Artificial particulate fouling of hyperfiltration membranes — II analysis and protection from fouling

An artificial fouling method was used to study the effect of suspended particulates on the performance of standard asymmetric cellulose acetate membranes in flat cells. Using a modified filtration theory developed here, membrane and cake resistances to permeation were individually monitored as a function of applied pressure, membrane curing temperature, and feed type. The membranes behaved according to the solution-diffusion model.A new previously unreported method of membrane protection — the protective cover method — for submicron colloidal solutions, resulted in as high as 78% increased fluxes and 43% decreased salt rejections, over the unprotected membrane. This development is applicable where low salt rejection and high fluxes are desired such as in industrial and municipal wastewater renovation.     

Membrane fouling and chemical cleaning in water recycling applications

Fouling and subsequent chemical cleaning are two important issues for sustainable operation of nanofiltration (NF) membranes in water treatment and reuse applications. Fouling strongly depends on the feed water quality, especially the nature of the foulants and ionic composition of the feed water. Consequently, appropriate selection of the chemical cleaning solutions can be seen as a critical factor for effective fouling control. In this study, membrane fouling and chemical cleaning under condition typical to that in water recycling applications were investigated. Fouling conditions were achieved over approximately 18 h with foulant cocktails containing five model foulants namely humic acids, bovine serum albumin, sodium alginate, and two silica colloids in a background electrolyte solution. These model foulants were selected to represent four distinctive modes of fouling: humic acid, protein, polysaccharide, and colloidal fouling. Three chemical cleaning solutions (alkaline solution at pH 11, sodium dodecyl sulphate (SDS), and a combination of both) were evaluated for permeate flux recovery efficiency. The results indicated that with the same mass of foulant, organic fouling was considerably more severe as compared to colloidal fouling. While organic fouling caused a considerable increase in the membrane surface hydrophobicity as indicated by contact angle measurement, hydrophobicity of silica colloidal fouled membrane remained almost the same. Furthermore, a mechanistic correlation amongst cleaning efficiency, characteristics of the model foulants, and the cleaning reagents could be established. Chemical cleaning of all organically fouled membranes by a 10 mM SDS solution particularly at pH 11 resulted in good flux recovery. However, notable flux decline after SDS cleaning of organically fouled membranes was observed indicating that SDS was effective at breaking the organic foulant—Ca²⁺ complex but was not able to effectively dissolve and completely remove these organic foulants. Although a lower permeate flux recovery was obtained with a caustic solution (pH 11) in the absence of SDS, the permeate flux after cleaning was stable. In contrast, the chemical cleaning solutions used in this study showed low effectiveness against colloidal fouling. It is also interesting to note that membrane fouling and chemical cleaning could permanently alter the hydrophobicity of the membrane surface.     

纤维素助滤剂的预敷过滤性能

对高黏度物料加入纤维素助滤剂的预敷过滤性能进行了研究,说明不同的预敷条件对预敷层的过滤性能会有很大影响。通过改变预敷压力和浓度,对纤维素滤饼层的过滤比阻、可压缩性系数和孔隙率进行了研究。实验表明,纤维素预敷层的比阻随压力增加而增加,随浓度增加而减小;孔隙率随压力增加而减小,随浓度增加而增加;可压缩性系数随浓度增加而增加。纤维素助滤剂为中等可压缩性物料,孔隙率大。研究结果可为高黏度物料用纤维素预敷过滤的工程应用提供参考。     

基于膜表面与界面作用的膜污染控制方法

膜污染控制是膜技术能否成功应用的关键因素之一。讨论了膜面粗糙度、亲水性和荷电性等性质对膜分离性能的影响,介绍了膜表面性质参数的表征方法,分析了颗粒污染物团聚、颗粒与胶体吸附、胶体大分子的变形等对膜污染形成的影响。最后对面向应用过程的陶瓷膜材料表面与界面控制方法进行了总结。膜污染的研究已从操作参数调节发展到表面与界面作用的控制,对提高膜分离性能,促进膜过程在更多领域的推广应用有重要意义。     

Colloidal fouling of reverse osmosis membranes

Small colloidal particles intermediate in size between suspended solids, and true dissolved solids have been causing serious problems to many reverse osmosis systems throughout the world.These tiny negatively charged particles become concentrated at the surface of the membrane during the R.O. process and begin to coagulate, plugging the membrane. If this fouling is allowed to go on unchecked, the quantity and quality of the product water will begin to deteriorate until the membranes will eventually need to be replaced.In order to prevent this from happening, it is suggested that the R.O. pretreatment system be designed properly. If the pretreatment is not adequate, it can lead to high maintenance costs and an increase in membrane replacements.     

Chemical control of colloidal fouling of reverse osmosis systems

Fouling of membranes by colloidal organic and inorganic particles continues to be documented as the most common and challenging obstacle in attaining stable continuous operation of reverse osmosis (RO) and ultrafiltration (UF) systems. Much current research is being conducted on physical parameters to mitigate such fouling. The focus has been on membrane synthesis and element design; microfiltration and ultrafiltration pretreatment; electromagnetic devices; correlation with physical factors such as Silt Density Index, zeta potential and critical flux; technique of direct observation of fouling process through a membrane; and classification of macromolecular organics for correlation with fouling characteristics. We report initial successes with chemical control of colloidal fouling. Through screening with a large number of observable coagulations of natural colloids, we have developed a group of proprietary anticoagulants and dispersants that would, at less than 10 ppm dosage to the RO feedwater, control various classes of colloidal foulants. Case studies of the control of humic matter, elemental sulfur and colloidal silicate in problematic RO systems that became stabilized are briefly presented. We conclude that a great need and potential exists in economically controlling the myriads of fouling interactions of colloidal particles during concentration within the brine channels of RO membrane elements. Low dosages of antifoulants can in many cases obviate the need for installation and maintenance of pretreatment unit or operations designed to remove such colloidal foulants from the process stream.     

UV-nanoparticles: Photopolymerized polymer colloids from aqueous dispersions of acrylated oligomers

Polymer colloids are usually prepared using thermally activated polymerization. Photo-induced techniques for the preparation of polymerized or crosslinked colloids have seldom been reported in the literature and to our knowledge none has found real application in industry. Here, we introduce a new approach to form insoluble crosslinked colloidal particles by in situ photopolymerization starting from aqueous dispersions of auto-dispersible acrylated oligomers originally developed for water-based UV-coating applications. Of particular interest is the relatively high rate at which particle polymerization takes place even in highly opaque conditions e.g. with particle loadings of more than 20 w/w% and particle sizes ranging from 40 up to 500 nm. In this paper, the preparation process is demonstrated with an immersion-type batch photoreactor and the properties of this new family of organic colloids are highlighted. Furthermore, we report the preparation of submicron sized containers by loading a hydrophobic compound (i.e. n-hexadecane) within a water-dispersible urethane acrylate shell followed by solidification of the container wall using radical photopolymerization with a suitable photoinitiator. The thermal and colloidal characteristics of these novel capsules are discussed in some detail and the potential application as nanoscaled phase-change material is highlighted.     

Discussion of silica speciation, fouling, control and maximum reduction

In the production of ultrapure water for the power and microelectronics industries, multiple pass reverse osmosis (RO) process is commonly the major step in the reduction of dissolved and suspended matter before polishing by ion exchange and other methods to attain the high purity requirements. With the diverse location of power plants and microelectronic manufacturing facilities around the world, silica concentrations in source waters can range between 1 and 60 ppm (mg/L) to even 300 ppm in some volcanic regions. High pressure steam generators and fine microelectronic structures now require water containing less than 1 ppb (ug/L) concentrations of silica. In designing purification processes, silica has presented issues not only as formidable challenges in many locations as RO membrane foulants, as well as a contaminant requiring efficient removal. Analyses of RO membrane foulants and correlation with water chemistry in the course of trouble-shooting numerous RO processes continues to offer us opportunities to understand silica chemistry, the patterns of silica fouling and methods by which we can chemically control the RO process. Such understanding is applicable to the operation of ion-exchange resin beds as well. In this paper we review the speciation of silica in feedwaters, and chemical approaches to controlling fouling and maximizing silica reduction. Silica and silicates are addressed in the three categories of reactive soluble, non-reactive soluble (colloidal, not filterable) and non-reactive insoluble (particulate, filterable) forms. A brief review of geochemistry, the chemical and biochemical dissolution and deposition of silica and silicates in nature is provided for insights and understanding of natural processes that can be applied to the task of process design and control in silica removal from water.     

Production of rare-earth orthoferrite ceramic fibres by aqueous sol-gel blow spinning process

Rare-earth orthoferrite, LnFeO₃ (Ln=La, Sm, Gd, Dy, Er and Yb) ceramic fibres were produced by aqueous sol-gel blow spinning process at low-temperatures. Stable, charge stabilised, colloidal precursor sols of orthoferrites were prepared by room temperature processing of inexpensive and commercially available metal salts. The average diameter (Z_av) of the colloidal sol particles was in the range 4–7 nm and had a narrow size distribution. The sols were concentrated, combined with spinning aids, and processed further to a viscous ‘spinning solution’. The gel fibres of about 6 μm diameter were blow spun, collected as random fibres, dried and heated to increasingly higher temperatures at a rate of 50 °C/h. The gel fibres converted to flexible ceramic fibres, and single-phase perovskite structure crystallised directly for all the LnFeO₃ (Ln=La, Sm, Gd, Dy, Er and Yb ) fibres on heating them to 700 °C. The ceramic fibres had mean diameter of about 3–4 μm, and consisted of randomly oriented submicron sized grains.     

覆盖式过滤器在凝结水回收中的应用

介绍覆盖过滤器工作原理、结构特点,通过现场实验表明,选用覆盖过滤器对凝结水中的铁杂质进行处理是有效的,能满足凝结水净化目的。该系统具有投资少、操作简单、运行灵活、耐高温等优点。     

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.     

Humic acid fouling in the membrane distillation process

This work investigates the extent of humic acid fouling during the membrane distillation process for water treatment. The effects of pH, ionic strength, and divalent ion on fouling were studied. The experiments were performed with a 0.22-μm PVDF flat-sheet membrane in a direct contact membrane distillation unit. Flux declines were negligible (less than 6%) for the ranges of humic acid concentration, ionic strength, and pH studied. The examination of the membrane surface by SEM revealed a thin deposit layer. The addition of divalent cations (Ca²⁺) into the solution considerably reduced flux when Ca²⁺ concentration exceeded the critical coagulation concentration. Ca²⁺ affected flux by forming complexes with humic acids and resulted in coagulation on the membrane surface. The normalized flux, J/J₀, was 0.57 after 18 h of operation when the CaCl₂ concentration was 3.775 mM. However, the deposit of humic acid coagulate on the membrane surface was loosely packed, and was rather easily removed. Rinsing of the fouled membrane with clean water and a 0.1 M NaOH solution gave 100% of flux recovery.     

Effects of a novel bimetallic catalytic biofilter-based pretreatment technique on the form of ultrafiltration membrane fouling

Pretreatments of influents using bimetallic catalytic biofilter (BC-biofilter) can help reduce transmembrane pressures. For ultrafiltration membranes coupled with a conventional biofilter pretreatment, the cake layer resistance accounts for 25.0% of the total resistance. However, for those coupled with BC-biofilter pretreatment, the cake layer resistance accounts only for 12.5% of the total resistance. Confocal laser scanning microscopy is employed to determine the porosity of cake layer. It is found that ultrafiltration membranes with BC-biofilter pretreatment show a cake layer porosity of up to 0.56 or greater, whereas those with a conventional biofilter pretreatment exhibit a cake layer porosity of only 0.36. This is because micro-flocculation occurs in the effluents of BC-biofilter. The flocs generated through flocculation deposit on membrane surfaces to create highly porous cake layer. Moreover, catalytic reduction can increase the zeta potentials of the biofilter effluents. This makes the deposition of colloidal particles and flocs on membrane surfaces difficult under electrostatic repulsion. Simultaneously, micro-flocculation after BC-biofilter pretreatment can remove colloidal particles with particle sizes of 200-350 nm in water. This can effectively prevent the blockage of ultrafiltration membrane pores. Furthermore, compared to conventional biofilter, BC-biofilter pretreatment can more effectively reduce the number of colloidal particles and the van der Waals forces of ultrafiltration membranes. They can also change the action directions of electric double layers and thereby mitigate ultrafiltration membrane fouling.     

Effects of a novel bimetallic catalytic biofilter-based pretreatment technique on the form of ultrafiltration membrane fouling

Pretreatments of influents using bimetallic catalytic biofilter (BC-biofilter) can help reduce transmembrane pressures. For ultrafiltration membranes coupled with a conventional biofilter pretreatment, the cake layer resistance accounts for 25.0% of the total resistance. However, for those coupled with BC-biofilter pretreatment, the cake layer resistance accounts only for 12.5% of the total resistance. Confocal laser scanning microscopy is employed to determine the porosity of cake layer. It is found that ultrafiltration membranes with BC-biofilter pretreatment show a cake layer porosity of up to 0.56 or greater, whereas those with a conventional biofilter pretreatment exhibit a cake layer porosity of only 0.36. This is because micro-flocculation occurs in the effluents of BC-biofilter. The flocs generated through flocculation deposit on membrane surfaces to create highly porous cake layer. Moreover, catalytic reduction can increase the zeta potentials of the biofilter effluents. This makes the deposition of colloidal particles and flocs on membrane surfaces difficult under electrostatic repulsion. Simultaneously, micro-flocculation after BC-biofilter pretreatment can remove colloidal particles with particle sizes of 200–350 nm in water. This can effectively prevent the blockage of ultrafiltration membrane pores. Furthermore, compared to conventional biofilter, BC-biofilter pretreatment can more effectively reduce the number of colloidal particles and the van der Waals forces of ultrafiltration membranes. They can also change the action directions of electric double layers and thereby mitigate ultrafiltration membrane fouling.     

Surface characterization and performance evaluation of commercial fouling resistant low-pressure RO membranes

This paper describes the characterization and evaluation of various RO/NF membranes for the treatment of seasonally brackish surface water with high organic contents (TOC ≈21 mg/L). Twenty commercially available RO and NF membranes were initially evaluated by performing controlled bench-scale flat-sheet tests and surface characterization. Based on the results, four low pressure RO membranes were selected for use in the pilot study. The surface characterization revealed that each of four selected membranes had unique surface characteristics to minimize membrane fouling. Specifically, the LFC1 membrane featured a neutral or low negative surface to minimize electrostatic interactions with charged foulants. The X20 showed a highly negatively charged surface, and thus, was expected to perform well with feed waters containing negatively charged organics and colloids. The BW30FR1 exhibited a relatively neutral and hydrophilic surface, which could be beneficial for lessening organic and/or biofouling. The SG membrane had a smooth surface that made it quite resistant to fouling, particularly for colloidal deposition. In the large-scale pilot study using single element, all of the four membranes experienced a gradual increase in specific flux over time, indicating no fouling occurred during the pilot study. The deterioration of permeate water quality such as TDS was also observed over time, suggesting that the integrity of the membranes might be compromised by the monochloramine used for biofouling control.     

Formulation and preparation of low-concentrated yttria colloidal dispersions

High-temperature creep resistant steels for nuclear applications consist of a steel matrix reinforced with a dispersion of nano-sized refractory ceramic oxide particles, e.g. yttria (Y₂O₃). In this study, the formulation and preparation of low-loaded (2, 3 and 4 vol.%) Y₂O₃ colloidal dispersions for its possible application as suspension precursor in the production of high-temperature creep resistant steels is discussed. The final end product is formulated as a low-concentrated and low-viscous colloidal suspension consisting of non-agglomerated Y₂O₃ nano-particles in the (sub)-100 nm range.The spherical as-received and submicron agglomerated powder necessitates ball-mill processing in order to reduce the as-received size distribution down to nano-level. A comparative study on the desagglomeration of Y₂O₃ nano-powder using conventional planetary ball-milling or nano-milling technology has been done. Formulation of stable colloidal suspensions by screening suitable dispersants has led to the selection of one specific dispersant for this application, Calgon N. Suspension characterization by the determination of particle size distribution (PSD), ζ-potential measurements, colloidal suspension visualization using transmission electron microscopy (TEM), rheology and suspension life-time is discussed in this paper.     

Effect of cake layer structure on colloidal fouling in reverse osmosis membranes

A series of reverse osmosis (RO) membrane filtration experiments was performed systematically in order to investigate the effects of various hydrodynamic and physicochemical operational parameters on a cake layer formation in colloidal and particulate suspensions. Bench-scale fouling experiments with a thin-film composite RO membrane were performed at various combinations of trans-membrane pressure (TMP), cross-flow velocity (CFV), particle size, pH, and ionic strength. In this study, silica particles with two different mean diameters of 0.1 and 3.0 μm were used as model colloids. Membrane filtration experiments with colloidal suspensions under various hydrodynamic operating conditions resulted that more significant permeate flux decline was observed as TMP increased and CFV decreased, which was attributed to the higher accumulative mass of particles on the membrane surface. Results of fouling experiments under various physicochemical operating conditions demonstrated that the rate of flux decline decreased significantly with an increase of the ionic strength as well as particle size, while the flux decline rate did not vary when solution pH changed. The experimentally measured cake layer thickness increased with a decrease in particle size and solution ionic strength. Furthermore, the model estimation of cake layer thickness by using a cake filtration theory based on the hydraulic resistance of membrane and cake layer was performed under various ionic strength conditions. The primary model parameters including accumulated mass and specific cake resistance were calculated from the cake layer resistance. This result indicated that the formation of cake layer could be closely related with solution water chemistry. The model estimated cake layer thickness values were in good agreement with the experimentally measured values.     

Synthesis,characterization and performance evaluation of an optimized ceramic membrane with physical separation and photocatalytic degradation capabilities

The main goal of the present study is synthesis, characterization and performance evaluation of an optimized photocatalytic ceramic membrane for wastewater treatment. It consists of three layers including alumina (Al₂O₃) macroporous support, colloidal titania (TiO₂) mesoporous intermediate layer and polymeric TiO₂ mesoporous top layer in order to obtain a pore gradient from the support through the top layer of membrane. The colloidal and polymeric TiO₂ layers were prepared via the sol-gel method and coated using sol dip-coating approach. In order to optimize the membrane, physical separation and photocatalytic degradation capabilities of each colloidal and polymeric layer as a function of time were evaluated using Rhodamine B (RhB) aqueous solution. Thus, optimum coating number of intermediate layer and top layer were determined. Also, the performance of the optimized membrane was investigated via oily wastewater treatment using crude oil and water emulsion. Based on the performance results, two consequence colloidal layers and one polymeric layer were considered as the optimum layer number. Also, RhB photocatalytic degradation was 24.7% and RhB physical separation and permeation flux were 40.4% and 25.7?kg?m^?2 h^?1, respectively. Furthermore, based on the oily wastewater treatment experiments, permeation flux and chemical oxygen demand (COD) rejection at the best-operating conditions (pressure of 5?bar, the temperature of 30?°C and cross flow of 600?l?h^?1) were 29.1?kg?m^?2 h^?1 and 78.4%, respectively. The prepared membrane was found efficient and exhibited high industrial potential due to its multifunctional capability and thus can be employed as an advanced material for wastewater treatment applications.     

微孔陶瓷膜过滤(三)——陶瓷膜的使用寿命、再生方法、预敷操作及其与有机膜的对比

以平板陶瓷膜为过滤介质的动态旋叶压滤机进行微滤实验，对陶瓷膜的使用寿命、阻塞情况、再生效率、预敷效果等进行了探讨，提出了有效的再生方法，并与有机膜进行了对比实验，得到了对工程实际有指导意义的一些初步结论，显示了用陶瓷膜作为动态旋叶压滤机过滤介质的优越性。