Modeling Dynamics of Colloidal Fouling of RO/NF Membranes with A Novel Collision-Attachment Approach

Enhanced properties of a positive-charged nanofiltration membrane containing quaternarized chitosan through second interfacial polymerization for the removal of salts and pharmaceuticals

Nanofiltration (NF) membrane technology has been widely used in the removal of salts and trace organic pollutants, such as pharmaceuticals and personal care products (PPCPs), due to its superiority. A positive-charged composite NF membrane with an active skin layer was prepared by polyethyleneimine (PEI), trimethyl benzene chloride, and quaternate chitosan (HTCC) through second interfacial polymerization on the polyethersulfone ultrafiltration membrane. The physicochemical properties of the nanocomposite membrane were investigated using surface morphology, hydrophilicity, surface charge, and molecular weight cut-off (MWCO). The influence of the concentration and reaction time of PEI and HTCC was documented. The optimized membrane had a MWCO of about 481 Da and possessed a pure water permeability of 25.37 L·m-2·h-1·MPa-1. The results also exhibited salt rejection ability as MgCl2 > CaCl2 > MgSO4 > Na2SO4 > NaCl > KCl, showing a positive charge on the fabricated membrane. In addition, the membrane had higher rejection to atenolol, carbamazepine, amlodipine, and ibuprofen at 89.46, 86.02, 90.12, and 77.21%, respectively. Moreover, the anti-fouling performance and stability of the NF membrane were also improved.

Collision-attachment simulation of membrane fouling by oppositely and similarly charged colloids

The fouling propensity of oppositely charged colloids (OCC) and similarly charged colloids (SCC) on reverse osmosis (RO) and nanofiltration (NF) membranes are systematically investigated using a developed collision-attachment approach. The probability of successful colloidal attachment (i.e., attachment efficiency) is modelled by Boltzmann energy distribution, which captures the critical roles of colloid-colloid/membrane interaction and permeate drag. Our simulations highlight the important effects of ionic strength Is, colloidal size dp and initial flux J0 on combined fouling. In a moderate condition (e.g., Is =10 mM, dp=50 nm and J0= 100 L/m2h), OCC mixtures shows more severe fouling compared to the respective single foulant owing to electrostatic neutralization. In contrast, the flux loss of SCC species falls between those of the two single foulants but more closely resembles that of the single low-charged colloids due to its weak electrostatic repulsion. Increased ionic strength Is leads to less severe fouling for OCC but more severe fouling for SCC, as a result of the suppressed electrostatic attraction/repulsion. At a high Is (e.g., 3-5 M), all the single and mixed systems show the identical pseudo-stable flux Js. Small colloidal size leads to the drag-controlled condition, where severe fouling occurs for both single and mixed foulants. On the contrary, better flux stability appears at greater dp for both individual and mixed species, thanks to the increasingly dominated role of energy barrier and thus lowered attachment efficiency. Furthermore, higher J0 above limiting flux exerts greater permeate drag, leading to elevated attachment efficiency, and thus more flux losses for both OCC and SCC. Our modelling gains deep insights into the role of energy barrier, permeate drag, and attachment efficiency in governing combined fouling, which provides crucial guidelines for fouling reduction in practical engineering.

Relating critical and limiting fluxes to metastable and long-term stable fluxes in colloidal membrane filtration through collision-attachment theory

In membrane technology for water/wastewater treatment, the concepts of critical flux (J_C) and limiting flux (J_L) suggest the existence of a threshold flux below which no fouling occurs. However, their important roles on stable flux duration have not been sufficiently understood. This work adopts a collision-attachment approach to clarify the relationship of J_C, J_L to metastable (i.e., short-term stable) and long-term stable fluxes based on their dependence on initial flux (J₀), foulant-clean-membrane energy barrier (E_f-m), and foulant-fouled-membrane energy barrier (E_f-f). When J₀ is below J_L, water flux remains stable over a long time even for the case of J₀ over J_C, thanks to the strongly repulsive E_f-f. At J₀ > J_L and J₀ > J_C, the water flux is unstable at the beginning of filtration, and the flux ultimately decreases to J_L as the long-term stable flux. Under the condition of J_L < J₀ ≤ J_C, an initial metastable flux appears owing to the high E_f-m, with longer metastable period observed at lower J₀ and for more hydrophilic/charged membrane or colloids. Nevertheless, rapid flux decline occurs subsequently due to the energy barrier shifting to weak E_f-f, and the water flux eventually degenerates to J_L in long-term fouling duration. Our results provide significant guidelines for fouling control strategies with respect to membrane design, feedwater pretreatment, and operational optimization.

A new insight into the low membrane fouling tendency of liquid-liquid hollow fiber membrane contactor capturing ammonia from human urine

To unravel the low membrane fouling tendency and underlying membrane fouling mechanism of liquid-liquid hollow fiber membrane contactor (LL-HFMC) capturing ammonia from human urine, the ammonia flux decline trend, membrane fouling propensity, foulant-membrane thermodynamic interaction energy and microscale force analysis at different feed urine pH were comprehensively investigated. The 21-d continuous experiments showed that the ammonia flux decline trend and membrane fouling propensity significantly strengthened with the decrease of feed urine pH. The calculated foulant-membrane thermodynamic interaction energy decreased with the decreasing feed urine pH and agreed with the ammonia flux decline trend and membrane fouling propensity. The microscale force analysis showed that the absence of hydrodynamic water permeate drag force resulted in the foulant located at long distance from the membrane were difficult to approach the membrane surface, thus considerably alleviating membrane fouling. Additionally, the vital thermodynamic attractive force near the membrane surface increased with the decrease of feed urine pH, which made the membrane fouling further relieved at high pH condition. Therefore, the absence of water permeate drag force and operating at high pH condition minimized the membrane fouling during the LL-HFMC ammonia capture process. The obtained results provide a new insight into the low membrane tendency mechanism of LL-HFMC.

A facile green "wastes-treat-wastes" strategy: Electrogenerated chloramines for nanofiltration concentrate recirculation

Use of nanofiltration (NF) membrane to reuse the secondary wastewater suffers from severer biofouling and refractory concentrate. To realize sustainable NF membrane processes in water purification, the electro-oxidation (EO) process using boron-doped diamond (BDD) anodes was applied in current study to treat the NF concentrate for removal of organic contaminates and nutrients using simultaneously controllable in-situ generation of chloramines. The electrolytic effluent would be mixed with the raw secondary wastewater as the feed of subsequent NF process for conducting chloramination to mitigate membrane biofouling. It was found that under a constant current density of 30 mA/cm², the chloramine formed with the electrolysis while its concentration reached the maximum at 30 min of electrolysis when NH₃-N was 7 mg/L and Cl^- concentration was below 500 mg/L. The complete elimination of antibiotics and bacteria can be attained in the hybrid NF-EO process thanks to the in-situ simultaneous generation of large amount of chloramine. In particular, the membrane biofouling was effectively alleviated to maintain a stable permeate flux during the 160-h period of sustainable operation. Our study provides a promising "wastes-treat-wastes" strategy for sustainable reuse of secondary wastewater.

Nanofiltration Membranes with Crumpled Polyamide Films: A Critical Review on Mechanisms, Performances, and Environmental Applications

Nanofiltration (NF) membranes have been widely applied in many important environmental applications, including water softening, surface/groundwater purification, wastewater treatment, and water reuse. In recent years, a new class of piperazine (PIP)-based NF membranes featuring a crumpled polyamide layer has received considerable attention because of their great potential for achieving dramatic improvements in membrane separation performance. Since the report of novel crumpled Turing structures that exhibited an order of magnitude enhancement in water permeance ( Science 2018, 360 (6388), 518-521), the number of published research papers on this emerging topic has grown exponentially to approximately 200. In this critical review, we provide a systematic framework to classify the crumpled NF morphologies. The fundamental mechanisms and fabrication methods involved in the formation of these crumpled morphologies are summarized. We then discuss the transport of water and solutes in crumpled NF membranes and how these transport phenomena could simultaneously improve membrane water permeance, selectivity, and antifouling performance. The environmental applications of these emerging NF membranes are highlighted, and future research opportunities/needs are identified. The fundamental insights in this review provide critical guidance on the further development of high-performance NF membranes tailored for a wide range of environmental applications.

A unified thermodynamic fouling mechanism based on forward osmosis membrane unique properties: An asymmetric structure and reverse solute diffusion

Fouling mechanism of the forward osmosis membrane, which was peculiarly featured by the asymmetric membrane structure and reverse solute diffusion, was investigated at the molecular level and from the energy perspective. Two noteworthy fouling behaviors were observed in batch fouling tests conducted in AL-FS mode (active layer facing feed solution) and AL-DS mode (active layer facing draw solution) after filtering foulants with identical volume: 1) after filtering 100 mL of foulants, the flux decline rate in AL-DS mode was 1.78 times faster than that in AL-FS mode, but the flux decline behaviors of the two modes were similar in the subsequent filtration stages; 2) although the foulant layer weight of the same mode increased linearly in middle and late stages, the flux loss rate was distinctly different. Thermodynamic analysis indicated that the attractive interaction energy between the foulants and the support layer was about 5 times higher than that between the foulants and the active layer, well interpreting the higher flux decline rate of AL-DS mode in initial stage. Meanwhile, a non-invasive microscope observed that the structure of the fouling layer remarkably changed from loose to dense in the middle stage, and stabilized in the late stage. Furthermore, quantum chemistry calculation proved that the reverse diffusion of NaCl brought alginate molecular chains closer, whereas the distance between them tended to be constant as the continuous increase of NaCl. Based on these findings, the thermodynamic fouling mechanism proposed by combining the structure change process of the fouling layer with Flory-Huggins lattice theory satisfactorily interpreted the noteworthy fouling behaviors caused by reverse NaCl diffusion in middle and late stages. The revealed fouling mechanism unifies the adhesion and filtration behaviors related to the unique properties of FO membrane, deepening understanding of membrane fouling in the dynamic and complex ternary system of the FO process.

Insights into the roles of membrane pore size and feed foulant concentration in ultrafiltration membrane fouling based on collision-attachment theory

Membrane property and feed characteristics play critical roles in membrane fouling. This paper aims to clarify the roles of membrane pore size (φ) and feed foulant concentration (C_b ) in ultrafiltration fouling induced by polysaccharides. The fouling behaviors were expounded by collision-attachment theory, where the rate of membrane fouling is mainly determined by collision frequency (JC_b ) and attachment efficiency (γ). At the initial fouling stage, rapid flux decline was observed at large φ or high C_b due to the great JC_b and/or γ. At the later fouling stage, there existed a nearly identical maximum stable flux attributing to the same JC_b and γ, which was independent of φ and C_b . Moreover, the smaller φ can lead to less foulants passed through the membrane and thus more foulants attaching on the membrane, while the higher C_b can give rise to more foulants on both the membrane surface and in the permeate. The results presented in current study provide fundamental basis in understanding membrane fouling. PRACTITIONER POINTS: Collision-attachment theory was employed to expound the UF fouling behavior. Rapid flux decline occurred at large membrane pore size or high feed foulant concentration in the initial fouling stage. Membranes with different pore size or feed foulant concentration had an identical flux at the latter fouling stage. Lowering membrane pore size or increasing feed foulant concentration can lead to more foulants attaching on the membrane surface.

Stochastic Collision-Attachment-Based Monte Carlo Simulation of Colloidal Fouling: Transition from Foulant-Clean-Membrane Interaction to Foulant-Fouled-Membrane Interaction

The initial behavior of colloidal fouling is governed by foulant-clean-membrane interaction (F-M), and its long-term behavior is determined by foulant-fouled-membrane interaction (F-F). Nevertheless, the transitional fouling behavior from F-M to F-F has not been fully understood. This study reports a novel collision attachment (CA)-Monte Carlo (MC) approach, with the stochastic colloid-membrane collision events modeled by MC and the probability of colloidal attachment to the membrane determined by the interplay of flux and the energy barrier arising from colloid-membrane interaction (E_m for F-M and E_f for F-F). The long-term membrane flux remains stable for large E_f, whereas severe fouling occurs when both E_m and E_f are small. Our study reveals the existence of a metastable flux behavior for the combination of large E_m but small E_f. The time evolution of flux behavior and colloidal deposition patterns shows a nearly constant flux for an extended period, with the high energy barrier E_m retarding initial colloidal deposition. However, accidental random deposition of a colloidal particle could reduce the local energy barrier (toward the smaller E_f), seeding for further colloidal deposition in its vicinity. This initiates an uneven patch-wise fouling and eventually leads to a complete transition to F-F-dominated behavior. The metastable period can be effectively extended by increasing the energy barrier (E_m or E_f) or lowering flux, which provides important implications to membrane design and operation.

Intelligent mitigation of fouling by means of membrane vibration for algae separation: Dynamics model, comprehensive evaluation, and critical vibration frequency

Vibration membrane filtration has been confirmed as an effective method to improve algae separation from water. However, the fouling evolution process and the antifouling mechanism are not well understood. In this study, a novel hybrid method based on a dynamics model was proposed, a comprehensive evaluation was conducted, and the critical vibration frequency for accurate analysis and prediction of membrane fouling was developed. The dynamics model was studied with an improved collision-attachment model by considering all the concurrent and synergistic effects of the hydrodynamic interactions acting on algae. From the perspective of potential energy, the improved model systematically elucidated the reason why the antifouling performance was enhanced when the vibration frequency varied from 1 Hz to 5 Hz. In addition, the Technique for Order Preference by Similarity to Ideal Solution-grey relational analysis (TOPSIS-GRA) method with combined weights was incorporated for the first time to provide direct comprehensive evaluation evidence to determine the effect of the vibration frequency on membrane fouling. It was found that increasing the vibration frequency could not alleviate membrane fouling caused by extracellular organic matter. Moreover, the concept of a critical vibration frequency was proposed using genetic algorithm optimized back propagation neural network, and the energy consumption was analyzed. This combination could provide an effective means to choose the most appropriate vibration frequency, thereby improving the efficiency of the vibration membrane system in the algae separation process.

Analysis of organic foulants in the coagulation-microfiltration process for the treatment of Taihu Lake

This paper analysed organic foulants in the coagulation-microfiltration process for Taihu Lake treatment. High-performance size-exclusion chromatography (HPSEC) and fluorescence excitation-emission matrices (EEM) were applied to elucidate the influence of characteristics of organics on microfiltration (MF) membrane fouling. Results showed that coagulation pretreatment could extend the operation duration of MF based on the fact that pretreatment could effectively remove macromolecular substances as well as a portion of small molecular weight (MW) organics. The analysis of foulants indicated that organics of strong hydrophobic acids (SHA) and neutral hydrophilic (Neut) fractions (based on hydrophobicity) and medium and small MW components (based on MW distribution) contributed greatly to irreversible fouling. EEM fluorescence analysis of chemical solutions exhibited that aromatic proteins and soluble microbial products were mainly a response to irreversible fouling.

Understanding the Antifouling Mechanism of Zwitterionic Monomer-Grafted Polyvinylidene Difluoride Membranes: A Comparative Experimental and Molecular Dynamics Simulation Study

The antifouling process of the membrane is very vital for the highly efficient treatment of industrial wastewater, especially high salinity wastewater containing oil and other pollutants. In the present work, the dynamical antifouling mechanism is explored via molecular dynamics simulations, while the corresponding experiments about surface properties of the zwitterionic monomer-grafted polyvinylidene difluoride membrane are designed to verify the simulated mechanism. Water can form a stable hydration layer at the grafted membrane surface, where all the simulated radial distribution function of water/membrane, hydrogen bond number, water diffusivity, and experimental oil contact angles are stable. However, the water flux across the membrane will increase first and then decrease as the grafting ratio increases, which not only depends on the reduced pore size of the zwitterionic monomer-grafted membrane but also results from water diffusion. Furthermore, the dynamical fouling processes of pollutants (taking sodium alginate as an example) on the grafted membrane in water and brine solution are investigated, where both the high grafting ratio and electrolyte CaCl₂ can enhance the fouling energy barrier of the pollutant. The results show that both the enhanced hydrophilic property and the electrostatic repulsion can affect the antifouling capability of the grafted membrane. Finally, the ternary synergistic antifouling mechanisms among the zwitterionic membrane, electrolyte, and pollutant sodium alginates are discussed, which could be helpful for the rational design and preparation of new and highly efficient zwitterionic antifouling membranes.

Antifouling performance of polytetrafluoroethylene and polyvinylidene fluoride ultrafiltration membranes during alkali/surfactant/polymer flooding wastewater treatment: Distinctions and mechanisms

Alkali/surfactant/polymer (ASP) flooding wastewater is highly caustic, and membrane fouling is the main obstacle during ASP ultrafiltration (UF) treatment. To maintain favorable filtration performance, polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF) membranes were implemented here, and their antifouling properties and mechanisms were investigated based on the threshold flux theory. Compared with the PVDF membranes, the PTFE membranes exhibited superior antifouling properties with lower reductions in flux and smaller hydraulic resistance, and they presented a nearly identical pseudo-stable fouling rate at a later time point. In the fouling layers of the PTFE and PVDF membranes, anion polyacrylamide (APAM) was observed along with divalent/trivalent metal ions. The thermodynamic and molecular mechanisms of membrane fouling by APAM were elucidated using the Extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory and atomic force microscopy (AFM), respectively. The calculated total interfacial free energy (mJ/m²) of adhesion between the APAM and PTFE membranes was positive, and the value between the APAM and PVDF membranes was negative. Furthermore, the values and interaction distances of the measured intermolecular rupture and approaching forces were larger for APAM-PTFE than for APAM-PVDF. For the PTFE membranes, the positive free energies and smaller intermolecular interaction resulted in weaker APAM-PTFE adhesion and adsorption and therefore the lower levels of flux decline and the later achievement of the pseudo-stable fouling rate. Additionally, the total flux recoveries observed after physical cleaning reached 0.78-0.80 and 0.32-0.39 for the PTFE and PVDF membranes, respectively, which showed that the PTFE membranes can be cleaned easily. The PTFE membranes have considerable potential for extensive application in UF treatments for ASP wastewater. These results should promote understanding the essence of the threshold flux and the fouling control of UF membranes.