeate drag that causes the foulants to move towards the membrane. Cross-flow is used to induce shear at the membrane surface to mitigate the fouling. This study describes the flow-field mitigation of membrane fouling (FMMF) technique that reduces the fouling based on a judicious manipulation of the flow-field by inclining the channel walls to counter the permeate drag experienced by the foulants. The key mechanism is the additional transverse (i.e., orthogonal to the fluid flow direction) fluid vector caused by a small inclination of the channel walls (on the order of one to a few degrees). Both simulation and experiments confirm the efficacy of FMMF in mitigating fouling at a reduced energy requirement compared with the conventional channels with parallel walls. Simulations over a range of permeate fluxes indicated that a slight inclination angle of 1.15° can give a deposition factor less than that in the conventional channel with parallel walls even at twice the permeate flux. Direct observation through the membrane (DOTM) experiments for fouling using two particle diameters over a range of power inputs showed that the critical fluxes were significantly increased in the FMMF module compared with the conventional module. This study provides a proof-of-concept of the principle underlying FMMF and underscores the potential benefits. The direction for further development and scale-up is suggested.

Effect of fluidized granular activated carbon (GAC) on critical flux in the microfiltration of particulate foulants

The fluidization of Granular Activated Carbon (GAC) particles has received much attention in recent years as a promising method for mitigating membrane fouling particularly in membrane bioreactors (MBRs). In particular, the fluidized GAC particles are acknowledged to be beneficial in terms of adsorption of organic foulants, and mechanical scouring and surface shear of the membrane surface. Both lab-scale and pilot-scale studies have affirmed the efficacy of GAC fluidization in maintaining a depressed extent of membrane fouling in anaerobic fluidized bed - membrane bioreactors (AnFMBRs). More in-depth studies have also correlated the hydrodynamics of fluidized GAC particles with the effectiveness of membrane fouling mitigation. This study aimed at understanding the improvement of fine (micron-sized) particle critical flux in the presence of fluidized GAC via the Direct Observation Through the Membrane (DOTM) technique. The model foulant was a suspension of polystyrene particles (5µm sized), and the parameters investigated included GAC particle diameter (dp), superficial liquid velocity (Ul) and thereby power requirement (Pr), and height along the vertically aligned membrane. Results indicate that: (i) fluidized GAC increased the critical flux by an order-of-magnitude relative to that with tangential liquid shear alone; (ii) the overall critical flux (Jcritical, overall) expectedly increased with power input (Pr), but the relationship between local critical flux (Jcritical) and power input (Pr) depended on the position along the membrane height; (iii) at the same Pr, although the larger GAC particles were more effective locally (i.e., higher Jcritical) due to greater particle inertia, the smaller GAC particles were more effective overall (i.e., higher Jcritical, overall) due to greater bed expansion enabling scouring of more membrane heights; and (iv) a higher power input (Pr) was required for sufficient bed expansion to enable more consistent critical flux (Jcritical) values over the whole membrane height.

Effect of spacer and crossflow velocity on the critical flux of bidisperse suspensions in microfiltration

Crossflow microfiltration is a popular application spanning various industries. Although the impacts on fouling of feed bidispersity, crossflow velocity (CFV) and spacer, all of which are present in practical operations, are known separately, the understanding of the interplay of these three factors on fouling is lacking. Accordingly, this study used the Direct Observation Through the Membrane (DOTM) technique to characterize the critical flux of monodisperse and bidisperse polystyrene particles in both the absence and presence of a spacer over a range of CFV values. The results indicate that (i) the combined effects of both bidispersity (dp=3µm and 10µm) and spacer gave the highest Jcrit values for the smaller particles throughout the CFV range investigated; (ii) bidispersity was more effective in enhancing Jcrit at a lower CFV, while the presence of a spacer was more effective at a higher CFV; (iii) a higher CFV diminished the enhancement induced by bidispersity more than that by the spacer; and (iv) comparisons between models and experimental data reveal that shear-induced diffusion models based on monodisperse particles are deficient for bidisperse mixtures, because they cannot allow for particle size segregation effects that occur in flowing mixed systems.

Impact of the surface energy of particulate foulants on membrane fouling

Foulant-foulant and foulant-membrane interfacial interactions play an important role in dictating the extent of fouling. In order to understand the impact of the surface energy of particulate foulants on the fouling extent, the direct observation through the membrane (DOTM) technique was used to characterize critical flux and also assess the initial evolution of foulant deposition. Polystyrene and glass with diameters approximating 10μm were used as foulants because (i) the similar particle diameter eliminated the differences due to particle back-transport, and (ii) the Gibbs free energies (ΔG) of foulant-membrane and foulant-foulant interactions were both negative (i.e., attractive) for polystyrene and both positive (i.e., repulsive) for glass. Results indicate that: (1) because of the attractive and repulsive foulant-membrane interactions of the polystyrene and glass, respectively, (i) critical flux was lower for polystyrene than glass, (ii) the phenomenon of a flowing particle layer was observed at a lower cross-flow velocity (CFV) for glass than polystyrene, and (iii) relaxation was more effective for glass than polystyrene; (2) because of the attractive and repulsive foulant-foulant interactions of the polystyrene and glass, respectively, clustering was observed for the former but not latter; (3) out of the four particle deposition mechanisms (namely, convective, entrapment, dead zone and clustering) distinguished, the first three occurred for both attractive and repulsive interfacial interactions, but the forth (i.e., clustering) only occurred when foulant-foulant interactions were attractive.

Generalized criterion for the onset of particle deposition in crossflow microfiltration via DOTM – Modeling and experimental validation

The concept of a critical permeation flux for the onset of particle deposition in crossflow microfiltration (CFMF) is well-established. However, the critical flux is known to be a function of process parameters such as the particle size, bulk concentration and crossflow velocity. In the present study, the critical modified Peclet number (Pecrit) is explored instead as a generalized criterion for the onset of particle deposition that incorporates the effects of these process parameters as well as the axial position along the membrane. A proper determination of Pecrit requires an accurate prediction of the concentration polarization boundary layer thickness δc and shear-induced diffusion coefficient Ds. The classical Lévêque model is adapted to allow for the effect of the permeation flux on the velocity profile. Moreover, the assumptions of a constant concentration at the membrane surface cw and constant Ds that have been made in prior studies are relaxed in an improved numerical solution to the convective diffusion equation that is used to predict δc and Ds. The critical permeation flux is determined from particle deposition data taken for 6 and 10μm latex spheres via Direct Observation Through the Membrane (DOTM) characterization. A constant value of Pecrit=4.00±0.08 is found to characterize the effects of particle diameter, bulk concentration and crossflow velocity as well as axial position on the onset of particle deposition.

Microfiltration of algae ( Chlorella sorokiniana): Critical flux, fouling and transmission

Fouling and transmission studies of algae broth ( Chlorella sorokiniana) during cross-flow microfiltration were performed using direct microscopic observation technique. The flux stepping test was conducted for approximately 15 min interval and images were captured at the beginning and at the end of each flux step for further analysis of membrane surface coverage. The rate of membrane surface coverage was correlated to the rate of transmembrane pressure (TMP) rise. Results suggested that Direct Observation Through the Membrane (DOTM) technique was more sensitive to indicate the membrane fouling than TMP monitoring method. Deposition of microalgal cells started to occur even at a very low permeate flux. The layer of algal cells would eventually build up and might result in severe fouling. Despite this early deposition, TMP values remained low and would only increase when the fouling had become more pronounce. Lower fouling rate was achieved at higher cross-flow velocity. However, higher cross-flow velocity (CFV) also generated greater shear force which enhanced the transmission of extracellular polymeric substances (EPS). The particle removal due to bubble-induced shear was visualised with DOTM to provide better understanding. A significant reduction in fouling rate and near complete removal of microalgae could be achieved by applying continuous air bubbling. Based on TMP measurement, an intermittent aeration mode could achieve approximately identical performance as continuous bubbling mode. However, the recorded DOTM images indicated that intermittent bubbling mode could not completely maintain a cell-free surface.

Critical flux and particle deposition of bidisperse suspensions during crossflow microfiltration

Bidisperse suspensions of latex beads with a discrete distribution were investigated in the crossflow microfiltration process. The critical fluxes of single-sized latex particles (3.0, 5.0 and 10.0 μm) and mixed suspensions (3.0 + 5.0, 5.0 + 10.0 and 3.0 + 10.0 μm) were determined using a non-invasive, in situ, direct observation through the membrane (DOTM) technique. While the critical fluxes of the single-sized particles were consistent with past studies in the literature, the critical fluxes of the mixed suspensions were found to increase in the presence of the bigger particles. In addition, only the smaller size particles deposited on the membrane surface even when the imposed flux was higher than the critical fluxes for the combined and individual components in the bidisperse system. The most plausible explanation is that the enhancement of critical flux is due to augmentation of shear-induced diffusivity of the smaller particles by the bigger particles.

Observation of deposition and removal behaviour of submicron bacteria on the membrane surface during crossflow microfiltration

The deposition and removal of submicron bacteria (SW8) has been examined by the direct observation through the membrane (DOTM) technique. The original DOTM was modified to incorporate fluorescence microscopy to better visualise the submicron material. The flux at which deposition commenced, the so-called critical flux, was visually identified before the transmembrane pressure indicated cake formation. After supercritical operation for about 15 min the flux was reduced to subcritical and then zero; slow cake removal was observed as distinct cylindrical rolling floc (about 50 μm diameter) and aggregates. The extent of cake removal varied from about 90 to <5% depending on the ionic environment with low ions resulting in better removal. Cake formed over longer periods (upto 60 min) showed negligible removal. The critical fluxes of SW8 measured with DOTM increased with crossflow, but exhibited higher values than expected from their primary particle size.

The effect of filament orientation on critical flux and particle deposition in spacer-filled channels

Experiments have been conducted using the direct observation through the membrane (DOTM) technique to determine the effect of the presence and orientation of spacers on the critical flux of particles. It was found that spacers significantly enhance (up to 2 times) critical flux of 6.4 μm latex beads. The degree of enhancement depended on spacer orientation. When the attached filament was perpendicular to flow (90° orientation), deposition occurred in a transverse band across the entire spacer cell, set back from the transverse filament by a zone of no deposition. With the reverse orientation (0°—transverse filaments opposite the membrane), deposition occurred at higher fluxes and was concentrated around the attached filament parallel to flow. Whilst with the attached filaments arranged at 45° to flow, deposition occurred particularly in the centre of the cell. The critical fluxes and deposition patterns are in qualitative agreement with the anticipated fluid flow patterns through the spacer-filled channels. A discussion of the errors and implications of these results for industrial systems is also included.