Interplay of different NOM fouling mechanisms during ultrafiltration for drinking water production

Ultrafiltration is an emerging technology for drinking water production, but a main challenge remains the lack of understanding about fouling. This paper investigates the impact of molecular interactions between different natural organic matter (NOM) compounds on ultrafiltration fouling mechanisms. We performed dead-end filtration experiments with individual and mixed humic acid and alginate (polysaccharide). Alginate showed detrimental, but mostly reversible, flux decline and high solute retention. Our results indicate that this was caused by pore blocking transformed into cake building and weak molecular foulant-membrane and foulant-foulant interactions. In the presence of calcium, aggravated fouling was observed, related to complexation of alginate and its subsequently induced gel formation. With humic acid, more severe irreversible fouling occurred due to humic acid adsorption. Minor adsorption of alginate onto the membrane was also observed, which probably caused the substantial irreversible flux decline. The fouling characteristics in the mixtures reflected a combination of the individual humic acid and alginate experiments and we conclude, that the individual fouling mechanisms mutually influence each other. A model elucidates this interplay of the individual fouling mechanisms via hydrophobic and electrostatic interactions. In our study such an interplay resulted in an alginate cake, or gel in the presence of calcium, which is relatively irreversibly adsorbed onto the membrane by humic acid associations. This study shows the importance of mutual influences between various foulants for improved understanding of fouling phenomena. Furthermore it shows that substances with a minor individual influence might have a large impact in mixed systems such as natural water.     

Influence of the characteristics of natural organic matter on the fouling of microfiltration membranes

Natural organic matter (NOM) plays a significant role in fouling microfiltration membranes in drinking water treatment processes even though the NOM is retained only to a small extent. The aim of this study was to obtain a better understanding of the interactions between the fractional components of NOM and microfiltration membranes. Filtration experiments were performed using 0.22 μm hydrophobic and hydrophilic polyvinylidene fluoride (PVDF) membranes in a stirred-cell system on the NOM isolated from three Australian surface waters. As expected, the fouling rate for the hydrophobic membrane was considerably greater than for the hydrophilic membrane. Focusing on the hydrophobic membrane, it was shown that the high molecular weight fraction of NOM (>30 kDa) was responsible for the major flux decline. Filtration tests on the four fractions of NOM isolated on the basis of hydrophobicity and charge using non-functionalised and anionic resins revealed that the fouling potential for the three waters was hydrophilic neutral>hydrophobic acids>transphilic acids>hydrophilic charged. The low-aromatic hydrophilic neutral compounds were the main determinant of the rate and extent of flux decline. This was linked to the colloidal size fraction (>30 kDa) and to the selective concentration of calcium in the fraction leading to organics-Ca²⁺ bridging. It was also shown that the higher the aromaticity of the NOM the greater the flux decline, and the aromatics mainly resided in the hydrophobic acids fraction. Overall, the fouling mechanism controlling the flux decline involved the combined effects of adsorptive and colloidal fouling by the hydrophilic neutral fraction in the internal pore structure of the membrane.     

Studies on the effect of humic acids and phenol on adsorption-ultrafiltration process performance

Application of pressure-driven membrane processes, such as ultrafiltration (UF) and microfiltration (MF) for surface water treatment have become very popular during last decades. Membrane fouling by humic substances (HS) is one of the major limiting factors in these processes. In order to alleviate the unfavorable effects of the presence of HS in the feed on the process performance UF and MF are often combined with adsorption on powdered activated carbon (PAC). The main goal of the present study was to evaluate the effect of humic acid (HA) on membrane fouling during UF. Moreover, the effect of PAC addition to the feed on UF process, especially on flux decline was determined. The applicability of the adsorption-ultrafiltration (PAC/UF) system to purification of water containing low (phenol) and high molecular (HA) was also investigated. Three different polymer UF membranes, prepared from polysulfone (PSF), cellulose acetate (CA) or polyacrylonitrile (PAN) were applied. It was found that the membranes prepared from PSF and CA are very susceptible to fouling caused by HA. The permeate flux decreased for ca. 50% during UF of HA solution through the PSF membrane and for ca. 45%-through the CA membrane. In the case of the PAN membrane, a negligible effect of HA on the flux was observed. On the basis of the FTIR spectra it was found that the drop in the permeate flux through these membranes may result from interactions between the negatively charged functional groups present on the membrane surface, such as carboxyl groups (CA) and sulfone groups (PSF) with HA, which results in coating of the membrane surface with HA. When PAC was added to the feed containing HA, the permeate flux through the CA and PAN membranes was maintained on a practically unchanged level. However, in case of the PSF membrane, a 50% drop in the permeate flux in comparison with the flux value, when process was conducted without PAC addition was observed. That was supposed to be due to attractive forces among hydrophobic PAC particles, HA molecules and PSF membrane surface. The performed studies showed that the application of PAC/UF system was very effective in the removal of organic substances having both, low and high molecular weights. The role of PAC suspended in a feed in the PAC/UF system is the adsorption of low molecular organic compounds, which cannot be removed by UF alone.     

Identification and understanding of fouling in low-pressure membrane (MF/UF) filtration by natural organic matter (NOM)

An understanding of natural organic matter (NOM) as a membrane foulant and the behavior of NOM components in low-pressure membrane fouling are needed to provide a basis for appropriate selection and operation of membrane technology for drinking water treatment. Fouling by NOM was investigated by employing several innovative chemical and morphological analyses.

Source (feed) waters with a high hydrophilic (HPI) fraction content of NOM resulted in significant flux decline. Macromolecules of a relatively hydrophilic character (e.g. polysaccharides) were effectively rejected by low-pressure membranes, suggesting that macromolecular compounds and/or colloidal organic matter in the hydrophilic NOM fraction may be a problematic foulant of low-pressure membranes. Moreover, the significant organic fouling that is contributed by polysaccharides and/or proteins in macromolecular and/or colloidal forms depends on molecular shape (structure) as well as size (i.e. molecular weight). More significant flux decline was observed in microfiltration (MF) compared to ultrafiltration (UF) membrane filtration. MF membrane fouling may be caused by pore blockage associated with large (macromolecular) hydrophilic molecules and/or organic colloids. In the case of UF membranes, the flux decline may be caused by sequential or simultaneous processes of surface (gel layer) coverage during filtration. Morphological analyses support the notion that membrane roughness may be considered as a more important factor in membrane fouling by controlling interaction between molecules and the membrane surface, compared to the hydrophobic/hydrophilic character of membranes. Membrane fouling mechanisms are not only a function of membrane type (MF versus UF) but also depend on source (feed) water characteristics.     

Effects of mass retention of dissolved organic matter and membrane pore size on membrane fouling and flux decline

Ultrafiltration (UF) fouling has been attributed to concentration polarization, gel layer formation as well as outer and inner membrane pore clogging. It is believed that mass of humic materials either retained on membrane surface or associated with membrane inner pore surface is the primary cause for permeate flux decline and filtration resistance build-up in water supply industries. While biofilm/biofouling and inorganic matter could also be contributing factors for permeability decline in wastewater treatment practices. The present study relates UF fouling to mass of dissolved organic matter (DOM) retained on membrane and quantifies the effect of retained DOM mass on filtration flux decline. The results demonstrate that larger pore membranes exhibit significant flux decline in comparison with the smaller ones. During a 24-h period, dissolved organic carbon mass retained in 10 kDa membranes was about 1.0 g m⁻² and that in 100 kDa membranes was more than 3 times higher (3.6 g m⁻²). The accumulation of retained DOM mass significantly affects permeate flux. It is highly likely that some DOMs bind or aggregate together to form surface gel layer in the smaller 10 kDa UF system; those DOMs largely present in inner pore and serving as pore blockage on a loose membrane (100 kDa) are responsible for severe flux decline.     

Ultrafiltration membrane fouling by extracellular organic matters (EOM) of Microcystis aeruginosa in stationary phase: influences of interfacial characteristics of foulants and fouling mechanisms

This paper focused on the membrane fouling caused by extracellular organic matters (EOM) which was extracted from lab-cultured Microcystis aeruginosa in stationary phase. The characteristics of EOM such as molecular weight distribution, hydrophobicity and fluorescence were measured. It was found that high molecular weight (MW) and hydrophilic organics accounted for the major parts of algal EOM which was comprised of protein-like, polysaccharide-like and humic-like substances. Ultrafiltration (UF) experiments were carried out in a stirring cell and hydrophobic polyethersulfone (PES) membranes which carried negative charge were used. Prefiltration, calcium addition and XAD fractionation were employed to change the interfacial characteristics of EOM. Then the effects of these interfacial characteristics on flux decline, reversibility and mass balance of organics were compared. Algal EOM proved to cause serious membrane fouling during UF. The fraction of algal EOM between 0.45 μm and 100 kDa contributed a significant portion of the fouling. Hydrophobic organics in EOM tended to adhere to membrane surface causing irreversible fouling, while the cake layer formed by hydrophilic organics caused greater resistance to water flow due to hydrophilic interaction such as hydrogen bond and led to faster flux decline during UF. The results also indicated that the algal EOM was negatively charged and the electrostatic repulsion could prevent organics from adhering to membrane surface. In term of fouling mechanisms, cake layer formation, hydrophobic adhesion and pore plugging were the main mechanisms for membrane fouling caused by algal EOM.     

大型超滤水厂PVC和PVDF膜运行特性差异与优化

超滤技术已广泛应用于城镇给水处理厂,使得饮用水水质得到显著改善。不同材质超滤膜的运行周期、膜通量、跨膜压差(TMP)等的变化规律会有显著的差异,需要根据各自的特点对超滤系统进行运行参数调控与优化,以保证超滤系统的长期稳定运行。分析了山东某大型超滤水厂超滤系统长期运行的特点,对比了PVC膜和PVDF膜孔结构特征、膜通量、跨膜压差变化趋势及膜过滤阻力特性,调控和优化超滤系统运行参数,并进行了长时间的运行验证。结果表明,膜孔径和膜孔结构不同造成的膜污染是PVC膜与PVDF膜过滤特性差异的主要因素,恒定过滤周期运行模式下PVC膜通量加速衰减时段为82~220 min,造成PVC膜通量衰减了9. 14%,并形成了约5%的永久衰减膜通量,而PVDF膜的通量衰减并不明显,使得水厂超滤系统在恒定过滤周期(180min)运行模式下,出现了PVC膜的污染速率明显高于PVDF膜、系统的运行工况出现显著差异的现象。按照恒定过滤阻力模式运行时,PVC膜和PVDF膜的最佳过滤周期范围分别为82~108 min和96~155 min。水厂超滤系统在恒定过滤阻力运行模式下将PVC膜与PVDF膜在高温期和低温期的过滤周期分别调整为110、90 min和150、120 min,TMP的增长速率和化学维护清洗周期均基本一致,超滤系统实现了长期稳定运行。研究成果为我国超滤水厂中不同材质超滤膜的运行参数优化及协同稳定运行提供了参考。     

Effect of functional groups of humic substances on uf performance

The role of different functional groups present in humic substances on the membrane flux is unclear. This study is undertaken to (1) separate the carboxyl and phenolic groups from a humic solution, and (2) evaluate the effect of each fractionated humic substances on the ultrafiltration (UF) performance. A weak-base amine resin was used for the adsorption (pH 7) and the subsequent desorption (pH 13) of the phenolic groups from a commercial humic solution. These fractions were evaluated qualitatively (via Fourier transform infrared spectroscopy) and quantitatively (titration); they were further subjected to the analyses of the trihalomethane formation potential (THMFP) and ultrafiltration performance. Although, a complete separation of the phenolic and carboxyl groups is not possible, the results nevertheless provide useful information about their effects on UF performance. The fraction with a higher content of the phenolic OH group exhibits the highest THMFP (190 microg/mg C), whereas the fraction with a higher content of the carboxyl groups exhibits more flux decline. The UF system evaluated is unable to remove a significant portion of THM precursors, resulting in high THMs in permeate. The use of powdered activated carbon for the pretreatment of these fractions fails to improve membrane fouling. The pore size of UF membrane does not appear to affect the membrane flux, and the switch from the hydrophobic to hydrophilic membrane only slight improves the permeate flux.     

Identification and quantification of major organic foulants in treated domestic wastewater affecting filterability in dead-end ultrafiltration

Ultrafiltration (UF) membranes can be used after conventional wastewater treatment to produce particle free and hygienically safe water for reuse. However, membrane fouling affects the performance of UF to a large extent. Stirred cell tests with UF membrane show high flux decline filtering treated domestic wastewater. Investigation on the impact of size fractioned substances indicates that dissolved substances are major foulants affecting water filterability. Dissolved organic substances in feed and permeate samples of the stirred cell tests are analyzed by liquid chromatography with online organic carbon detection (LC-OCD). The resulting chromatograms displayed a significant difference of feed and permeate samples in the range of large molecules identified as biopolymer peak. The substances detected in this peak (mostly macro polysaccharide-like and protein-like molecules) are almost completely retained by UF membranes. Quantified investigation shows that biopolymer concentration influences filterability of corresponding water sample proportionally. The apparent magnitude of delivered biopolymer to membrane has a striking correlation with fouling resistance. The relationship was verified to be reproducible using different water samples. Mechanism analysis demonstrates that based on the delivered biopolymer load to membrane pore blocking or cake/gel fouling is the main fouling mechanism in the present experiment conditions.     

Cost factors and chemical pretreatment effects in the membrane filtration of waters containing natural organic matter

This paper compares the membrane processes available for water treatment. Membranes have the advantage of currently decreasing capital cost, a relatively small footprint compared to conventional treatment, generally a reduction in chemicals usage and comparably low maintenance requirements. Three membrane processes applicable to water treatment, micro- (MF), ultra- (UF), and nanofiltration (NF), are compared in terms of intrinsic rejection, variation of rejection due to membrane fouling and increase in rejection by ferric chloride pretreatment. Twelve different membranes are compared on the basis of their membrane pore size which was calculated from their molecular weight cut-off. A pore size of < 6 nm is required to achieve substantial (> 50%) organics removal. For a fouled membrane this pore size is about 11 nm. UV rejection is higher than DOC rejection. Coagulation pretreatment allows a higher rejection of organics by MF and UF and the cut-off criterion due to initial membrane pore size is no longer valid. A water quality parameter (WQP) is introduced which describes the product water quality achieved as a function of colloid, DOC and cation rejection. The relationship between log (pore size) and WQP is linear. Estimation of membrane costs as a function of WQP suggests that open UF is superior to MF (similar cost at higher WQP) and NF is superior to tight UF. Chemical pretreatment could compensate for the difference between MF and UF. However, when considering chemicals and energy costs, it appears that a process operated at a higher energy is cheaper at a guaranteed product quality (less dependent on organic type). This argument is further supported by environmental issues of chemicals usage, as energy may be provided from renewable sources.     

粉末活性炭吸附对超滤膜污染的影响研究

进行了粉末活性炭(PAC)吸附缓解浸没式和内压式超滤膜污染试验研究,并探讨了PAC吸附缓解膜污染的机理。浸没式超滤膜试验结果表明:PAC通过吸附溶解小分子有机物,减少了膜孔堵塞和膜孔内吸附污染,缓解了运行初期通量的快速下降;但PAC在膜表面的累积会降低浸没式超滤膜的起始通量。内压式超滤膜试验结果表明:膜前PAC吸附预处理能提高对有机物的去除效能,降低膜表面的污染负荷,从而降低TMP及其增长速度。     

Effect of NOM characteristics and membrane type on microfiltration performance

Efforts to understand and predict the role of different organic fractions in the fouling of low-pressure membranes are presented. Preliminary experiments with an experimental apparatus that incorporates automatic backwashing and filtration over several days has shown that microfiltration (MF) of the hydrophilic fractions leads to rapid flux decline and the formation of a cake or gel layer, while the hydrophobic fractions show a steady flux decline and no obvious formation of a gel or cake layer. The addition of calcium to the weakly hydrophobic acid (WHA) fraction led to the formation of a gel layer from associations between components of the WHA. The dominant foulants were found to be neutral and charged hydrophilic compounds, with hydrophobic and small pore size membranes being the most readily fouled. The findings suggest that surface analyses such as FTIR will preferentially identify hydrophilic compounds as the main foulants, as these components form a gel layer on the surface while the hydrophobic compounds adsorb within the membrane pores. Furthermore, coagulation pre-treatment is also likely to reduce fouling by reducing pore constriction rather than the formation of a gel layer, as coagulants remove the hydrophobic compounds to a large extent and very little of the hydrophilic neutral components.     

Assessing PAC contribution to the NOM fouling control in PAC/UF systems

This paper investigates the powdered activated carbon (PAC) contribution to the fouling control by natural organic matter (NOM) in PAC/UF hybrid process, as well as the foulant behaviour of the PAC itself. Solutions of NOM surrogates (humic acids, AHA, and tannic acid, TA) and AOM/EOM (algogenic organic matter/extracellular organic matter) fractions from a Microcystis aeruginosa culture were permeated through an ultrafiltration (UF) hollow-fibre cellulose acetate membrane (100 kDa cut-off). The greatest impairment on flux and the poorest rejection were associated with polysaccharide-like EOM substances combined with mono and multivalent ions. PAC, either in the absence or in the presence of NOM, did not affect the permeate flux nor the reversible membrane fouling, regardless of the NOM characteristics (hydrophobicity and protein content) and water inorganics. However, PAC controlled the irreversible membrane fouling, minimising the chemical cleaning frequency. Furthermore, PAC enhanced AHA and TA rejections and the overall removal of AOM, although it was apparently ineffective for the highly hydrophilic EOM compounds.     

混凝/PAC/超滤工艺去除水中天然有机物的研究

在饮用水处理过程中如何去除天然有机物是一个亟待解决的问题。混凝能降低水中污染物的浓度,避免这些物质进入膜孔内部,改善沉积在膜表面滤饼层的过滤性能和水中颗粒、胶体的迁移性能,提高膜通量。投加粉末活性炭(PAC)吸附溶解性有机物,利用超滤(UF)膜截留粉末炭,可达到提高出水水质的目的,还能防止膜污染。试验结果表明,混凝/PAC/UF组合工艺对水中UV254、UV410、TOC有较好的去除效果,平均去除率分别为92%、95%、84%,同时能改善膜透水通量、降低膜的吸附阻力、延长过滤周期、有效减少膜污染。     

Characterization of dissolved extracellular organic matter (dEOM) and bound extracellular organic matter (bEOM) of Microcystis aeruginosa and their impacts on UF membrane fouling

Extracellular organic matter (EOM) of cyanobacteria was classified into the dissolved EOM (dEOM) which was released into culture solution and the bound EOM (bEOM) which surrounded the cells. The dEOM and bEOM extracted from Microcystis aeruginosa in stationary phase were used to study their characteristic differences and then their impacts on ultrafiltration (UF) membrane fouling. Component analyses showed that dEOM was comprised of proteins, polysaccharides and humic-like substances, while that bEOM contained only proteins and polysaccharides. Additionally, polysaccharides dominated in dEOM with a polysaccharide/DOC ratio of 1.11 mg mg⁻¹, while proteins were the primary components of bEOM with a protein/DOC ratio of 1.08 mg mg⁻¹. Results of size fractionation and XAD resin fractionation revealed that bEOM was mainly distributed in the high-MW and hydrophobic fractions, while that dEOM was more hydrophilic. Result of UF experiments indicated that dEOM which had a higher organic content and stronger hydrophilicity caused more severe flux decline and reversible fouling, and that bEOM led to slower flux decline but more irreversible fouling due to less electrostatic repulsive and more hydrophobic adhesion. The impacts of these two kinds of EOM on the UF fouling caused by cyanobacterial cells were also investigated. It was found that both flux decline and irreversible membrane fouling caused by the cells were aggravated when cells were together with EOM, especially for bEOM which might increase the surface hydrophobicity of the cells.     

Adsorption combined with ultrafiltration to remove organic matter from seawater

Organic fouling and biofouling are the major severe types of fouling of reverse osmosis (RO) membranes in seawater (SW) desalination. Low pressure membrane filtration such as ultrafiltration (UF) has been developed as a pre-treatment before reverse osmosis. However, UF alone may not be an effective enough pre-treatment because of the existence of low-molecular weight dissolved organic matter in seawater. Therefore, the objective of the present work is to study a hybrid process, powdered activated carbon (PAC) adsorption/UF, with real seawater and to evaluate its performance in terms of organic matter removal and membrane fouling. The effect of different PAC types and concentrations is evaluated. Stream-activated wood-based PAC addition increased marine organic matter removal by up to 70% in some conditions. Moreover, coupling PAC adsorption with UF decreased UF membrane fouling and the fouling occurring during short-term UF was totally reversible. It can be concluded that the hybrid PAC adsorption/UF process performed in crossflow filtration mode is a relevant pre-treatment process before RO desalination, allowing organic matter removal of 75% and showing no flux decline for short-term experiments.     

混凝-超滤去除腐殖酸的试验研究

进行了直接超滤和混凝－超滤组合工艺对腐殖酸的去除效果和膜污染状况的试验。结果表明，超滤前另混凝土预处理对腐殖酸的DOC和UV254的去除率分别从28％、40％提高到53％、78％。同时高效液相色普（HPLC）分析表明，混凝－超滤组合工艺对分子质量＜6000u的有机物的去除率提高幅度更为显著。混凝－超滤组合工艺的最佳pH值条件pH=7,通过卷扫混凝使小分子有机物结合成微絮体，降低了膜孔污染，使超滤保持了较同的渗透通量。     

High-performance thin-layer hydrogel composite membranes for ultrafiltration of natural organic matter

Thin-layer hydrogel composite (TLHC) ultrafiltration (UF) membranes were synthesized by photo-grafting of either poly(ethylene glycol) methacrylate (PEGMA) or N,N-dimethyl-N-(2-methacryloyloxyethyl-N-(3-sulfopropyl) ammonium betaine (SPE) onto commercial polyethersulfone (PES) UF membranes. The performance of TLHC UF membranes was evaluated for natural organic matter (NOM) filtration and compared to commercial PES UF membranes. The fouling evaluation was done by investigation of membrane-solute interactions (adsorptive fouling) and membrane-solute-solute interactions (UF). The results suggest that the TLHC membranes convincingly displayed a higher adsorptive fouling resistance than unmodified PES UF membranes. In long-term stirred dead-end UF, a much lower fouling was observed for TLHC membranes than for commercial membranes with the same flux and rejection. Further, water flux recovery was also much higher. An analysis using an existing blocking model was performed in order to elucidate the effect of a polymer hydrogel layer on fouling mechanism as well as cake layer characteristics. The TLHC membranes synthesized by photo-grafting of PEGMA (40 g/L) and PEGMA with a low concentration of cross-linker monomer in the reaction mixture (ratio: 40/0.4 (g/L)/(g/L)) showed a much better performance than the other composite membranes. Those membranes could reduce the cake resistance on the membrane surface. This work has relevance for the design of high-performance UF membranes for applications in water treatment.     

Reversible and irreversible low-pressure membrane foulants in drinking water treatment: Identification by principal component analysis of fluorescence EEM and mitigation by biofiltration pretreatment

With the increased use of membranes in drinking water treatment, fouling - particularly the hydraulically irreversible type - remains the main operating issue that hinders performance and increases operational costs. The main challenge in assessing fouling potential of feed water is to accurately detect and quantify feed water constituents responsible for membrane fouling. Utilizing fluorescence excitation-emission matrices (EEM), protein-like substances, humic and fulvic acids, and particulate/colloidal matter can be detected with high sensitivity in surface waters. The application of principal component analysis to fluorescence EEMs allowed estimation of the impact of surface water constituents on reversible and irreversible membrane fouling. This technique was applied to experimental data from a two year bench-scale study that included thirteen experiments investigating the fouling potential of Grand River water (Ontario, Canada) and the effect of biofiltration pre-treatment on the level of foulants during ultrafiltration (UF). Results showed that, although the content of protein-like substances in this membrane feed water (= biofiltered natural water) was much lower than commonly found in wastewater applications, the content of protein-like substances was still highly correlated with irreversible fouling of the UF membrane. In addition, there is evidence that protein-like substances and particulate/colloidal matter formed a combined fouling layer, which contributed to both reversible and irreversible fouling. It is suggested that fouling transitions from a reversible to an irreversible regime depending on feed composition and operating time. Direct biofiltration without prior coagulant addition reduced the protein-like content of the membrane feed water which in turn reduced the irreversible fouling potential for UF membranes. Biofilters also decreased reversible fouling, and for both types of fouling higher biofilter contact times were beneficial.     

Fouling characteristics of NF and RO operated for removal of dissolved matter from groundwater

A pilot study had been performed for about 6 months in order to investigate the removal efficiency of dissolved matter and its fouling potential during nanofiltration (NF) and reverse osmosis (RO) of local groundwater that was pretreated with an ultrafiltration (UF) membrane system. After pilot plant operation, autopsy tests were performed to identify the characteristics of foulants that were attached to the membrane surface. In the autopsy tests, the flux recovery for each specific cleaning scheme (hydraulic washing, acid cleaning, and alkaline cleaning) was also measured using a dead-end filtration cell unit. The washing solution used in each chemical cleaning was analyzed to identify major components of the foulants, and the membrane surface was observed using the scanning electron microscopy (SEM).Among three kinds of membranes tested, one NF and two RO membranes, the NF and RO1 membranes showed a rapid flux decline after 100 days of operation. Especially, the RO1 membrane showed the more serious flux decline than the NF membrane. The RO2 membrane, with the lowest recovery rate, demonstrated a gradual flux decline. The removal efficiency of dissolved inorganic matter (as conductivity) for each NF, RO1 and RO2 membrane was 76.3%, 88.2% and 95.3%, respectively. The removal of dissolved organic matter (as total organic carbon) was found to be about 80% for both NF and RO membranes used in this study. During the membrane autopsy tests, five sections of the fouled membrane were cut along each NF and RO membrane module from the feed inlet side to the concentrate outlet side, the specific flux for each membrane section was measured before and after each cleaning step. As expected, the degree of fouling was intensified along the membrane surface as the feed flow approached the outlet. Based on the analysis results of wash water used in each cleaning step, the major foulants attached to the membrane surface appeared to be Ca bound with inorganic matter and Si bound with organic matter. Fe seemed to be a great contributor to irreversible fouling. The SEM analysis indicated that the organic matter was forming the first fouling layer close to the membrane and that the inorganic matter was layered top of the organic fouling layer in a tetragonal shape. Any evidence of biofouling was not observed in this study because most of microorganisms had been already removed by the UF pretreatment.