Deposition of powdered activated carbon (PAC) on ultrafiltration (UF) membrane surface: influencing factors and mechanisms

Hybrid Adsorption-Microfiltration Process for the Pretreatment of Sulfide-Containing Seawater: A Promising Strategy to Mitigate Membrane Fouling

The presence of dissolved sulfides in feed seawater causes severe elemental sulfur fouling in the reverse osmosis (RO) process. However, current pretreatment methods suffer from large footprint, high energy consumption, and limitations in effluent quality. In this study, adsorption and microfiltration are merged into a single process for the pretreatment of sulfide-containing seawater. Powdered activated carbon (PAC) was selected for its superior adsorption capacity (14.6-fold) and faster kinetics (3.9-fold) for sulfide removal compared to granular activated carbon. The high surface area and multiple pore structures of PAC facilitate surface and intraparticle diffusion, as well as anion-π conjugation likely occur between PAC and sulfide. Polypropylene microporous membranes, capable of tolerating high PAC dosages, were used in the hybrid process. Long-term pilot tests demonstrated that the effluent (turbidity < 1 NTU and SDI15 ≈ 2.50) met the quality requirements for RO unit feedwater, achieving 100% sulfide removal efficiency over 101 h, with no risk of PAC leakage throughout the entire operation process. The formation of a loose, porous PAC cake layer alleviates membrane fouling and enhances the retention and adsorption of metal(loid)s and sulfide. Moreover, the low permeate flux of the polymeric membranes significantly mitigates filter cake formation. The hybrid system adapts to variations in feedwater quality, making it highly suitable for desalination plants with limited space and budget. These findings offer valuable insights and practical guidance for advancing seawater desalination pretreatment.

Carbon-Based Nanocomposite Membranes for Membrane Distillation: Progress, Problems and Future Prospects

The development of an ideal membrane for membrane distillation (MD) is of the utmost importance. Enhancing the efficiency of MD by adding nanoparticles to or onto a membrane's surface has drawn considerable attention from the scientific community. It is crucial to thoroughly examine state-of-the-art nanomaterials-enabled MD membranes with desirable properties, as they greatly enhance the efficiency and reliability of the MD process. This, in turn, opens up opportunities for achieving a sustainable water-energy-environment nexus. By introducing carbon-based nanomaterials into the membrane's structure, the membrane gains excellent separation abilities, resistance to various feed waters, and a longer lifespan. Additionally, the use of carbon-based nanomaterials in MD has led to improved membrane performance characteristics such as increased permeability and a reduced fouling propensity. These nanomaterials have also enabled novel membrane capabilities like in situ foulant degradation and localized heat generation. Therefore, this review offers an overview of how the utilization of different carbon-based nanomaterials in membrane synthesis impacts the membrane characteristics, particularly the liquid entry pressure (LEP), hydrophobicity, porosity, and membrane permeability, as well as reduced fouling, thereby advancing the MD technology for water treatment processes. Furthermore, this review also discusses the development, challenges, and research opportunities that arise from these findings.

Removal of glyphosate (GLY) and aminomethylphosphonic acid (AMPA) by ultrafiltration with permeate-side polymer-based spherical activated carbon (UF-PBSAC)

Glyphosate (GLY) is the most commonly used herbicide worldwide, and aminomethylphosphonic acid (AMPA) is one of its main metabolites. GLY and AMPA are toxic to humans, and their complex physicochemical properties present challenges in their removal from water. Several technologies have been applied to remove GLY and AMPA such as adsorption, filtration, and degradation with varied efficiencies. In previous works, an ultrafiltration membrane with permeate-side polymer-based spherical activated carbon (UF-PBSAC) showed the feasibility of removing uncharged micropollutants via adsorption in a flow-through configuration. The same UF-PBSAC was investigated for GLY and AMPA adsorption to assess the removal of charged and lower molecular weight micropollutants. The results indicated that both surface area and hydraulic residence time were limiting factors in GLY/AMPA adsorption by UF-PBSAC. The higher external surface of PBSAC with strong affinity for GLY and AMPA showed higher removal in a dynamic process where the hydraulic residence time was short (tens of seconds). Extending hydraulic residence times (hundreds of seconds) resulted in higher GLY/AMPA removal by allowing GLY/AMPA to diffuse into the PBSAC pores and reach more surfaces. Enhancement was achieved by minimising both limiting factors (external surface and hydraulic residence time) with a low flux of 25 L/m2.h, increased PBSAC layer of 6 mm, and small PBSAC particle size of 78 µm. With this configuration, UF-PBSAC could remove 98 % of GLY and 95 % of AMPA from an initial concentration of 1000 ng/L at pH 8.2 ± 0.2 and meet European Union (EU) regulation for herbicides (100 ng/L for individuals and 500 ng/L for total herbicides). The results implied that UF-PBSAC was able to remove charged micropollutants to the required levels and had potential for application in wastewater treatment and water reuse.

Chemical-free vacuum ultraviolet irradiation as ultrafiltration membrane pretreatment technique: Performance, mechanisms and DBPs formation

Membrane fouling induced by natural organic matter (NOM) has seriously affected the further extensive application of ultrafiltration (UF). Herein, a simple, green and robust vacuum ultraviolet (VUV) technology was adopted as pretreatment before UF and ultraviolet (UV) technology was used for comparison. The results showed that control effect of VUV pretreatment on membrane fouling was better than that of UV pretreatment, as evidenced by the increase of normalized flux from 0.27 to 0.38 and 0.73 after 30 min UV or VUV pretreatment, respectively. This is related to the fact that VUV pretreatment exhibited stronger NOM degradation ability than UV pretreatment owing to the formation of HO•. The steady-state concentration of HO• was calculated as 3.04 × 10-13 M and the cumulative exposure of HO• reached 5.52 × 10-10 M s after 30 min of VUV irradiation. And the second-order rate constant between NOM and HO• was determined as 1.36 × 104 L mg-1 s-1. Furthermore, fluorescence EEM could be applied to predict membrane fouling induced by humic-enriched water. Standard blocking and cake filtration were major fouling mechanisms. Moreover, extension of UV pretreatment time increased the disinfection by-products (DBPs) formation, the DBPs concentration was enhanced from 322.36 to 1187.80 μg/L after 210 min pretreatment. However, VUV pretreatment for 150 min reduced DBPs content to 282.57 μg/L, and DBPs content continued to decrease with the extension of pretreatment time, revealing that VUV pretreatment achieved effective control of DBPs. The variation trend of cytotoxicity and health risk of DBPs was similar to that of DBPs concentration. In summary, VUV pretreatment exhibited excellent effect on membrane fouling alleviation, NOM degradation and DBPs control under a certain pretreatment time.

Role of inorganic foulants in the aging and deterioration of low-pressure membranes during the chemical cleaning process in surface water treatment: A review

Low-pressure membrane (LPM) filtration, including microfiltration (MF) and ultrafiltration (UF), is a promising technology for the treatment of surface water for drinking and other purposes. Various configurations and operational sequences have been developed to ensure the sustainable provision of clean water by overcoming fouling problems. In the literature, various periodic physical and/or chemical approaches to the cleaning of LPMs have been reported, but little data is available on the aging of MF/UF membranes that results from the interaction between the foulants and the cleaning agent. Periodic physical cleaning of the membrane is expected to return the membrane to its original performance capacity, but it only recovers to a certain level because the remaining foulants cause irreversible fouling. Chemical cleaning can then be employed to recover the membrane from this irreversible fouling but, in the process, it can cause irrecoverable damage to the membrane. In this review, the foulants responsible for irrecoverable damage to MF/UF membranes are summarized, and their interaction with cleaning agents and other foulants is described. The impact of these foulants on various membrane parameters, including filtration efficiency, flux decline, permeability, membrane characterization, and membrane integrity are also summarized and discussed in detail. In addition, mitigation options and future prospects are also discussed with regard to increasing the operational life span of a membrane in a cost-effective manner. Ultimately, this review suggests an advanced control system based on membrane-foulant interactions under the impact of various operational parameters to mitigate the integrity loss of membranes.

Evaluation of Organic and Inorganic Foulant Interaction Using Modified Fouling Models in Constant Flux Dead-End Operation with Microfiltration Membranes

The goal of this study was to elucidate the interaction of complex feed solutions under modified membrane fouling models for constant flux operation. The polyvinylidene fluoride membrane (PVDF) was tested for three types of solutions containing inorganic foulants (Al, Mn, and Fe), organic foulants, and suspended solids at 0.5 mM Ca2+ ionic strength. The membrane's performance was evaluated by measuring the increase in transmembrane pressure (TMP) during two different filtration scenarios: continuous filtration lasting 1 h and cyclic filtration lasting 12 min, with 3 min backwashing cycles included. Statistical analysis (linear regression results (R2), p-value) was used to verify the fouling model propagation along with the determination of the contributing constant of each fouling model. An increasing TMP percentage of 164-302%, 155-300%, and 208-378% for S1 (HA + Ca2+), S2 (inorganics + kaolin + Ca2+), and S3 (HA + inorganics + kaolin + Ca2+) was recorded for 1 h filtration, respectively. Furthermore, a five percent increase in irreversible resistance was noted for the S3 solution due to the strong adsorption potential of foulants for the PVDF membrane caused by the electrostatic and hydration forces of foulants. In addition to that, the participation equation elucidated the contribution of the fouling model and confirmed that complete blocking and cake layer contribution were dominant for the S1 and S3 solutions, while standard blocking was dominant for the S2 solution with a high significance ratio. Moreover, R2 and cyclic filtration analysis also confirmed the propagation of these fouling models. The statistical confirmation and regression results analysis of the modified model gave comparative results and satisfied the filtration mechanism and can be used for the constant flux dead filtration analysis of water treatment.

Controlling ultrafiltration membrane fouling in surface water treatment via combined pretreatment of O3 and PAC: Mechanism investigation on impacts of technological sequence

In this research, the effects of combined powdered activated carbon (PAC)-ozone (O3) pretreatment on ultrafiltration (UF) performance were comprehensively examined and compared with the conventional O3-PAC pretreatment. The performance of pretreatments on mitigating membrane fouling caused by Songhua River water (SHR) was evaluated by specific flux, membrane fouling resistance distribution, and membrane fouling index. Moreover, the degradation of natural organic matter in SHR was investigated by UV absorbance at 254 nm (UV254), dissolved organic carbon (DOC), and fluorescent organic matter. Results showed that the 100PAC-5O3 process was the most effective in improving the specific flux, with 82.89 % and 58.17 % reductions in the reversible fouling resistance and irreversible fouling resistance respectively. Additionally, the irreversible membrane fouling index was reduced by 20 % relative to 5O3-100PAC. The PAC-O3 process also exhibited superior performance in the degradation of UV254, DOC, three fluorescent components, and three micropollutants in the SHR system compared to O3-PAC pretreatment. The O3 stage played a major role in mitigating membrane fouling, while PAC pretreatment enhanced the oxidation in the subsequent O3 stage during the PAC-O3 process. Furthermore, the Extended Derjaguin-Landau-Verwey-Overbeek theory and pore blocking-cake layer filtration model fitting analysis were employed to explain the mechanisms of membrane fouling mitigation and fouling patterns transformation. It was found that PAC-O3 significantly increased the repulsive interactions between the foulants and the membrane, which restrained the formation of the cake layer filtration stage. Overall, this study evidenced the potential of PAC-O3 pretreatment in surface water treatment applications, providing new insights into the mechanism of controlling membrane fouling and improving the permeate quality.

Advanced water treatment process by simultaneous coupling granular activated carbon (GAC) and powdered carbon with ultrafiltration: Role of GAC particle shape and powdered carbon type

In current ultrafiltration systems, limited removal for small-sized contaminants and membrane fouling remain longstanding obstacles to overcome. Herein, a novel process by simultaneous coupling powered carbon (PC) and fluidized granular activated carbon (GAC) with ultrafiltration was proposed aiming to achieve high effluent quality and mitigated membrane fouling. This study conducted mechanistic explorations on the performances of different-shaped GAC particles on fouling control and PC release during fluidization, meanwhile comparing the utilizations of powdered activated carbon (PAC) and biochar in terms of their adsorption, deposition and interactions with aquatic contaminants during filtration. The results showed that the effluent COD of biochar-UF was slightly higher than PAC-UF attributed to lower specific surface area and pore volume present on biochar. Compared with PAC-UF, the biochar-UF without fluidized GAC exhibited higher fouling propensity due to more organics attached on membranes via bridging with Ca²⁺ released by the biochar. Concurrently, distinct morphologies were found for PAC and biochar depositions, where PAC uniformly dispersed on membranes but biochar tended to agglomerate. Interestingly, fluidized spherical GAC (RGAC) with highest particle momentum and least energy consumption appeared highly effective in reducing fouling associated with biochar, and the overall fouling rate of RGAC-biochar-UF was even lower than RGAC-PAC-UF system. More importantly, substantial amount of small-sized PC was released by two cylindrical-shaped GACs, which were determined to be around 12-16 mg/L in contrast to merely 3.4 mg/L produced from RGAC. Consequently, the RGAC-biochar-UF system achieved commensurate effluent quality but better permeability than RGAC-PAC-UF along with a 20% expenditure saved, which might be a promising water treatment system more suitable for large-scale applications.

Reduction of Ultrafiltration Membrane Fouling by the Pretreatment Removal of Emerging Pollutants: A Review

Ultrafiltration (UF) processes exhibit high removal efficiencies for suspended solids and organic macromolecules, while UF membrane fouling is the biggest obstacle affecting the wide application of UF technology. To solve this problem, various pretreatment measures, including coagulation, adsorption, and advanced oxidation, for application prior to UF processes have been proposed and applied in actual water treatment processes. Previously, researchers mainly focused on the contribution of natural macromolecular pollutants to UF membrane fouling, while the mechanisms of the influence of emerging pollutants (EPs) in UF processes (such as antibiotics, microplastics, antibiotic resistance genes, etc.) on membrane fouling still need to be determined. This review introduces the removal efficiency and separation mechanism for EPs for pretreatments combined with UF membrane separation technology and evaluates the degree of membrane fouling based on the UF membrane's materials/pores and the structural characteristics of the cake layer. This paper shows that the current membrane separation process should be actively developed with the aim of overcoming specific problems in order to meet the technical requirements for the efficient separation of EPs.

UV/Fe(II) synergistically activated S(IV) per-treatment on HA-enhanced Ca2+ scaling in NF filtration: Fouling mitigation, mechanisms and correlation analysis of membrane resistance in different filtration stage

The aim of this study was to investigate the feasibility and fouling mitigation mechanisms of UV/Fe(II) synergistically activated sulfite (S(IV)) (UFS) pretreatment to alleviate membrane fouling caused by HA-enhanced Ca²⁺ scaling. After UFS treatment, hydrophobic substances and carboxyl groups structure were destroyed by the in-situ-generated SO•-4, resulted in a significant reduction of hydrophobic substances ratio and carboxyl group concentration. Due to the formation of more electropositive in-situ-generated Fe(III), the complexation between Ca²⁺ and carboxyl groups was weakened so that the bulk crystallization size on the membrane surface was greatly reduced. The filter cake enhanced osmotic pressure effect (CEOP) and concentration polarization effect were hence alleviated, as well as the surface roughness. At the microcosmic perspective, as the energy barrier between the membrane and foulants was increased significantly after pretreatment, the anti-foulants adsorption ability of the membrane was enhanced. Correlation analysis showed that the carboxyl concentration and density, HPO ratio, larger particle size (>100 nm) ratio, the Ca²⁺ concentration in the scaling layer and energy barrier all had a good correlation with the membrane resistance. This research not only provides an advanced oxidation technology that can effectively alleviate the synergistically-fouling effect of HA and Ca²⁺ of nanofiltration process, but also proposes a guidance for the UV/Fe(II) synergistically activated sulfite.

The role of PAC adsorption-catalytic oxidation in the ultrafiltration performance for treating natural water: Efficiency improvement, fouling mitigation and mechanisms

Powdered activated carbon (PAC) has turned out to be an efficient adsorbent in drinking water treatment, whereas its application integrated with membrane filtration is still controversial because of the combined fouling effect between organic pollutants and PAC. To this end, an integrated process of combining PAC adsorption-catalytic oxidation and membrane filtration was proposed for natural surface water treatment. The synergistic effect of PAC and peroxymonosulfate (PMS) was confirmed through the generation of reactive oxidation species, and both radical oxidative pathways (^•OH, SO₄^•- and O₂^•-) and nonradical (¹O₂ and PMS) pathways involved in the process. The removal efficiency of DOC and UV₂₅₄ was significantly strengthened by PAC/PMS, with removal rates of 56.1% and 64.9%, respectively. The integration of PAC and PMS could significantly enhance the reduction of fluorescent organics, and pollutants with varying molecular weights. The fouling condition of membrane was dramatically alleviated, with the flux increased by 38.9%, and the reversible and irreversible resistances declined by 79.7% and 48.3%, respectively. The major fouling mechanism was significantly changed, and complete pore blocking always played a dominant role, rather than cake filtration. The effectiveness of PAC/PMS was further verified by the characterization of membrane surface morphologies and functional groups. Moreover, the attractive interactions between foulants and membrane were converted to repulsive interactions with the pretreatment of PAC/PMS. The proposed synergistic process was efficient and convenient, which could significantly improve the purification efficiency of conventional PAC-UF system in drinking water treatment.

Combined Effect of Activated Carbon Particles and Non-Adsorptive Spherical Beads as Fluidized Media on Fouling, Organic Removal and Microbial Communities in Anaerobic Membrane Bioreactor

The combined effect of acrylonitrile butadiene styrene (ABS) spherical beads and granular activated carbon (GAC) particles as fluidized media on the performance of anaerobic fluidized bed membrane bioreactor (AFMBR) was investigated. GAC particles and ABS beads were fluidized together in a single AFMBR to investigate membrane fouling and organic removal efficiency as well as energy consumption. The density difference between these two similarly sized media caused the stratified bed layer where ABS beads are fluidized above the GAC along the membrane. Membrane relaxation was effective to reduce the fouling and trans-membrane pressure (TMP) below 0.25 bar could be achieved at 6 h of hydraulic retention time (HRT). More than 90% of soluble chemical oxygen demand (SCOD) was removed after 80 d operation. Biogas consisting of 65% of methane was produced by AFMBR, suggesting that combined use of GAC and ABS beads did not have any adverse effect on methane production during the operational period. Scanning Electron Microscope (SEM) examinations showed the adherence of microbes to both media. However, 16S rRNA results revealed that fewer microbes attached to ABS beads than GAC. There were also compositional differences between the ABS and GAC microbial communities. The abundance of the syntrophs and exoelectrogens population on ABS beads was relatively low compared to that of GAC. Our result implied that syntrophic synergy and possible occurrence of direct interspecies electron transfer (DIET) might be facilitated in AFMBR by GAC, while traditional methanogenic pathways were dominant in ABS beads. The electrical energy required was 0.02 kWh/m3, and it was only about 13% of that produced by AFMBR.

Effect and mechanism of reduced membrane bioreactor fouling by powdered activated carbon

Powered Activated Carbon - Membrane Bioreactors (PAC-MBRs) have been used with good results for slightly polluted water treatment. Our batch experiments showed that the transmembrane pressure of a PAC-MBR was 25% less than that of a MBR in one period of test, which indicated that PAC did help control the fouling in MBRs. Based on this observation, several mechanisms of membrane fouling of MBRs and PAC-MBRs were investigated to have some insight into how PAC brought a positive impact. The total resistances decreased by 60% and different resistances were redistributed after adding PAC. The dominant one changed from filtration resistance to cake resistance. These smaller cake resistances resulted from the PAC because, showing in the scanning electron microscopy pictures, it made the cake layer looser and rougher than that on a normal membrane. Meanwhile, the analysis of the membrane eluent showed that the addition of PAC changed the microbial species and its metabolites on the membrane and effectively reduced the adsorption of hydrophilic organic molecules on the membrane surface. Additionally, PAC prevented polypeptide compounds from being trapped inside the pores of membranes, so the cake on the PAC-MBR contaminated membrane surface was easier to scrape off. In the test of cleaning methods, alkaline cleaning removed the most organics from contaminated membranes to restore membrane performance.

Application of UV/chlorine pretreatment for controlling ultrafiltration (UF) membrane fouling caused by different natural organic fractions

In this study, the effects of UV/chlorine pretreatment on ultrafiltration (UF) membrane fouling derived from different fractions of natural organic matter (NOM) were studied and compared. Three model organic compounds including humic acid (HA), sodium alginate (SA) and bovine serum albumin (BSA) were employed to represent different NOM fractions in natural surface water. The results suggest that membrane fouling induced from HA, SA and HA-SA-BSA mixture could be effectively mitigated by UV/chlorine pretreatment, which could be further improved by increasing the chlorine dose. Although UV irradiation alone severely aggravated BSA fouling, the addition of chlorine (0.0625 mM) to the pretreatment process could effectively avoid the fouling. The alleviation of membrane fouling is primarily ascribed to the reduction of molecular weight (MW) of organic compounds, and the decomposition of unsaturated organic species, thereby reducing the accumulation of organics on the membrane surface and pores. This is confirmed by the reduction of UV₂₅₄ and fluorescent components in the feed solution and the increase of DOC in the permeate after UV/chlorine pretreatment. Membrane fouling during the filtration of untreated HA, SA, and HA-SA-BSA mixture was occupied by cake filtration and intermediate pore blocking, while UV/chlorine pretreatment led to the exacerbation of pore blocking at the initial filtration stage. The initial fouling mechanism of untreated BSA was mainly governed by complete blocking, which shifted to intermediate pore blocking after UV/chlorine pretreatment.

Identification of scaling during clean-in-place (CIP) in membrane water treatment process

The goal of this study was to identify the scaling from the chemical cleaning of a polyvinylidene fluoride (PVDF) membrane, fouled by treating a solution containing inorganic foulants (Al, Fe, and Mn) in the presence of kaolin and humic acid as a natural organic matter at Ca⁺² strength of 0.5 mMole. Chemical cleaning of the membrane was conducted using solutions prepared in deionized water and permeate water (PW), and the accumulation of insoluble salts on the membrane during cleaning were evaluated. Energy dispersive spectroscopy analysis was used to verify the presence inorganic foulants, and field emission scanning electron microscopy confirmed the changes in membrane symmetry from the accumulation of the foulants. A Fourier-transformed infrared spectroscopy analysis indicated the presence of new functional groups, i.e., C-Cl and C-O with bond vibrations at 542 cm ^-1 and 1,026 cm^-1, respectively, on the membrane surface. The adsorbed mass of HA in the presence of inorganic foulants decreased from 3.54 ± 0.045 mg to 2.24 ± 0.095 mg and 1.71 ± 0.075 mg, and the flux recoveries decreased from 93.2% to 85.69% and 81.92%, for the pristine to chemically DI and PW cleaned membrane, respectively. However, the membrane characterization results confirmed that Al was the major contributor to the accumulation of inorganic salts on the membrane during chemical cleaning and its role was more severe in the presence of Mn. The fitting results of Hermia's fouling models and a specific fouling analysis confirmed the contribution of complete blocking model with increase in irreversible fouling was observed after chemical cleaning.

Control of organic and surfactant fouling using dynamic membranes in the separation of oil-in-water emulsions

HYPOTHESIS

A superhydrophilic membrane with rough and hierarchical structures is possibly fouled by surfactant-stabilized oil and organic foulants, because these foulants could not be hindered by the water layer formed on superhydrophilic membrane surface. A dynamic membrane was possibly an effective method to address this fouling problem.

EXPERIMENTS

A microfiltration membrane, a nanofiber membrane, and a dynamic membrane were used for the separation of surfactant-free emulsions, surfactant-stabilized emulsions, and the surfactant-stabilized emulsions containing typical organic foulants. The oil rejection and membrane fouling were compared.

FINDINGS

The microfiltration membrane, nanofiber membrane, and dynamic membrane had high resistances to the fouling by surfactant-free emulsions because these membranes were underwater superoleophobic. However, these membranes showed low resistances to the fouling by surfactant-stabilized oil droplets and organic foulants. For the dynamic membrane, the oil droplets and organic foulants trapped in the separation layer could be readily removed in the detachment-washing-recoating steps; therefore, almost no physically irreversible fouling was observed in the multi-cycle filtration. With the size distributions of oil droplets in the emulsions and the particle of the dynamic membrane, the rejection of oil by the dynamic membrane could be calculated by simply assuming that the particle was spherical, uniform, and tightly packed.

Reuse of shale gas flowback and produced water: Effects of coagulation and adsorption on ultrafiltration, reverse osmosis combined process

The shale gas flowback and produced water (FPW) from hydraulic fracturing in the Sichuan province of China has relatively low to moderate levels of total dissolved solids (<20 g/L) and organics (<50 mg/L of dissolved organic carbon). As such, a combined ultrafiltration (UF), reverse osmosis (RO) system can be successfully applied to desalinate this feed water with the goal of reuse. However, the concentration of influent organic matter and particulates in the UF and RO stage is high, and the overall ionic and organics composition is highly complex, so that the membrane processes do not perform well, also due to fouling. To ensure the long-term and efficient operation of the UF-RO stages, a combined pretreatment of the FPW with coagulation and adsorption was investigated. The effect of different parameters on the performance on the system was studied in detail. Overall, the coagulation-adsorption pre-treatment greatly reduced fouling of the membrane processes, thanks to the high removal rate of turbidity (98.8%) and dissolved organic carbon (86.3%). The adsorption of organic matter by powdered activated carbon was best described by the Freundlich equilibrium model, with a pseudo second-order model representing the adsorption kinetics. Also, the various ions had competitive removal rates during the adsorption step, a phenomenon reported for the first time for FPW treatment. Also, an optimal dose of activated carbon existed to maximize fouling reduction and effluent quality. The overall treatment system produced a high-quality water streams, suitable for reuse.

Membrane fouling by the aggregations formed from oppositely charged organic foulants

Due to the lack of robust ways to quantify aggregations, fouling of two-foulant aggregations is poorly understood. This work systematically reports the ultrafiltration membrane fouling by aggregations formed from two oppositely charged organic foulants (i.e., humic acid (HA) and lysozyme (LYS)) with the aid of resonance light scattering (RLS) technique. RLS provides an effective approach to detecting the aggregation concentration and reveals that the HA-LYS aggregations were formed at a mass ratio of m(LYS)/m(HA) = 2.77. During the filtration of the mixture of HA and LYS, aggregations over individual foulants were identified to be the main substances deposited on the membrane surface, where the mass of deposition had a good linear relationship with the feed concentration of the aggregations. The HA-LYS aggregations might decrease the total fouling due to their large size, but reduce the fouling reversibility. In the pH range of 5.5-9.2, the pH value had limiting effects on the concentration of HA-LYS aggregations, as well as the consequent fouling. At low ionic strength, the membrane fouling by HA-LYS aggregations decreased as the ionic strength increased due to the reduction of the aggregation concentration. Oppositely, at high ionic strength, this tendency was reversed due to the electrical double layer compression effect. These results suggest that RLS is a simple and effective way to quantify the aggregations of foulants, and the aggregations of foulants have distinct fouling behaviors compared with the individual foulants.

Algal fouling and extracellular organic matter removal in powdered activated carbon-submerged hollow fiber ultrafiltration membrane systems

In this work, the effect of powdered activated carbon (PAC) on fouling by algal solution during ultrafiltration using two different PAC dosing strategies: pre-depositing PAC onto the membrane surfaces or the conventional addition of PAC to the bulk feed. The addition of PAC by either mode improved the removal of extracellular organic matter (EOM) from the algal solution. However, for the pre-deposition mode, increasing the PAC amount from 0 to 2.1 g caused a steady increase in the membrane fouling rate (from 0.4 to 1.4 kPa/h), whereas the opposite result (from 0.4 down to 0.1 kPa/h) was found for the conventional PAC dosing mode. This is likely due to the differences in the initial arrangement of algal cells and PAC along the cake layer depths. The pre-deposited PAC avoided contact between cells and membranes, but aggravated the deformation of the cells and hindered their back-transport to the bulk solution. Furthermore, although the effect of PAC on the EOM fouling was marginal, there were highly synergistic effects when cells and EOM were present together in the PAC pre-deposition mode. Changes in the PAC dosing mode also altered the PAC-membrane interactions, inducing a higher cleaning efficiency of backwash for the conventionally-dosed PAC from membrane surfaces than that for the pre-deposited PAC.

Effect of peroxymonosulfate oxidation activated by powdered activated carbon for mitigating ultrafiltration membrane fouling caused by different natural organic matter fractions

Powdered activated carbon (PAC) adsorption has been widely applied prior to ultrafiltration membrane for potable water production. However, the impact of PAC adsorption on membrane fouling was still controversial. To solve this problem, combined PAC and peroxymonosulfate (PMS) pretreatment was proposed in this study. The application of PAC/PMS for mitigating membrane fouling by natural organic matter (NOM) has been evaluated, and compared with PMS oxidation or PAC adsorption alone. The influence of NOM fractions on the control efficiency was also investigated using humic acid (HA), bovine serum albumin (BSA), sodium alginate (SA), and their mixture (HA-BSA-SA). The performance was examined through normalized flux decline, fouling resistances analysis, scanning electron microscopy, and model fits. The results indicated that PAC and PMS exhibited a remarkable synergistic effect in the reduction of NOM, with the DOC reduction rates of 53.6%, 24.3%, 27.1% and 31.4% for HA, BSA, SA and HA-BSA-SA, respectively. PAC adsorption exhibited limited influence on mitigating membrane fouling, and the co-existence of PAC and HA even exacerbated fouling due to the synergistic fouling effect between them. By contrast, PAC/PMS pretreatment efficiently reduced both reversible and irreversible fouling resistances. The control efficiency was closely associated with the NOM fractions in the feed water, and followed the order of SA > HA-BSA-SA > BSA > HA. The fouling mitigation by PAC/PMS was attributed to both PAC adsorption and oxidation with SO₄^- and OH. The experimental results are expected to provide a feasible strategy of PAC/PMS for fouling mitigation, and simultaneously solve the problem faced by PAC adsorption.

Regeneration of 4-chlorophenol from spent powdered activated carbon by ultrasound

Ultrasound-assisted regeneration of powdered activated carbon (PAC) saturated with 4-chlorophenol (4-CP) was investigated in this work. The variables, i.e., acoustic frequency, density levels, regenerating solution species, spent PAC mass, and the number of repeated cycle, were optimized. The UV-Vis spectra, thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET), and pore structure analysis were used to characterize the regenerated PAC as compared to the virgin and spent PAC. The adsorption experiments indicated that the adsorption kinetics of 4-CP fitted well with the pseudo-second-order model, and the main chemisorption process was nonlinear and heterogeneous. The desorption results showed that the optimized regenerated conditions were identified as 40 kHz of frequency, 0.18 W/mL of sonication intensity, 0.1 M NaOH and 50% (v/v) ethyl alcohol mixture of regeneration solution, and 1 g/L of saturated PAC mass. Under this condition, the regeneration efficiency reached up to 86.81%. Hydroxylated oxidation products especially benzoquinone and hydroquinone were formed during ultrasound regeneration. Ultrasound mainly acted on surface functionalities, mesopores, and macropores of PAC through the hydroxyl radical oxidization resulted from the cavitation effect.

Immobilized microalgae for anaerobic digestion effluent treatment in a photobioreactor-ultrafiltration system: Algal harvest and membrane fouling control

A photobioreactor (PBR) coupled with ultrafiltration (UF) system was developed with goals of microalgae cultivation, harvest, and membrane fouling control in the anaerobic digestion effluent purification. Firstly, three-sequencing batch PBRs were started-up with suspended Chlorella vulgaris (C. vulgaris, SCV), immobilized C. vulgaris (ICV) and immobilized C. vulgaris with powdered activated carbon (ICV + PAC). The results exhibited high DOC degradation (66.61%-84.35%) and completely nutrients (nitrogen and phosphorus) removals were attained in PBRs. This indicated bacterial-microalgal consortiums enhanced biodegradation and PAC adsorption accelerated photodegradation. During the microalgae harvest by UF, immobilized microalgae beads protected cells integrity with less debris and intracellular/extracellular organic matters lysis. Moreover, the cake layer in ICV + PAC could even serve as a dynamic layer to entrap the residual pollutants and control membrane fouling. Hence, membrane fouling mitigation and ADE purification were realized during the microalgae harvest process in the ICV + PAC.