Quantifying the effect of ionic strength on colloidal fouling potential in membrane filtration

Ultrafiltration experiments were conducted to study the fouling potential of colloidal suspensions under different ionic strengths and colloid concentrations. A linear relationship was found relating the colloidal fouling potential to the logarithm of the Debye-Huckel parameter, a characteristic for electrical double layers of colloids. This finding provided a useful quantitative linkage between the colloidal fouling potential and the water chemistry. Considering the linear dependence of colloidal fouling potential on the colloid concentration, a bilinear model was proposed to explain the coupling effects of colloid concentration and ionic strength of the suspension on the fouling potential. The model predictions of fouling potential were found to fit accurately with experimentally determined fouling potential values. Further analysis of the model showed that ionic strength can significantly affect colloidal fouling, for example, a 10-fold increase in ionic strength from 0.001 to 0.01 M for a given feed concentration has the same membrane fouling effect as doubling the feed concentration. The model allows for a quick and reliable assessment of fouling potential without even performing any experiments. This could then be used to design the membrane process or pretreatment stages required to mitigate membrane fouling.     

Irreversible membrane fouling in microfiltration membranes filtering coagulated surface water

Chemical coagulation has been widely used as a method to mitigate membrane fouling in MF/UF membranes used for drinking water treatment. Optimization of coagulation as pre-treatment of membrane processes has not been achieved yet: the optimum condition of coagulation for conventional treatment systems is not necessarily applicable to membrane-based treatment systems. This study investigated (physically) irreversible membrane fouling in an MF membrane used with pre-coagulation by aluminum salt. In a series of bench-scale filtration tests, feed water containing commercially available humic acid or organic matter isolated from surface water was coagulated with polyaluminum chloride (PACl) under various conditions and subsequently filtered with an MF membrane with the nominal pore size of 0.1 μm. It was found that coagulation conditions had great impacts on the degree of physically irreversible fouling. Acidic conditions improved the quality of treated water but generally caused greater physically irreversible fouling than did neutral or alkaline conditions. Also, dosage of coagulant was found to be influential on the degree of membrane fouling: high dosage of coagulant frequently caused more severe irreversible fouling. Sizes of flocs seemed to become small under acidic conditions in this study, which was indicated by high concentrations of aluminum in the permeate under acidic conditions. It is thought that small flocs produced under acidic conditions could migrate into micropores of the membrane and caused physically irreversible fouling by plugging or adsorption. These findings obtained in the bench-scale tests were verified in a long-term pilot-scale test.     

Concentrating the extract of traditional Chinese medicine by direct contact membrane distillation

This study applies direct contact membrane distillation (DCMD) to concentrating the extract of traditional Chinese medicine (TCM). The trans-membrane flux under various operation conditions was measured in real-time during concentration process. By decoupling the factors affecting the trans-membrane flux decline, it was found that the observed flux decline throughout the process could be attributed to the membrane fouling, the reduction of water vapor pressure and the increase of transport resistance at feed side. Analysis of the combined factors was given to show in detail the mechanism of flux decline. Factors that may affect the flux level, such as feed velocity, feed temperature and pretreatment were experimentally examined. Gas bubbling or sparging was introduced into DCMD system for reducing membrane fouling, and it was found that both gas–liquid two-phase flow at the feed side and gas back-washing within membrane module are effective ways to control membrane fouling.     

Blocking of capillaries as fouling mechanism for dead-end ultrafiltration

In this paper plugging of capillaries in the potting is investigated. A lot of research has been done on fouling of the membrane surface (pore blocking, cake filtration) but research on other types of membrane fouling like plugging of capillaries is not so common. Experiments were performed with a lab-scale test installation under constant flux conditions with synthetic feed water containing ferric hydroxide flocks as a fouling component. The experiments showed that during operation capillaries became blocked by fouling plugs. The presence of blockages, especially in the potting at the concentrate side of the capillaries, could not be detected by measuring the clean water resistance. However such blockages did result in an increased forward flush pressure. A combination of the clean water resistance and the forward flush pressure is suitable for determining the fouling of a membrane and the effectiveness of a cleaning procedure. The part of the capillaries in the potting is not backwashed and therefore the hydraulic as well as the chemical cleaning is not efficient at this place.     

Influence of various monovalent cations and calcium ion on the colloidal fouling potential

The influence of various monovalent cations and of divalent calcium ions on colloidal fouling strength was investigated quantitatively on a bench-scale ultrafiltration device. A higher colloidal fouling potential (k) was consistently observed with lithium chloride compared to the same ionic strengths of chlorides of other monovalent cations (Na+, K+, and Cs+). This observation was attributed to the formation of an impervious layer around the colloidal particle by lithium ions that prevented the repulsive forces due to the interaction of the silica hairs formed on the particles in the presence of water. The impact of the divalent calcium ion on the fouling potential was more complex. The fouling potential first increased with calcium ion concentration and then decreased. The maximum value of fouling potential occurred at the ionic strength corresponding to the critical coagulation concentration, which decreased with increasing colloid concentration. The colloidal fouling potential was well correlated by a bilinear relationship with colloid concentration and ionic strength for all salts tested under the critical coagulation concentration.     

Enhancement of electrodialysis performances using pulsing electric fields during extended period operation

Many methods have been considered for mitigating and minimizing fouling potentials in the electrodialysis process, because fouling of ion exchange membranes is one of the significant considerations in process design and operation. In the observation of foulant behaviors, it was observed that the humate was deposited and formed a loosely packed fouling layer on the anion-exchange membrane surfaces, thus having reversible fouling effects on the process. In order to investigate the effects of the frequencies on the electrodialysis performance during fouling experiments in the presence of humate, the square-wave powers having various frequencies in the electric fields were employed. The results showed that the pulsing electric fields mitigated the fouling potential and that there exists an optimal frequency for the minimization of the fouling potential. Also, the pulsation of the electric field with an optimal frequency reduced the fouling potential of the already fouled membrane systems in the continuous batch runs. It was suggested that the electric field with pulsing effects enhanced the electrophoretic mobilities of the charged foulants, thus decreasing fouling potentials.     

Colloidal interactions and fouling of NF and RO membranes: a review

Colloids are fine particles whose characteristic size falls within the rough size range of 1-1000 nm. In pressure-driven membrane systems, these fine particles have a strong tendency to foul the membranes, causing a significant loss in water permeability and often a deteriorated product water quality. There have been a large number of systematic studies on colloidal fouling of reverse osmosis (RO) and nanofiltration (NF) membranes in the last three decades, and the understanding of colloidal fouling has been significantly advanced. The current paper reviews the mechanisms and factors controlling colloidal fouling of both RO and NF membranes. Major colloidal foulants (including both rigid inorganic colloids and organic macromolecules) and their properties are summarized. The deposition of such colloidal particles on an RO or NF membrane forms a cake layer, which can adversely affect the membrane flux due to 1) the cake layer hydraulic resistance and/or 2) the cake-enhanced osmotic pressure. The effects of feedwater compositions, membrane properties, and hydrodynamic conditions are discussed in detail for inorganic colloids, natural organic matter, polysaccharides, and proteins. In general, these effects can be readily explained by considering the mass transfer near the membrane surface and the colloid-membrane (or colloid-colloid) interaction. The critical flux and limiting flux concepts, originally developed for colloidal fouling of porous membranes, are also applicable to RO and NF membranes. For small colloids (diameter?100 nm), the limiting flux can result from two different mechanisms: 1) the diffusion-solubility (gel formation) controlled mechanism and 2) the surface interaction controlled mechanism. The former mechanism probably dominates for concentrated solutions, while the latter mechanism may be more important for dilute solutions. Future research needs on RO and NF colloidal fouling are also identified in the current paper.     

Fouling in direct contact membrane distillation process

The formation of deposit on the membrane surface (fouling) is one of the major operating problems of membrane distillation process. The influence of fouling on the performance of this process was investigated during the concentration of wastewater with proteins, bilge water, brines, and the production of demineralized water. The experiments were performed with polypropylene capillary membranes. The morphology and composition of the fouling layer were studied using Fourier transform infrared with diffuse reflectance spectroscopy and scanning electron microscopy coupled with the energy dispersing spectrometry. Fouling with various intensity was observed in most of the studied cases. Permeate flux decline was mainly caused by an increase in the heat resistance of the fouling layer. However in the case of non-porous deposit, a magnitude of the permeate flux was also determined by a resistance of water transport through the deposit layer. It was found the deposits were formed not only on the membrane surface, but also inside the pores. Salt crystallization in the membrane pores besides their wetting, also caused the mechanical damage of the membrane structure. The intensity of the fouling can be limited by the pretreatment of feed and a selection of the operating conditions of membrane distillation.     

Biofouling in spiral wound membrane systems: Three-dimensional CFD model based evaluation of experimental data

A three-dimensional (3D) computational model describing fluid dynamics and biofouling of feed channels of spiral wound reverse osmosis and nanofiltration membrane systems was developed based on results from practice and experimental studies. In the model simulations the same feed spacer geometry as applied in practice and the experimental studies was used. The 3D mathematical model showed the same trends for (i) feed channel pressure drop, (ii) biomass accumulation, (iii) velocity distribution profile, resulting in regions of low and high liquid flow velocity also named channeling. The numerical model predicted a dominant biomass growth on the feed spacer, consistent with direct in situ observations on biofouling of spiral wound membrane modules and monitors using Magnetic Resonance Imaging (MRI). The model confirms experimental results that feed spacer fouling is more important than membrane fouling. The paper shows that mathematical modeling techniques have evolved to a stage that they can be used hand-in-hand with experiments to understand the processes involved in membrane fouling.     

Mitigated membrane fouling in a vertical submerged membrane bioreactor (VSMBR)

In a laboratory-scale study, characteristics of membrane fouling in an A/O (anoxic/oxic) series membrane bioreactor (MBR) and in a vertical submerged membrane bioreactor (VSMBR) treating synthetic wastewater were compared under the same operating conditions. Accordingly, fouling characteristics of a pilot-scale VSMBR treating municipal wastewater were studied under various operating conditions. Various physical, chemical, and biological factors were used to describe membrane resistances. As a result, it was concluded that high concentrations of extracellular polymeric substances (EPS), high viscosity and a high sludge volume index (SVI) corresponded to high membrane resistance indicating severe membrane fouling in both the laboratory-scale MBRs and the pilot-scale VSMBR. In addition, high fouling potential was observed in the pilot-scale VSMBR at 60-day sludge retention time (SRT). In this case, as hydraulic retention time (HRT) decreased from 10 to 4 h, EPS concentrations increased and the average particle size increased, leading to reduced settling of the sludge and increased membrane fouling. To mitigate fouling, two different methods using air bubble jets were adopted in the pilot-scale VSMBR. As a result, it was found that air backwashing was more efficient for fouling mitigation than was air scouring.     

Crossflow microfiltration of oily water

Two α-alumina ceramic membranes (0.2 and 0.8 μm pore sizes) and a surface-modified polyacrylonitrile membrane (0.1 μm pore size) were tested with an oily water, containing various concentrations (250–1000 ppm) of heavy crude oil droplets of 1–10 μm diameter. Significant fouling and flux decline were observed. Typical final flux values (at the end of experiments with 2 h of filtration) for membranes at 250 ppm oil in the feed are ≈30–40 kg m⁻² h⁻¹. Increased oil concentrations in the feed decreased the final flux, whereas the crossflow rate, transmembrane pressure, and temperature appeared to have relatively little effect on the final flux. In all cases, the permeate was of very high quality, containing <6 ppm total hydrocarbons. The addition of suspended solids increased the final membrane flux by one order of magnitude. It is thought that the suspended solids adsorb the oil, break up the oil layer, and act as a dynamic or secondary membrane which reduces fouling of the underlying primary membrane. Resistance models were used to characterize the type of fouling that occurs. Both the 0.2 μm and the 0.8 μm ceramic membranes appeared to exhibit internal fouling followed by external fouling, whereas external fouling characterized the behavior of the 0.1 μm polymer membrane from the beginning of filtration. Examination of the external fouling layer showed a very thin hydrophobic oil layer adsorbed to the membrane surface. This oil layer made the membrane surface hydrophobic, as demonstrated by increased water-contact angles. The oil layer proved resistant to removal by hydrodynamic (shear) methods. By extracting the oil layer with tetrachloroethylene, followed by IR analysis, its average thickness at the end of a 2 h experiment under typical conditions was determined to be 60 μm for the 0.2 μm ceramic membrane and 30 μm for the 0.1 μm polymer membrane. These measured amounts of oil associated with the membrane at the end of the experiments are in good agreement with those determined from a simple mass balance, in which it is assumed that all of the oil associated with the permeate collected is retained on or in the membrane, indicating that the tangential flow did not sweep the rejected oil layer to the filter exit.     

In-situ manipulation of gel layer fouling into gel layer membrane formation on porous supports for water treatment

The inaccessibility of clean water is one of the growing issues of this era. Indeed, cost-effective and sustainable methods for recycling wastewater are essential. Although membrane separation is an efficient technology for the recycling and purification of water, membrane fouling is still a major drawback of this technology. This work is aimed to develop a dynamic method to form gel layer membranes (GLMs) by manipulating the irreversible fouling process itself as a problem-solving approach. A microporous polyvinylidene fluoride (PVDF) support is subjected to gel layer formation by applying a supernatant of industrial aerobic sludge (containing soluble extracellular polymeric substances EPS) as a feed. Retention of polysaccharides and calcium during the filtration and the topographical analysis after the filtration show that EPS uniformly formed a gel layer on the PVDF support. No further decline in permeability is observed (i.e. remained around 27–33 L/m² hr) when the formed GLM is subjected to fouling under similar conditions. Moreover, the percent flux recovery ratio (FRR) of the GLM is also significant (i.e. 90.1 ± 2.71). The retention ability, hydrophilicity, porosity, and water uptake capability of the formed GLM also increased significantly. The optimal performance and stability of GLMs are observed at room temperature (RT) under neutral pH and sub-critical trance membrane pressures (TMP). Based on these results it is suggested that the in-situ manipulation of gel layer fouling is a viable approach for preparing fouling resistant GLMs with high retention efficiency, potentially applicable to wastewater treatment under normal conditions.     

Membrane fouling and cleaning in ultrafiltration of wastewater from banknote printing works

The hollow fiber blend membrane, fouled by plant wastewater from banknote printing works, was characterized with SEM and the fouling elements were investigated by EDX. Based on analysis results, fouling process in ultrafiltration was simulated by using the model substances, which exist in the wastewater from banknote printing works, such as Turkey red oil, sodium hydroxide and calcium ion. It is observed that the reaction between Turkey red oil and calcium ion forms sediments, which leads to the beginning of membrane fouling. Furthermore, a four-step cleaning method, including de-ionized water cleaning, hydrochloric acid (0.1N) aqueous solution cleaning, second de-ionized water cleaning and sodium hydroxide (1 wt.%) aqueous solution cleaning, was used to clean the seriously fouled membrane in both lab and plant scale (membrane areas were 0.0157 and 80 m², respectively) experiments. The results show that the cleaning method is effective. The membrane surface after cleaning was also analyzed by SEM/EDX and the foulants in the cleaning solutions were identified by TOC and ICP. According to these experimental results, the mechanisms of membrane fouling and cleaning were proposed. The four-step cleaning method has been widely used in the ultrafiltration of wastewater from banknote printing works.     

Modelling of submerged membrane bioreactor: Conceptual study about link between activated slugde biokinetics,aeration and fouling process

A mathematical model was developed to simulate filtration process and aeration influence on submerged membrane bioreactor (SMBR) in aerobic conditions. The biological kinetics and the dynamic effect of the sludge attachment and detachment from the membrane, in relation to the filtration and a strong intermittent aeration, were included in the model. The model was established considering soluble microbial products (SMP) formation-degradation. The fouling components responsible of pore clogging, sludge cake growth, and temporal sludge film coverage were considered during calculation of the total membrane fouling resistance. The influence of SMP, transmembrane pressure, and mixed liquor suspended solids on specific filtration resistance of the sludge cake was also included. With this model, the membrane fouling under different SMBR operational conditions can be simulated. The influence of a larger number of very important process variables on fouling development can be well quantified. The model was developed for evaluating the influence on fouling control of an intermittent aeration of bubbles synchronized or not with the filtration cycles, taking into account the effects of shear intensity on sludge cake removal.     

Effect of surface morphology on membrane fouling by humic acid with the use of cellulose acetate butyrate hollow fiber membranes

A serious problem faced during the application of membrane filtration in water treatment is membrane fouling by natural organic matter (NOM). The hydrophilicity, zeta potential and morphology of membrane surface mainly influence membrane fouling. The aim of the present study is to reveal the correlation between membrane surface morphology and membrane fouling by use of humic acid solution and to investigate the efficiency of backwashing by water, which is applied to restore membrane flux. Cellulose acetate butyrate (CAB) hollow fiber membranes were used in the present study. To obtain the membranes with various surface structures, membranes were prepared via both thermally induced phase separation (TIPS) and nonsolvent-induced phase separation (NIPS) by changing the preparation conditions such as polymer concentration, air gap distance and coagulation bath composition. Since the membrane material is the same, the effects of hydrophilicity and zeta potential on membrane fouling can be ignored. More significant flux decline was observed in the membrane with lower humic acid rejection. For the membranes with similar water permeability, the lower the porosity at the outer surface, the more serious the membrane fouling. Furthermore, the effect of the membrane morphology on backwashing performance was discussed.     

Surface modification of phenolphthalein poly(ether sulfone) ultrafiltration membranes by blending with acrylonitrile-based copolymer containing ionic groups for imparting surface electrical properties

Asymmetric ultrafiltration membranes were fabricated from the blends of phenolphthalein polyethersulfone (PES-C) and acrylonitrile copolymers containing charged groups, poly(acrylonitrile-co-acrylamido methylpropane sulfonic acid) (PAN-co-AMPS). From the surface analysis by XPS and ATR-FTIR, it was found that the charged groups tend to accumulate onto the membrane surface. This result indicated that membrane surface modification for imparting surface electrical properties could be carried out by blending charged polymer. Furthermore, with the help of a relatively novel method to measure membrane conduction, the true zeta potentials calculated on the basis of the streaming potential measurements were used to reflect the charge state of membrane surface. In addition, it was noteworthy that, from the profiles of zeta potential versus pH curves and the magnitude of zeta potentials, the determination of zeta potential was dependent not only on the electrical properties of membrane surface but also on its hydrophilicity. At last, based on a relatively elaborate study on the electrostatic interaction between the membrane surface and protein, it was found that these charged membranes could meet different demands of membrane applications, such as resisting protein fouling or protein separation, through adjusting solution pH value.     

Atomic Force Microscopy Studies of Membranes: Effect of Surface Roughness on Double-Layer Interactions and Particle Adhesion

Atomic force microscopy in conjunction with the colloid (silica) probe technique has been used to quantify the variations in electrical double-layer interactions and adhesion at different locations on a rough reverse osmosis membrane (AFC99) surface in NaCl solutions. Prior scanning of the membrane surface with the colloid probe allowed precise location for force measurements. The membrane surface was composed entirely of peaks and valleys with a surface roughness substantially greater than that of most other types of polymeric membranes. The magnitude of the electrical double-layer repulsion between the colloid probe and the membrane at peaks on the membrane surface was greatly reduced compared to that in the valleys. Nevertheless, adhesion of the colloid probe was lower at the peaks on the membrane surface than in the valleys with the difference increasing with decreasing salt concentration, and reaching a factor of more than 20 in 10(-3) M solution. The study shows that minimization of membrane fouling by colloids could be achieved by choosing membranes with a roughness periodicity preventing penetration of foulants into valleys on the surface. Copyright 2000 Academic Press.     

Impact of gel layer formation on colloid retention in membrane filtration processes

Colloidal particles in the feed streams of membrane filtration processes control membrane fouling rate in many instances. In this study, the non-gelling colloidal Na-alginate and the gelling colloidal Ca-alginate are employed to investigate the significance of gel layer formation in membrane filtration processes in terms of contribution to membrane fouling and supplementary impurity removal. The results show that contribution of colloidal particles to membrane fouling depends on the gelling propensity of the colloids and the operational mode (constant pressure or constant flux) implemented. A small dose of either Na-alginate or Ca-alginate was found to greatly increase membrane fouling rate during constant pressure filtration. Both the resistance to removal by application of shear and the lower susceptibility of the concentration polarization layer to shear resulted in more severe fouling during constant flux filtration in the presence of Ca-alginate assemblages than in the presence of Na-alginate. Apparent channel sizes of the Ca-alginate gel layer were calculated from the material properties of the fouling layer. Incomplete catalase retention highlighted the likely heterogeneity in size of liquid transport pathways. Adsorption also contributed to the trapping of colloidal particles according to the retention behaviour of BSA by the Ca-alginate gel layer. Gel layer formation propensity should be seriously considered for the operation of membrane filtration processes. Two simple methods based on (i) a permeability recovery experiment and (ii) comparison of dead-end filtration behaviour with and without shear application are proposed for evaluation of the gelling propensity of colloidal dispersions.     

Response surface modelling and optimization in pervaporation

Both the conventional method of experimentation, in which one of factors is varied maintaining the other factors fixed at constant levels and the statistically designed experimental method, in which all factors are varied simultaneously are carried out for organic removal from water by pervaporation. Binary acetonitrile–water mixtures are considered. The effects of the operating parameters on the pervaporation performance of the membrane system have been investigated. The overall mass transfer coefficients have been determined for different conditions of feed temperature and initial organic concentration. In addition, the activation energy associated to the permeation process has been determined and discussed for each feed organic mixture. Statistical experimental design and response surface methodology, RSM, have been applied to optimize the operational conditions of pervaporation process in order to maximize the output responses, which are permeate flux ratio and concentration of organic in permeate. The input variables employed for experimental design were the feed temperature, initial concentration of organic in feed and operational downstream pressure. Based on the design of experiment the quadratic response surface models have been developed to link the output responses with the input variables via mathematical relationships. The constructed response models have been tested using the analysis of variance and the canonical analysis. The obtained optimal point by means of Monte Carlo simulation method and desirability function corresponds to a feed temperature of 57.69 °C, a feed acetonitrile concentration of 6.96 wt% and a downstream pressure of 28.95 kPa. The maximal values of the permeate flux ratio and the concentration of organic in permeate obtained under optimal process conditions have been confirmed experimentally.     

Direct-flow microfiltration of aquasols: II. On the role of colloidal natural organic matter

Natural organic matter (NOM) has been considered a major contributor to the fouling of microfiltration (MF) and ultrafiltration (UF) membranes employed in water treatment. However, the fouling potential of NOM has often been assessed in terms of its size or chemical composition. The colloid’s chemical properties have often been ignored. In this study, a chemical attachment-based (CAB) model established previously was used in conjunction with a variety of analytical techniques to investigate the existence of three major components of an aquatic NOM and their role in the fouling of a polyvinylidene fluoride MF membrane. The results suggest that colloidal NOM relevant to membrane fouling has a broader size distribution and variations in chemical properties than proposed previously. For the model aquatic NOM used in this research, fouling was primarily contributed by both non-humic and humic colloidal fractions. The non-humic colloids were larger in size and probably adhered to the membrane regardless of the solution chemistry, while humic colloids had variable size and stickiness depending on solution chemistry. The fouling caused by organic colloids was mostly hydraulically irreversible, as a consequence of favorable surface interactions. The CAB model provided a useful way to understand the role of organic colloids in membrane fouling.