Natural Organic Matter Rejection Research Articles

Coupling effect of 17β-estradiol and natural organic matter on the performance of a PAC adsorption/membrane filtration hybrid system

A powdered activated carbon/submerged microfiltration (PAC/SMF) system was investigated to examine the removal efficiency of 17β-estradiol (E2) as well as the membrane filterability in the presence of natural organic matter (NOM). The removal of E2 was approximately 92%, but the presence of NOM did not significantly affect the adsorption efficiency of E2 on PAC. When E2 was spiked in the raw water containing NOM, the coupling phenomena did not occur in the rejection of E2 and NOM, but took place in the membrane fouling. Although the reduction of filterability by E2 itself was not so significant, membrane fouling by the river water in the presence of E2 significantly deteriorated in comparison to that generated by filtrating only the river water without E2. This was because E2 induced the additional adsorption of weakly-adsorptive NOM. This coupling phenomenon was related to the initial contact concentration of E2 with the membrane surface. The higher the initial contact concentration, the worse the fouling by NOM.

Natural organic matter fouling using a cellulose acetate copolymer ultrafiltration membrane

The low molecular weight cut-off ultrafiltration process has become acceptable for drinking water treatment; however, irreversible fouling curtails the economic viability of such process. The objective of this study was to evaluate the effectiveness of an ultrafiltration membrane on natural organic matter rejection and the components of natural water that contribute to fouling. Membranes with different molecular weight cut-off were employed. Experimental solutions consist of natural organic matter isolated from natural water or humic substances. The experimental solutions were prefiltered and diluted to prevent cake formation on membrane and change the fouling mode to pore blockage. The aggregation rejection caused irreversible fouling of the 100 kDa membrane, presumably a result of pore size reduction due to internal deposition aggregates. The solution showed differences in rejection, flux decline and membrane resistance.

Nanofiltration of pretreated Lake Houston water: Disinfection by-product speciation, relationships, and control

Water quality results related to disinfection by products (DBPs) from a bench-scale study designed to evaluate surface water nanofiltration (NF) for the City of Houston are analyzed to provide insights into mechanisms of DBP formation and control. One contribution of this manuscript is that we report on DBP speciation following NF of Lake Houston water containing very low concentrations of the bromide ion (< ∼50 μg/L or 0.6 μM). Rejection of natural organic matter (NOM) and DBP precursors decreased with increasing recovery indicating diffusion-limited transport of these macromolecular contaminants under conditions typical of drinking water NF. Additional evidence of diffusivity effects on transport was provided by decreasing permeation coefficients of polyethylene glycols with molecular weight. Haloacetic acid (HAA) molar concentrations were higher than corresponding trihalomethane (THM) concentrations in NF permeates contrary to expectations from the DBP rule where lower mass-based maximum contaminant levels are enforced for HAAs. Similar to conventional treatment, ultraviolet absorbance at 254 nm (UV 254), dissolved organic carbon (DOC), and chlorine consumption were found to be excellent surrogates for aqueous DBP concentrations even in chlorinated nanofiltered waters. Mole fractions of mono-, di-, and trihalogenated HAA species were invariant with respect to changes in Br −/DOC molar ratio suggesting that the brominated HAAs are formed through the same mechanisms as the chlorinated ones in NF permeate waters. Additionally, bromine incorporation into both THMs and HAAs increased with increasing Br/DOC ratio suggesting that the brominated THM and HAA species form first consuming the reactive precursor sites and in the process restricting formation of the chlorinated species. Bromide utilization was observed to decrease with Br −/DOC molar ratio demonstrating that nanofilter permeate waters were substantially limited with respect to NOM and DBP precursors.

Membrane interactions with NOM and an adsorbent in a vibratory shear enhanced filtration process (VSEP) system

The effects of torsional membrane vibration and the addition of heated aluminum oxide particles (HAOPs) on the interactions of natural organic matter (NOM) with UF membranes were investigated using a vibratory shear enhanced filtration process (VSEP) laboratory module. Torsional vibration reduced fouling in both dead-end and crossflow modes. Incorporating crossflow into a system with vibration increased NOM removal efficiency. In systems with crossflow, increasing the vibration amplitude decreased fouling up to a point, and provided negligible benefit thereafter. On the other hand, it had a negligible effect on NOM rejection up to the same point, and improved permeate quality thereafter. Although a mass balance suggested that a negligible portion of the NOM remained on the membrane surface at the end of the crossflow tests, foulant aggregates were observed on the membrane both visually and in SEM images. The SEM images indicated that vibration changed the morphology of the foulants from a continuous layer into scattered clumps, and that higher vibration amplitude facilitated this transformation. Deposition of a HAOPs layer on the membrane surface relieved fouling only initially, after which the HAOPs/UF system fouled almost as fast as a bare UF membrane. However, intermittent vibration to re-mix HAOPs particles during the HAOPs/UF operation kept permeate flux relatively high for much longer, thereby significantly improving the performance.

Factors affecting nanofiltration performances in natural organic matter rejection and flux decline

A crossflow bench-scale test cell was used to investigate factors (i.e. NOM concentration, ionic strength, and solution pH) affecting natural organic matter (NOM) rejection and flux decline during nanofiltration (NF). Experimental results revealed that increased NOM concentration increased permeate flux decline, salt rejection, and NOM rejection, enhancing NOM accumulation on membrane surface. At high concentration of NOM, permeate flux curve corresponded to cake formation model. Increased ionic strength from 0.004 M to 0.1 M illustrated higher flux decline, possibly as a result of increasing osmotic pressure from higher concentration of salt. Solutions possessing high ionic strength (0.05 M) showed greater flux decline and NOM rejection than those having low ionic strength (0.01 M). Increased solution pH from 4 to 10 exhibited greater flux decline, caused by increasing salt rejection and enhancing salt concentration on membrane surface.

Fabrication, fouling and foulant analyses of asymmetric polysulfone (PSF) ultrafiltration membrane fouled with natural organic matter (NOM) source waters

The fouling behaviours and membrane autopsy protocol for polysulfone (PSF) ultrafiltration membrane fouled with natural organic matter source waters were studied. Samples from Ulu Pontian river which has a relatively hydrophilic NOM source water and Bekok Dam river which has a relatively hydrophobic NOM source water have been used as the case study. Fouling characteristics of the NOM source waters were assessed by filtering the feed water with an immersed hydrophobic PSF ultrafiltration membrane. The asymmetric hollow fiber PSF membrane was spun by a dry–wet phase inversion spinning process. The membrane autopsy protocol was performed to identify the nature of the deposited foulants and their relative effects on membrane characteristics. Results for the relatively hydrophilic NOM source water (Ulu Pontian river) exhibited greater flux decline but lesser NOM removal considerably due to pore adsorption, indicating that the low molecular weight, aliphatic linear structure and neutral/base organic matter contained within the hydrophilic fraction were the prime foulants. In contrast, relatively hydrophobic NOM source water (Bekok Dam water) that possessed higher charge density, greater molecular weight and bulky aromatic structure has exhibited lesser flux decline and better NOM rejection noticeably due to cake deposition, despite filtering through a hydrophobic membrane, thus suggesting that the electrostatic repulsion was more influential than the steric hindrance mechanisms. In comparison a non-charged model compound (polyethylene glycol) of similar molecular weight was used to quantify the role of electrostatic charge repulsion on NOM rejection. Moreover, analyses on the permeate characteristics revealed that the hydrophobic NOM was preferentially removed by the negatively charged PSF membrane as opposed to the hydrophilic NOM, hence, suggesting that the charge interactions, in addition to size exclusion were more crucial to NOM removal. The membrane autopsies analyses confirmed the flux decline results and permeate analyses as the filtered-membrane was mainly fouled by the hydrophilic NOM components rather than humic compounds. Distinctive changes were observed in membrane characteristics in terms of ionizable functional groups, membrane wettability and zeta potential. ATR-FTIR analysis revealed that hydrophilic components such as the polysaccharides-like substances, alcoholic compounds and aliphatic amide of protein groups as the responsible materials covering the membrane surface. Morphological analyses using SEM indicated different fouling mechanisms occur for both NOM sources associated with differences in the relative NOM constituent distributions, NOM structural variations and NOM removal mechanisms.

Effect of membrane properties and pretreatment on flux and NOM rejection in surface water nanofiltration

Nanofiltration (NF) is a compact process that has potential for removing organic and inorganic pollutants for drinking water. In this study, various NF membranes for surface water treatment were compared using a batch-stirred cell and crossflow equipment in terms of solute rejection and membrane flux. The rejection of natural organic matter (NOM) was closely related to ion rejection and membrane flux. To better control NOM, NF membranes having an effective MWCO of less than 200 Da were required due to the bimodal distribution of NOM in surface water. The flux of NF membranes was related to their hydrophobicity and charges. Fouling by NOM adsorption was an important factor in hydrophobic and positively charged membranes, whereas, it was negligible for hydrophilic and negatively charged membranes. Ozone pretreatment was ineffective for controlling NF fouling, whereas, pretreatment by means of PAC or UF showed a remarkable improvement in flux, indicating that the removal of colloids is crucial for preventing NF fouling.

Ultrasonic control of ceramic membrane fouling caused by natural organic matter and silica particles

The ultrasonic control of membrane fouling caused by natural organic matter (NOM) and silica particles was investigated under various solution conditions. Twenty kilohertz ultrasound was applied to a cross-flow ultrafiltration system with γ-alumina membranes. Experimental results indicated that solution chemistry affected the ability of ultrasound to control membrane fouling. More effective control of membrane fouling occurred at high pH, low ionic strength, and in the absence of divalent cations, consistent with the expectation of stronger electrostatic repulsion between foulant–foulant and foulant–membrane components. Fourier transform infrared spectra of the NOM foulant showed the presence of aliphatic carbon and carboxylate functional groups, both of which decreased in magnitude with increasing pH, in the absence of Ca 2+, or the presence of ultrasound. A decrease in the NOM rejection rate during ultrasound was observed. This likely occurred due to decreased steric exclusion of NOM through the fouled membrane, as scanning electron microscope images indicated that ultrasound partially removed the foulant layer from the membrane surface allowing larger NOM molecules to pass through the membrane. No damage to the membrane by ultrasound was observed.

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

Two natural source waters containing natural organic matter (NOM) with different physical and chemical characteristics were dead-end filtered using five types of membranes having different material and geometric properties. In this study retained dissolved organic carbon (DOC) per unit membrane area is introduced as a better parameter compared to permeate volume, time, and delivered DOC to provide a reasonable comparison of NOM rejection and flux-decline trends. Retained DOC/NOM was calculated, which influences NOM concentration polarization at the membrane interface, and transport measurements of NOM rejection and flux decline were made. Molecular weight (MW) distribution measurements (by size exclusion chromatography) were used to calculate the average MW of the NOM. This persuasively demonstrated that the nominal molecular weight cut-off (MWCO) of a membrane is not the unique predictor of rejection characteristics for NOM composites. The charge density of NOM from the source waters was measured to estimate its effects on NOM rejection and flux decline during filtration. The contact angle of the membranes was used to determine hydrophobic interactions between NOM and membrane. All filtration measurements were performed at approximately the same permeate flow rate in order to minimize artifacts from mass transfer at the membrane interface. ESNA having a nominal MWCO of 200 Daltons showed NOM rejection greater than 95% and flux decline lower than 10% under a condition of a retained DOC of 0.5 mg C/cm 2 for the feed source waters. The other membranes having larger membrane pores (nominal MWCOs ranging from 8,000 to 20,000 Daltons) than the ESNA showed NOM rejection ranging from 68% to 86% and flux decline ranging from 5% to 17% at the same retained DOC for the waters.

NOM removal from different raw waters by membrane filtration

In this study natural organic matter (NOM) removal by ultra- and nanofiltration from four different raw waters was investigated. The experiments were carried out with a flat channel cross flow unit and with a pilot unit equipped with two parallel spiral wound modules. NOM rejection was characterized using 13CMAS-NMR spectroscopy, scanning electron microscopy (SEM) and size exclusion chromatography with online DOC detection (LC-OCD). Additionally measurements, such as DOC, UVA (254nm and 436nm), AOX- and THM-precursors, ion concentration, turbidity, etc. were made on raw waters, permeates and concentrates. The results indicate that NOM removal by nanofiltration for all water sources is almost complete (&amp;gt;90%), while NOM removal by ultrafiltration depends on raw water chemistry, especially ionic strength. Solid state 13C-NMR spectra of freeze-dried NOM deposits on membrane surfaces suggest the presence of structural polysaccharides (such as chitin and cellulose). SEM images confirm the NMR-results: after filtration, inorganic deposits and algae like structures were found on membrane surfaces.

Nanofiltration of natural organic matter with H 2O 2/UV pretreatment: fouling mitigation and membrane surface characterization

This research investigated the application of H 2O 2/UV oxidation for source water pretreatment, and membrane cleaning to improve the performance of nanofiltration processes. It further examined the nature and mechanisms of membrane fouling by natural organic matter (NOM), and membrane cleaning using different chemical agents, by employing several surface characterization techniques. These techniques included attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and scanning electron microscopy (SEM). The study revealed that significant improvement could be achieved in the efficiency and economics of nanofiltration for removing NOM and synthetic organic chemicals (SOCs) by employing source water pretreatment and membrane cleaning strategies. The H 2O 2/UV oxidation of source water prior to nanofiltration showed potential for the following: (i) mitigation of flux decline due to membrane fouling, (ii) removal of the pesticide alachlor and hydrogen sulfide, and (iii) improvement in membrane cleanability. Nonetheless, careful control of the preoxidation conditions was exercised to arrive at a reasonable compromise between fouling mitigation and NOM rejection.

Low fouling synthetic membranes by UV-assisted graft polymerization: monomer selection to mitigate fouling by natural organic matter

A UV-assisted photochemical graft polymerization technique was used to produce modified poly(ether sulfone) (PES) ultrafiltration (UF) membranes that exhibit reduced interaction with natural organic matter (NOM), as a route to reduce the fouling caused by NOM. We evaluated six different hydrophilic monomers for their ability to reduce fouling by NOM: two are neutral monomers, N-vinyl-2-pyrrolidinone (NVP) and 2-hydroxyethyl methacrylate (HEMA); two are weakly acidic (carboxylic) monomers, acrylic acid (AA) and 2-acrylamidoglycolic acid (AAG); and two are strongly acidic (sulfonic) monomers, 3-sulfopropyl methacrylate (SPMA) and 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS). Grafting increased membrane surface wettability and shifted the membrane pore size distribution to smaller sizes, which increased natural organic matter rejection (except in the case of NVP). Total fouling appeared to depend primarily on solute rejection, and varied in a complex way that could be interpreted in the context of the NOM molecular weight distribution. Reversible fouling resulting from cake formation was only weakly dependent on membrane surface chemistry; in contrast, irreversible fouling exhibited a marked dependence on surface chemistry. Membranes modified with the weak acid AA monomer were able to reduce irreversible fouling to zero, in contrast to other strongly hydrophilic monomers such as HEMA and AAG, which increased irreversible fouling relative to the unmodified membrane. We conclude that wettability (or hydrophilicity) is not an appropriate parameter for estimating reduced fouling potential for NOM feeds, as it is for feeds containing protein. We hypothesize that this is a consequence of the structural and chemical heterogeneity of NOM. The AA-modified membranes exhibited excellent filtration performance over multiple runs, and outperformed a regenerated cellulose (RC) membrane having similar initial NOM rejection.

Effects of molecular weight cutoff, f/k ratio (a hydrodynamic condition), and hydrophobic interactions on natural organic matter rejection and fouling in membranes

All the possible mechanisms and factors affecting flux decline, natural organic matter (NOM) rejection, and NOM transport were studied in this paper. Electrostatic exclusion (charge repulsion) was determined to be an influential factor in minimizing flux decline and membrane fouling, and in maximizing NOM rejection; the effective MWCO (membrane molecular weight cutoff) concept was used to support the notion of electrostatic interactions and to describe an apparent MWCO by a charged membrane. This allowed for a negatively-charged tight ultrafiltration (UF) membrane to be comparatively compared to a nanofiltration (NF) membrane in terms of NOM rejection. The f/k ratio, relating relative back-diffusional transport to convective transport, was identified as a major influential factor in terms of flux decline and NOM rejection. The effect of hydrophobicity on flux decline was not significant when the same f/k ratio was used for different NOM-membrane pairs. The convectional and diffusional transportation of NOM was also evaluated for NF and UF

Predictive models and factors affecting natural organic matter (NOM) rejection and flux decline in ultrafiltration (UF) membranes

Prediction equations for NOM rejection were formulated using parameters, including specific UV absorbance [SUVA = UV absorbance at 254 nm/dissolved organic matter (DOC)] and a f k ratio [a ratio of water permeability ( f) to the mass transfer coefficient ( k)], which have been found to influence aspects of NOM and operating conditions, respectively. Attempts were made to formulate relationships between adsorption resistance ( R a ) and interfacial membrane concentration ( C m ), and specifically between the amount of NOM absorbed (mg C/cm 2) and the R a . Flux decline was also formulated in two ways: (1) by the (so-called) empirical flux-decline equation with three flux-decline coefficients, and (2) by the adsorption flux-decline model with two NOM adsorption terms between bulk, the NOM and the membrane surface and an existing NOM adsorption layer.

Cleaning strategies for flux recovery of an ultrafiltration membrane fouled by natural organic matter

One of the most common problems encountered in water treatment applications of membranes is fouling. Natural organic matter (NOM) represents a particularly problematic foulant. Membranes may be fouled by relatively hydrophilic and/or hydrophobic NOM components, depending on NOM characteristics, membrane properties, and operating conditions. To maximize flux recovery for an NOM-fouled ultrafiltration membrane (NTR 7410), chemical cleaning and hydraulic rinsing with a relatively high cross-flow velocity were investigated as cleaning strategies. The modification of the membrane surface with either an anionic or a cationic surfactant was also evaluated to minimize membrane fouling and to enhance NOM rejection. Foulants from a hydrophobic NOM source (Orange County ground water (OC-GW)) were cleaned more effectively in terms of permeate flux by acid and caustic cleanings than foulants from a relatively hydrophilic NOM source (Horsetooth surface water (HT-SW)). An anionic surfactant (sodium dodecyl sulfate (SDS)) was not effective as a cleaning agent for foulants from either hydrophobic or hydrophilic NOM sources. High ionic strength cleaning with 0.1 M NaCl was comparatively effective in providing flux recovery for NOM-fouled membranes compared to other chemical cleaning agents. Increased cross-flow velocity and longer cleaning time influenced the efficiency of caustic cleaning, but not high ionic strength cleaning. The membrane was successfully modified only with the cationic surfactant; however, enhanced NOM rejection was accompanied by a significant flux reduction.

Membrane filtration of natural organic matter: comparison of flux decline, NOM rejection, and foulants during filtration with three UF membranes

Three ultrafiltration membranes [thin-film composite, polyethersulfone (PES), and sulfonated PES] which have different chemistries and nominal relative molecular mass cut-offs (MWCO), were compared in terms of their characteristics of flux decline, rejection of natural organic matter (NOM), and the adsorbed foulants, with two very different (relatively hydrophilic and hydrophobic) NOM-containing source waters. To facilitate comparison, the ratio ( J o k ) of the initial pure water flux J o to the estimated boundary layer mass transfer coefficient k was used to define similar initial hydrodynamic operating conditions for the three different membranes. The membranes showed differences in NOM rejection when filtering hydrophilic NOM-source water, and also exhibited differences in flux decline with the hydrophobic NOM-source water. Flux decline and NOM rejection were quantified using a resistances-in-series model and effective molecular mass cut-off, respectively. Non-charged NOM fractions (hydrophilic and hydrophobic neutrals/bases) were found to be significant foulants for these negatively charged membranes.

Membrane filtration of natural organic matter: factors and mechanisms affecting rejection and flux decline with charged ultrafiltration (UF) membrane

We studied natural organic matter (NOM) rejection and the membrane’s flux decline during natural water filtration using a charged ultrafiltration membrane based on thin-film-composite technology. NOM rejection mechanisms such as steric exclusion and aromatic/hydrophobic and charge interactions were considered. Water composition factors affecting NOM rejection and flux decline were investigated, including ionic strength, pH, and calcium ion concentration. The membrane’s effective relative molecular mass cutoff for the NOM in our study was between 1500 and 2300 (significantly lower than the manufacturer’s nominal value of 8000) and depended on the NOM characteristics in the source water. In particular the ratio of UV absorbance at 254 nm to dissolved organic carbon (related to the humic content) correlated with the rejection. Comparison of relative molecular mass distributions between fractionated NOM and recovered membrane foulants indicates that the foulants are the larger-sized neutral and/or basic NOM components, and not the humic substances that were efficiently rejected by this membrane.

Interactions Between Natural Organic Matter (NOM) and Membranes: Rejection and Fouling

This work demonstrates that interactions between membranes and natural organic matter (NOM) include rejection by both steric and electrostatic exclusion, and fouling by adsorption. NOM rejection and fouling are influenced by both NOM characteristics (molecular weight, aromaticity/humic content, and charge) and membrane properties (molecular weight cutoff (MWCO), hydrophobicity, and charge) as well as hydrodynamic operating conditions represented by a f/k ratio (the ratio of permeate flux (f) to a diffusional back-transport mass transfer coefficient (k)).

Membrane filtration of natural organic matter: initial comparison of rejection and flux decline characteristics with ultrafiltration and nanofiltration membranes

Two source waters containing natural organic matter (NOM) with different physical and chemical characteristics were crossflow-filtered using four types of membranes having different material and geometric properties. Transport measurements of NOM rejection and flux decline were made. A resistances-in-series model was used to represent and quantitatively compare membrane flux decline and recovery. As anticipated, the resistance due to specific adsorption depended on the concentration at the membrane interface. For the two membranes showing evidence of NOM adsorption, reducing the initial flux (which we infer to also reduce the interfacial NOM concentration) also lowered the measured resistance assigned to adsorption in our protocol. Relative molecular mass (RMM) distribution measurements (by size exclusion chromatography) were used to calculate the average RMM of the NOM and persuasively illustrated that the nominal relative molecular mass cut-off (MWCO) of a membrane is not the unique predictor of rejection characteristics for NOM compounds. Size exclusion, electrostatic repulsion, and NOM aromaticity all influenced the NOM rejection. For a given water composition (including pH and ionic strength), membrane characteristics (such as the surface charge, hydrophobicity and nominal MWCO) can be combined with the NOM properties (such as total dissolved organic carbon, specific UV absorbance at 254nm and humic content) to provide a consistent qualitative rationale for the transport results.

Interactions between natural organic matter (nom) and membranes: Rejection and fouling

This work demonstrates that interactions between membranes and natural organic matter (NOM) include rejection by both steric and electrostatic exclusion, and fouling by adsorption. NOM rejection and fouling are influenced by both NOM characteristics (molecular weight, aromaticity/humic content, and charge) and membrane properties (molecular weight cutoff (MWCO), hydrophobicity, and charge) as well as hydrodynamic operating conditions represented by a f/k ratio (the ratio of permeate flux (f) to a diffusional back-transport mass transfer coefficient (k)).