Variations in backwash efficiency during colloidal filtration of hollow-fiber microfiltration membranes

A series of filtration experiments was performed systematically to investigate physical and chemical factors affecting the efficiency of backwashing during microfiltration of colloidal suspensions. In this study, all experiments were conducted in dead-end filtration mode utilizing an outside-in, hollow-fiber module with a nominal pore size of 0.1 μm. Silica particles (mean diameter = 0.14 μm) were used as model colloids. Using a flux decline model based on the Happel's cell for the hydraulic resistance of the particle layer, the cake structure was determined from experimental fouling data and then correlated to backwash efficiency. Modeling of experimental data revealed no noticeable changes in cake layer structure when feed particle concentration and operating pressure increased. Specifically, the packing density of the cake layer (1-cake porosity) in the cake layer ranged from 0.66 to 0.67, which corresponds well to random packing density. However, the particle packing density increased drastically with ionic strength. The results of backwashing experiments demonstrated that the efficiency of backwashing decreased significantly with increasing solution ionic strength, while backwash efficiency did not vary when particle concentration and operating pressure increased. This finding suggests that backwash efficiency is closely related to the structure of the cake layer formed during particle filtration. More densely packed cake layers were formed under high ionic strength, and consequently less flux was recovered per given backwash volume during backwashing.     

Investigations of the coupled effect of cake-enhanced osmotic pressure and colloidal fouling in RO using crossflow sampler-modified fouling index ultrafiltration

The aim of this study was to investigate the potential of using the resistivity and deposition rate data from Crossflow Sampler-Modified Fouling Index Ultrafiltration (CFS-MFI_UF) measurements to determine the coupled effects of colloidal fouling and cake enhanced osmotic pressure (CEOP) effect. Cake filtration derived from CFS-MFI_UF was combined with a CEOP model to predict the crossflow RO fouling profile under constant flux filtration. The prediction based on resistivity, I′ from CFS-MFI_UF measurement alone was found to underestimate the RO fouling for high salinity solutions. However, when incorporating the mass information from the CFS-MFI_UF test to account for the CEOP effect, the prediction showed good agreement with the TMP profile of the RO system. The results indicated that the CFS-MFI_UF test which includes the CEOP effect is a very promising technique to provide an estimation of the RO colloidal fouling profile. When the changes of cake thickness and porosity throughout the filtration were considered, the predicted TMP profile based on the model clearly indicated a two-stage of fouling profile which agreed well with the experimental data. Additional studies on the effects of cake thickness and porosity on CEOP highlighted the important influence of cake structure on CEOP.     

Calcium sulphate fouling of reverse osmosis membranes: Flux decline mechanism

The cake filtration fouling model for reverse osmosis (RO) was examined and found to be physically unrealistic for flux decline due to calcium sulphate precipitation fouling. Continuous flow RO experiments on a saturated solution of calcium sulphate were performed to elucidate the flux decline mechanism. The calcium sulphate deposit on the membrane was characterized by SEM and computer-aided image analysis. It is shown that flux decline is due to blockage of the membrane surface by lateral growth of the deposit rather than the hydraulic resistance of a cake building up at the membrane surface. The importance of deposit characterization for the verification of fouling mechanisms is emphasized.     

Permeate flux decline in cross-flow microfiltration at constant pressure

Microfiltration processes are frequently used to separate solids from aqueous suspensions. The rejection of suspended matter is facilitated by means of a size exclusion mechanism and is affected by membrane properties, characteristics of the suspension and operating conditions. Therefore, the filtration performance of a single polymeric hollow-fibre membrane was investigated by monitoring the permeate flux decline for a filtration at constant transmembrane pressure (TMP). For these bench-scale experiments, a model suspension consisting of silica particles in xanthan gum solutions was used in order to represent the characteristics of biological suspensions such as activated sludge properly. In the framework of this study, it was confirmed that the permeate flux declines rapidly during the first stage of filtration until an equilibrium of particle deposition and entrainment is reached. The steady-state permeate flux was found to increase with an increase in cross-flow velocity, a decrease in solid concentration, a decrease in particle size (for this ratio of particle to pore diameter) and a decrease in apparent viscosity of the suspension. However, the equilibrium permeate flux was not affected by variations in TMP, which is in agreement with the limiting flux theory.     

颗粒体系错流过滤过程的数学模拟

以流体力学理论为基础,通过颗粒在水溶液中的受力分析,结合早期的阻塞过滤模型和近期的滤层过滤模型,建立了一种新的颗粒体系错流过滤过程的数学模拟方法,在该模拟过程中同时考虑了膜孔径和颗粒大小两种分布,并对具有相同平均孔径而孔分布不同的情形对膜渗透通量的影响进行了模似,经实验验证,模拟结果与实验值能够较好地吻合。     

MODELING OF MEMBRANE FOULING AND FLUX DECLINE IN MICROFILTRATION OF OILY WASTEWATER USING CERAMIC MEMBRANES

One of the major problems in pressure-driven membrane processes is reduction of flux far below the theoretical capacity of the membrane. The results of an experimental study of fouling mechanisms of ceramic membranes in separation of oil from synthesized oily wastewaters are presented. Mullite microfiltration (MF) membranes were synthesized from kaolin clay as MF ceramic membranes. The rejection of total organic carbon (TOC) for the synthetic feeds was found to be more than 94% by these membranes. Hermia's models were used to investigate the fouling mechanisms of membranes. The effect of pressure, cross flow velocity (CFV), temperature, oil concentration, and salt concentration on flux decline were investigated. The results showed that the cake filtration model can well predict the flux decline of mullite ceramic membranes; average error of this model is less than 7%. The results show that by increasing pressure from 0.5 to 4 bar, porosity of the cake layer on the mullite membranes decreases from 25.68% to 14.98%. After the cake filtration model, the intermediate pore blocking model was found to well predict the experimental data with an average error less than 10.5%.     

Development of the MFI-UF in constant flux filtration

The MFI-UF, based on cake filtration, was developed to measure and predict the particulate fouling potential of feedwater to membrane filtration installations. The MFI-UF is determined in constant pressure filtration with the flux deceasing during the test. However, many membrane systems, e.g., reverse osmosis (RO), operate at constant flux with pressure increasing when fouling occurs. As both pressure and flux contribute to cake compression, determining the MFI-UF in constant flux with correction to the flux of an RO system is expected to more closely simulate particulate fouling Therefore, this research investigated the development of the MFI-UF test in constant flux filtration applying low (tap water) and high fouling (diluted canal water) feedwater. Preliminary experiments were promising; the fouling index (I) (and hence the MFI-UF) of all feedwater could be determined within 2 h under constant flux filtration. Cake filtration was demonstrated as (1) a minimum in the fouling indexvs time plot and (2) by linearity of the fouling index with feedwater particulate concentration. The fouling index increased with increasing applied flux due to cake compression. Further investigation at higher and lower applied flux is required to identify a reference test flux and to develop a method to correct the fouling index to the reference test flux and/or the flux of a membrane filtration system The fouling index can then be applied in a model to predict fouling.     

Influence of colloidal fouling and feed water recovery on salt rejection of RO and NF membranes

The influence of colloidal fouling and feed water recovery (or concentration factor, CF) on salt rejection of thin-film composite reverse osmosis (RO) and nanofiltration (NF) membranes was investigated. Fouling experiments were carried out using a laboratory-scale crossflow test unit with continuous permeate disposal to simulate the CF and recovery as commonly observed in full-scale RO/NF systems. For feed waters containing only salt (NaCl), permeate flux declined linearly as CF was increased and salt rejection was nearly constant for both RO and NF membranes. On the other hand, a sharp decrease in permeate flux and significant decline in salt rejection with increasing CF were observed under conditions where colloidal fouling takes place. For both RO and NF membranes, the marked permeate flux decline was attributed to the so-called “cake-enhanced osmotic pressure”. The decline in salt rejection when colloidal fouling predominated was much more substantial for NF than for RO membranes. In all cases, the decline in salt rejection was higher under conditions of more severe colloidal fouling, namely at higher ionic strength and initial permeate flux.     

An Investigation of the Mechanism of Critical Flux in Membrane Filtration Using Electron Microscopy

The critical fluxes for colloidal latex suspensions of 1.0 µm and 0.1 µm and their mixture were measured. The similar fluxes were observed for latex suspensions and pure water. Electron microscopy was used as a tool for critical flux mechanism investigation.For all latex suspensions tested, there is no evidence of cake formation for flux below the critical value. A small deposition of the particles may form on the membrane surface which can be established during the intermittent blocking of the pores. This evidence supports the model of intermittent obstruction and blocking of pores. Another possibility is that the local fluxes in parts of the membrane may be higher than the critical value. Above the critical flux deposition of latex beads or formation of a cake is evident. The deposits grow and the cake thickened during time. This is usually accompanied by a drop in delivered flux.The results of working below critical flux suggest a possible membrane-feed interaction which is not a fouling cake layer. One possibility is that of a concentrated polarised layer with intermittent pore blockage or obstruction. A surface interaction effect also plays a role. The particle content of the feed could influence flow through the pores and may involve intermittent obstruction of pores without permanent fouling.The general picture of pumping permeate operation below the critical flux is non-formation of a stagnant cake layer. However a flowing cake layer might be established.     

Fouling of reverse osmosis membranes by hydrophilic organic matter: implications for water reuse

Effluent organic matter (EfOM) is suspected as a major cause of fouling of reverse osmosis (RO) membranes in advanced wastewater reclamation. Among the main constituents in EfOM, polysaccharides are the most ubiquitous. The influence of solution chemistry and hydrodynamics on RO membrane fouling with alginate — a model for polysaccharides in secondary wastewater effluent — was systematically investigated. Results of fouling runs with alginate demonstrate that RO membrane fouling increases with decreasing pH, increasing ionic strength, and addition of calcium ions. At fixed solution ionic strength and pH, the presence of divalent calcium ions, at concentrations typical of those found in secondary wastewater effluent, had a dramatic effect on membrane fouling. However, for similar concentrations of divalent magnesium ions, fouling was negligible. The severe fouling in the presence of calcium is attributed to the formation of a thick, dense alginate gel layer on the membrane surface via calcium-alginate complexation and crosslinking (bridging) of alginate macromolecules by calcium. In addition to solution chemistry, hydrodynamic operating conditions — initial permeate flux and crossflow velocity — were also shown to influence RO membrane fouling with alginate.     

颗粒粒径和膜孔径对陶瓷膜微滤微米级颗粒悬浮液的影响

通过测定颗粒悬浮液通过陶瓷微滤膜时的参透通量及污染阻力,确定了陶瓷膜处理微米级颗粒悬浮液时,颗粒粒径和膜孔径对微滤过程的影响和膜污染机理,获得了微米级颗粒悬浮液微滤过程中膜孔径的选择方法。     

Effect of Fouling Conditions and Cake Layer Structure on the Ultrasonic Cleaning of Ceramic Membranes

Abstract

Homogeneous alumina membranes fouled by polystyrene latex particles at different pH values and ionic strengths were subjected to ultrasonic cleaning. Cleaning was more effective at high and low pH than at neutral pH. At low pH values, less repulsive particle‐particle interactions resulted in the removal of millimeter‐scale aggregates and highly effective cleaning. At near‐neutral pH, stronger repulsive particle‐particle interactions caused detachment to occur as individual particles from the cake layer rather than as flocs, which was a slightly less effective cleaning mechanism. Ultrasonic cleaning of cake layers formed at high ionic strength (>0.3 M KCl) was less effective than cleaning at lower ionic strength (<0.3 M KCl). High ionic strength caused particles to coagulate in solution and deposit as flocs on the membrane surface forming a highly permeable fouling layer. This fouling layer was resistant to ultrasound at the sub‐optimal cleaning conditions used in this study, perhaps due to particle attachment occurring within a primary energy minimum. Membrane cleaning experiments performed with particles of varying size showed that particle size was less important than the surface potential of the particles. For a given mass, particles that possessed the largest surface potential formed the thickest fouling layer, irrespective of particle size, and showed the greatest improvement in flux with ultrasonic cleaning. These results demonstrate that solution conditions influence ultrasonic cleaning of membranes primarily by modifying particle‐particle and particle‐membrane interactions as well as cake layer structure, rather than by impacting the extent or magnitude of cavitation events.     

The Ultrafiltration of Very Dilute Colloidal Gold Suspensions

The ultrafiltration behaviour of very dilute colloidal suspensions has been investigated in terms of transmembrane pressure and pH in a batch cell with and without stirring using PM30 and SKIP membranes.For small (10 nm) colloidal gold particles the unstirred flux was higher than the stirred. An increase in concentration polarization due to the lack of stirring shields the electrostatic repulsion between particles, resulting in aggregation of particles that are retained at the membrane surface. This loosely packed layer can be responsible for the higher flux in the unstirred condition. For large (50 nm) gold particles, greater flux was achieved in stirred condition due to the decrease in concentration polarization.While the SKIP membranes showed a complete retention for larger colloidal gold particles, for smaller particles the retention was complete after around 1 minute. The PM30 membrane completely retained colloidal gold particles of both sizes.Lower flux with higher flux decline was obtained for the smaller colloidal gold particles compared to the larger ones. The cross section micrographs show that the larger sol forms a less densely packed deposit layer on the membrane surface. Changing the pH of the colloidal gold suspension resulted in a substantial change to the flux and retention. However the level of local ionic concentration at the membrane surface appears to be of utmost importance as it affects the degree of colloidal aggregation and packing of the deposit layer, thus influencing flux and retention.Analysis of the filtration data coupled with electron microscopy showed that cake filtration was the dominant mechanism during the course of ultrafiltration as well as microfiltration of very dilute colloidal gold suspensions.     

A comparison of hydrodynamic methods for mitigating particle fouling in submerged membrane filtration

Hydrodynamic methods are used for mitigating particle fouling and for enhancing the filtrate flux in submerged membrane filtration. In the comparison membrane blocking-cake formation filtration system, the effects of filtration pressure, aeration intensity, backwash duration and stepwise increasing pressure on the filtration resistances and filtration flux are measured and discussed. Aeration is helpful for reducing particle deposition on the membrane surface, while stepwise increasing pressure can mainly mitigate internal fouling of the membrane. Periodic backwash can significantly reduce both the resistance caused by the membrane internal fouling and by cake formation; consequently, it can effectively recover the filtrate flux. In contrast, increasing the pressure in constant pressure filtration leads the flux to be decreased due to more severe membrane blockage. According to the comparison of the long-term flux and the received filtrate volume, among these hydrodynamic methods, the periodic backwash with longer duration is the optimal strategy for the filtration.     

Optimizing the coagulant dose to control membrane fouling in combined coagulation/ultrafiltration systems for textile wastewater reclamation

Optimal coagulation conditions need to be re-examined when coagulation is coupled to membrane filtration for wastewater treatment. This work focused on the optimization of coagulant dosing in order to control membrane fouling in ultrafiltration (UF), following coagulation for the reclamation of textile wastewater. The effects of pore size and coagulant types and dosages on flux decline were investigated using a stirred-cell UF unit. The flux was greatly enhanced for the UF membrane when a coagulant was added, whereas for the microfiltration (MF) membrane the flux decreased. This could be attributed to changes in the size of coagulated particles and their interaction with membrane pores. At a low dosage (e.g., 0.0371 mM as Al), the polyaluminum chloride (PACl) coagulant was found to control the flux decline most effectively for low ionic-strength wastewater. The optimal dose minimized the fouling and cake layer resistances, although it was sharp and dependent on influent composition. The cake layer protected the membrane from fouling, but it provided additional resistance to permeation. Analyses of turbidity, particle size, and membrane surface exhibited the characteristics of coagulated particles and their cake structures that are closely associated with flux behavior.     

Effect of Hydraulic Retention Time on Sludge Properties,Cake Layer Structure,and Membrane Fouling in a Thermophilic Submerged Aerobic Membrane Bioreactor

Sludge properties, cake layer structure, and membrane fouling in a membrane bioreactor were studied under various hydraulic retention times (HRT). A decrease in HRT resulted in an increase in extracellular polymeric substance production in bulk sludge and changes in cake layer structure from gel layer to one or two cake layers. Particle size distribution in cake sludge changed with respect to HRT. An evolution in cake layer thickness and porosity was observed with trans-membrane pressure (TMP) jump. The change in cake layer structure might bear more responsibility for the TMP jump than the quantity of cake layer.     

Study of the separation of yeast by microsieves: In situ 3D characterization of the cake using confocal laser scanning microscopy

In situ 3D characterization of Aquamarijn microsieves fouling was achieved using Confocal Laser Scanning Microscopy (CLSM). A filtration chamber allowing direct microscopic observation of microbial cell deposition and cake characterization, specially designed for in situ observations, was used. Fluorescent dyed Saccharomyces cerevisiae yeast suspensions were filtered through 0.8 μm and 2 μm pore diameters silicon nitride microsieves under constant flow rate. The on-line yeasts deposition was recorded and the cake construction was followed layer by layer. Based on the 3D image processing, cake properties (particle arrangement, homogeneity, thickness and porosity). The compressibility of the yeast cake was analyzed. Finally, cake removal efficiency was also studied during microsieve cleaning operation.     

Interactions controlling biopolymer fouling of reverse osmosis membranes

Laboratory experiments and model calculations were performed to elucidate the fundamental interactions that control organic fouling in reverse osmosis (RO) processes. Bovine serum albumin and alginic acid were selected as model aquatic organic macromolecules (organic foulants). An extended Derjaguin-Landau-Verwey-Overbeek (DLVO) characterization analysis was used to elucidate mechanisms of organic matter fouling on a commercial, polyamide composite RO membrane. Surface tension parameters derived from contact angle analyses are used to demonstrate that the apparent thermodynamic stability of macromolecules determines and adhesive free energy between membranes and macromolecules explained the observed differences in flux decline. Further, foulant–membrane and foulant–foulant interfacial forces helped explain why hydrophilic macromolecules formed polarization layers causing minimal flux decline, while hydrophobic macromolecules formed gel (or cake) layers that led to severe flux decline.     

A Review of Membrane Fouling in MBRs: Characteristics and Role of Sludge Cake Formed on Membrane Surfaces

Abstract

Membrane bioreactor (MBR) has been deemed to be a promising technology for wastewater treatment and reclamation; however, the MBR filtration performance inevitably decreases with filtration time attributed to the deposition of soluble and particulate materials onto and into the membrane under the interactions between activated sludge components and the membrane. Cake layer formation on membrane surfaces has been a major challenge in the operation of MBRs under supra-critical flux operation, and/or caused by uneven distribution of aeration intensities, etc.; however, it was argued that a thin cake layer might improve filtration operation by some researchers. This paper provides a critical review on the formation mechanisms, properties, the role of sludge cake in membrane filtration, and the corresponding strategies of controlling cake fouling in MBRs. Drawbacks and benefits of the formation of sludge cake were also discussed in order to better understand the characteristics and role of sludge cake formation in MBRs.     

RO membrane solute rejection behavior at the initial stage ofcolloidal fouling

This study investigated the rejection of salt and inert organic compounds by reverse osmosis membranes during the initial stage of colloidal fouling. Results of laboratory-scale experiments showed that colloidal fouling caused a marked decrease in flux, salt rejection and rejection of organics with molecular weight (MW) smaller than about 100 g/mol. Removal of neutrally charged organics was mainly by size or steric exclusion. Rejection of xylose, which has MW >100 g/mol, was not affected much by colloidal fouling. The decrease in salt and low MW organic rejections during the initial stage of colloidal fouling was attributed to cake-enhanced concentration polarization, whereby the colloidal cake layer hindered back diffusion of solutes from the membrane surface to the bulk solution, resulting in higher solute concentration gradient across the membrane. At higher channel wall shear rate, the rates of colloidal deposition, flux decline, decrease in salt rejection, and decrease in low MW organic rejection were lower.