Effects of retained natural organic matter (NOM) on NOM rejection and membrane flux decline with nanofiltration and ultrafiltration

Two natural source waters containing natural organic matter (NOM) with different physical and chemical characteristics were dead-end filtered using five types of membranes having different material and geometric properties. In this study retained dissolved organic carbon (DOC) per unit membrane area is introduced as a better parameter compared to permeate volume, time, and delivered DOC to provide a reasonable comparison of NOM rejection and flux-decline trends. Retained DOC/NOM was calculated, which influences NOM concentration polarization at the membrane interface, and transport measurements of NOM rejection and flux decline were made. Molecular weight (MW) distribution measurements (by size exclusion chromatography) were used to calculate the average MW of the NOM. This persuasively demonstrated that the nominal molecular weight cut-off (MWCO) of a membrane is not the unique predictor of rejection characteristics for NOM composites. The charge density of NOM from the source waters was measured to estimate its effects on NOM rejection and flux decline during filtration. The contact angle of the membranes was used to determine hydrophobic interactions between NOM and membrane. All filtration measurements were performed at approximately the same permeate flow rate in order to minimize artifacts from mass transfer at the membrane interface. ESNA having a nominal MWCO of 200 Daltons showed NOM rejection greater than 95% and flux decline lower than 10% under a condition of a retained DOC of 0.5 mg C/cm² for the feed source waters. The other membranes having larger membrane pores (nominal MWCOs ranging from 8,000 to 20,000 Daltons) than the ESNA showed NOM rejection ranging from 68% to 86% and flux decline ranging from 5% to 17% at the same retained DOC for the waters.     

Evaluation of the performance of tight-UF membranes with respect to NOM removal using effective MWCO, molecular weight, and apparent diffusivity of NOM

Concepts for the effective MWCO of tight-UF membranes, and apparent diffusion coefficients for NOM, were introduced to determine the mechanisms influencing NOM removal and to explain the various behaviors of NOM removal by UF membranes with different hydrophobicities, permeability, and surface charges. Colloidal NOM (COM) and non-colloidal hydrophobic NOM (NCD HP) constituents were chosen for the evaluation of two different UF membranes. For a relatively hydrophobic, relatively high permeability, and less negatively charged UF membrane, the hydrophobic fractions of COM were preferentially removed and were also removed by a size exclusion mechanism (i.e., both hydrophobic interaction and size exclusion mechanisms). The NCD HP exhibited no such preferential removal of the hydrophobic fractions, but could be removed by a size exclusion mechanism (i.e., only size exclusion mechanism). With a relatively hydrophilic, relatively low permeability, and more negatively charged UF membrane, COM exhibited no preferential removal of the hydrophobic fractions, but could be removed by a size exclusion mechanism (i.e., only size exclusion mechanism). Whereas the hydrophobic fractions of the NCD HP were preferentially removed, these could not be removed by a size exclusion mechanism (i.e., only hydrophobic interaction mechanism). The apparent diffusion coefficients of NOM, as determined from NOM diffusion experiments using a diffusion cell equipped with a regenerated cellulose membrane, were much lower than those calculated by the Stokes-Einstein relation. The diffusion coefficient of NOM is expected to be used to predict and explain NOM transport behaviors in tight-UF membranes.     

陶瓷膜超滤薏苡仁混合油脱胶

研究了陶瓷膜在薏苡仁混合油脱胶中的应用。研究了不同截留相对分子质量陶瓷膜的脱胶效果以及过膜压力、料液温度和运行时间对膜通量的影响,并对陶瓷膜清洗工艺进行探索。选择截留相对分子质量15 000的超滤膜,在50 ℃和0.5 MPa下能够去除混合油中90%磷脂。用碱洗和次氯酸钠清洗能够彻底去除膜污染,恢复膜通量。     

Influence of sodium bromide additive on polyethersulfone ultrafiltration membranes

The effect of sodium bromide (NaBr) on performance and characteristics of ultrafiltration (UF) membranes was studied. Asymmetric UF membranes were prepared by phase inversion technique from a multicomponent dope polymer solution consisting of the polymer; polyethersulfone (PES), solvent; N, N‐dimethylformamide (DMF) and NaBr as micromolecular additive. The dissolution of PES‐DMF‐NaBr was carried out using microwave irradiation technique to induce rapid dissolution through minimal heating time. Various concentrations of NaBr were mixed with PES in the range of 1–5 wt % and its influence on membrane characteristics such as surface hydrophilicity was measured by contact angle and the performance in terms of water flux and rejection rates were evaluated using micromolecular test substances. The morphology and streaming potential of PES UF membranes were analyzed using scanning electron microscopy (SEM) and ζ‐potential measurement, respectively. Overall, the results suggest that the membrane consisting of 1 wt % NaBr exhibits the best performance in terms of rejection and flux rates with molecular weight cutoff (MWCO) of 45 kDa and mean pore size of 6 nm. The membrane with the 1 wt % addition of NaBr demonstrates most negative charge which indicates less fouling characteristics and displays approximately three times higher permeation. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

超滤去除水溶液中聚乙二醇的研究

采用截留分子量为10kDa和50kDa的超滤膜,对不同分子量的聚乙二醇(PEG)溶液进行了截留实验。考察了超滤对不同分子量PEG的去除效果,膜的截留分子量、操作条件及料液浓度对PEG去除效果的影响,讨论了操作压力和过滤时间对超滤膜过滤通量的影响,分析了超滤(UF)对PEG的去除机理和超滤膜的污染机理。     

Comparative analysis of the processes of collagen concentration by ultrafiltration using different types of membranes

Tangential flow filtration of the collagen protein solutions with a molecular weight 12, 14, and 24 kDa is investigated using flat sheet membranes. The effects of tangential ultrafiltration (UF) on the permeate properties using two regenerated celluloses (RCs) and two polyethersulfone (PES) membranes with molecular weight cut-off (MWCO) of 5 and 10 kDa are reported. The permeate and concentrate obtained in the UF experiments are characterized from a physical–chemical point of view by determining the temperature, pH, electrical conductivity, nitrogen content, and protein concentration. In addition, the experimental data are modeled using Hermia's model. The UF experiments demonstrated that permeate flux declined with increasing molecular weight of collagen at constant concentration (1%). Regardless of the molecular weight of collagen, the rejections decrease in the following order: PES 5 kDa > RC5kDa > RC10kDa > PES10kDa. In case of membrane with higher MWCO, the clogging phenomenon is mainly due to the blockage of the internal pores of the membrane than the formation of a polarization layer. Morphologies and characteristics of the membranes are characterized using scanning electron microscopy.     

pH adjustment for seasonal control of UF fouling by natural waters

Natural surface waters in Algarve, Portugal, have important seasonal variations in natural organic matter (NOM) content, that influences ultrafiltration (UF) performance. This paper addresses the evaluation of the pH adjustment for seasonal control of UF fouling at a laboratory scale, using a plate and frame polysulphone membrane of 47 kDa MWCO. Results of two types of natural water (clear water, 3-5 NTU, and turbid water, 33-34.6 NTU) and three different water pH values (acid, neutral and basic) demonstrated that the pH adjustment could be used for seasonal control of UF fouling: when the water has less NOM (in dry periods, clear water), the acid pH will improve the UF performance, while during and after intense rainfall periods (turbid water with high NOM concentration) basic pH will be advantageous, because it minimizes membrane fouling. This behaviour is explained for clear water in terms of charge effects on membrane size. For turbid water, the electrostatic repulsion between membrane surface and NOM and turbidity particles is reduced at pH 4.13 and protonation of the NOM functional groups decreases the hydrodynamic radii of humic substances while increasing their hydrophobicity and their tendency to adsorb. Therefore, a dense fouling layer develops and flux is lower at pH 4.13 than at pH 8.33. These results together with the observed raw water feed concentrations decline and rejection decrease with WRR confirm the extensive adsorption on the membrane enhanced by the moderate hardness cation of this water.     

Ceramic membrane behavior in textile wastewater ultrafiltration

Textile industries are rated as the most polluting among all industrial sectors taking into account both wastewater volume and composition. In order to be able to reuse these effluents, membrane technologies have been proven as a viable alternative. However, these technologies show some drawbacks, such as the retentate stream management and the worsening of their performance due to membrane fouling. Therefore, effluents must be pre-treated in order to prevent fouling. Membrane technologies also provide some alternatives for these pre-treatments by means of the application of ceramic ultrafiltration among others.This work studies the performance of ceramic membranes by carrying out experiments with three commercial ceramic membranes with molecular weight cut-offs (MWCO) of 30, 50 and 150 kDa, respectively. Moreover, the effect of cross-flow velocity (CFV) was studied by performing the essays at different flow velocities of 3, 4 and 5 m/s.According to the obtained results, ceramic UF membranes proved to be a feasible pre-treatment alternative. Permeate flux increased as flow velocity was increased for most of the cases, owing to the fact that the cake layer formation was limited. Furthermore, flux decline along operating time was negligible for the higher flow velocities, whereas it was noticeable for the lowest flow velocity tested. For the lowest MWCO analyzed, lower cross-flow velocities were needed in order to limit the cake layer formation and reach the steady-state. This implies that, although fouling was significant for all the three molecular weight cut-off studied, it was much more noticeable when MWCO was increased. Slightly better results in terms of permeate quality were achieved as MWCO was lower. In addition, higher chemical oxygen demand (COD) and conductivity retention coefficients were obtained for the lowest cross-flow velocity. Turbidity and color removals seemed to be more influenced by water composition than CFV, with rejections higher than 99% and between 84 and 98%, respectively.     

Colloidal organic matter fouling of UF membranes: role of NOM composition & size

The aim of this study was to fractionate pre-filtered surface water using a 3.5 and a 10 kDa dialysis membrane, and to compare the rate of fouling and the fouling reversibility/irreversibility of the NOM fractions. Trial dialyses (3.5 and 10 kDa) were carried out for 6 and 21 days with pre-filtered surface water using synthetic surface water as dialysate. The aim of the trials was to optimize the dialysis process for NOM fractionation. DOC, Ca²⁺, Mg²⁺, soluble silica and bacteria were monitored at intervals during the dialysis process. Thereafter, the various NOM fractions (with low and high Ca²⁺) were fed to a miniature UF system operated at a constant flux of 138.5 L/m2 h, filtration cycle times of 31.5 min and backwash duration of 1.75 min. A PES/PSV hollow fiber UF membrane (MWCO 100 kDa) with a surface area of 0.0125 m² was employed for the filtration tests (X-Flow). Transmembrane pressure (TMP) and UF feed and permeate (LC-OCD) were monitored at regular intervals. For a dialysate recirculation of 95 L/h, sample to dialysate ratio of 5.2:80 L and a dialysate change frequency of 3 times per 24 h, the shortest duration of dialysis was about 6–7 days for both 3.5 and 10 kDa dialyses membranes. The removal of organic carbon (OC) increased with dialysis duration and MWCO of the bags. The biopolymer fraction increased from 120% to 240% when the duration of dialysis was increased from 6 days (1.1 mg DOC/L, 151 mg Ca/L) to 21 days (0.82 mg DOC/L, 133 mg Ca/L) with the 10 kDa dialysis membrane. The increased biopolymer fraction in the NOM sample that was dialyzed for 21 days resulted in a doubling of the fouling rate from 3.5 to 6.6 mbar/min per mg DOC/L. The other NOM fractions (humics and building blocks) and the Ca/DOC ratio was more or less the same in both NOM samples suggesting that biopolymers were the major cause of UF fouling.     

Ultrafiltration of partially hydrolyzed rice bran protein to recover value-added products

Rice brans were treated with a protease together with a disulfide bond-breaking agent (Na₂SO₃) to achieve a 2–4% peptide bond hydrolysis (DH). Ultrafiltration (UF) with 3 kDa molecular weight cut-off (MWCO) membrane led to substantial loss of feed protein due to permeation. Using 1 kDa MWCO membrane increased protein yields, but it was not effective in purifying the protein hydrolysates despite the increase in membrane area and operating time. The efficiency of this UF process can be improved using a larger MWCO membrane (e.g., 2 kDa MWCO), which may facilitate complete removal of phytate. Based on disparity of molecular sizes, use of phytase may also increase purity of protein retentates and allow the recovery of functional inositol phosphates in permeates. The presence of Na₂SO₃ during proteolysis to 2% DH of preheated bran (100°C, 10 min) repaired the damage caused by preheat treatment by increasing protein recovery but increased the concentration of small peptides in hydrolysates, i.e., <1 kDa, particularly for highly aggregated proteins. Heat treatment is necessary to stabilize rice bran, but the sulfite treatment may be avoided to increase UF yield and purity of protein retentates and allow higher DH values for hydrolysis of stabilized brans. Accordingly, this UF process can be an efficient method for recovering high-value components from rice bran, an underutilized rice milling co-product, for many industrial applications. Presented in part at the American Oil Chemists' Society Annual Meeting, Seattle, Washington, May 11–14, 1997.     

Natural organic matter fouling due to foulant–membrane physicochemical interactions

An extended DLVO (XDLVO) force analysis was introduced to predict natural organic matter (NOM) fouling in ultrafiltration (UF) membrane processes. Ultrafiltration membrane fouling experiments were performed using two NOM extracts from real waters and two commercial polymeric UF membranes. The hydrodynamic force by permeation drag and three interfacial forces of XDLVO (van der Waals, electrostatic, acid–base energy) were used for the force analysis. Acid–base interaction forces between NOM and UF membranes were dominant in short range (separation distances < 5 nm) and appear to determine the potential of NOM deposition. Relative extents of flux decline were successfully predicted using the short-range force analyses.     

中空纤维超滤膜分离聚乙二醇条件的研究

采用截留相对分子质量为6000的中空纤维超滤膜,对不同相对分子质量的聚乙二醇(PEG)溶液进行了截留实验。讨论了超滤对不同相对分子质量聚乙二醇的去除效果,研究了过滤时间、操作压力及料液质量浓度对聚乙二醇去除效果的影响。最佳分离条件为:操作压力0.05 MP,PEG相对分子质量20000,料液质量浓度100 mg/L,过滤时间5 min。     

Foulants analyses for NF membranes with different feed waters: coagulation/sedimentation and sand filtration treated waters

Two different NF membranes were operated to remove natural organic matter (NOM) originating from Dongbok Lake in Korea. Coagulation/sedimentation and sand filtration treated waters as membrane feed waters were used. The tested NF membranes were autopsied to compare the fouling propensity from different feed waters using pure water and a NaOH solution. Organic/inorganic foulants onto membrane surface were analyzed in terms of molecular weight (MW) distribution, structure, and IR analysis, and fouled membranes were also characterized in terms of pore size distribution, surface charge, and SEM–EDS analysis. Polysaccharides and/or N-acetyl aminosugar groups with MW ranging from 30,000 to 50,000 g/mol were identified using HP-SEC and IR analysis. Inorganic foulants (i.e., Si and Al) were also fouled onto the membrane surface and/or pores, and it is effectively removed by caustic cleaning, not pure water. Caustic cleaning was proven to be effective to remove both fouled NF membranes as a basis of flux recovery, and it could efficiently desorb the hydrophobic NOM constituents or protein-like substances from the relatively hydrophilic and less negatively charged NF membranes.     

多孔陶瓷膜用于碳量子点的分离与纯化

尺寸分布均一的碳量子点由于其良好的光学特性,在光电设备、离子检测、纳米传感器、生物成像和催化剂等领域具有广阔的应用前景。采用陶瓷膜“超滤-纳滤”双膜法,对微波合成的碳量子点进行分离和纯化。研究了pH对碳量子点料液荧光强度和粒径分布的影响。在pH=3时,碳量子点分散较好,荧光强度较高。陶瓷超滤膜可以有效截留碳量子点料液中的大颗粒杂质,渗透侧的碳量子点平均粒径约为2 nm,分散良好,无团聚现象。陶瓷纳滤膜对碳量子点具有良好的截留性能,在浓缩和水洗过程中可以进一步去除料液中的小分子杂质。经双膜法处理后,发射光谱由多峰分布变为单峰分布,且峰宽变窄,碳量子点的发光纯度得到了明显提高。     

A convenient method for the determination of molecular weight cut-off of ultrafiltration membranes

With the rapid increase of water contamination,membrane separation technology and their corresponding molecular weight cut-off (MWCO) evaluation method become more necessary.In this study,Panax notoginseng saponins was used as a new standard marker to determinate ultrafiltration (UF) membrane MWCOs,series of Millipore membranes were selected as control group to analyze and calculate the relationship between retention rate and MWCOs with exponential or logarithmic equation.A new and convenient method was provided for determining the membrane MWCO by modeling analysis retention rate with MWCOs,and the regression coefficients ≥0.990.The feasibility and practicability of established method was verified by different manufactures' membrane and dextrans.In the detection progress,as the main ingredient of Panax notoginseng saponins,Notoginsenoside R1,Ginsenoside Rg1,Ginsenoside Rb1and Ginsenoside Rd with different surface activity,the MWCO range of UF membranes can be divided into two zones mainly due to the retention rate difference among Notoginsenoside R1,Ginsenoside Rg1,Ginsenoside Rb1 and Ginsenoside Rd.Zone Ⅰ,1000-10000;and Zone Ⅱ,10000-100000.Thus,the new method would be helpful to improve the applicability of UF membrane in separation technology.     

Thin-film nanofibrous composite membranes containing cellulose or chitin barrier layers fabricated by ionic liquids

The barrier layer of high-flux ultrafiltration (UF) thin-film nanofibrous composite (TFNC) membranes for purification of wastewater (e.g., bilge water) have been prepared by using cellulose, chitin, and a cellulose-chitin blend, regenerated from an ionic liquid. The structures and properties of regenerated cellulose, chitin, and a cellulose-chitin blend were analyzed with thermogravimetric analysis (TGA) and wide-angle X-ray diffraction (WAXD). The surface morphology, pore size and pore size distribution of TFNC membranes were determined by SEM images and molecular weight cut-off (MWCO) methods. An oil/water emulsion, a model of bilge water, was used as the feed solution, and the permeation flux and rejection ratio of the membranes were investigated. TFNC membranes based on the cellulose-chitin blend exhibited 10 times higher permeation flux when compared with a commercial UF membrane (PAN10, Sepro) with a similar rejection ratio after filtration over a time period of up to 100 h, implying the practical feasibility of such membranes for UF applications.     

Treatment of an Oil/Grease Wastewater Using Ultrafiltration: Pilot-Scale Results

Abstract

Wastewater containing about 0.5% oil and grease (O/G) from a metal industry was treated by tubular ultrafiltration using membranes having a molecular weight cutoff (MWCO) of 120,000 and a negative surface charge (P membrane) and of 100,000 and no surface charge (M membrane). Permeate flux decreased dramatically during the first several hours of operation and then leveled-off for the remainder of semibatch operation. The average P membrane flux was significantly higher than the M membrane (38 versus 27 gal/ft ^2.d) because of its higher MWCO and negative surface charge. Increasing the transmembrane pressure and crosstlow velocity increased the permeate flux for both membranes. O/G concentrations less than 50 mg/L and total suspended solids (TSS) levels less than 25 mg/L were common for both membranes. O/G removal efficiencies (rejections) averaged 98% for the M membrane and 97% for the P membrane. TSS rejections were approximately 97% for both membranes. Effluent O/G concentration and turbidity from the P membrane were slightly higher than the M membrane because of the P membrane's higher MWCO and the larger flux. The average volume reduction and residual production were 97% and 32 gal/1000 gal. respectively. Acid cracking of the concentrate with sulfuric acid was marginally successful.     

电渗析-超滤耦合技术研究进展

电渗析-超滤耦合技术（EDUF）是一项用于分离带电有机物的新型技术,用超滤膜替换一部分离子交换膜,或将超滤膜嵌入到传统电渗析器中,从而形成一种超滤与电渗析内部结合的新型分离技术。该技术一方面利用超滤膜的孔径差异性分离不同分子量的带电有机物,另一方面利用电驱动原理克服压力驱动式超滤膜的膜污染问题,在营养食品、生物制药等领域用于分离提纯活性有机物组分,展示出了良好的应用优势。本文聚焦于电渗析-超滤耦合技术的产生和发展,详细介绍了该技术的技术原理及应用优势,重点阐述了影响其分离效率的主要因素,包括pH、超滤膜切割分子量（MWCO）、电场强度、膜对结构等。最后,从提高分离效率、降低系统能耗、超滤膜材质、水解液预处理、系统泄漏等角度提出了未来的研究方向,为该技术的研究和应用提供指导。     

Characterization of anion exchange membranes with natural organic matter (NOM) during electrodialysis

Natural organic matter (NOM) is thought to be a major source of fouling during membrane filtration of natural waters. The organic matter present in surface waters was characterized in terms of its molecular weight distribution, acidity and electrokinetic properties. The fouling potentials of anion exchange membranes were predicted by the characterization. Changes in the physicochemical properties of anion exchange membranes were also examined during electrodialysis (ED) process of solutions containing NOM. The ED performances were evaluated for the three anion exchange membranes (AMX, AM-1 and ACM) in the presence of NOM. Fouling phenomena in terms of current efficiency and NaCl flux were in good agreement with the fouling potentials predicted by the characterization results. Observations of the molecular weight distribution and the constituents of NOM revealed that the hydrophobic NOM fraction with high molecular weights deposited mainly on the membrane surface, providing fouling effects on the anion exchange membrane.     

Ultrafiltration of Coagulation-Pretreated Serratia marcescens Fermentation Broth: Flux Characteristics and Prodigiosin Recovery

The dead-end ultrafiltration (UF) of coagulation-pretreated fermentation broth of Serratia marcescens SMΔR for prodigiosin recovery was studied. Experiments were performed using different types (regenerated cellulose, YM; polyethersulfone, PES) and molecular weight cut-offs (MWCOs, 1–10 kDa) of the membranes, feed concentrations of prodigiosin (300–1000 mg/L), applied pressures (68.9–206.8 kPa), and stirring speeds (200–400 rpm). With the same MWCO, the YM membrane had a higher retention of prodigiosin and a lower flux than the PES membrane. A two-fold concentration of prodigiosin was observed in the retentate using a 1-kDa YM membrane compared to the concentration in the permeate using a 10-kDa YM membrane. In addition, the extent of membrane fouling was quantitatively analyzed in terms of the modified fouling index. Flux decline in the present batch UF process was mainly due to cake layer formation and partly due to pore blocking. A two-stage UF process was proposed for this purpose, with 81% recovery yield and four-fold concentration.