Low-pressure membrane (MF/UF) fouling associated with allochthonous versus autochthonous natural organic matter

Natural organic matter (NOM) isolates/fractions; organic colloids, and hydrophobic (HPO), transphilic (TPI), and hydrophilic (HPI) fractions; isolated from a natural surface water as an allochthonous source, and in the form of algal organic matter (AOM) derived from blue green algae as an autochthonous source, were investigated in low-pressure membrane filtration. The most significant flux decline was caused by organic colloids, with an intermediate flux decline caused by AOM derived (isolated) from ground and sonicated blue green algae. 3D fluorescence excitation-emission matrix (EEM) analyses revealed that colloids and AOM contain protein-like substances, and FTIR analyses showed overlapping peaks associated with the peptide bonds in proteins and alcohols in polysaccharides originating from extra- and/or intra-cellular materials. HP-SEC results also support a high content of apparently macromolecular compounds in the colloid fraction. The presence of a divalent cation (Ca(2+)), hypothesized to enhance fouling by NOM acids by a reduction in molecular charge, showed little effect. Morphological analyses indicated that the surface topography of fouled UF membranes was elevated, presumably due to deposition of NOM on the membrane surface. The pores of MF membranes were reduced, suggesting pore blockage and/or constriction by NOM aggregates.     

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.     

Identification of effluent organic matter fractions responsible for low-pressure membrane fouling

Anion exchange resin (AER), powder activated carbon (PAC) adsorption and ozonation treatments were applied on biologically treated wastewater effluent with the objective to modify the effluent organic matter (EfOM) matrix. Both AER and PAC led to significant total organic carbon (TOC) removal, while the TOC remained nearly constant after ozonation. Liquid Chromatography-Organic Carbon Detection (LC-OCD) analysis showed that the AER treatment preferentially removed high and intermediate molecular weight (MW) humic-like structures while PAC removed low MW compounds. Only a small reduction of the high MW colloids (i.e. biopolymers) was observed for AER and PAC treatments. Ozonation induced a large reduction of the biopolymers and an important increase of the low MW humic substances (i.e. building blocks).Single-cycle microfiltration (MF) and ultrafiltration (UF) tests were conducted using commercially available hollow fibres at a constant flux. After reconcentration to their original organic carbon content, the EfOM matrix modified by AER and PAC treatments exhibited higher UF membrane fouling compared to untreated effluent; result that correlated with the higher concentration of biopolymers. On the contrary, ozonation which induced a significant degradation of the biopolymers led to a minor flux reduction for both UF and MF filtration tests. Based on a single filtration, results indicate that biopolymers play a major role in low pressure membrane fouling and that intermediate and low MW compounds have minor impact. Thus, this approach has shown to be a valid methodology to identify the foulant fractions of EfOM.     

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.     

Formation of assimilable organic carbon (AOC) and specific natural organic matter (NOM) fractions during ozonation of phytoplankton

Ozonation of natural surface water increases the concentration of oxygen-containing low molecular weight compounds. Many of these compounds support microbiological growth and as such are termed assimilable organic carbon (AOC). Phytoplankton can contribute substantially to the organic carbon load when surface water is used as source for drinking water treatment. We have investigated dissolved organic carbon (DOC) formation from the ozonation of a pure culture of Scenedesmus vacuolatus under defined laboratory conditions, using a combination of DOC fractionation, analysis of selected organic acids, aldehydes and ketones, and an AOC bioassay. Ozonation of algae caused a substantial increase in the concentration of DOC and AOC, notably nearly instantaneously upon exposure to ozone. As a result of ozone exposure the algal cells shrunk, without disintegrating entirely, suggesting that DOC from the cell cytoplasm leaked through compromised cell membranes. We have further illustrated that the specific composition of newly formed AOC (as concentration of organic acids, aldehydes and ketones) in ozonated lake water differed in the presence and absence of additional algal biomass. It is therefore conceivable that strategies for the removal of phytoplankton before pre-ozonation should be considered during the design of drinking water treatment installations, particularly when surface water is used.     

Granular iron oxide adsorbents to control natural organic matter and membrane fouling in ultrafiltration water treatment

Fine iron oxide particles (IOPs) are effective in removing natural organic matter (NOM) that causes membrane fouling in water treatment, but the separation of used IOPs is problematic. This study focused on the fabrication and use of granular iron oxide adsorbents, in combination with ultrafiltration (UF) membranes while investigating the NOM removal efficiency and fouling control. Sulfonated styrene-divinylbenzene copolymer beads were coated with two types of iron oxides (ferrihydrite and magnetite) and their performances were compared to that of fine IOPs. A significant amount of iron oxide coating (52–63 mg of Fe per g bead) was achieved by means of electrostatic binding and hydrolysis of iron ions. Iron oxide coated polymer (IOCP) beads were able to remove some amounts (～20%) of dissolved organic carbon (DOC) comparable to that achieved by IOPs within a short period of time (<15 min). Regenerated IOCPs exhibited the same sorption capacity as the fresh ones. The integrated IOCP/UF system operation with a 15-min empty bed contact time and 10-h cyclic regeneration maintained the 20% DOC removal with no sign of significant membrane fouling. In contrast, a sharp transmembrane pressure buildup occurred in the UF system when no iron oxide pretreatment was applied, regardless of the types of membranes tested. Iron oxide adsorbed the NOM fraction with molecular weights of >1000 kDa which is believed to be responsible for severe UF fouling.     

Natural organic matter fouling of low-pressure, hollow-fiber membranes: Effects of NOM source and hydrodynamic conditions

Effects of natural organic matter (NOM) source and hydrodynamic conditions on both hydraulically reversible and irreversible fouling of low-pressure, hollow-fiber (LPHF) membranes were systematically investigated using representative sources of natural waters and wastewater effluents. It was found that NOM source plays a primary role in determining the fouling of these membranes. Increase in permeate flux promoted membrane fouling, but to a lesser extent than NOM source. Permeate backwash flux appeared to restore permeability more effectively for the polyether sulfone (PES) membranes than to the polyvinylidene fluoride (PVDF) membranes used. NOM characterization revealed that organic colloids contributed predominantly to the hydraulically reversible fouling, and potentially to the irreversible fouling. Overall, this study demonstrated the importance of NOM source and the presence of organic colloids in the fouling of LPHF membranes, as well as the relevance of hydrodynamic operating conditions on the hydraulic reversibility of the fouling.     

Removal of natural organic matter by ion exchange

Ion exchange is an effective method for removing humic substances from drinking water supplies. We have explored a range of anion exchangers for removal of natural organic matter (NOM), both as isolated from surface waters and after fractionation into four fractions based on hydrophobic and hydrophilic properties. Resins of open structure and high water content are confirmed as the better performers, being very efficient at removal of any charged material, especially that of smaller molecular size. Quaternary ammonium resins containing polar groups are especially effective. The presence of a neighbouring OH group close to the quaternary nitrogen, heteroatoms in the bridge between the exchange site and the polymer backbone, a secondary amino group as the exchange site, or a low ratio of carbon to quaternary nitrogen is beneficial. A suitable balance of polar and non-polar regions in the resin structure appears to be required. Weakly basic amino groups may have a greater affinity for hydrophilic counter ions than quaternary ammonium groups, but generally there are fewer charged sites in the resin at neutral pH. Nevertheless, weak base resins have NOM uptakes nearly as high as strong base resins of similar water content. Water content was found to be the most important parameter, though the effect was less pronounced for strong base resins. For weak base resins of low charge density a non-electrostatic mechanism involving hydrogen bonding of the undissociated acidic species in the NOM to the unprotonated amino groups on the resins is proposed.     

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.     

Comparison of two fractionation strategies for characterization of wastewater effluent organic matter and diagnosis of membrane fouling

Two fractionation strategies were compared for characterizing organic components in effluent organic matter (EfOM) and natural organic matter (NOM). The first method is widely used and requires sample acidification and then re-neutralization during sequential organic removals onto resins. The second method uses a different suite of separation methods, does not require pH manipulation, and sequentially removes particles, colloids, organic acids, and hydrophobic neutrals without the need for adjusting pH. The NOM samples were dominantly organic acids while EfOM contained a broader distribution of organic functionalities so further evaluation was focused on EfOM. The new method completely removed colloidal matter from EfOM while the conventional fractionation method resulted in an increase in the percentage of EfOM >100 kDa with each fractionation step after filtration. Organic acids were removed in one fractionation step using the new method instead of three steps with the conventional method. The conventional method resulted in increased fouling after the final separation step apparently caused by production of inorganic colloids. The new fractionation method provided a clearer diagnosis that organic acids were the primary cause of fouling even though they were only 14% of EfOM organic carbon. We suggest that the new fractionation method should be considered for diagnosing the effects of NOM or EfOM on the performance of membrane filtration.     

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.     

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.     

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.     

Characterization of natural organic matter adsorption in granular activated carbon adsorbers

The removal of natural organic matter (NOM) from lake water was studied in two pilot-scale adsorbers containing granular activated carbon (GAC) with different physical properties. To study the adsorption behavior of individual NOM fractions as a function of time and adsorber depth, NOM was fractionated by size exclusion chromatography (SEC) into biopolymers, humics, building blocks, and low molecular weight (LMW) organics, and NOM fractions were quantified by both ultraviolet and organic carbon detectors. High molecular weight biopolymers were not retained in the two adsorbers. In contrast, humic substances, building blocks and LMW organics were initially well and irreversibly removed, and their effluent concentrations increased gradually in the outlet of the adsorbers until a pseudo-steady state concentration was reached. Poor removal of biopolymers was likely a result of their comparatively large size that prevented access to the internal pore structure of the GACs. In both GAC adsorbers, adsorbability of the remaining NOM fractions, compared on the basis of partition coefficients, increased with decreasing molecular size, suggesting that increasingly larger portions of the internal GAC surface area could be accessed as the size of NOM decreased. Overall DOC uptake at pseudo-steady state differed between the two tested GACs (18.9 and 28.6 g-C/kg GAC), and the percent difference in DOC uptake closely matched the percent difference in the volume of pores with widths in the 1-50 nm range that was measured for the two fresh GACs. Despite the differences in NOM uptake capacity, individual NOM fractions were removed in similar proportions by the two GACs.     

Ultrafiltration of natural organic matter and black carbon: Factors influencing aggregation and membrane fouling

There are concerns about black carbon (BC), due to its potential for sorption of toxic pollutants and inevitably entering drinking water sources. This study aimed to evaluate factors affecting BC aggregation and membrane fouling in the ultrafiltration of river water. Hydrophilic carbon black (CB) was selected as a surrogate of submicron BC in natural waters. Calcium, pH, and natural organic matter (NOM) were found to influence CB aggregation. Calcium induced interparticle interactions in a pH range of 4.3-7.7. In river water at 0.3 mM Ca²⁺, CB remained as fine aggregates (<300 nm) that caused substantial filtration resistance. At 1.3 mM Ca²⁺, CB size increased to 2.2-3.3 μm and membrane fouling was reduced. Interactions between particles and NOM enhanced organic rejection and eliminated irreversible membrane fouling. BC in water resources is a noxious substance, but it was easily aggregated in hard waters and could enhance NOM removal in the ultrafiltration process.     

In-situ identification of iron electrocoagulation speciation and application for natural organic matter (NOM) removal

In this work, iron speciation in electrocoagulation (EC) was studied to determine the impact of operating parameters on natural organic matter (NOM) removal from natural water. Two electrochemical EC parameters, current density (i) and charge loading rate (CLR), were investigated. Variation of these parameters led to a near unity current efficiency (φ = 0.957 ± 0.03), at any combination of i in a range of 1–25 mA/cm² and CLR in a range of 12–300 C/L/min. Higher i and CLR led to a higher bulk pH and limited the amount of dissolved oxygen (DO) reduced at the cathode surface due to mass transfer limitations. A low i (1 mA/cm²) and intermediate CLR (60 C/L/min) resulted in low bulk DO (<2.5 mg/L), where green rust (GR) was identified by in-situ Raman spectroscopy as the primary crystalline electrochemical product. Longer electrolysis times at higher i led to magnetite (Fe₃O₄) formation. Both higher (300 C/L/min) and lower (12 C/L/min) CLR values led to increased DO and/or increased pH, with lepidocrocite (γ-FeOOH) as the only crystalline species observed. The NOM removal of the three identified species was compared, with conditions leading to GR formation showing the greatest dissolved organic carbon removal, and highest removal of the low apparent molecular weight (<550 Da) chromophoric NOM fraction, determined by high performance size exclusion chromatography.     

Tracking natural organic matter (NOM) in a drinking water treatment plant using fluorescence excitation-emission matrices and PARAFAC

Natural organic matter (NOM) in water samples from a drinking water treatment train was characterized using fluorescence excitation emission matrices (F-EEMs) and parallel factor analysis (PARAFAC). A seven component PARAFAC model was developed and validated using 147 F-EEMs of water samples from two full-scale water treatment plants. It was found that the fluorescent components have spectral features similar to those previously extracted from F-EEMs of dissolved organic matter (DOM) from diverse aquatic environments. Five of these components are humic-like with a terrestrial, anthropogenic or marine origin, while two are protein-like with fluorescence spectra similar to those of tryptophan-like and tyrosine-like fluorophores. A correlation analysis was carried out for samples of one treatment plant between the maximum fluorescence intensities (F_max) of the seven PARAFAC components and NOM fractions (humics, building blocks, neutrals, biopolymers and low molecular weight acids) of the same sample obtained using liquid chromatography with organic carbon detection (LC-OCD). There were significant correlations (p < 0.01) between sample DOC concentration, UVA₂₅₄, and F_max for the seven PARAFAC components and DOC concentrations of the LC-OCD fractions. Three of the humic-like components showed slightly better predictions of DOC and humic fraction concentrations than UVA_254. Tryptophan-like and tyrosine-like components correlated positively with the biopolymer fraction. These results demonstrate that fluorescent components extracted from F-EEMs using PARAFAC could be related to previously defined NOM fractions and that they could provide an alternative tool for evaluating the removal of NOM fractions of interest during water treatment.     

Modeling monochloramine loss in the presence of natural organic matter

A comprehensive model describing monochloramine loss in the presence of natural organic matter (NOM) is presented. The model incorporates simultaneous monochloramine autodecomposition and reaction pathways resulting in NOM oxidation. These competing pathways were resolved numerically using an iterative process evaluating hypothesized reactions describing NOM oxidation by monochloramine under various experimental conditions. The reaction of monochloramine with NOM was described as biphasic using four NOM specific reaction parameters. NOM pathway 1 involves a direct reaction of monochloramine with NOM (k_doc1=1.05×10⁴-3.45×10⁴ M⁻¹ h⁻¹). NOM pathway 2 is slower in terms of monochloramine loss and attributable to free chorine (HOCl) derived from monochloramine hydrolysis (k_doc2=5.72×10⁵-6.98×10⁵ M⁻¹ h⁻¹), which accounted for the majority of monochloramine loss. Also, the free chlorine reactive site fraction in the NOM structure was found to correlate to specific ultraviolet absorbance at 280 nm (SUVA₂₈₀). Modeling monochloramine loss allowed for insight into disinfectant reaction pathways involving NOM oxidation. This knowledge is of value in assessing monochloramine stability in distribution systems and reaction pathways leading to disinfection by-product (DBP) formation.     

Estimation of the hydrophobic fraction of dissolved organic matter in water samples using UV photometry

In this study, we tested a simple and rapid method for the estimation of carbon in the hydrophobic fraction of dissolved organic matter (DOM) of different origin (spruce, pine, and beech litter) in soil water. The method is based on the fact that the hydrophobic fraction of DOM contains almost entirely the aromatic moieties of DOM. Thus, it showed a clearly distinct light absorption at 260 nm compared to the hydrophilic fraction. This light absorption was directly proportional to the concentration of the hydrophobic fraction. Moreover, it was independent of the concentration of the hydrophilic fraction. We compared the concentrations of hydrophobic DOM estimated by the UV method with those of the conventional fractionation using chromatographic columns of XAD-8 macroporous resin and found an excellent agreement between the two methods for both solutions from laboratory sorption experiments and field samples of forest floor leachates and subsoil porewaters. In addition, the absorption at 260 nm of hydrophobic DOM proved to be independent of pH values ranging from 2.0 to 6.5. Compared to the conventional chromatographic fractionation, the method using the UV absorption at 260 nm is less time consuming, needs a much smaller sample volume, and showed a better reproducibility. However, its use is restricted to water samples of low nitrate (<25 mg L⁻¹) and Fe (<5 mg L⁻¹) concentrations and, probably, with the hydrophobic fraction dominated by aromatic compounds deriving from degradation of lignin.     

A comparison of membrane fouling under constant and variable organic loadings in submerge membrane bioreactors

The aim of this study is to compare the effect of constant and variable influent organic loadings on membrane fouling in submerged membrane bioreactors (sMBRs). Two identical lab-scale sMBRs were operated for 162 days at an SRT of 30 days, whereas the influent organic loading was kept constant in one MBR, and varied in another. The microbial characteristics of sludge in terms of MLSS, bound EPS, EPS in the supernatant and particle size distribution were investigated in order to evaluate their respective effect on membrane fouling. During the start-up period, membrane fouling in the MBR fed with variable loadings was more serious than that in the MBR with the constant loading. However, at the stable state, the fouling tendency was clearly reversed with less membrane fouling for variable feed strength. It was shown that the contents of polysaccharides in the supernatant and particle size of the bioflocs were responsible for the observed differences in the fouling tendencies of the two MBRs.