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.     

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.     

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.     

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.     

Autopsy of high-pressure membranes to compare effectiveness of MF and UF pretreatment in water reclamation

A pilot-plant study was designed to compare the effectiveness of microfiltration (MF) and ultrafiltration (UF) as pretreatment for high-pressure membranes in reclamation of biologically treated wastewater effluent. Granular media, filtered secondary effluent from a full-scale wastewater treatment plant, was fed to MF and UF units that operated in parallel. Each of these filtrates served as the feedwater to two reverse osmosis (RO) units and one nanofiltration (NF) unit that operated in parallel. The decline in specific flux was substantially lower for high-pressure membranes receiving UF than MF pretreatment over the course of each of four pilot plant runs that lasted from 1 to 7 weeks. The removal of organic matter as measured by dissolved organic carbon (DOC) was somewhat higher by UF than MF pretreatment (about 15% by UF compared with 11% by MF). Addition of ferric chloride ahead of the UF unit, but not ahead of the MF unit, may account for this additional removal of organic matter. However, the additional DOC removal appeared insufficient to explain the differential in foulant accumulation between high-pressure membranes receiving UF and MF pretreatment. Extensive autopsy analyses of these high-pressure membranes showed from 35% to 56% less organic carbon on those receiving UF rather than MF pretreatment. A more specific indicator of a differential in organic fouling was the accumulation of polysaccharides and this showed from 27% to 38% less on UF- than on MF-pretreated membranes. Yet another possible source of foulants is inorganic material given that the inorganic and organic weight percentages were nearly equal (56% vs. 44%) on the membrane surface. One specific source was aluminum added for phosphorus removal. Less fouling of high-pressure membranes pretreated by UF than MF could be due to the following: (1) a small, but very important, colloidal fouling fraction may have passed through MF but was rejected by UF pretreatment; (2) organic fouling was not related to organics in either the MF or UF filtrates but rather to organics that are generated in situ by microbial activity on the membrane surface; and/or (3) less passage of colloidal Al-P that carried over from secondary wastewater treatment.     

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.     

Biopolymer fouling in dead-end ultrafiltration of treated domestic wastewater

Ultrafiltration (UF) is considered as a suitable treatment process after conventional wastewater treatment to produce reuse water. Nevertheless, fouling affects the performance of UF to a large extent. As biopolymers (mostly macro polysaccharide-like and protein-like molecules) have been identified as major foulants affecting the filterability of water in dead-end UF, the present study focuses on investigating the reversibility of biopolymer fouling occurring at different biopolymer mass loads to the membrane and under different compression conditions. UF-membrane stirred cell tests using five cycles show that filtering treated domestic wastewater leads to a significant permeability reduction due to the accumulation of biopolymers on the membrane surface and/or in the membrane pores. Although they can be removed by hydraulic backwashing, an increased mass load of biopolymers reduces the removal efficiency. This correlation was verified using a UF pilot plant filtering treated wastewater (secondary effluent or slow sand filtrate). The effect of biopolymer fouling layer deformation on its reversibility was studied using multi-cycle membrane filtration tests under different filtration pressures. The results showed that higher filtration pressures result in more compact biopolymer fouling which is more difficult to be hydraulically backwashed. This phenomenon was also confirmed by pilot-scale UF experiments.     

UF膜组件的有机污染及清洗

近年来,超滤（UF）膜组件的污染问题主要集中在有机物污染的范围。利用几种化学药剂对聚砜超滤膜有机污染造成的堵塞的清洗方式进行了试验研究,研究了UF件的有机污染行为及膜组件污染物的去除。实验证明NaClO,H2O2和NaOH对于UF膜的有机污染去除都有一定的效果,其中H2O2的去除效果最好。控制原水的PH值有利于减轻膜污染。同时,以NaOH／NaClO及NaOH／H2O2水溶液相结合的方式对于膜的蛋白质污染的去除也有很好的效果,能够使超滤膜的渗透水通量恢复到初始值。同时还对UF膜的有机污染形成基理进行了分析。     

Application of ceramic membranes with pre-ozonation for treatment of secondary wastewater effluent

Membrane fouling is an inevitable problem when microfiltration (MF) and ultrafiltraion (UF) are used to treat wastewater treatment plant (WWTP) effluent. While historically the use of MF/UF for water and wastewater treatment has been almost exclusively focused on polymeric membranes, new generation ceramic membranes were recently introduced in the market and they possess unique advantages over currently available polymeric membranes. Ceramic membranes are mechanically superior and are more resistant to severe chemical and thermal environments. Due to the robustness of ceramic membranes, strong oxidants such as ozone can be used as pretreatment to reduce the membrane fouling. This paper presents results of a pilot study designed to investigate the application of new generation ceramic membranes for WWTP effluent treatment. Ozonation and coagulation pretreatment were evaluated to optimize the membrane operation. The ceramic membrane demonstrated stable performance at a filtration flux of 100 gfd (170 LMH) at 20 °C with pretreatment using PACl (1 mg/L as Al) and ozone (4 mg/L). To understand the effects of ozone and coagulation pretreatment on organic foulants, natural organic matter (NOM) in four waters - raw, ozone treated, coagulation treated, and ozone followed by coagulation treated wastewaters - were characterized using high performance size exclusion chromatography (HPSEC). The HPSEC analysis demonstrated that ozone treatment is effective at degrading colloidal NOMs which are likely responsible for the majority of membrane fouling.     

An autopsy study of a fouled reverse osmosis membrane element used in a brackish water treatment plant

The fouling of a spiral wound reverse osmosis (RO) membrane after nearly 1 year of service in a brackish water treatment plant was investigated using optical and electron microscopic methods, Fourier transform infrared spectroscopy (FTIR) and inductively coupled plasma atomic emission spectrometry (ICP-AES). Both the top surface and the cross-section of the fouled membrane were analysed to monitor the development of the fouling layer. It has been found that the extent of fouling was uneven across the membrane surface with regions underneath or in the vicinity of the strands of the feed spacer being more severely affected. Fouling appeared to have developed through different stages. In particular, it consisted of an initial thin fouling layer of an amorphous matrix with embedded particulate matter. The amorphous matrix comprised organic-Al-P complexes and the particulate matter was mostly aluminium silicates. Subsequently, as the fouling layer reached a thickness of about 5-7microm, further amorphous material, which is suggested to include extracellular polymeric substances such as polysaccharides, started to deposit on top of the existing fouling layer. This secondary amorphous material did not seem to contain any particulate matter nor any inorganic elements within it, but acted as a substrate upon which aluminium silicate crystals grew exclusively in the absence of other foulants, including natural organic matter (NOM).     

Characterization of organic membrane foulants in a submerged membrane bioreactor with pre-ozonation using three-dimensional excitation-emission matrix fluorescence spectroscopy

This study focuses on organic membrane foulants in a submerged membrane bioreactor (MBR) process with pre-ozonation compared to an individual MBR using three-dimensional excitation-emission matrix (EEM) fluorescence spectroscopy. While the influent was continuously ozonated at a normal dosage, preferable organic matter removal was achieved in subsequent MBR, and trans-membrane pressure increased at a much lower rate than that of the individual MBR. EEM fluorescence spectroscopy was employed to characterize the dissolved organic matter (DOM) samples, extracellular polymeric substance (EPS) samples and membrane foulants. Four main peaks could be identified from the EEM fluorescence spectra of the DOM samples in both MBRs. Two peaks were associated with the protein-like fluorophores, and the other ones were related to the humic-like fluorophores. The results indicated that pre-ozonation decreased fluorescence intensities of all peaks in the EEM spectra of influent DOM especially for protein-like substances and caused red shifts of all fluorescence peaks to different extents. The peak intensities of the protein-like substances represented by Peak T₁ and T₂ in EPS spectra were obviously decreased as a result of pre-ozonation. Both external and internal fouling could be effectively mitigated by the pre-ozonation. The most primary component of external foulants was humic acid-like substance (Peak C) in the MBR with pre-ozonation and protein-like substance (Peak T₁) in the individual MBR, respectively. The content decrease of protein-like substances and structural change of humic-like substances were observed in external foulants from EEM fluorescence spectra due to pre-ozonation. However, it could be seen that ozonation resulted in significant reduction of intensities but little location shift of all peaks in EEM fluorescence spectra of internal foulants.     

预处理对东江原水超滤过程中膜污染的控制作用

以珠江流域东江水作为原水,研究不同预处理(混凝、吸附、氧化)及其组合对水体中有机污染物的去除效果及对超滤膜污染的控制作用。试验结果表明,针对东江原水中天然有机物的去除,聚合氯化铝(PACl)、粉末活性炭(PAC)和高锰酸钾(KMnO4)的最佳投加量分别为20、30、0. 1 mg/L;三种单一预处理方法能够在一定程度上缓解膜通量衰减,而两两组合预处理则能够进一步提高膜运行通量;对于聚偏氟乙烯膜,PACl+PAC组合预处理对膜污染的控制作用最好。对于UV254和蛋白质,PACl和KMn O4对其去除效果优于PAC;对于多糖,三种预处理方法对其去除效果均不佳(<40%),其中PAC略好于PACl和KMn O4。此外,三种单一预处理方法对腐殖酸类荧光物质的去除效果高于蛋白质类荧光物质,而组合预处理能够更加显著地降低这两类荧光物质的响应强度,其中PACl+PAC组合预处理对有机物各荧光组分的去除效果最佳。通过对膜污染物成分的识别分析可知,东江原水中造成超滤膜污染的物质有腐殖酸类、多糖类和蛋白质类物质,而化学不可逆污染物主要为多糖类物质及少量的腐殖酸类物质,化学可逆污染物主要为蛋白质类物质及部分腐殖酸类物质。     

Characterisation of initial fouling in aerobic submerged membrane bioreactors in relation to physico-chemical characteristics under different flux conditions

The initial fouling characteristics of aerobic submerged membrane bioreactors (MBRs) were analysed under different flux conditions. Physico-chemical analyses of the mixed liquor hinted that carbohydrates were more important to membrane fouling than proteins. However, this contrasted with the characterisation of foulants on the membrane surfaces. Micro-structural analyses of the foulants on the membrane surfaces showed that the dominant foulants were different under different flux conditions. Membrane fouling occurred through a biofilm-dominated process under lower flux conditions, but the mechanism shifted towards a non-biofilm, organic fouling process as the flux was increased. In spite of the differences in fouling mechanisms, it was found that the protein fraction on the membrane surfaces, in the initial stages of MBR operations, had the greatest impact in the rise of transmembrane pressure.     

Effects of magnetic ion exchange pretreatment on low pressure membrane filtration of natural surface water

Magnetic ion exchange (MIEX) pretreatment has been increasingly employed by water treatment plants for removal of dissolved organic carbon (DOC). In this study, the effects of MIEX pretreatment on low pressure membrane filtration of natural surface water were investigated under different feedwater qualities, membrane properties, and MIEX dosing conditions. Regardless of feedwater DOC, moderate decrease in the total and hydraulically irreversible fouling was observed for a polyvinylidene fluoride (PVDF) microfiltration membrane and a polyethersulfone ultrafiltration (UF) membrane after MIEX pretreatment, which was coincident with moderate removals of high molecular weight DOC in the feedwaters. Comparatively, the fouling of a PVDF UF membrane did not decrease after MIEX pretreatment, revealing the impact of membrane properties on membrane fouling in the presence of MIEX pretreatment. Reuse of virgin or regenerated MIEX resulted in similar membrane fouling as observed with single use of the virgin MIEX. The level of DOC removal by MIEX was similar to the removal of MS2 bacteriophage spiked in the feedwater, suggesting a potential similarity in the removal of organic and microbial particles. In conclusion, MIEX pretreatment was effective for DOC removal, but less effective in controlling short-term membrane fouling or removing viruses.     

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.     

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.     

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.     

Fouling mechanisms in the integrated system with softening and ultrafiltration

Softening is designed to remove hardness ions, but it can also remove NOM and particles, yielding the possibility to use the process as a pretreatment for ultrafiltration. The objectives of this research were to understand the nature of the fouling mechanisms for ultrafiltration when used for waters that either require softening or have been softened, and to use that understanding to determine promising options for the use of softening as a pretreatment before ultrafiltration. To understand fouling mechanisms in the integrated system with softening and ultrafiltration, three different levels of softening performance in terms of removal of inorganics and organic matter were selected. Experiments were performed with both natural waters and synthetic waters with similar (but separable) inorganic, organic, and particulate characteristics. The synthetic waters were used to distinguish among inorganic fouling by precipitates, organic fouling, particulate fouling, and combined fouling by particles and organic matter. The results showed that organic matter played a major role in fouling, either by itself or by adsorption onto particles, and that softening pretreatment effectively reduced the foulants prior to ultrafiltration.     

Selection of NF membrane to improve quality of chemically treated surface water

The requirement for higher quality drinking water necessitates the application of more efficient water treatment techniques. Nanofiltration is one promising option for enhanced water treatment, for example, in enhanced organic matter removal. The characteristics of different nanofiltration membranes vary remarkably, and the selection of a membrane has to be made according to the requirements of an application. In this study six nanofiltration membranes (NF70, NF255, NTR-7450, NTR-7410, Desal-5 and TFC-S) were evaluated in improving the quality of chemically pre-treated surface water in a pilot-scale process. The results indicate that the membrane with high organics removal and slightly reduced ion removal characteristics (NF255) performed best in terms of product water quality as well as membrane productivity and fouling. The most permeable membrane (NTR-7410) suffered intensive fouling and insufficient product water quality. An interesting finding was that the permeates of all the tested membranes possessed a significant potential for microbial growth, despite the low nutrient contents.