Influence of membrane structure on fouling layer morphology during apple juice clarification

The flux behavior of 0.2 μm nylon, polysulfone (PS), polyvinylidene fluoride (PVDF) and polyethersulfone (PES) membranes was examined during dead-end microfiltration of commercial apple juice. On nylon membranes, a 0.1 μm thick surface fouling layer rapidly formed that acted as a secondary membrane. The colloidal particles retained by this surface layer aggregated to form a thick loose gel structure, producing an anisotropic fouling structure. In contrast, the 4 μm thick surface fouling layer of PES was slower to form and had a more open structure with a lower flux resistance per unit thickness. The morphology of the PES surface layer also did not differ dramatically from the loose gel structure that subsequently formed on top of this secondary membrane. The PS surface fouling layer was similar in structure to nylon whereas the PVDF layer more closely resembled that found with PES. The density of the surface fouling layer did not directly correlate to membrane surface hydrophobicity or pure water flux. Atomic force microscopy (AFM) indicated that surface roughness strongly influenced surface fouling layer morphology. The membrane surface appears to act as a template for the fouling process; therefore, smooth membranes (nylon and PS) produce a dense surface fouling layer whereas this same layer on rough membranes (PES and PVDF) is much more open. Consequently, the fluxes of PES and PVDF membranes are less affected by fouling formation.     

Direct visual observation of yeast deposition and removal during microfiltration

Periodic reverse flow through membranes is an effective technique to remove foulants from microfiltration (MF) membrane surfaces. This work explored direct visual observation (DVO) of yeast deposition and subsequent removal via backwashing and single backpulses using microvideo photography with cellulose-acetate (CA) and Anopore anodised-alumina (AN) MF membranes. Foulant deposited less uniformly on the surfaces of the CA membranes than on the AN membrane surfaces during forward filtration. Foulant cake layers of approximately 30 μm thickness formed on both membranes after forward filtration for 1–2 h, leading to fouled-membrane fluxes of only 15–25% of the clean-membrane fluxes.Foulant was removed by reverse flow from the CA membrane surfaces in clumps. The time constant for foulant removal was determined from photomicrographs to be approximately 0.2 s, and 95% of the membrane surface was cleaned within 1 s of backpulsing, resulting in 95% recovery of the initial flux. The foulant cake was also removed from the AN membranes in clumps, though much of the membrane remained covered in a monolayer of yeast. The flux through the membrane covered with a full monolayer was determined during forward filtration to be about 70% of the clean membrane flux.A model for flux recovery is proposed which takes into account the fraction of the membrane surface which is completely cleaned as well as the fraction which remains covered in a foulant monolayer. The predicted and experimentally-determined recovered fluxes as a function of backpulse duration are in very good agreement.     

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.     

Transmission of bovine albumin under controlled flux ultrafiltration

This work reports the transmission of bovine albumin using 50k MWCO, 100k MWCO and 0.2 μm membranes under controlled fluxes and low transmembrane pressures. With the 50k MWCO membrane, the transmission remained low and when the flux was increased step by step, there was a sharp increase in transmission as the flux reached 50 lm⁻² h⁻¹. The concentration of bovine albumin was estimated at the membrane surface by the classical film theory and did not increase sharply. It is suggested that increasing extensional shear at the higher flux might change the conformation of BSA molecules. This sudden change in rejection was not observed when a 0.2 μm membrane was used. It was also observed that the transmission at iso-electric point of bovine albumin (pH 4.9) was much higher than that at either pH 3.5 or pH 8 under the same operating conditions.     

Influence of polypropylene membrane surface porosity on the performance of membrane distillation process

Two kinds of polypropylene capillary membranes were used in the membrane distillation (MD). These membranes exhibited a similar morphology, but one of them has an additional low porosity layer on the internal surface of capillaries. The changes of membrane performance during MD process of tap water were investigated. The presence of low porosity layer (thickness below 1 μm) caused that the air permeability was reduced from 1.365 to 0.863 dm³/m² s kPa, whereas the MD permeate flux was decreased only by 15%. A significantly larger decline of the flux was caused by CaCO₃ deposit formed during distillation of tap water. This deposit was removed every 30–70 h by rinsing the modules with a 2–5 wt.% HCl. Unfortunately, a repetition of this operation several times resulted in a gradual decline of the maximum permeate flux (distilled water as a feed). However, the module efficiency with the membranes covered by a surface layer of low porosity was found to decreases twice as slowly. The investigations revealed that a low surface porosity does not limit the possibility of surface wetting of polypropylene membranes, but hindered the scale formation inside the pores.     

Effect of calcium and carbonate concentrations on anionic membrane fouling during electrodialysis

A previous study on electrodialysis of calcium and carbonate high concentration solutions demonstrated that calcium migrated through the cation-exchange membrane (CEM) was blocked by the anion-exchange membrane (AEM) where it formed another fouling. The aim of the present work was to complete the identification of the deposit formed on AEM during electrodialysis and to characterize its physical structure at the interface of the membrane. No fouling was found on the anionic membranes treated without calcium chloride in presence of sodium carbonate, while membranes used during ED process of solutions containing calcium chloride and sodium carbonate were slightly fouled. A thin layer of precipitates was observed on the anionic membrane surface. The appearance of precipitates was typical of a crystalline substance. The size and form of crystal increased in proportion to the concentration of calcium chloride in solution. Large and cubic crystals were the best defined on the membrane treated at 1600 mg/L of CaCl2. The precipitate was identified as calcium hydroxide. However, this fouling was not found to affect significantly the electrical conductivity and the thickness of the membranes. Furthermore, the fouling formed was reversible.     

Efficiency of various chemical cleanings for nanofiltration membrane fouled by conventionally-treated surface water

Nanofiltration systems are generally cleaned chemically. The optimal choice of the cleaning agent is a function of membrane material and foulant in a complex manner. This study evaluated the cleaning efficiency and effects of several cleaning agents on NF255 nanofiltration membrane. The nanofiltration pilot plant was fed with conventionally-treated surface water from a water treatment plant in southern Finland. Fouled membranes were cleaned weekly with different chemicals and procedures, and the cleaning efficiencies were compared in terms of flux recoveries and foulant removals. On the basis of the cleaning chemical analysis, the fouling material consisted of biofouling, organic deposits and metal complexes. In these circumstances, alkaline cleaners with chelatants resulted in the most efficient cleaning both in terms of flux recovery and foulant removal. Alkaline cleaning modified the membrane and improved the flux substantially in comparison to the virgin state. The results demonstrate that the choice of chemical cleaning agent is critical to cleaning efficiency, both technically and economically. The same flux recovery could be reached either by a single cleaning phase or by three sequential cleaning phases.     

Electric mass transport through homogeneous and surface-modified heterogeneous ion-exchange membranes at a rotating membrane disk

Polarization characteristics of the homogeneous MF-4SK perfluorinated sulfonated cation-exchange membrane and the heterogeneous MK-40 sulfonic acid membrane with its surface modified by a homogeneous film of Nafion are studied at a rotating membrane disk in 0.1 and 0.001 M sodium chloride solutions. Partial current-voltage curves (CVC) are obtained for sodium and hydrogen ions, and limiting current densities in the electromembrane systems (EMS) under study are calculated as a function of the rotation rate of the membrane disk. Contribution from different mechanisms (electrodiffusion, electroconvection, dissociation of water, and the effect of the limiting-current exaltation) to the total ion flow is estimated experimentally and theoretically under conditions that the diffusion layer in the EMS has stabilized in thickness. It is established that surface modification of the heterogeneous MK-40 membrane with a 7 μm layer of a modifying agent almost completely eliminates the dissociation of water molecules, and the properties of the heterogeneous MK-40 membrane approximate those of the homogeneous Nafion membrane. From IR spectra and potentiometric titration curves of the MK-40 and MF-4SK membranes, it is shown that the acidity of the sulfonate groups in these membranes is nearly identical, but a difference in the dissociation rate of water at these membranes is determined by a different character of charge-density distribution and potential near the membrane-solution interphase boundary. By means of the theory of the overlimiting state in EMS, the internal parameters of the systems under investigation are calculated: distribution of space-charge density and electric-field potential in the diffusion layer and in the membrane. Partial CVC are calculated for H⁺ ions for the space-charge region in the phase of the MF-4SK and MK-40/Nafion ion-exchange membranes. Partial CVC with similar characteristics are compared for the heterogeneous monopolar MK-40 and the bipolar MB-2 membranes, which contain sulfonate groups. It is concluded that the membrane surface layer, where the space charge is localized, plays a dominant role in speeding up the dissociation of water in EMS.     

A comparative study of diffusion samplers for the determination of hexavalent chromium by sequential injection spectrophotometry

A method is described for determining hexavalent chromium in high particulate-containing surface waters by sequential injection (SI). The relative performance of two membrane-based methods for sampling is compared. The first membrane approach is based on a commercial design known as the ‘supported capillary membrane sampler (SCMS)’ (Wolcott, D.K., US Pat. 5 317 932 (1995)) that uses tubular membranes; the second approach is based on a conventional parallel-plate dialyzer (PPD) design that uses planar membranes. The membranes are evaluated using the well-known colorimetric method for the determination of hexavalent chromium by complexation with 1,5-diphenylcarbazide (DPC). Thin-walled (∼200 μm) microporous (pore size ∼0.2 μm) polypropylene membranes are equilibrated with DPC during each sampling period. Formation of the DPC-Cr(VI) complex allows for efficient membrane transport; without the membrane, Cr(VI) transport decreases ∼90%. Factors optimized included reagent concentrations, sampling time, flow rate, and spectrophotometric conditions. Optimal conditions were 2.00 mM DPC and 0.100 M nitric acid for the reagent, and 600 and 900 s sampling times for the planar and tubular designs, respectively. The planar (PPD) design increased the sensitivity relative to the tubular (SCMS) design by ∼225%. The PPD-SI method was applied to the determination of dissolved Cr(VI) in high particulate-containing surface water samples. Figures of merit included a detection limit of <20 μg/l, precision of 1.1% R.S.D. at 100 μg/l (n=4), and selectivity for dissolved Cr(VI) in several surface water samples with high levels of particulate matter.     

Fabrication of blend hydrophilic polyamide imide (Torlon®)-sulfonated poly (ether ether ketone) hollow fiber membranes for oily wastewater treatment

Blend hydrophilic polyamide imide (PAI)-sulfonated poly (ether ether keton) (SPEEK) hollow fiber membranes were fabricated for oil-water emulsion separation. The structure and performance of the membranes were examined by FESEM analysis, N₂ permeation, overall porosity, collapsing pressure, water contact angle, pure water flux, molecular weight cutoff (MWCO), and oil rejection tests. By studying ternary phase diagrams of polymer/solvent-additive/water system, the higher phase-inversion rate was confirmed for the solutions prepared at higher PAI/SPEEK ratio. A more open structure with larger finger-likes was observed by increasing PAI/SPEEK ratio. Mean pore size of 81 nm, overall porosity of 79% and water contact angle of 58° were obtained for the improved membrane prepared by PAI/SPEEK ratio of 85/15. Increasing SPEEK ratio resulted in lower mechanical stability in terms of collapsing pressure. Pure water flux of about 2.5 times of the plain PAI membrane was found for the improved membrane. MWCO of 460 kDa was found for the improved blend membrane. From oil rejection test, all the membranes demonstrated an oil rejection of over 95%. The improved membrane showed a lower rate of permeate flux reduction compared to the plain membrane which was related to the smaller fouling possibility. Less fouling resistance of the improved membrane was related to the higher flux recovery ratio (about 92%). For all the membranes, the dominant fouling mechanism was found to be the cake filtration. The improved PAI-SPEEK hollow fiber membranes was found to be practical for ultrafiltration of oily wastewaters.     

Comparison of polysulfone and ceramic membranes for the separation of phenol in micellar-enhanced ultrafiltration

Micellar-enhanced ultrafiltration (MEUF) of phenol and a cationic surfactant, cetylpyridinium chloride (CPC), is studied using two polysulfone membranes of 5- and 50-kDa molecular weight cutoff (MWCO) and two ceramic membranes of 15- and 50-kDa MWCO. Filtrations are run under laminar cross-flow and steady-state conditions. The effect of operation variables (pressure and retentate flux) and membrane properties (nature and MWCO) on permeate flux, surfactant, and phenol rejections is analyzed. The permeate flux depends, among other variables, on the fouling favored by membrane-micelle interactions, which are strongest in the 50-kDa MWCO ceramic membrane. On the other hand, surfactant rejection is mainly determined by the pore size and influenced by the pressure for both 50-kDa MWCO membranes. An equilibrium distribution constant, K(s), of phenol between surfactant micelles and water is calculated. Its value is not significantly affected by operation conditions and membrane type. K(s) is also approximately 20% lower than the value determined in a previous work with batch dead-end ultrafiltration.     

Alginate-coated microporous PTFE membranes for use in the osmotic distillation of oily feeds

Osmotic distillation (OD) has two main advantages over thermally driven concentration processes attributable to its ambient temperature operation. These are maintenance of the integrity of thermally labile components and minimisation of the loss of volatile flavour/fragrance components. However, a major disadvantage of osmotic distillation is the potential for wet-out of the hydrophobic membrane when fouled by surface-active agents such as citrus oils. In this work, sodium alginate hydrogel coatings were applied to PTFE membranes for protection against wet-out. The coating technique developed for this purpose resulted in a 10-fold increase in adhesion strength over that achievable by simple casting. This was effected by increased intrusion of the coating solution meniscus into the porous PTFE structure by surface tension adjustment with ethanol, precipitation of sodium alginate by the selective removal of water, and finally alginate crosslinking. Precipitation occurred both on the surface and in the void spaces between the PTFE fibres, thereby providing better anchorage for the coating. The coating decreased the overall OD mass transfer coefficient by less than 5%. OD flux measurements using coated membranes with 0.2, 0.4 and 0.8 wt.% orange oil–water mixtures over a period of 300 min indicated that the coating was successful in protecting the membrane against wet-out. An uncoated membrane was immediately wet out by a 0.2 wt.% orange oil–water OD feed. In a separate trial, a coated membrane retained its integrity after contact with a 1.2 wt.% oil–water mixture for 72 h.     

木质素磺酸钠改性聚砜膜的制备及其在正渗透膜中的应用

采用木质素磺酸钠作为亲水添加剂,通过浸没沉淀相转化法制备了木质素磺酸钠共混改性聚砜膜,以改善聚砜膜的亲水性,并用作正渗透膜的支撑层,以降低内浓差极化效应.利用扫描电子显微镜、衰减全反射傅里叶变换红外光谱仪、水接触角仪等研究了不同木质素磺酸钠添加量对聚砜膜的结构和表面性质的影响.结果表明,添加木质素磺酸钠后,聚砜膜的指状孔变得规整且狭长.水接触角实验证实添加木质素磺酸钠能改善聚砜膜的亲水性,当木质素磺酸钠含量为0.4 wt%时,聚砜膜的表面水接触角可降低至65°.正/反渗透测试装置分别用于表征正渗透膜的传质性质和结构参数.结果表明,以0.4 wt%木质素磺酸钠改性聚砜膜为支撑层的正渗透膜的水渗透性能(A=3.12×10~(-5) LMH×Pa~(-1))优于纯聚砜基底正渗透膜(0.76×10~(-5)LMH×Pa~(-1)),而且前者的结构参数(S=2010mm)远小于后者(3450mm),说明木质素磺酸钠改性聚砜膜有效弱化了正渗透膜的内浓差极化效应.     

Removal of Natural Organic Matter by Forward Osmosis Membrane: Flux Behaviour and Foulant Characterization

Fouling of cellulose triacetate(CTA) forward osmosis(FO) membranes by natural organic matter(NOM) was studied by means of a cross-flow flat-sheet forward osmosis membrane system. The NOM solution was employed as the feed solution(FS), and a sodium chloride solution(3 mol/L) was used for the draw solution(DS). The process was conducted at various temperatures and cross-flow velocities. The flux decline was investigated with 3 h forward osmosis operation. The substances absorbed on the membranes were cleaned by ultrasonic oscillation of the fouled membranes and were characterized by methodologies including fluorescence excitation-emission matrices (EEMs) and liquid chromatography with an organic carbon detector(LC-OCD), and the variations of membrane properties were also investigated by Fourier transform infrared spectrometer(FTIR) and a contact angle meter. It was noted that the rejection efficiency of NOM is remarkable and that ultrasonic oscillation is an effective method to extract the NOM fouled on the CTA membranes after FO process. A higher cross-flow velocity and lower temperature benefit the anti-fouling capacity of the membrane significantly. Although humic substances accounted for the majo- rity of the NOM, aromatic proteins and amino acids were the main fouling components on the membranes, with symbolic FTIR peaks at 2355, 1408 and 873 cm^-1. The present surface foulant made the membranes becoming more hydrophilic, as demonstrated by a significant decrease in contact angle(ranging from 20% to 46%) under all the operation conditions.     

Visualisation of an ultrafiltration membrane by non-contact atomic force microscopy at single pore resolution

Non-contact atomic force microscopy (AFM) has been used to investigate the furface pore structure of a polyethersulfone ultrafitration membrane of specified molecular weight cut off (MWCO) 25 000 (ES625, PCI Membrane Systems). Excellent images at up to single pore resolution were obtained. This is the first time that AFM images of a membrane at such high resolution have been presented. Analysis of the images gave a mean pore size of 5.1 nm with a standard deviation of 1.1 nm. The results have been compared to previously published studies of membranes of comparable MWCO using contact AFM and electron microscopy. Non-contact AFM is a powerful means of studying the surface pore characteristics of ultrafiltration membranes.     

Development of octanol membranes for drug screening

Most drugs permeate biological membranes via passive diffusion and it is generally assumed that the main barrier is the lipophilic structure of the membranes. However, it has been observed that an unstirred water layer adjacent to the membrane surface can in some cases be just as effective barrier as the lipophilic membrane itself. Hydrophilic cyclodextrins can enhance drug delivery through biological membranes by increasing the availability of dissolved drug molecules immediate to the membrane surface, i.e., by increasing drug delivery through the unstirred water layer. We have developed an artificial octanol membrane that is cheap and simple to prepare. The novel membrane consists of a hydrated semi-permeable cellophane membrane with a molecular weight cut off (MWCO) of 12,000–14,000 that mimics the unstirred water layer and a lipophilic membrane of pure n-octanol in a nitrocellulose matrix. The membrane was used to investigate the effects of 2-hydroxypropyl-β-cyclodextrin (HPβCD) on the flux of hydrocortisone through the membrane. In aqueous HPβCD saturated with hydrocortisone the drug concentration gradient, over the unstirred water layer, increased with increasing HPβCD concentration which resulted in enhanced drug delivery through the membrane. When excess HPβCD was present in the donor phase the octanol/water partition coefficient decreased with increasing HPβCD concentration that lead to decreased drug delivery through the membrane.     

Zwitterionic microgel based anti(-bio)fouling smart membranes for tunable water filtration and molecular separation

Tunable gating polymeric nanostructured membrane with excellent water permeability and precise molecular separation is highly advantageous for smart nanofiltration application. Polymeric nanostructures such as microgels with functionalizable cross-linkable moieties can be an excellent choice to construct membranes with a thin separation layer, functionality, and tunable transport properties. In the present work, we prepared switchable anti(bio)fouling membranes using zwitterionically functionalized antibacterial thermoresponsive aqueous core-shell microgels with a thin separation layer for controlled filtration and separation applications. The microgels were synthesized using a one-step graft copolymerization of poly(N-isopropylacrylamide) and polyethyleneimine (PEI) followed by zwitterionization of free amine groups of PEI chains with 1,3-propane sultone. Microgel synthesis and zwitterionization were confirmed by spectroscopic and elemntal analysis. The obtained microgels were thoroughly characterized to analyze their thermoresponsive behavior, morphology, charge, and antibacterial properties. After that, characterizations were performed to elucidate the surface properties, water permeation, rejection, and flux recovery of the microgel membranes prepared by suction filtration over a track-etched support. It was observed that zwitterionic membrane provides better hydrophilicity, lower bovine serum albumin (BSA) adsorption, and desirable antimicrobial activity. The pure water permeability was directly related to the microgel layer thickness, applied pressure, and temperature of the feed solution. The novel nanostructured membrane leads to an excellent water permeance with a high gating ratio, high flux recovery rate with low irreversible fouling, better rejection for various dyes, and foulant. Most importantly, the long-term performance of the membrane is appreciable as the microgel layer remains intact and provides excellent separation up to a longer period. Owing to easy preparation and well control over thickness, the zwitterionic microgel membranes constitute unique and interactive membranes for various pressure-driven separation and purification applications.     

Effects of polyether–polyamide block copolymer coating on performance and fouling of reverse osmosis membranes

The effects of a water-permeable polymer coating on the performance and fouling of high-flux (ESPA1 and ESPA3) and low-flux (SWC4) polyamide reverse osmosis (RO) membranes were investigated. It was anticipated that the coating would create a smoother hydrophilic surface that would be less susceptible to fouling when challenged with a motor-oil/surfactant/water feed emulsion (used as a model foulant). AFM and FT-IR analyses confirm that a 1 wt.% polyether–polyamide (PEBAX^® 1657) solution applied to ESPA and SWC4 membranes produces a continuous polymer coating layer and, thereby, provides smoother membrane surfaces. However, pure-water permeation data combined with a series-resistance model analysis reveal that the coating does not only cover the surface of the polyamide membrane, but also penetrates into its porous ridge-and-valley structure. During a long-term (106-day) fouling test with an oil/surfactant/water emulsion, the rate of flux decline was slower for coated than for uncoated membranes. This improvement in fouling resistance compensated for the decrease in permeate flux for SWC4 over a period of approximately 40 days. However, the coating material is believed to penetrate more deeply into the polyamide surface layer of the high flux, high surface area ESPA membranes relative to the low-flux SWC4, resulting in significant water flux reduction.     

Fouling of reverse osmosis membranes by biopolymers in wastewater secondary effluent: Role of membrane surface properties and initial permeate flux

Reverse osmosis (RO) is being increasingly used in treatment of domestic wastewater secondary effluent for potable and non-potable reuse. Among other solutes, dissolved biopolymers, i.e., proteins and polysaccharides, can lead to severe fouling of RO membranes. In this study, the roles of RO membrane surface properties in membrane fouling by two model biopolymers, bovine serum albumin (BSA) and sodium alginate, were investigated. Three commercial RO membranes with different surface properties were tested in a laboratory-scale cross-flow RO system. Membrane surface properties considered include surface roughness, zeta potential, and hydrophobicity. Experimental results revealed that membrane surface roughness had the greatest effect on fouling by the biopolymers tested. Accordingly, modified membranes with smoother surfaces showed significantly lower fouling rates. When Ca²⁺ was present, alginate fouled RO membranes much faster than BSA. Considerable synergistic effect was observed when both BSA and alginate were present. The larger foulant particle sizes measured in the co-existence of BSA and alginate indicate formation of BSA-alginate aggregates, which resulted in greater fouling rates. Faster initial flux decline was observed at higher initial permeate flux even when the flux was measured against accumulative permeate volume, indicating a negative impact of higher operating pressure.     

A new technique for membrane characterisation: direct measurement of the force of adhesion of a single particle using an atomic force microscope

An Atomic Force Microscope (AFM) has been used to quantify directly the adhesive force between a colloid probe and two polymeric ultrafiltration membranes of similar MWCO (4000 Da) but different materials (ES 404 and XP 117, PCI Membrane Systems (UK)). The colloid probe was made from a polystyrene sphere (diameter 11 μm) glued to a V shaped AFM cantilever. Measurements were made in 10⁻² M NaCl solution at pH 8. It was found that the adhesive force at the ES 404 membrane was more than five times greater than that at the XP 117 membrane. As it allows direct quantification of particle/membrane interactions, this technique should be invaluable in the development of new membrane materials and in the elucidation of process behaviour.