Identification of Fouling Mechanism During Ultrafiltration of Stevia Extract

Stevioside is one of the naturally occurring sweetener, which can be widely applied in food, drinks, medicine, and daily chemicals. Membrane separation has potential application in clarification of stevioside from pretreated stevia extract by ultrafiltration. In the present study, namely 5-, 10-, 30-, and 100-kDa molecular weight cutoff membranes have been used. Quantification of membrane fouling during ultrafiltration is essential for improving the efficiency of such filtration systems. A systematic analysis was carried out to identify the prevailing mechanism of membrane fouling using a batch unstirred filtration cell. It was observed that the flux decline phenomenon was governed by cake filtration in almost all the membranes. For 100 kDa membrane, both internal pore blocking and cake filtration are equally important. Resistance in series analysis shows that the cake resistance is several orders of magnitude higher than the membrane resistance. The cake resistance is almost independent of transmembrane pressure drop, which indicates the incompressible nature of the cake. A response surface analysis was carried out to quantify the development of cake resistance with time during ultrafiltration of various membranes. Quality parameters show that the 30-kDa membrane is better suited for clarification purposes. Identification of the fouling mechanism would aid in the process of design and scaling up of such clarification setup in future.     

超滤技术在各领域的应用与发展

膜分离技术在分离领域发展迅速,超滤作为一种新型的膜分离技术具有条件温和、能耗低、效率高、操作简便,无需添加任何化学试剂等优点,其应用十分广泛。该文综述了超滤技术在制药工业、食品工业、环境保护以及饮用水处理等诸多领域中的应用,探讨了超滤膜的污染及膜前预处理技术,以及超滤技术的优点及不足,应用中所面临的问题及未来的研究发展方向。     

Effects of palm oil-based fatty acids on fouling of ultrafiltration membranes during the clarification of glycerin-rich solution

Ultrafiltration (UF) is an alternative option that can be used to clarify glycerin-rich solutions in the oleochemical industry. However, the UF membranes used in the process were observed to have been seriously fouled in our preliminary study. Thus, the ultrafiltration of a glycerol-water-fatty acid mixture was carried out to investigate the membrane binding properties of the components and their contributions to membrane fouling. Palmitic, stearic, and oleic acid were used as fouling substances in the glycerol-water mixture. The flux decline experiments were carried out using polyvinylidenefluoride (PVDF) as well as polyethersulphone (PES) membranes with a MWCO of 30 and 25 kDa, respectively at specific conditions (T = 40 °C; ΔP = 2 bar). It was found that the PES membrane exhibited severe fouling for all types of fatty acids in solution with glycerol-water. As the pH decreased to acidic conditions, the PES membrane tended to exhibit more severe fouling than the PVDF membrane.     

Clarification of Apple Juice Using Polymeric Ultrafiltration Membranes: a Comparative Evaluation of Membrane Fouling and Juice Quality

The aim of this study was to evaluate the performance of three different commercial polymeric ultrafiltration (UF) membranes during clarification of raw apple juice, comparatively. The influence of membrane pore size, roughness, and hydrophobicity on flux profile and fouling was investigated. The initial flux was simultaneously decreased at the beginning of the process, and quite steady flux was obtained in the membranes with rougher surface and more hydrophobic nature. As the pore size and hydrophobicity increased, the reversible fouling became the major resistance, while cake formation was more prominent for the membranes with narrower pore size. The overall quality results revealed that the main quality characteristics of the raw juice can be better retained by using the membranes that have higher resistance to fouling.     

Recovery and purification of potato proteins from potato starch wastewater by hollow fiber separation membrane integrated process

Potato starch wastewater contains high-concentration potato proteins which have great potential in the fields of food and health care. Most researches on potato protein recovery by membrane separation technique are focused on flat sheet or tubular ultrafiltration (UF) and reverse osmosis (RO) membranes and lack the further protein purification and the in-depth discussions on the fouling behavior. In this laboratory-scale study, potato proteins were recovered and purified from the simulated potato starch wastewater by the self-made hollow fiber (HF) UF and nanofiltration (NF) separation membrane integrated process. 85.62% potato proteins with high molecular weight in the potato starch wastewater could be retained by UF membrane and 92.1% potato proteins with low molecular weight were rejected by NF membrane. The concentrated solution after UF and NF filtration was desalinated and purified by diluting the solution eight times and filtering the diluted solution with UF membrane. Both types of HF membranes, UF and NF, suffered the inevitable membrane fouling. After the traditional physical washing and chemical cleaning, water flux of UF and NF membranes can be effectively recovered. The corresponding recovery rates of UF and NF membranes can reach 93.5% and 84.7%, respectively. The hollow fiber UF-NF separation membrane integrated process was proved to be a promising technique of high-purity potato protein recovery from potato starch wastewater.     

Fouling Behavior of Polyphenols during Model Juice Ultrafiltration: Effect of Membrane Properties

Fruit juice is a complex mixture, in which polyphenols are active compounds for human health. In this paper, the effects of membrane properties, such as materials and molecular weight cutoff (MWCO), on fouling behavior of four typical polyphenols in model fruit juice ultrafiltration process were investigated. Zeta potential, contact angle, SEM image, and fouling resistances were determined. Static adsorption and rejection content of polyphenols on membranes were measured. Quantitative structure-activity relationship (QSAR) model has been developed to demonstrate the relationships between rejection and molecular parameters. Results showed that materials, MWCOs, hydrophobic properties, electrical parameters, and molecular parameters are critical factors. Membranes which were hydrophobic and positively charged have higher permeate flux and lower fouling resistance. Polyphenols were adsorbed by membranes as irreversible fouling and also had an important contribution in cake layer fouling. Retention of polyphenols on polyethersulfon 5-kDa membrane was the largest, while that on PVDF 50-kDa membrane was the least. According to QSAR analysis, rejections of polyphenols were higher when the molecules have smaller dipole moment, larger connectivity index ³X_p^v, and smaller ⁴X_pc^v.     

Membrane Processing Technology in the Food Industry: Food Processing,Wastewater Treatment,and Effects on Physical,Microbiological, Organoleptic,and Nutritional Properties of Foods

Membrane processing technology (MPT) is increasingly used nowadays in a wide range of applications (demineralization, desalination, stabilization, separation, deacidification, reduction of microbial load, purification, etc.) in food industries. The most frequently applied techniques are electrodialysis (ED), reverse osmosis (RO), nanofiltration (NF), ultrafiltration (UF), and microfiltration (MF). Several membrane characteristics, such as pore size, flow properties, and the applied hydraulic pressure mainly determine membranes’ potential uses. In this review paper the basic membrane techniques, their potential applications in a large number of fields and products towards the food industry, the main advantages and disadvantages of these methods, fouling phenomena as well as their effects on the organoleptic, qualitative, and nutritional value of foods are synoptically described. Some representative examples of traditional and modern membrane applications both in tabular and figural form are also provided.     

Fouling of nanofiltration, reverse osmosis, and ultrafiltration membranes by protein mixtures: the role of inter-foulant-species interaction

Protein fouling of nanofiltration (NF), reverse osmosis (RO), and ultrafiltration (UF) membranes by bovine serum albumin (BSA), lysozyme (LYS), and their mixture was investigated under cross-flow conditions. The effect of solution chemistry, membrane properties, and permeate flux level was systematically studied. When the solution pH was within the isoelectric points (IEPs) of the two proteins (i.e., pH 4.7-10.4), the mixed protein system experienced more severe flux decline compared to the respective single protein systems, which may be attributed to the electrostatic attraction between the negatively charged BSA and positively charged LYS molecules. Unlike a typical single protein system, membrane fouling by BSA-LYS mixture was only weakly dependent on solution pH within this pH range, and increased ionic strength was found to enhance the membrane flux as a result of the suppressed BSA-LYS electrostatic attraction. Membrane fouling was likely controlled by foulant-fouled-membrane interaction under severe fouling conditions (elevated flux level and unfavorable solution chemistry that promotes fouling), whereas it was likely dominated by foulant-clean-membrane interaction under mild fouling conditions. Compared to nonporous NF and RO membranes, the porous UF membrane was more susceptible to dramatic flux decline due to the increased risk of membrane pore plugging. This study reveals that membrane fouling by mixed macromolecules may behave very differently from that by typical single foulant system, especially when the inter-foulant-species interaction dominates over the intra-species interaction in the mixed foulant system.     

Ultrafiltration fouling of amylose solution: Behavior, characterization and mechanism

Applications of ultrafiltration membrane often deal with feed streams containing amylose starch. This paper describes a detailed investigation of amylose fouling during ultrafiltration. Commercial membranes made of polysulfone and fluoro polymer were used. Both adsorptive and ultrafiltration fouling were investigated. Experiments using different membrane characteristics, feed concentrations and trans-membrane pressures were carried out. The resulting fouling was characterized by water flux and contact angle measurements and was visualized by scanning electron microscopy (SEM). The results suggest that solute adsorption has occurred as noticed by significant water flux reductions as well as changes in membrane characteristics. Further, both reversible and irreversible fouling have occurred during ultrafiltration with irreversible fouling was more dominant. Apparently, cake layer formation initiated by either adsorption due to hydrophobic–hydrophobic interactions or pore blocking is the dominant fouling mechanism. However, pore narrowing instead of pore blocking was also observed for the membrane having large and relative uniform pore structure or for the ultrafiltration using low trans-membrane pressure or low solute concentration. Membrane autopsy using SEM confirmed the formation of solute layer on the membrane surface.     

中药用膜的污染及防治

分析了膜污染的机理及其影响因素,同时从膜分离过程前、膜分离过程中、膜分离过程后3个阶段采取相应的措施来改善膜的污染。     

Fouling of microfiltration and ultrafiltration membranes by natural waters

Membrane filtration (microfiltration and ultrafiltration) has become an accepted process for drinking water treatment, but membrane fouling remains a significant problem. The objective of this study was to systematically investigate the mechanisms and components in natural waters that contribute to fouling. Natural waters from five sources were filtered in a benchtop filtration system. A sequential filtration process was used in most experiments. The first filtration steps removed specific components from the water, and the latter filtration steps investigated membrane fouling by the remaining components. Particulate matter (larger than 0.45 microm) was relatively unimportant in fouling as compared to dissolved matter. Very small colloids, ranging from about 3-20 nm in diameter, appeared to be important membrane foulants based on this experimental protocol. The colloidal foulants included both inorganic and organic matter, but the greatest fraction of material was organic. When the colloidal fraction of material was removed, the remaining dissolved organic matter (DOM), which was smaller than about 3 nm and included about 85-90% of the total DOM, caused very little fouling. Thus, although other studies have identified DOM as a major foulant during filtration of natural waters, this work shows that a small fraction of DOM may be responsible for fouling. Adsorption was demonstrated to be an important mechanism for fouling by colloids.     

Fouling Analysis and Performance of Tubular Ultrafiltration on Pretreated Olive Mill Waste Water

This study deals with the performance of a tubular ultrafiltration system on sieved and centrifuged olive mill waste water. A generalized statistical model was developed describing the impact and the relative importance of major experimental parameters (membrane pore size, transmembrane pressure, feed flow rate, and feed temperature) on permeate flux. According to this model, process pressure appeared to have the largest impact on permeate flux, followed by process temperature. As membrane treatment of such a difficult material largely depends on fouling, a systematic analysis of prevailing fouling mechanisms was also run. Despite sieving and centrifugation of the original waste, membrane fouling caused a flux decline of 60–65% within 15–20 min. Internal fouling, pore blocking, and cake layer formation were all responsible for membrane fouling during the first 40 min of operation. After that period, cake formation appeared to play a predominant role. Based on the proposed generalized model, the relative importance of process parameters can be evaluated and process performance can be improved by proper interventions. Independent of membrane size, fouling is a serious problem to be resolved. The qualitative performance of this process, including chemical oxygen demand distribution, polyphenol profile, and antioxidant capacity, is discussed in a separate paper.     

中空纤维超滤膜在水处理中的应用

为了测试不同的中空纤维超滤膜在污水处理方面的性能,本文采用三种不同材料的中空纤维超滤膜：聚醚砜、聚丙烯和聚丙烯腈超滤膜,分别过滤了自来水和污水。讨论了过滤前后,水的色度、浊度、PH值、电导率、氨氮含量、化学需氧量和水流量的变化,并通过扫描电镜观察过滤前后三种超滤膜的表面和横截面微观形态的变化。试验结果表明：在三种超滤膜中,聚醚砜超滤膜在污水处理中表现出了最好的综合应用性能。三种超滤膜对污水中的氨氮和化学需氧量的去除率有限,采用二级或三级过滤可以提高过滤效果。电镜实验还说明,指状结构比孔状结构的防污性能好。     

Effect of Ultrafiltration on Anthocyanin and Flavonol Content of Black Currant Juice (Ribes nigrum L.)

This paper evaluates the efficiency of ultrafiltration and the effects of processing on the total anthocyanin and flavonol contents of black currant juice at chosen operational conditions. Ultrafiltration of black currant juices was carried out using Biomax 100?kDa polyethersulfone membrane. Ultrafiltration was used to process the juice prior concentration by reverse osmosis; with the aim to enhance the efficiency of the concentration process in terms of permeate flux. To avoid the fouling of the membrane, the juices were depectinized with Panzym Super E liquid enzyme preparation. The ultrafiltration was carried out at a transmembrane pressure of 2?bars and the operating temperature of 25?°C. The effect of processing on the valuable anthocyanin and flavonol content of the juices was evaluated based on the results of high-performance liquid chromatography analyses. The article includes detailed analyses of anthocyanin and flavonol compounds of the enzyme treated and ultrafiltered juice as compared with the original juice. The results indicate that, due to the enzymatic treatment, the valuable compound content of the juice increases. However, the ultrafiltration process resulted in a significant loss of a valuable content; 54% of total flavonol and 50% of total anthocyanins maintained in ultrafiltered juice when compared to the feed samples.     

不同预处理方式对柠檬果汁超滤澄清效果和膜污染情况的影响

利用超声辅助果胶酶法和超滤技术相结合对柠檬果汁进行澄清。以柠檬汁的透光率、果胶含量为指标,讨论不同预处理方式对柠檬果汁超滤澄清效果和膜污染情况的影响。结果表明,采用切割分子量为50 kDa的聚丙烯腈(PAN)超滤膜澄清果汁,不同预处理方式对超滤膜的稳定通量大小的影响规律是酶+超声处理>酶处理>未经酶+超声处理,其稳定通量分别为30、33.5和13 L·m-2·h-1左右。较优的操作压力可确定为0.1 MPa,此时柠檬汁的果胶含量为0.5%,透光率约99.6%,处理效果较好。超声辅助果胶酶法澄清果汁,有利于提高果汁的透光率,并且在后续膜处理过程中,对于提高超滤膜的通量和缓解膜污染都有显著的效果。     

超滤技术在果酒澄清中的应用

超滤是一种可用于果酒澄清处理的有效分离技术。本文首先介绍了我们设计的板框式超滤机结构和组件特点,以及工厂规模的工艺操作流程,其次测定了具有不同孔径分布的膜对青梅酒渗透通量随时间的变化,评述了减小膜污染和恢复通量的三种清洗方法;最后讨论了在食品,饮料工业应用中需考虑的一些工程因素。     

菜籽蛋白超滤液反冲对超滤膜污染的控制研究

超滤是制备植物油料蛋白的先进工艺技术，但膜的污染问题严重影响超滤技术的推广应用。本文在研究了菜籽蛋白水溶液在中空纤维超滤装置中的超滤行为之后，研究了膜面透过液反冲对膜污染的控制作用。实验结果表明：菜籽蛋白水溶液的超滤浓缩速率随时间延长而下降；对工作中的超滤膜进行定期间歇反冲（反冲时间20s、反冲频率为1次／15min、反冲压差为0．2MPa），是控制膜污染程度的有效措施，这样可以使超滤膜的透水速率恢复90％以上。     

Improvement of a sustainable hybrid technology for caseins isoelectric precipitation (electrodialysis with bipolar membrane/ultrafiltration) by mitigation of scaling on cation-exchange membrane

Electroacidification of milk by electrodialysis with bipolar membranes (EDBM) can be performed to produce isoelectric precipitated casein. In spite of advantages of EDBM, a problem of fouling hampers the industrial application of this technology. There are two types of fouling occurring during milk electroacidification such as protein fouling inside the EDBM stack and scaling on cation-exchange membrane (CEM). Recent studies demonstrated that protein fouling can be avoided by coupling an EDBM module with an ultrafiltration (UF) module. The present study aims the mitigation of scaling on the CEM. In order to attain this goal, two approaches were tested: 1) addition of KCl in the milk reservoir and 2) use of a UF membrane allowing higher permeate flux and consequently higher flow rates of solutions in the EDBM stack. Results of this study demonstrate the reasonableness of both approaches resulting in the significant decrease of CEM scaling (more than 30%).Industrial RelevanceCaseins are the major milk proteins largely produced in the world (around 330 000 t per year) and widely applied in the food and pharmaceuticals industries. There are several relevant features of proposed method (electrodialysis with bipolar membranes coupled with ultrafiltration (EDBM-UF)) for the casein production such as

• 1)absence of the hazardous reagents;
• 2)high purity of caseins due to the demineralization during EDBM;
• 3)no waste generation;
• 4)the base stream from EDBM module (co-product) can be used for the formation of caseinates.

The present work demonstrates the significant decrease of scaling on the cation-exchange membrane (more than 30%), which is the major problem of the EDBM-UF process. Indeed, during milk acidification, there is a liberation of minerals (Ca^2 + and Mg^2 +), which form the membrane deposit. This deposit negatively affects the process performance and leads to the membrane replacement. The cost of ion-exchange membranes in ED module comprises a major part of the total cost of EDBM-UF process. Thus, control and prevention of the membrane deposit will allow to increase a lifetime of ion-exchange membranes and to decrease a price of EDBM-UF treatment.     

食品工业中膜污染与膜清洗研究

膜技术由于其优异的分离性能，近年来在食品工业中得到广泛应用。但在膜分离过程中存在着膜污染现象，使膜的渗透通量及截留率等性能发生改变，使用寿命缩短，极大地限制了膜技术在食品工业中的实际应用。介绍了引起膜污染的机理。分析了影响膜污染的因素，并对防止膜污染的措施及其新进展进行了探讨。     

Flux evaluation based on fouling mechanism in acoustic field-assisted ultrafiltration for cold sterilization of tender coconut water

Membrane-based processing was attempted as a clean, green and energy-efficient method for cold sterilization of tender coconut water (TCW). Ultrafiltration of TCW was carried out at four different pressures employing two different polyethersulfone membranes, using a stirred cell. Permeate flux was evaluated based on different fouling mechanisms which are analysed employing Hermia's model. The objective of the study was to examine the efficacy of ultrafiltration for cold sterilization of TCW while retaining good sensory quality. The effect of acoustic field (ultrasonication) on the performance of ultrafiltration has been evaluated. Permeate flux increased on the application of ultrasonication. Enzyme activities in TCW permeate were found to reduce after ultrafiltration with good sensorial acceptability. Microbial load of TCW after ultrafiltration was observed to decrease from 4.16 to 0.0 log CFU/mL, achieving cold sterilization. Sensorial quality of ultrafiltered TCW was found to be good after three months storage period with zero microbial count.