Membrane-foulant Interactions Research Articles

Enhanced performance of aerobic granular sludge-membrane system through the utilization of different iron-based nano-metal oxides for mitigating membrane fouling

Fe3O4, γ-Fe2O3, and α-Fe2O3 nanoparticles play a crucial role in influencing the adsorption capacity and pretreatment efficacy as iron-based nano-metal oxides due to their distinct structural and physicochemical properties. However, the impact of these adsorbents on membrane filtration efficiency remains unexplored. This study aimed to investigate the influence of these three adsorbents on membrane fouling in an aerobic granular sludge–membrane system (AGSMS) used for municipal wastewater reuse treatment. The results demonstrated that under optimal conditions, γ-Fe2O3 exhibited superior effectiveness in removing low and medium molecular weight contaminants from wastewater, resulting in a higher normalized flux (0.59) and reduced total fouling resistance (2.66 × 1011 m−1). It also showed the highest removal rate for organic matter and suspended solids, making it particularly effective in mitigating AGSMS membrane fouling. However, excessive addition (2 g/L) of Fe3O4, γ-Fe2O3, and α-Fe2O3 could potentially hinder the proper operation of the AGSMS. Pre-adsorption facilitated the formation of a porous cake layer while inhibiting standard blocking behavior. Additionally, the extended Derjaguin–Landau–Verwey–Overbeek theory analysis revealed a substantial energy barrier between membrane-foulant interactions (125.65 kT) as well as foulant-foulant interactions (62.06 kT) within the γ-Fe2O3 system, confirming its exceptional antifouling characteristics. This study offers valuable insights into the selection of iron oxide adsorbents for membrane fouling mitigation and wastewater reuse.

Unraveling the intricate fouling behaviors of landfill leachate components during membrane distillation concentration toward zero liquid discharge

Membrane distillation (MD) holds promise for cost-effectively concentrating wastewater toward zero liquid discharge (ZLD), while membrane fouling problems still hinder its practical application, especially for high-strength wastewater containing complex constituents. This study investigated the effects of different foulant components in actual landfill leachate (LFL) on MD deep concentration performances. The fouling behaviors of LFL components were revealed by comprehensively characterizing and analyzing the physicochemical properties of the fouling layers and membrane-foulant interactions. Results illustrated that suspended solids (SS) and macromolecular organics (MO) played significant yet distinct roles in fouling layer formation. SS (>0.45 μm, ∼24 % TOC) possessed strong adhesion energies with membrane surfaces and constructed the main skeleton of the fouling layer at the preliminary concentration stage; while MO (>20 kDa, 0.36 % TOC) deposition resulted in a densely packed fouling layer, influencing the salt crystallization potential and interfering with water and ammonia transport. In contrast, the low-molecular organics with the highest content (<20 kDa, ∼75 % TOC) were mainly electron-donor monopolar substances weakly adsorbed on the nonpolar membrane surface. Removing SS and MO from LFL notably prevented fouling layer formation and maintained high membrane efficacies (water flux decline mitigated from ∼50 % to ∼15 % and ammonia rejection increased from 71 % to 88 %), even as water recovery increased to 95 %. The findings of this work imply conventional anti-fouling strategies (e.g., complete removal or degradation of organic components) may have gone beyond what is necessary, and will inspire the development of simpler and greener methods to maintain membrane efficacy during high-strength wastewater treatment toward ZLD.

Eliminating adverse effects of fouling layer using composite membranes with dense surface layers for sustainable membrane distillation of high-strength wastewater

This study explored composite membranes (CMs) with dense and water-permeable surface layers for the deep-concentration of high-strength wastewater using membrane distillation (MD). Compared to conventional microporous MD membranes, coating a dense PVA surface layer avoided the detrimental effects of landfill leachate (LFL) foulants (e.g., membrane pore blockage and deformation) during MD concentration. The hydrophilicity and water/salt separation capability of surface layers enhanced the dispersion of water diffusion on membrane surfaces and altered the solute distribution on membrane cross-sections, resulting in inorganic crystallization mitigation and transforming the fouling layer structure to a more porous and water-permeable state. This ensured the membrane efficiency was little affected by membrane fouling even as LFL was highly concentrated (water flux decline: PVDF ∼49 % vs. CM ∼14 % as concentration factor increased to 20). The resistance of foulant intrusion and in-pore deposition, which easily occurred on microporous membranes, and the decrease in membrane-foulant interactions by the surface layer endowed the CM with a high foulant-release capability (clean efficiency: ∼97 %). The surface layer also exhibited an ammonia/water sieving effect which enhanced volatile ammonia rejection (∼1.6 times). This work provides insight into the construction of high-sustainable MD membranes for high-strength wastewater treatment, promoting the practical implementation of MD.

Engineering nanofiltration membranes based on a two-step modification process with enhanced antifouling and salt separation performance

Nanofiltration (NF) membranes with impenetrable hydration layers and hydrophobic inserts can integrate multiple antifouling properties. This work used a two-step modification process to construct NF membranes with hydrophilic surfaces and low-surface energy inserts innovatively. The construction of antifouling surface consisted of two steps, including sequential grafting of triethanolamine (TEOA) and perfluorooctyltrimethoxysilane (POTS). Due to modification process by adjusting the pore size distribution and reducing the membrane surface negative charge, the Cl-/SO42- separation selectivity (α) of the modified membrane P-T-PA/PSF reached 49.53, which was 2.31 times higher than the virgin membrane. During the filtration of multiple modelled contaminants, irreversible flux decreases were reduced, and flux recovery after pure water backwashing was above 95% in all cases. Operational stability tests confirmed that P-T-PA/PSF membrane has superior pH stability, chlorine resistance, stability under high contaminant loading, and recoverability. Interfacial thermodynamic analysis quantitatively described the antifouling mechanism that increases the energy barrier of membrane-foulant interaction. This work lays the foundation for preparing NF membranes with enhanced antifouling and salt separation.

Efficacies and mechanisms of different cleaning strategies for NF and RO membranes in a full-scale zero liquid discharge system

Membrane fouling conditions can be distinct for multi-step nanofiltration (NF) and reverse osmosis (RO) membrane processes in zero liquid discharge (ZLD) system. Customizing cleaning procedures according to fouling characteristics is imperative to improve the overall cleaning efficiency of the system. Herein, we performed four different “alkaline + acid” cleaning procedures, including alkaline solution (NaOH), acid solution (citric acid), metal chelating agents (Na2EDTA), and surfactants (SDS), on five fouled NF and RO membranes from a full-scale ZLD system for cold-rolling wastewater brine treatment. The“alkaline + acid” cleaning procedures were effective for organic and inorganic combined fouling removal in two-stage NF process, but failed to remove SiAl complex scaling completely for the 2nd RO process. Due to the slight membrane fouling, both performance and surface properties of 1st and 3rd pass RO membranes did not change significantly after the cleaning. The high pH-induced solubilization were the major mechanism for the removal of the SiAl complex. Na2EDTA/SDS addition caused the decrease of foulant-foulant and foulant-membrane interactions. Customizing membrane cleaning procedures based on membrane fouling characteristics can facilitate the stable operation of hybrid membrane system for the practical industrial wastewater ZLD treatment.

Designing anti(-bio)fouling membranes with synergistic grafting of quaternized and zwitterionic polymers through surface initiated atom transfer radical polymerization

Membrane-based filtration technologies have significant potential to address the global problem of freshwater scarcity. However, fouling, caused by nonspecific adsorption of foulants and microbiological growth during filtration, remains a considerable concern. To address this, a rational surface design method comprising bifunctional coating with both defensive and attacking techniques has been devised, which can successfully decrease fouling and biofouling. To generate a bifunctional membrane, a single-step surface-initiated atom-transfer radical polymerization (SI-ATRP) of monomers with zwitterionic and quaternized imidazolium moieties was done on a polydopamine (PDA) primed polyvinylidene fluoride (PVDF) membrane. The PDA layer’s catechol functionalities provided abundant anchoring sites, allowing the grafting of a dense polymer layer on the membrane. The addition of active zwitterionic and quaternized imidazolium moieties in the grafted polymer chains resulted in very low fouling and microbiological species death. Extensive antifouling studies revealed that the functionalized membranes have excellent flux recovery (>93%), low irreversible fouling (<8.5%), and long-term filtration stability. This surface functionalization strategy based on a bifunctional unit represents a significant advancement in fouling mitigation by minimizing membrane-foulant interactions and killing adhered bacteria, with the potential to significantly broaden the range of membrane-based filtration applications and contribute to more efficient and sustainable water purification processes.

Surfactant pretreatment as a novel fouling mitigation strategy for membrane distillation

Membrane distillation (MD) has emerged as a promising thermal desalination technology for hypersaline wastewater reclamation. However, the practical implementations of MD face a critical challenge of membrane fouling. Conventional strategy to address membrane fouling is to emphasize the progress in developing composite MD membranes, but these membranes are commercially unavailable. In this study, we introduce a new operation strategy to mitigate fouling by using surfactant pretreatment. We demonstrate that after being pretreated by 0.05 mM Triton X-100 feed solution for 10 min, a commercial polyvinylidene fluoride (PVDF) membrane can exhibit outstanding fouling resistance in MD process. Further, we elucidate the underlying working mechanism using quartz crystal microbalance (QCM) experiments, oil contact angle measurements, and force spectroscopy determinations. Specifically, in the pretreatment process, the amphiphilic surfactant adsorbs onto the hydrophobic membrane, altering the membrane surface to be hydrophilic. During the MD operations, a hydration layer forms on the hydrophilic surface, eliminating the foulant-membrane interaction and thereby achieving fouling mitigation. For effective fouling mitigation, a surfactant exhibiting a moderate hydrophilic-lipophilic balance (HLB) value (e.g., Triton X-100 with an HLB of ∼14) is required. As an easy-to-implement but effective strategy to mitigate membrane fouling, surfactant pretreatment can substantially improve the fouling resistances of commercial hydrophobic membranes, greatly facilitating the practical applications of MD and membrane contactor technologies.

Bioinspired N-Oxide-Based Zwitterionic Polymer Brushes for Robust Fouling-Resistant Surfaces

Fouling-resistant surfaces are needed for various environmental applications. Inspired by superhydrophilic N-oxide-based osmolytes in saltwater fish, we demonstrate the use of a trimethylamine N-oxide (TMAO) analogue for constructing fouling-resistant surfaces. The readily synthesized N-oxide monomer of methacrylamide is grafted to filtration membrane surfaces by surface-initiated atom transfer radical polymerization (SI-ATRP). Successful grafting of the amine N-oxide brush layer as confirmed by material characterization endows the surface with increased hydrophilicity, reduced charge, and decreased roughness. Notably, the introduction of the N-oxide layer does not compromise transport properties, i.e., water permeability and water-salt selectivity. Moreover, the modified membrane exhibits improved antifouling properties with a lower flux decline (32.1%) and greater fouling reversibility (18.55%) than the control sample (45.4% flux decline and 3.26% fouling reversibility). We further evaluate foulant-membrane interaction using surface plasmon resonance (SPR) to relate the reduced fouling tendency to the synergic effects of surface characteristic changes after amine N-oxide modification. Our results demonstrate the promise and potential of the N-oxide-based polymer brushes for the design of fouling resistance surfaces for a variety of emerging environmental applications.

Photothermally-activated peroxymonosulfate (PMS) pretreatment for fouling alleviation of membrane distillation of surface water: Performance and mechanism

Membrane distillation (MD) has demonstrated its efficacy for seawater desalination, which prompted researchers to investigate its performance for the treatment of surface water containing natural organic matter (NOM). However, membrane fouling remains a significant technological constraint in the acceptability and implementation of MD modules. Herein, for the first time, a photothermally-activated PMS pre-treatment process is coupled to a direct contact (DC)-MD module for effective membrane fouling control. Photothermally-activated PMS (2 mM) pre-treatment achieved 69 % of UV254 removal and 30.9 % of dissolved organic carbon removal and allowed stable operation of the DCMD without any reduction in water permeance vs 50 % reduction obtained without the pretreatment step. The analysis of NOM fractions after pre-treatment using Parallel Factor Analysis and molecular weight distribution indicated changes in NOM composition, which facilitated membrane fouling control. The photothermally-activated PMS pre-treatment seems to alleviate membrane fouling by limiting the foulant-membrane interaction, which was observed to result in the formation of the fouling layer containing aromatic proteins, microbial byproducts-like substances, and inorganic (e.g., calcium) foulants. Finally, characterization of the used MD membranes revealed that the MD membrane properties could be kept unchanged by the photothermally-activated PMS pre-treatment step, thereby making the integrated DCMD system sustainable and attractive for widespread applications.

How and why does time matter - A comparison of fouling caused by organic substances on membranes over adsorption durations

This study investigated the effect of time on the severity of adsorptive fouling on polyvinylidene fluoride (PVDF) membrane surface. Sodium alginate (SA), bovine serum albumin (BSA), and humic acid (HA) were selected as representative membrane foulants. We examined the fouling behavior of these three selected model foulants over different adsorption durations (i.e., ~2300 and ~20,000 s). The fouling experiments were performed under conditions with and without the presence of Ca2+. For the SA-Ca2+ system, a longer adsorption duration slightly increased adsorption amount of SA but sharply reduced the reversibility (from 86.8 % to 12.9 %). For BSA-Ca2+, extended time did not change the deposition amount of BSA on the membrane surface, but led to more residual BSA after cleaning (reversibility decreased from 11.3 % to 4.5 %). Similarly, in the HA-Ca2+ system, adsorption duration barely influenced the adsorption amount of HA, while reduced its reversibility from 39.4 to 32.2 %. Therefore, time duration significantly influenced the amount and reversibility of membrane fouling depending on their chemical property. Corresponding results can be well reflected by a selected mathematical model. Further investigation on relevant mechanisms was conducted, quartz crystal microbalance with dissipation (QCM-D) and atomic force microscope (AFM) measurements indicated that longer adsorption duration resulted in more compacted fouling layer and stronger foulant-membrane interaction force. Our results suggest that time (adsorption duration) plays an important role in determining the reversibility of membrane fouling, while the severity is related to the inherent characteristics of foulants.

Choice of DLVO approximation method for quantifying the affinity between latex particles and membranes

The Derjaguin-Landau-Verwey-Overbeek (DLVO) model, as well as the extended model (XDLVO), is popularly employed to quantify the interfacial interactions underlying membrane fouling. However, disagreements between membrane fouling extent and the interaction energies derived from DLVO or XDLVO models have been reported, which suggests gaps in the understanding. This study demonstrates that the DLVO approximation methods for predicting the interfacial foulant-membrane interaction are sensitive to the boundary condition assumptions (e.g., constant charge versus constant potential). In particular, while both the Poisson-Boltzmann (P–B) and linear superposition approximation (LSA) equations can quantify the electrostatics (EL) interaction energy component, the former assumes constant potential, while the latter additionally considers constant charge scenarios. The relative accuracy of these two equations were evaluated here. For dead-end filtration tests, flux decline trends, OCT analysis results and fouling model parameters were obtained. Regarding fouling of pristine PCTE membrane by latex particles of opposite charge signs, both the P–B and LSA equations contribute to predict relative fouling extents correctly. However, for the case of the BPEI-coated membrane, the P–B equation failed whereas the LSA equation gave good agreements. For cross-flow filtration tests in organic solvents, LSA also out-performed the P–B equation in providing more accurate predictions of membrane fouling. The results here highlight the shortcomings in the commonly used P–B equation and are expected to be potentially valuable in the development of better approximations for quantifying interfacial interaction energies.

Porous substrate affects fouling propensity of thin-film composite nanofiltration membranes

Fouling is a critical consideration for the design of thin-film composite (TFC) nanofiltration membranes. Traditional wisdom believes that fouling propensity is primarily dictated by membrane surface properties while porous substrates play little role (on the basis on the latter have no effect on the foulant-membrane interaction). Nevertheless, porous substrates can regulate the water transport pathways, resulting in uneven water flux distribution over the membrane surface. For the first time, we experimentally investigated the micro-scale water flux distribution for nanofiltration membranes with different substrate porosities and the impact of such flux distribution pattern on fouling. With gold nanoparticles as tracers, we demonstrated more evenly distributed water flux at increasing substrate porosity. This was found to effectively alleviate membrane fouling by eliminating localized hot spots of high flux. Furthermore, higher substrate porosity also effectively enhanced the membrane water permeance due to the optimized water transport pathways. Our study reveals the fundamental relationship between the micro-scale transport behavior and the membrane fouling propensity, which provides a firm basis for the rational design of TFC membranes toward better separation performance.

Effect of operation mode on membrane fouling from traditional Chinese medicine water extracts

Membrane technology is a viable alternative to many conventional methods currently used to process traditional Chinese medicine (TCM). However, membrane fouling is a significant obstacle to the application of membrane technology in TCM. In this study, the ultrafiltration (UF) membrane fouling from TCM water extracts under constant transmembrane pressure (TMP) and constant flux operation was studied in connection with critical and threshold flux determinations. Meanwhile, the physicochemical foulant–membrane interactions were analyzed using the extended Derjaguin, Landau, Verwey, and Overbeek (XDLVO) theory. At constant TMP operation, the high pressure resulted in a rapid drop in permeate flux at the start of the filtration period, but the steady-state fluxes did not increase significantly with an increase in TMP. At constant flux operation, the TMP increased somewhat when the permeate flux was below the threshold flux. Fouling was severe above the threshold flux, and the TMP rapidly increased. By displaying fouling resistance as a function of permeate volume, the constant flux operation was compared to a constant TMP operation beginning at the same flux imposed during the constant flux operation. Below the threshold flux, fouling resistance was similar for constant TMP and constant flux operations. However, above the threshold flux, fouling resistance under constant flux operation was much greater than fouling resistance under constant TMP operation. The results of physicochemical interactions showed that there are attractive interactions between hydrophobic membrane and foulants in the TCM water extracts. These interactions should be considered a key factor leading to serious membrane fouling.

Mechanistic Insights of a Thermoresponsive Interface for Fouling Control of Thin-Film Composite Nanofiltration Membranes.

In spite of extensive research, fouling is still the main challenge for nanofiltration membranes, generating an extra transport resistance and requiring a larger operational pressure in practical applications. We fabricated a highly antifouling nanofiltration membrane by grafting poly(N-isopropylacrylamide) (PNIPAM) chains on a bromine-containing polyamide layer. The resulting membrane was found to have a double permeance compared to the pristine membrane, while the rejection of multivalent ions remained the same. In addition, PNIPAM chains yielded a better deposition resistance and adhesion resistance, thereby mitigating the increase of fouling and promoting the recovery of flux during the filtration and traditional cleaning stages, respectively. Moreover, PNIPAM chains shrank when the water temperature was above the lower critical solution temperature (LCST), indicating the formation of a buffer layer between the membrane and pollutants. The buffer layer would eliminate the membrane-foulant interaction energy, thus further enhancing the detachment of pollutants. This simple and efficient cleaning method could act as an enhanced cleaning procedure to remove irreversible fouling. This provides new insights into the fabrication of enhanced antifouling membranes using smart responsive polymer chains.

Assessing the role of cold temperatures on irreversible membrane permeability of tertiary ultrafiltration treating municipal wastewater

A comprehensive assessment of the effects of temperature on hydraulically irreversible membrane permeability in tertiary ultrafiltration of municipal wastewater was conducted through bench scale testing. The influence of temperature was differentiated into impacts on SBR effluent characteristics, water rheological properties, and intrinsic membrane properties, which all subsequently affected the development of hydraulically irreversible permeability. When filtration tests were conducted at 20 °C with the effluents from sequencing batch reactors (SBRs) that were operated at 8, 14 and 20 °C, hydraulically irreversible permeability declined by 38%, 22% and 17%, respectively, during the steady period. The greater decline was attributed to higher generation of effluent organic matter (EfOM) at low SBR operating temperatures. When filtration tests were conducted at the corresponding SBR operating temperature, the hydraulically irreversible permeability with the effluents from SBRs operated at 14 and 8 °C further decreased. The water viscosity and intrinsic membrane resistance were observed to be responsible for 20–29% of the total decrease in hydraulically irreversible permeability. These declines established the upper limit to which fouling mitigation strategies could enhance hydraulically irreversible permeabilities at low temperature. The hydraulically irreversible permeability decline with low filtration temperature beyond that associated with changes in viscosity and intrinsic membrane resistance was attributed to narrowed membrane pores that retained additional EfOM fractions and modified membrane-foulant interactions. The novel approach in differentiating the effects of temperature on secondary treatment and filtration processes developed in the present study provides insights into hydraulically irreversible fouling development and the results from the present study can be employed to understand the limits of tertiary fouling control under challenging conditions.

Dissect the role of particle size through collision-attachment simulations for colloidal fouling of RO/NF membranes

Colloidal size affects the whole process of particle transport and membrane filtration. However, its compound effect on fouling remains controversial. In the present study, we adopt a collision-attachment approach to systematically investigate the role of colloidal size on fouling. Our study highlights the critical importance of four competing mechanisms: reduced specific cake resistance and enhanced foulant-membrane interaction of larger particles tend to mitigate flux decline, while the simultaneously increased hydrodynamic drag and reduced particle back-transport tend to promote fouling. The net effect of particle size on fouling is governed by the competition among these mechanisms. When strong foulant-membrane repulsion prevails, we show enhanced flux stability for larger particles as a result of a greatly increased energy barrier to resist particle deposition. Nevertheless, this trend could be reversed for weak foulant-membrane interaction. Our study reconciles the contradictory experimental observations of the effect of particle size on colloidal fouling and provide important insights for effective fouling mitigation.

Electrically conductive hydrophobic membrane cathode for membrane distillation with super anti-oil-fouling capability: Performance and mechanism

This study mainly focused on enhancing oil-membrane electrostatic repulsion towards anti-oil-fouling via electrically conductive hydrophobic membrane in electricity-assisted membrane distillation (MD). Carbon nanotubes (CNTs) were coated on commercial membranes to fabricate membrane cathode. For concentrated hexadecane-in-saline water emulsions, the modified membrane exhibited significantly less flux decline, < 5% in dealing with extreme high-concentration oil emulsion (2000 ppm) at cell potential of 3.0 V. The anti-oil-fouling robustness was further confirmed over a continuous three-cycle operation with in-situ DI rinse. The anti-fouling mechanism was systematically discussed regarding the hydrophilicity of the membrane interface, charge repulsion between the oil and membrane cathode as well as slippery property at the liquid-gas-solid triple-phase interface. A modeled fouling rate constant was negatively associated with the calculated capacitive surface charge of the membrane cathode. Thermodynamics analysis suggested enhanced foulant-membrane electrostatic repulsion leads to a significant energy barrier which favored anti-fouling performance. Interestingly, the sliding dynamics of oil droplet along the interface of membrane cathode was found, tuned by the herein weak cell potential, which could also contribute to the fouling mitigation. Our results insight that the electrically conductive membrane cathode could modulate the foulant-membrane interaction which plays an important role in mitigating fouling/wetting occurred at the triple-phase interface.

The synergistic fouling of ceramic membranes by particles and natural organic matter fractions using different surface waters in South Africa

This study demonstrates the fundamental differences in fouling development and mechanisms of unfiltered and 0.45 µm pre-filtered water samples on ceramic membranes. Robust characterization of the feed waters was conducted using gravimetric analysis, optical methods and modeling techniques. UV254 removal and suspended solids (SS) for the unfiltered samples presented a strong correlation (R2 = 0.87). Further, SS exhibited strong correlations with fluorescent fractions (R2 = 0.82; 0.81 and 0.74 for C1; C2 and C3, respectively). This observation confirmed the significance of inorganic particles in the development of a combined fouling layer with fluorescent organic components. The fouling development rate for water sampled from Plattenburg Bay (PL) was higher than the rest of the 0.45 µm pre-filtered samples. This was attributed to the low conductivity (175 µS.m-1) of the water sample, translating to a low ionic strength environment. Samples collected from Hermanus River (HL) and Lepelle River (OL) had similar SS quantity (87.6 mg/L and 88.4 mg/L, respectively), and modified fouling index (MFI) values for raw samples were 6625 and 8060 s/L2 , respectively, despite a very large difference in the content of organic matter (22.67 mg/L.C and 9.81 mg/L.C). This could be due to organic matter attaching onto the surface of particles and reducing the adsorption of NOM within membrane pores and/or onto the membrane active layer. This study demonstrated the extent of in situ background electrolytes, foulant concentration, foulant-foulant interactions, foulant-membrane interaction and physicochemical properties of feed stream on fouling development and mechanisms.

Modelling the critical roles of zeta potential and contact angle on colloidal fouling with a coupled XDLVO - collision attachment approach

A coupled XDLVO-collision attachment (CA) model is proposed to quantitatively evaluate the critical roles of zeta potential ζ and contact angle θ of foulant on colloidal fouling. In this model, the XDLVO theory is applied to relate ζ and θ to foulant-membrane interaction energy barrier (ΔEb). The latter is further bridged to membrane fouling rate (dmf/dt) by the CA theory, recognizing fouling as foulant colliding with a membrane surface followed by its attachment. XDLVO-CA model predictions agree well with the experimental water flux declines under various solution pHs. Modelling results show that severe fouling occurs at low ζ2, and increasing ζ2 can substantially lower dmf/dt as a result of the increased ΔEb. Interestingly, a negative linear relationship is observed between ζ2 and logarithm of dmf/dt at high ζ2. For the role of contact angle for water, glycerol and diiodomethane, there exists a critical θ, below (or above) which little fouling occurs owing to the dramatically increased ΔEb and thus decreased dmf/dt by orders of magnitude. Furthermore, the threshold θ corresponds to the acid-base interaction energy transiting from attraction to repulsion. Our modelling results reveal the importance of optimally tailoring foulant ζ and θ for fouling control.

Nanofiltration fouling behaviors with different membrane materials induced by residual natural organics left over after ultrafiltration unit encountered with divalent cations

It is of practical significance to explore the fouling mechanism of nanofiltration membrane caused by residual natural organics left over after ultrafiltration unit. This paper systematically investigates nanofiltration membrane fouling behaviors induced by preserved humic acid components after ultrafiltration filtration (PHACUF) in the existence of Mg2+/Ca2+ ion from micro- and macro-level analyses. Those similarities and disparities in the roles of polyethersulfone or polyamide active layer to these issues were also studied comparatively. The results showed that there existed unneglectable standard blocking for polyethersulfone membrane when permeating bare PHACUF solution. However, with the addition of divalent cations, the main fouling mechanism changed to intermediate blocking. Fewer Mg2+ ions could fully link with the carboxylic groups of PHACUF molecules, leading to larger and more dense flocs by the “bridging” effect and the aggravated nanofiltration membrane fouling with more compact layer structure, especially for more hydrophobic polyethersulfone membrane. It could be inferred that in Mg2+/Ca2+ environment, actually, PHACUF-caused nanofiltration membrane fouling was mainly formed and constituted by hydrophobic acids and neutral fractions with much higher both hydrophobicity and content percentage, whether for the membrane-foulant interaction in the early stage or the foulant-foulant interaction in subsequent permeation stage.