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.     

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.     

Use of surfactant modified ultrafiltration for perchlorate (Cl(O)(4-)) removal

Determinations of perchlorate anion (ClO(4)(-)) transport and rejection were performed using a surfactant modified ultrafiltration (UF) membrane. Perchlorate anion (at a concentration of 100 microg/L of ClO(4)(-), spiked with KClO(4)) was introduced to the membrane as a pure component, in binary mixtures with other salts, cationic and anionic surfactants, and at various ionic strength conditions (conductivity). Also, a natural source water was spiked with perchlorate in the presence of cationic and anionic surfactants and used to determine the effects of a complex mixture (including natural organic matter (NOM)) on the observed rejection. All filtration measurements were performed at approximately the same permeate flow rate in order to minimize artifacts from mass transfer at the membrane interface. The objective of this study was to modify a negatively charged UF membrane in terms of the fundamental mechanisms, steric/size exclusion and electrostatic exclusion and to enhance perchlorate rejection, with synthetic water and a blend of Colorado River water and State Project water (CRW/SPW). Previous work suggested that perchlorate was dominantly rejected by electrostatic exclusion for charged nanofiltration (NF) and UF membranes (Rejection of perchlorate by reverse osmosis, nanofiltration and ultrafiltration (UF) membranes: mechanism and modeling. Ph.D. dissertation, University of Colorado, Boulder, USA, 2001). In that research, perchlorate rejection capability was quickly lost in the presence of a sufficient amount of other ions. However, this study showed that ClO(4)(-) was excluded from a (negatively) charged UF membrane with pores large with respect to the size of the ion. Although perchlorate rejection capability due to apparent electrostatic force was reduced in the presence of a cationic surfactant, a desired amount of the ClO(4)(-) was excluded by steric exclusion. The steric exclusion was due to decreasing membrane pore size caused by the adsorption of the cationic surfactant.     

Identification of nanofiltration membrane foulants

The Mery-sur-Oise plant (France) has been using nanofiltration (NF) membranes (NF200) to produce safe drinking water since 1999. However, significant fouling has been occasionally observed according to seasonal conditions, even with various pre-treatments including conventional surface water treatment followed by ozonation, acid addition to pH 6.9, anti-scalant addition, and microfiltration (6mum). Pilot-scale filtration experiments were performed to determine the effects of natural organic matter (NOM) character and ozonation on NF membrane fouling under constant operating conditions. Two parallel pilot units were operated with sand-filtered water (SFW) and sand-filtered-ozonated water (SFOW) for 3-month periods corresponding to spring and fall seasons. To identify NF foulants, Fourier transform infrared spectroscopy, fluorescence excitation emission matrix, scanning electron microscope, energy-dispersive spectrophotometry, and HPSEC-UVA-DOC-fluorescence chromatography have been used. Even though the dissolved organic carbon (DOC) and ultraviolet (UVA) levels of spring samples were lower than those of winter season, these feed waters showed higher fouling presumably due to a higher hydrophilic fraction of NOM and the presence of microorganisms. In addition, for both seasons, ozonation increased the degree of fouling mainly by a change in NOM characteristics and by the promotion of bacterial cell growth conditions. The hydrophilic NOM is not expected to be easily rejected by the relatively hydrophilic and negatively charged NF200 membrane due to its non-charged (or oppositely charged) properties, indicating a high fouling potential by NOM associated with spring samples. The adhesion of bacteria and accumulation of microalgae on the membrane may be due to the role of extracellular biopolymers released by algae upon ozonation, promoting adhesion between microorganisms and the membrane surface. Protein- and polysaccharide-like substances were found as major foulants. The reason for the minor fouling by humic substances on membranes fed with SFOW during the spring season might be a loss of membrane surface charge due to screening by significant subsequent fouling on the base of the fouling layer of extracellular materials.     

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.     

Rejection of micropollutants by clean and fouled forward osmosis membrane

As forward osmosis (FO) gains attention as an efficient technology to improve wastewater reclamation processes, it is fundamental to determine the influence of fouling in the rejection of emerging contaminants (micropollutants). This study focuses on the rejection of 13 selected micropollutants, spiked in a secondary wastewater effluent, by a FO membrane, using Red Sea water as draw solution (DS), differentiating the effects on the rejection caused by a clean and fouled membrane. The resulting effluent was then desalinated at low pressure with a reverse osmosis (RO) membrane, to produce a high quality permeate and determine the rejection with a coupled forward osmosis - low pressure reverse osmosis (FO-LPRO) system. When considering only FO with a clean membrane, the rejection of the hydrophilic neutral compounds was between 48.6% and 84.7%, for the hydrophobic neutrals the rejection ranged from 40.0% to 87.5%, and for the ionic compounds the rejections were between 92.9% and 96.5%. With a fouled membrane, the rejections were between 44.6% and 95.2%, 48.7%-91.5% and 96.9%-98.6%, respectively. These results suggest that, except for the hydrophilic neutral compounds, the rejection of the micropollutants is increased by the presence of a fouling layer, possibly due to the higher hydrophilicity of the FO fouled membrane compared to the clean one, the increased adsorption capacity of hydrophilic compounds and reduced mass transport capacity, membrane swelling, and the higher negative charge of the membrane surface, related to the foulants composition, mainly NOM acids (carboxylic radicals) and polysaccharides or polysaccharide-like substances. However, when coupled with RO, the rejections in both cases increased above 96%. The coupled FO-LPRO system was an effective double barrier against the selected micropollutants.     

Effect of membrane character and solution chemistry on microfiltration performance

To help understand and predict the role of natural organic matter (NOM) in the fouling of low-pressure membranes, experiments were carried out with an apparatus that incorporates automatic backwashing and long filtration runs. Three hollow fibre membranes of varying character were included in the study, and the filtration of two different surface waters was compared. The hydrophilic membrane had greater flux recovery after backwashing than the hydrophobic membranes, but the efficiency of backwashing decreased at extended filtration times. NOM concentration of these waters (7.9 and 9.1mg/L) had little effect on the flux of the membranes at extended filtration times, as backwashing of the membrane restored the flux to similar values regardless of the NOM concentration. The solution pH also had little effect at extended filtration times. The backwashing efficiency of the hydrophilic membrane was dramatically different for the two waters, and the presence of colloid NOM alone could not explain these differences. It is proposed that colloidal NOM forms a filter cake on the surface of the membranes and that small molecular weight organics that have an adsorption peak at 220nm but not 254nm were responsible for "gluing" the colloids to the membrane surface. Alum coagulation improved membrane performance in all instances, and this was suggested to be because coagulation reduced the concentration of "glue" that holds the organic colloids to the membrane surface.     

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.     

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.     

Combined coagulation-disk filtration process as a pretreatment of ultrafiltration and reverse osmosis membrane for wastewater reclamation: an autopsy study of a pilot plant

The effects of the combined coagulation-disk filtration (CC-DF) process on the fouling characteristics and behavior caused by interactions between effluent organic matter (EfOM) and the membrane surfaces of the ultrafiltration (UF) and reverse osmosis (RO) membranes in a pilot plant for municipal wastewater reclamation (MWR) were investigated. The feed water from secondary effluents was treated by the CC-DF process used as a pretreatment for the UF membrane to mitigate fouling formation and the permeate from the CC-DF process was further filtered by two UF membrane units in parallel arrangement and fed into four RO modules in a series connection. The CC-DF process was not sufficient to mitigate biofouling but the UF membrane was effective in mitigating biofouling on the RO membrane surfaces. Fouling of the UF and RO membranes was dominated by hydrophilic fractions of EfOM (e.g., polysaccharide-like and protein-like substances) and inorganic scaling (e.g., aluminum, calcium and silica). The desorbed UF membrane foulants included more aluminum species and hydrophobic fractions than the desorbed RO membrane foulants, which was presumably due to the residual coagulants and aluminum-humic substance complexes. The significant change in the surface chemistry of the RO membrane (a decrease in surface charge and an increase in contact angle of the fouled RO membranes) induced by the accumulation of hydrophilic EfOM onto the negatively charged RO membrane surface intensified the fouling formation of the fouled RO membrane by hydrophobic interaction between the humic substances of EfOM with relatively high hydrophobicity and the fouled RO membranes with decreased surface charge and increased contract angle.     

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.     

A hybridized photocatalysis-microfiltration system with iron oxide-coated membranes for the removal of natural organic matter in water treatment: Effects of iron oxide layers and colloids

A photocatalysis/microfiltration (MF) hybrid system, with the coating of a membrane using iron oxide particles (IOPs), was investigated with respect to natural organic matter (NOM) removal and membrane permeability during the treatment of various surface waters. A comparison of the performance between bare (uncoated) and IOP-coated membranes employed for the photocatalytic hybrid system was made. Due to the additional adsorption of NOM onto IOPs on the membrane surface, the IOP-coated membrane system always achieved greater DOC removal efficiencies during photocatalysis/MF. Particularly, the influence of colloidal particles that were present in different water sources with respect to membrane fouling was explored. Colloidal fouling occurred to both bare and IOP-coated membranes, but the interaction of colloids with IOP coating layers was in close association with the characteristics of colloids, such as size distribution, resulting in opposing fouling behaviors with varying water sources. The IOP-coated membrane was able to control fouling properly when a relatively large size of colloidal particles existed in raw water, but not for the case of small colloids. The IOP coat layer may become denser as small colloids penetrate into it, therefore leading to further fouling. The analysis of the hydraulic filtration resistances revealed that such fouling was virtually reversible in being removed by backwashing processes. Scanning electron microscopic observations, however, visualized the existence of several foulants remaining at the membrane surface after backwashing when feed water, containing a relatively large portion of small-sized colloids, was supplied.     

MBR、MCR处理微污染水的膜污染比较

膜污染是影响膜反应器稳定运行的重要原因之一，为此考察了膜生物反应器(MBR)和膜混凝反应器(MCR)处理微污染地表水时的运行状况，并对膜比通量的变化进行了比较，发现MBR的膜污染情况比MCR的严重。MCR和MBR的膜组件经物理、化学清洗后膜比通量分别恢复至新膜比通量的99．7％和76．9％，物理清洗对此的贡献较大。经分析发现，MCR中无机污染占优势，主要污染元素是Fe；MBR中微生物和有机物是膜污染的主要组成，而无机污染物则主要是铁盐和磷酸盐。     

Membrane and solution effects on solute rejection and productivity

Limited understanding of the physical and chemical processes involved in membrane processes affects their widespread application in drinking water treatment. Insight into these processes was attained through a systematic manipulation of solution chemistry in membrane filtration with three ‘loose’ nanofiltration membranes. Eighteen known solutions were created varying pH, ionic strength, and major cation valence in the presence of a commercial humic acid. The membrances varied as well, including a non-ionic hydrophobic, a non-ionic hydrophilic, and an anionic hydrophilic membrane surface. Specific membrane productivity and TOC and conductivity rejection were monitored.

In all cases, the presence of divalent cations decreased the rejection of both conductivity and organic matter. Divalent cations also greatly increased the rate of productivity decline over equivalent tests in solutions with monovalent cations. The most hydrophobic membrane had the greatest productivity decline rate under all solution conditions. The lowest ionic strength solutions showed the greatest TOC and conductivity rejection and the greatest rate of productivity decline for each of the membranes.     

Combined influence of membrane surface properties and feed water qualities on RO/NF mass transfer, a pilot study

The impact of membrane surface characteristics and NOM on membrane performance has been investigated for varying pretreatment and membranes in a field study. Surface charge, hydrophobicity and roughness varied significantly among the four membranes used in the study. The membranes were tested in parallel following two different pretreatment processes, an enhanced Zenon ultrafiltration process (ZN) and a compact CSF process (Superpulsator (SP)) prior to RO membrane treatment for a total of eight integrated membrane systems. All membrane systems were exposed to the similar temperature, recovery and flux as well as chemical dosage. The membrane feed water qualities were statistically equivalent following ZN pretreatment and SP pretreatment except for NOM and SUVA. Membrane surface characteristics, NOM and SUVA measurements were used to describe mass transfer in a low-pressure RO integrated membrane system. Solute and water mass transfer coefficients (MTCs) were investigated for dependence on membrane surface properties and NOM mass loading. Inorganic MTCs were accurately described by a Gaussian distribution curve. Water productivity decreased with NOM loading and increased with contact angle and roughness. The negative effects of NOM loading on productivity were reduced as the negative charge on the membrane surface increased. Inorganic MTCs were also correlated to surface hydrophobicity and surface roughness. The permeability change of identical membranes was related to NOM loading, hydrophobicity and roughness. Organic fouling as measured by water, organic and inorganic mass transfer was less for membranes with higher hydrophilicity and roughness.     

Characterizing algogenic organic matter (AOM) and evaluating associated NF membrane fouling

Occasional algal blooms, comprised of blue-green algae and/or green algae, cause significant challenges in drinking water treatment due to the release of algogenic organic matter (AOM) into water extracellularly and, upon cell lysis, intracellularly. AOM has been extracted from blue-green algae (cyanobacteria) by various means and analyzed by UV absorbance scanning, HPSEC-UV-fluorescence-DOC, UV absorbance ratio index (URI), FTIR, and fluorescence excitation emission matrix (EEM). AOM extracted in water as a solvent exhibited a high hydrophilic fraction (57.3%) with a low SUVA (1.0 L/m-mg). The molecular weight (MW) distribution showed a significant heterogeneity (high value of polydispersivity) and high protein content (as indicated by specific fluorescence). Significant amounts of proteinaceous components such as mycosporine-like amino acids (MAAs, UV-screening components) and phycobilins (light-harvesting pigment) were detected by UV/visible absorption. The presence of proteins was confirmed by FTIR (at 1661 and 1552 cm(-1)), EEM spectra (EX:278-282 nm and EM:304-353 nm), and high URI values (3.1-6.0). A bench-scale cross-flow unit, employing a flat-sheet membrane specimen, was used to examine nanofiltration (NF) membrane fouling and removal of natural organic matter (NOM) derived from different blends of Suwannee River humic acid (SRHA) and AOM: SRHA 10 mgC/L, AOM 3mg C/L + SRHA 7 mgC/L, AOM 7 mgC/L + SRHA 3 mgC/L, and AOM 10 mgC/L. The study focused mainly on the effects of two different sources of organic matter on NF (NF 200) membrane fouling under otherwise similar conditions. Flux decline and organic matter rejection as a function of delivered DOC (cumulative mass of feed DOC per unit area) showed significantly different results depending on the organic matter composition of samples even though the test conditions were the same (organic matter concentration, pH, temperature, inorganic salt composition and concentration, and recovery). A higher flux decline was observed with increasing proportions of AOM. Organic matter rejections also decreased with higher AOM contributions to the samples, indicating that lower molecular weight (MW) AOM components were not well rejected by the NF 200 membrane having a 360 Da MWCO. However, SRHA that exhibited a relatively high MW (1000-5000 Da range) and high SUVA (7.4 L/m-mg) was preferentially rejected through electrostatic repulsion/size exclusion by the NF 200 membrane, having a high negative charge, low MWCO, and relatively low hydrophobicity. Even though the DOC concentration of feed water is a decisive factor for membrane fouling along with membrane properties and operating conditions, the characteristics of organic matter are more influential in fouling potential. Protein-like and polysaccharide-like substances were found as major foulants by FTIR.     

Analysis of filtration characteristics in submerged microfiltration for drinking water treatment

Hollow fiber membranes have been widely employed for water and wastewater treatments. Nevertheless, understanding the filtration characteristics of hollow fiber membranes is complicated by the axial distributions of transmembrane pressure (TMP) and flux, which are key factors for both fouling control and module design. In this study, model equations to account for different fouling mechanisms were derived to analyze the performance of submerged hollow fiber systems with different conditions in terms of feed water characteristics and membrane material. A series of experiments with synthetic feed and raw water were carried out using hydrophilic and hydrophobic membrane modules. The model successfully fits the experimental results for synthetic feed as well as raw water. The major fouling mechanisms for filtration of raw water using hydrophilic and hydrophobic membranes are identified as cake formation and standard blocking, respectively. The model calculations indicate that the distributions of flux and cake (fouling) resistance are sensitive to the fiber length of the membrane.     

Rejection of pharmaceutically active compounds and endocrine disrupting compounds by clean and fouled nanofiltration membranes

Rejections of 9 pharmaceuticals and 5 endocrine disruptors by clean and fouled nanofiltration membranes were investigated in this study. Waters containing a cocktail of compounds were filtered by clean and pre-fouled membranes. The rejection of hydrophilic neutral compounds by the clean NF-200 membrane varied from 35 to 70% under steady state conditions while that of NF-90 membrane was in the range of 62-96%. The clean NF-90 membrane rejected nearly all of the hydrophobic neutral compounds (95-98%) predominantly due to size exclusion. Nevertheless, electrostatic repulsion was the main mechanism of rejection of ionic compounds by both membranes (71-94% by NF-200 and 99% by NF-90). Fouling with sodium alginate deteriorated the performance of the NF-200 membrane in rejecting hydrophilic neutral compounds as well as hydrophilic and hydrophobic ionic compounds. In contrast, rejections of hydrophobic neutral compounds by the fouled NF-200 membrane increased by 5-38%. This may be attributed to the incipient interaction of the solutes with the membrane foulant layer resulting in less partitioning and diffusion across the membrane surface. On the other hand, rejections of hydrophobic compounds by NF-90 were not observed to be affected by fouling; however, hydrophilic neutral compounds showed increased rejections by 7-30%.     

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.     

Comparison of humic acid rejection and flux decline during filtration with negatively charged and uncharged ultrafiltration membranes

Increasingly stringent regulations for drinking water quality have stimulated the ultrafiltration (UF) to become one of the best alternatives replacing conventional drinking water treatment technologies. However, UF is not very effectively to remove humic acid due to the comparatively larger pore size compared to the size of humic acid. Fouling issue is another factor that restricts its widespread application. In this study, rejection of humic acid and flux decline were compared with essentially neutral, negatively charged version of a regenerated cellulose membrane, in which electrostatic interaction was explored for a better humic acid removal and less fouling. Solution environment, including ionic strength, pH and calcium ion concentration, affecting humic acid removal and flux decline on negatively charged and neutral membranes was also compared. Results indicated that the appropriate charge modification on the neutral UF membrane could be an effective way for better removal of NOM and reduction of the membrane fouling due to the electrostatic interactions with the combination effect of membrane pore size. Electrostatic interactions are significant important to achieve high humic acid removal and less fouling, and to improve the water quality and protect people’s health.