Applying ultraviolet/persulfate (UV/PS) pre-oxidation for controlling ultrafiltration membrane fouling by natural organic matter (NOM) in surface water

Nano-confined catalysis with Co nanoparticles-encapsulated carbon nanotubes for enhanced peroxymonosulfate oxidation in secondary effluent treatment: Water quality improvement and membrane fouling alleviation

Despite widespread deployment and investigation of ultrafiltration (UF) for secondary effluent purification, the challenge of membrane fouling due to effluent organic matter (EfOM) remains formidable. This study introduced a novel pretreatment method utilizing Co nanoparticles-encapsulated carbon nanotubes activated peroxymonosulfate (Co@CNT/PMS) to degrade EfOM and mitigate membrane fouling. Characterization of Co@CNT revealed the efficient encapsulation of Co nanoparticles within nanotubes, which notably enhanced the catalytic degradation of bisphenol A and typical organics. The tube-encapsulated structure increased the concentration of reactive species within the confined nanoscopic space, thereby improving the probability of collisions with pollutants and promoting their degradation. The Co@CNT/PMS pretreatment led to substantial reductions in aromatic compounds, fluorescent components, and both high and middle molecular weight substances. These changes proved crucial in diminishing the fouling potential in subsequent UF processes, where reversible and irreversible fouling resistances decreased by 97.1 % and 72.8 %, respectively. The transition volume from pore blocking to cake filtration markedly increased, prolonging the formation of a dense fouling layer. Surface properties analysis indicated a significant reduction of pollutants on membrane surfaces after the Co@CNT/PMS pretreatment. This study underscored the efficacy of confinement-based advanced oxidization pretreatment in enhancing UF performance, presenting a viable resolution to membrane fouling.

Comparison of UV/PS and VUV/PS as ultrafiltration pretreatment: Performance, mechanisms, DBPs formation and toxicity assessment

Ultrafiltration (UF) is widely used in drinking water plants, nevertheless, it still encounters challenges stemming from inevitable membrane fouling caused by natural organic matter (NOM). Herein, this work applied VUV/PS as UF membrane pretreatment and used UV/PS for comparison. VUV/PS system exhibited superior ability in removing NOM compared to UV/PS system. HO and SO4- played crucial roles in the degradation. [SO4-]ss was notably higher than [HO]ss in the systems, yet HO was of greater significance. [HO]ss and [SO4-]ss in the VUV/PS process were remarkably higher than those in the UV/PS process, due to the function of 185 nm photons. VUV/PS pretreatment basically recovered flux and effectively reduced fouling resistance, with better performance than UV/PS. Fouling mechanism was dominated by multiple mechanisms after UV/PS pretreatment, whereas it was transformed into pore blockage after VUV/PS pretreatment. Moreover, the UF effluent quality after VUV/PS pretreatment outperformed that of UV/PS but fell short of that without pretreatment, possibly due to the generation of abundant low MW substances under the action of HO and SO4-. After chlorine disinfection, UV/PS and VUV/PS pretreatments increased the DBPs production and cytotoxicity. Specifically, oxidant PS affected the membrane surface morphology and fouling behaviors, and had no obvious effect on interception performance and mechanical properties. In actual water treatment, VUV/PS and UV/PS pretreatments exhibited excellent performance in alleviating membrane fouling, improving water quality, and reducing DBPs formation and acute toxicity.

Control ultrafiltration membrane fouling in Chlorella-laden water treatment by integrated heat-activated peroxydisulfate pre-oxidation and coagulation treatment

The membrane fouling induced by algal extracellular organic matter (EOM) remain a bottleneck in restricting ultrafiltration (UF) application during harmful algal-water treatment. In current study, the application of heat-activated peroxydisulfate (PMS) and coagulation (Aluminum chlorohydrate, PACI) on membrane fouling behavior during Chlorella-laden water treatment was investigated. The membrane fouling mechanism was analyzed using the extended Derjaguin-Landau-Verwey-Over-beek (XDLVO) theory. The results revealed that separated heat-activated PMS could enhance the filtration flux of EOM at high PMS does >0.2 mM, whereas the membrane fouling was further alleviated by combined heat-activated PMS (0.2-1.0 mM) and PACI (20 mg/L) treatment, especially at low PMS dose. Combined heat-activated PMS and PACI pretreatment could effectively increase the adhesive repulsion between membrane and foulants and reduce the cohesion free energies between organic foulants than those by separated heat-activated PMS treatment, making the initial filtration flux reduced and the cake layer looser. Moreover, the organic foulants of proteins, polysaccharides, and humic-like organics were removed. Cake formation was the major fouling mechanism when EOM was treated with/without separated heat-activated PMS treatment, whereas the membrane fouling mechanism was changed from cake layer formation to pore blocking after combined heat-activated PMS and PACI treatment. Overall, this research provided a feasible method in membrane fouling control during Chlorella -laden water treatment.

Degradation of different fractions of natural organic matter in drinking water by the UV/persulfate process

The existence of natural organic matter (NOM) causes many problems in drinking water treatment processes. The degradation of different fractions of NOM in drinking water was studied using the ultraviolet/persulfate (UV/PS) process. The NOM was separated into hydrophobic (HPO), transition hydrophilic (TPI) and hydrophilic (HPI) fractions by reverse osmosis and XAD series resins. The effects of degradation were evaluated by dissolved organic carbon (DOC), UV254, three-dimensional fluorescence-parallel factor analysis (EEM-PARAFAC), and trihalomethane formation potential (THMFP). The results showed that UV/PS process could remove the three fractions of DOC, UV254, as well as the fluorescent components humic acid-like (C1 and C2) and protein-like (C3). The maximum removal rates of DOC of HPO, TPI, and HPI fractions were 34.6%, 38.4%, and 73.9%, respectively, and the maximum removal rates of UV254 were 72.1%, 86.3%, and 86.8%, respectively. The removal rate of the three fluorescent components can reach 100%, and C3 is easier to remove than C1 and C2 under the low PS dosage conditions. The order of kinetic degradation rate constant of UV254 first-order reaction is HPI > TPI > HPO. The optimum pH conditions for the degradation of HPO, TPI, and HPI fractions were acidic, basic, and neutral, respectively. The specific THMFP of HPO was higher than that of TPI and HPI. The specific THMFP of HPO and TPI fractions increased with the increase of radiation time, while the HPI fraction showed the opposite trend. THMFP has different degrees of correlation with DOC, UV254, C1, and C2. This study can provide a theoretical basis for the selection of the UV/PS process for drinking water sources containing NOM with different characteristics.

Enhancing microfiltration membrane performance by sodium percarbonate-based oxidation for hydraulic fracturing wastewater treatment

Low pressure membrane takes a great role in hydraulic fracturing wastewater (HFW), while membrane fouling is a critical issue for the stable operation of microfiltration (MF). This study focused on fouling mitigation by sodium percarbonate (SPC) oxidation, activated by ultraviolet (UV) and ferrous ion (Fe(II)). The higher the concentration of oxidizer, the better the anti-fouling performance of MF membrane. Unlike severe MF fouling without oxidation (17.26 L/(m2·h)), UV/SPC and Fe(II)/SPC under optimized dosage improved the final flux to 740 and 1553 L/(m2·h), respectively, and the latter generated Fe(III) which acted as a coagulant. Fe(II)/SPC oxidation enabled a shift in fouling mechanism from complete blocking to cake filtration, while UV/SPC oxidation changed it to standard blockage. UV/SPC oxidation was stronger than Fe(II)/SPC oxidation in removing UV254 and fluorescent organics for higher oxidizing capacity, but the opposite was noted for DOC removal. The deposited foulants on membrane surface after oxidation decreased by at least 88% compared to untreated HFW. Correlation analysis showed that UV254, DOC and organic fraction were key parameters responsible for membrane fouling (correlation coefficient＞0.80), oxidizing capacity and turbidity after oxidation were also important parameters. These results provide new insights for fouling control during the HFW treatment.

Molecular interaction mechanism for humic acids fouling resistance on charged, zwitterion-like and zwitterionic surfaces

Humic acids (HA) are ubiquitous in surface waters, leading to significant fouling challenges. While zwitterion-like and zwitterionic surfaces have emerged as promising candidates for antifouling, a quantitative understanding of molecular interaction mechanism, particularly at the nanoscale, still remains elusive. In this work, the intermolecular forces between HA and charged, zwitterion-like or zwitterionic monolayers in aqueous environments were quantified using atomic force microscope. Compared to cationic MTAC ([2-(methacryloyloxy)ethyl]trimethylammonium chloride), which exhibited an adhesion energy of ∼1.342 mJ/m2 with HA due to the synergistic effect of electrostatic attraction and possible cation-π interaction, anionic SPMA (3-sulfopropyl methacrylate) showed a weaker adhesion energy (∼0.258 mJ/m2) attributed to the electrostatic repulsion. Zwitterion-like MTAC/SPMA mixture, driven by electrostatic attraction between opposite charges, formed a hydration layer that prevented the interaction with HA, thereby considerably reducing adhesion energy to ∼0.123 mJ/m2. In contrast, zwitterionic MPC (2-methacryloyloxyethyl phosphorylcholine) and DMAPS ([2-(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl) ammonium hydroxide) displayed ultralow adhesion energy (0.06-0.07 mJ/m2) with HA, arising from their strong dipole moments which could induce a tight hydration layer that effectively inhibited HA fouling. The pH-mediated electrostatic interaction resulted in the increased adhesion energy for MTAC but decreased adhesion energy for SPMA with elevated pH, while the adhesion energy for zwitterion-like and zwitterionic surfaces was independent of environmental pH. Density functional theory (DFT) simulation confirmed the strong binding capability of MPC and DMAPS with water molecules (∼-12 kcal mol-1). This work provides valuable insights into the molecular interaction mechanisms underlying humic-substance-fouling resistance of charged, zwitterion-like and zwitterionic materials at the nanoscale, shedding light on developing more effective strategy for HA antifouling in water treatment.

Reduction of fouling of gravity-driven membrane by combined treatment of persulphate/nanoscale zero-valent iron/ultraviolet and dynamic dual coagulant flocs layer

In this study, persulphate and nanoscale zero-valent iron were activated by ultraviolet irradiation (PS/nZVI/UV), followed by formation of dynamic flocs with AlCl3-TiCl4 coagulant directly injected into a gravity-driven membrane (GDM) tank. Membrane fouling caused by typical organic matter fractions including humic acid (HA), HA together with bovine serum albumin (HA-BSA), HA combined with polysaccharide (HA-SA) and the HA-BSA-SA mixture at pH of 6.0, 7.5 and 9.0 were evaluated by specific flux and fouling resistance distribution. The results showed that GDM pre-layered with AlCl3-TiCl4 flocs exhibited the maximum specific flux, followed by AlCl3 and TiCl4. Pre-oxidation with 0.5 mM PS and 0.1 g nZVI under UV radiation for 20 min was beneficial to degrade HA and SA fraction with molecular weight >100 kDa and <30 kDa, and BSA fraction with <30 kDa. The presence of BSA attributed mostly to irreversible fouling, SA together with BAS could exacerbate irreversible fouling, while HA caused the least fouling. The irreversible resistance of a PS/nZVI/UV-GDM system was 62.79%, 27.27%, 58.03% and 49.68% lower than that of control GDM in the treatment of HA, HA-BSA, HA-SA and HA-BSA-SA, respectively. The PS/nZVI/UV-GDM system could achieve the highest foulants removal efficiency at pH of 6.0. Morphological observations confirmed the differences in biofouling layers in different water types. Over 30-day operation, the bacterial genera on the biofouling layer could affect the organic removals, while the type of organic matter that was present influenced the relative abundance of bacterial genera.

Degrading surface-water-based natural organic matter and mitigating haloacetonitrile formation during chlorination: Comparison of UV/persulfate and UV/hydrogen peroxide pre-treatments

Haloacetonitriles (HANs) are unregulated disinfection by-products that are more toxic than regulated species. Therefore, efficient decomposition of HAN precursors prior to disinfection is crucial for allaying the potential HAN-induced health risks. This study investigated the key roles of ultraviolet-activated persulfate (UV/PS) treatment in alleviating HAN formation. The effects of UV/PS treatment were evaluated by correlating with the characteristics of organic matter in surface water and comparing with conventional UV/H2O2 treatment. Upon irradiating raw water samples and a Suwannee River humic acid solution spiked with 10 mM PS or H2O2 with 254 nm UV light, UV/PS treatment was found to be more potent than UV/H2O2 in mitigating the HAN production and degrading organic substances; moreover, UV/PS treatment effectively decreased the dissolved organic nitrogen (DON) content. In contrast, UV/H2O2 treatment did not induce any noticeable reduction in DON level. Furthermore, both UV/PS and UV/H2O2 treatments reduced the dichloroacetonitrile (DCAN) formation potential (FP), leading to strong correlations with the degradation of aromatic and humic-acid-like compounds. Notably, UV/PS treatment efficiently decreased the FP of bromochloroacetonitrile (BCAN) and dramatically reduced that of dibromoacetonitrile (DBAN) after a sharp increase; however, UV/H2O2 treatment gradually increased the DBAN-FP. Bromide was activated by sulfate radicals during UV/PS treatment, negatively correlating with the BCAN-FP and DBAN-FP, indicating that the formation of reactive bromine species increased the DBAN-FP; however, excessive oxidation possibly led to the recovery of inorganic bromine for decreasing the BCAN-FP and DBAN-FP. Additionally, UV/PS treatment effectively suppressed toxicity owing to its high reduction rate for brominated HANs; in contrast, UV/H2O2 treatment resulted in less significant BCAN and DBAN reductions, leading to minimal net reduction in toxicity. Overall, UV/PS treatment was remarkably effective at diminishing the toxicity of brominated HANs, underscoring its potential to mitigate drinking-water-related health risks.

Critical fractions in reclaimed water responsible for membrane fouling: Isolation, fouling characteristics, quantitative and qualitative variations in practical application

Considering the different fouling characteristics between model foulants and organic components in real reclaimed water, it is of great importance to identify the critical foulants responsible for membrane fouling. This study identified and isolated the fraction with molecular weight (MW) > 100 kDa as the critical foulant in secondary effluent by MW cut-off membrane of 100 kDa with high efficiency. This fraction accounted for 92.2% membrane fouling of raw water, including 28.7%, 29.7% and 33.8% fouling contribution by subfractions with MW between 100-300, 300-500 and > 500 kDa. Specifically, the critical fraction with MW > 100 kDa were mainly distributed in two parts: < 0.22 μm and > 0.45 μm, corresponding to 41.9% and 56.9% fouling contribution of this fraction. Furthermore, both total organic carbon (TOC) and fouling potential of fraction with MW > 100 kDa were monitored, presenting about threefold increase from September to January in next year. Membrane fouling contribution of this critical fraction in raw secondary effluent were mainly distributed in 85∼95% throughout the 5 months, demonstrating its predominant fouling propensity. Moreover, the TOC concentration of fraction with MW > 100 kDa presented distinct positive correlation with the fouling potential of raw secondary effluent (R2 = 0.947), which was promising to be a surrogate for predicting membrane fouling in practical application.

A review on photocatalytic attribution and process of pyrolytic biochar in environment

Biochar has attracted significant attention due to its excellent environmental benefits and extensive applications. Recently, a consensus has been accepted that biochar can act as a photocatalyst and trigger effective photocatalytic reactions in the environment, which is important to energy conversion and the cycle of elements. However, its photocatalytic processes and the corresponding environmental impacts need to receive more and due attention. In this review, we provide a comprehensive summary of the underlying correlations among the pyrolytic evolution of biomass, the structure characteristic of biochar, and the resultant photocatalytic performance. Moreover, the photocatalytic processes and the influence of environmental factors were elaborately investigated on biochar. Finally, future tendencies and challenges in the photocatalysis of biochar have been prospected in the environmental field. This review has offered innovative insights into the photocatalytic essential of biochar and highly enhanced the understanding of its environmental impact.

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.

Membrane-based technology in water and resources recovery from the perspective of water social circulation: A review

In this review, the application of membrane-based technology in water social circulation was summarized. Water social circulation encompassed the entire process from the acquirement to discharge of water from natural environment for human living and development. The focus of this review was primarily on the membrane-based technology in recovery of water and other valuable resources such as mineral ions, nitrogen and phosphorus. The main text was divided into four main sections according to water flow in the social circulation: drinking water treatment, agricultural utilization, industrial waste recycling, and urban wastewater reuse. In drinking water treatment, the acquirement of water resources was of the most importance. Pressure-driven membranes, such as ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) were considered suitable in natural surface water treatment. Additionally, electrodialysis (ED) and membrane capacitive deionization (MCDI) were also effective in brackish water desalination. Agriculture required abundant water with relative low quality for irrigation. Therefore, the recovery of water from other stages of the social circulation has become a reasonable solution. Membrane bioreactor (MBR) was a typical technique attributed to low-toxicity effluent. In industrial waste reuse, the osmosis membranes (FO and PRO) were utilized due to the complex physical and chemical properties of industrial wastewater. Especially, membrane distillation (MD) might be promising when the wastewater was preheated. Resources recovery in urban wastewater was mainly divided into recovery of bioenergy (via anaerobic membrane bioreactors, AnMBR), nitrogen (utilizing MD and gas-permeable membrane), and phosphorus (through MBR with chemical precipitation). Furthermore, hybrid/integrated systems with membranes as the core component enhanced their performance and long-term working ability in utilization. Generally, concentrate management and energy consumption control might be the key areas for future advancements of membrane-based technology.

Membrane processes enhanced by various forms of physical energy: A systematic review on mechanisms, implementation, application and energy efficiency

Membrane technologies in water and wastewater treatment have been eagerly pursued over the past decades, yet membrane fouling remains the major bottleneck to overcome. Membrane fouling control methods which couple membrane processes with online in situ application of external physical energy input (EPEI) are getting closer and closer to reality, thanks to recent advances in novel materials and energy deliverance methods. In this review, we summarized recent studies on membrane fouling control techniques that depend on (i) electric field, (ii) acoustic field, (iii) magnetic field, and (iv) photo-irradiation (mostly ultraviolet or visible light). Mechanisms of each energy input were first reported, which defines the applicability of these methods to certain wastewater matrices. Then, means of implementation were discussed to evaluate the compatibility of these fouling control methods with established membrane techniques. After that, preferred applications of each energy input to different foulant types and membrane processes in the experiment reports were summarized, along with a discussion on the trends and knowledge gaps of such fouling control research. Next, specific energy consumption in membrane fouling control and flux enhancement was estimated and compared, based on the experimental results reported in the literature. Lastly, strength and weakness of these methods and future perspectives were presented as open questions.

Aquatic plant root exudates: A source of disinfection byproduct precursors in constructed wetlands

Aquatic plant-derived dissolved organic matter (DOM) in water bodies is an important source of disinfection byproduct (DBP) precursors. It is therefore very important to investigate DBP formation, and the main DBP precursors that enter drinking water during treatment processes. In this study, Lythrum salicaria root extract (LSRE) and Acorus calamus root extract (ACRE) were analyzed. The LSRE and ACRE were chlorinated and disinfected to generate trihalomethanes, haloacetic acids, haloketones, and haloacetaldehydes. The DBP formation potential of LSRE, dominated by humus, was higher than that of Suwannee River natural organic matter (SRNOM), and trichloroacetic acid was the main DBP. It was calculated that 2.09 % of the increased DOC brought by the surface flow wetland planted with emergent aquatic plants, and the contribution rates of TCMFP, DCAAFP and TCAAFP in effluent were 3.34 %, 3.23 % and 3.05 %, respectively. A total of 706 chlorinated-formula were detected by FTICR-MS, among which mono- and di-chlorinated formulae were the most abundant. Macromolecular hydrophobic organics and tannins were the main precursors for LSRE. Unlike LSRE, the DOM composition of ACRE was dominated by protein or aliphatic compounds; therefore, the risk of DBP formation was not as high as that for LSRE. This study is the first to determine the risk of DBP formation associated with aquatic plant root extracts, and confirmed that tannins in plant-derived DOM are more important DBP precursors than lignins.

Comparative study of UV/chlorine and VUV/chlorine as ultrafiltration membrane pretreatment techniques: Performance, mechanisms and DBPs formation

Membrane fouling, primarily resulting from natural organic matter (NOM) widely existing in water sources, has always been a chief hindrance for the prevalent application of ultrafiltration (UF). Thus, vacuum ultraviolet (VUV)/chlorine process was proposed as a strategy for UF membrane fouling control and ultraviolet (UV)/chlorine process was used for comparison. VUV/chlorine process exhibited more excellent performance on NOM removal than UV/chlorine process. [HO•]ss and [Cl•]ss were calculated as 1.26 × 10-13 and 3.06 × 10-14 M, respectively, and ClO• might not exist under the conditions of 0.08 mM chlorine and 30 min VUV irradiation. [HO•]ss, [Cl•]ss and [ClO•]ss were not available and the formation of reactive radicals was unsustainable in UV/chlorine system. Moreover, VUV/chlorine pretreatment also showed better performance on the reversible and irreversible membrane fouling control than UV/chlorine pretreatment. The dominated fouling mechanism in the final stage of filtration was cake filtration. Additionally, the amount of detected disinfection by-products (DBPs) in VUV/chlorine system was significantly lower than that in UV/chlorine system. During subsequent chlorination disinfection, the yield of DBPs with VUV/chorine pretreatment was higher than that with UV/chlorine pretreatment. VUV/chlorine pretreatment could effectively control DBPs formation when the pretreatment time was extended to 120 min. In summary, VUV/chlorine system presented a most excellent performance on membrane fouling control, NOM degradation and DBPs control.

Role of Titanium Dioxide-Immobilized PES Beads in a Combined Water Treatment System of Tubular Alumina Microfiltration and PES Beads

The membrane process has a limit to the decay of various pollutants in water. To improve the problem, the roles of backwashing media and titanium dioxide (TiO2) photocatalyst-immobilized-polyethersulfone (PES) beads' concentration were investigated in a combined system of tubular alumina MF and the PES beads for advanced drinking water treatment. The space between the outside of the MF membrane and the module inside was filled with the PES beads. UV at a wavelength of 352 nm was irradiated from outside of the acryl module. A quantity of humic acid and kaolin was dissolved in distilled water for synthetic water. Water or air intermittent backwashing was performed outside to inside. The membrane fouling resistance after 3 h process (Rf,180) was minimum at 30 g/L of the PES beads for water backwashing, and at 40 g/L for air backwashing when increasing the PES beads from 0 to 50 g/L. The irreversible membrane fouling resistance after physical cleaning (Rif) was at the bottom at 5 g/L of the PES beads for water backwashing, which was 3.43 times higher than minimal at 40 g/L of the PES beads for air backwashing. The treatment effectiveness of turbidity increased when increasing the PES beads' concentration from 0 to 50 g/L; however, it reached a maximum at 98.1% at 40 g/L and 99.2% at 50 g/L for water and air backwashing, respectively. The treatment effectiveness of UV254 absorbance, which was dissolved organic matter (DOM), increased dramatically when increasing the PES beads; however, it reached a peak of 83.0% at 40 g/L and 86.0% at 50 g/L for water and air backwashing, respectively. Finally, the best PES beads' concentration was 20~30 g/L to minimize the membrane fouling; however, it was 50 g/L to remove pollutants effectively. The water backwashing was better than the air at treating DOM; however, the air backwashing was more effective than the water at removing turbid matter and reducing membrane fouling.

UV/Fe(II)/S(IV) Pretreatment for Ultrafiltration of Microcystis aeruginosa-Laden Water: Fe(II)/Fe(III) Triggered Synergistic Oxidation and Coagulation

Ultrafiltration (UF) has been proven effective in removing algae during seasonal algal blooms, but the algal cells and the metabolites can induce severe membrane fouling, which undermines the performance and stability of the UF. Ultraviolet-activated sulfite with iron (UV/Fe(II)/S(IV)) could enable an oxidation-reduction coupling circulation and exert synergistic effects of moderate oxidation and coagulation, which would be highly preferred in fouling control. For the first time, the UV/Fe(II)/S(IV) was systematically investigated as a pretreatment of UF for treating Microcystis aeruginosa-laden water. The results showed that the UV/Fe(II)/S(IV) pretreatment significantly improved the removal of organic matter and alleviated membrane fouling. Specifically, the organic matter removal increased by 32.1% and 66.6% with UV/Fe(II)/S(IV) pretreatment for UF of extracellular organic matter (EOM) solution and algae-laden water, respectively, while the final normalized flux increased by 12.0-29.0%, and reversible fouling was mitigated by 35.3-72.5%. The oxysulfur radicals generated in the UV/S(IV) degraded the organic matter and ruptured the algal cells, and the low-molecular-weight organic matter generated in the oxidation penetrated the UF and deteriorated the effluent. The over-oxidation did not happen in the UV/Fe(II)/S(IV) pretreatment, which may be attributed to the cyclic redox Fe(II)/Fe(III) coagulation triggered by the Fe(II). The UV-activated sulfate radicals in the UV/Fe(II)/S(IV) enabled satisfactory organic removal and fouling control without over-oxidation and effluent deterioration. The UV/Fe(II)/S(IV) promoted the aggregation of algal foulants and postponed the shift of the fouling mechanisms from standard pore blocking to cake filtration. The UV/Fe(II)/S(IV) pretreatment proved effective in enhancing the UF for algae-laden water treatment.

Safe purification of rural drinking water by biological aerated filter coupled with ultrafiltration

Water resources of many rural areas are usually lakes or reservoirs, which can be easily affected by run-off, non-point source pollution and are often of poorer quality compared with urban water sources. Drinking water supply in remote rural areas usually suffers from various challenges, such as the high cost of construction and maintenance of centralized drinking water treatment plants and pipe networks, due to the dispersed nature of villages, which are often located in varied and complex topographies. In this study, a combined process comprising biological aerated filter (BAF) combined with ultrafiltration was developed to treat polluted reservoir water. Organic matter indexes, turbidity, and chroma were used as indicators for the evaluation of the system performance. In a long-term experiment lasting 260 days, the combined process was tested under different values of critical operational parameters, including filler types and empty bed contact time (EBCT). Furthermore, the microbial communities in different BAF reactors were carefully evaluated at different times, finding that microorganisms with specific functions were enriched in the various BAF reactors. The combined process reached 85.5 % removal rate of DOC with an EBCT of 45 min and using granule active carbon (GAC) as filler. Most of the effluents of BAF reactors met the requirements for drinking water in China. The combined system showed practical potential for polluted water treatment in some rural areas.

Comparison of UV-based advanced oxidation processes for the removal of different fractions of NOM from drinking water

This study examined the effectiveness for degradation of hydrophobic (HPO), transphilic (TPI) and hydrophilic (HPI) fractions of natural organic matter (NOM) during UV/H₂O₂, UV/TiO₂ and UV/K₂S₂O₈ (UV/PS) advanced oxidation processes (AOPs). The changing characteristics of NOM were evaluated by dissolved organic carbon (DOC), the specific UV absorbance (SUVA), trihalomethanes formation potential (THMFP), organic halogen adsorbable on activated carbon formation potential (AOXFP) and parallel factor analysis of excitation-emission matrices (PARAFAC-EEMs). In the three UV-based AOPs, HPI fraction with low molecular weight and aromaticity was more likely to degradate than HPO and TPI, and the removal efficiency of SUVA for HPO was much higher than TPI and HPI fraction. In terms of the specific THMFP of HPO, TPI and HPI, a reduction was achieved in the UV/H₂O₂ process, and the higest removal rate even reached to 83%. UV/TiO₂ and UV/PS processes can only decrease the specific THMFP of HPI. The specific AOXFP of HPO, TPI and HPI fractions were all able to be degraded by the three UV-based AOPs, and HPO content is more susceptible to decompose than TPI and HPI content. UV/H₂O₂ was found to be the most effective treatment for the removal of THMFP and AOXFP under given conditions. C1 (microbial or marine derived humic-like substances), C2 (terrestrially derived humic-like substances) and C3 (tryptophan-like proteins) fluorescent components of HPO fraction were fairly labile across the UV-based AOPs treatment. C3 of each fraction of NOM was the most resistant to degrade upon the UV-based AOPs. Results from this study may provide the prediction about the consequence of UV-based AOPs for the degradation of different fractions of NOM with varied characteristics.

An autopsy study of hollow fiber and multibore ultrafiltration membranes from a pilot-scale ultra high-recovery filtration system for surface water treatment

The organic fouling characteristics of hollow fiber ultrafiltration (HFUF) and multibore ultrafiltration (MBUF) membranes from long-term ultrafiltration (UF) membrane systems were systemically investigated in this study. The objective was to obtain insights into the fouling behavior of dissolved organic matter (DOM) in a pilot-scale ultra-high-recovery membrane filtration system (p-UHMS) used for surface water treatment. The pilot system consisted of a series of two different UF membranes (1st stage: polyvinylidene fluoride (PVDF) HFUF and 2nd stage: polyethersulfone (PES) MBUF). It was designed to feed the HFUF concentrate to the MBUF membranes to achieve ≥99.5 % total water recovery for surface water treatment, as these advances might enhance the production efficiencies of drinking water. The experimental results confirmed that hydrophobic DOM controlled the formation of HFUF membrane organic fouling, whereas hydrophilic DOM, including polysaccharide-like and protein-like matter, promoted MBUF membrane fouling. These opposing trends were attributed to the hydrophilic characteristics of the MBUF membrane surfaces (contact angle: PVDF = 90-130° and PES ≤ 80°), which reduced the hydrophobic interactions between the UF membrane surfaces and foulants. The performance declines of the MBUF membrane due to fouling layer formation was considerably severer than those of the HFUF membrane, decreasing total permeate water in the p-UHMS. Moreover, the quantity of the desorbed MBUF membrane foulants via 0.1 N NaOH was roughly 7.2 times larger than that of the desorbed HFUF membrane foulants through 0.1 N NaOH, indicating that alkaline-based cleaning agent could much more efficiently recover the performance of the fouled MBUF membranes. Hence, adequate cleaning strategies using alkaline-based agent for the MBUF membrane appeared to be essential for preventing the performance deterioration of the p-UHMS.

Removal mechanism of Microcystis aeruginosa in Fe2+/sodium percarbonate and Fe2+/sodium persulfate advanced oxidation-flocculation system

Advanced oxidation process (AOPs) can be used for the treatment of harmful algal blooms (HABs). In this study, two systems of Fe²⁺/sodium percarbonate (Fe²⁺/SPC system) and Fe²⁺/sodium persulfate (Fe²⁺/PS system) were established to explore the removal mechanism of Microcystis aeruginosa (M. aeruginosa). The results indicated that the Fe²⁺/SPC system catalyzed H₂O₂ to generate a large amount of [Formula: see text] for oxidation by Fe²⁺ and formed Fe³⁺ to promote the flocculation of M. aeruginosa. The persulfate was activated by Fe²⁺ to generate [Formula: see text] with super-oxidizing properties, and Fe³⁺ was generated to realize the oxidation and flocculation of M. aeruginosa in the Fe²⁺/PS system. Compared with the traditional method in which the pre-oxidation and flocculation processes are carried out separately, the method in this study effectively improves the utilization rate of the flocculant and the removal effect of M. aeruginosa. The absolute value of zeta potential of Fe²⁺/PS system (|ζ|= 0.808 mV) was significantly lower than that of Fe²⁺/SPC system (|ζ|= 21.4 mV) (P < 0.05), which indicated that Fe²⁺/PS system was more favorable for the flocculation of M. aeruginosa cells than the Fe²⁺/SPC system.

Existing Filtration Treatment on Drinking Water Process and Concerns Issues

Water is one of the main sources of life's survival. It is mandatory to have good-quality water, especially for drinking. Many types of available filtration treatment can produce high-quality drinking water. As a result, it is intriguing to determine which treatment is the best. This paper provides a review of available filtration technology specifically for drinking water treatment, including both conventional and advanced treatments, while focusing on membrane filtration treatment. This review covers the concerns that usually exist in membrane filtration treatment, namely membrane fouling. Here, the parameters that influence fouling are identified. This paper also discusses the different ways to handle fouling, either based on prevention, prediction, or control automation. According to the findings, the most common treatment for fouling was prevention. However, this treatment required the use of chemical agents, which will eventually affect human health. The prediction process was usually used to circumvent the process of fouling development. Based on our reviews up to now, there are a limited number of researchers who study membrane fouling control based on automation. Frequently, the treatment method and control strategy are determined individually.

Mitigation of ultrafiltration membrane fouling by a simulated sunlight-peroxymonosulfate system with the assistance of irradiated NOM

Oxidation pretreatments prior to ultrafiltration are hindered by the need for energy input and sludge disposal. Herein, a simulated sunlight-induced natural organic matter (NOM) for peroxymonosulfate (PMS) activation was used as pretreatment to alleviate ultrafiltration membrane fouling caused by NOM itself in the Songhua River water. When light intensity was over 100 mW/cm², the pretreatment removed NOM effectively, characterized with UV₂₅₄, dissolved organic carbon (DOC) and maximum fluorescent intensity (F_max), and improved filtration flux. At 200 mW/cm² light intensity and 0.5 mM PMS, 57.5% of UV₂₅₄ and 18.5% of DOC were removed, and humic-like fluorescent component was degraded by 84%-94% while ∼60% for protein-like substance. Membrane flux was increased by 94%, and reversible and irreversible fouling resistances were reduced by 62.4% and 51.9%, respectively. Both total fouling index (TFI) and hydraulic irreversible fouling index (HIFI) were moderately correlated with the DOC, whereas they prominently correlated with the UV₂₅₄ and the F_maxs of all fluorescence components, which could be served as key indicators to predict and control membrane fouling. Mathematical modeling showed that the pretreatment alleviated the fouling in the membrane pores and cake layer. The simulated sunlight-induced NOM (³NOM* and e_aq^¯) could activate PMS to form active species, which enabled to oxidize high molecular weight (MW) substances and mineralize low MW compounds in NOM as well as hinder their linking with inorganic cations, thereby reducing organic and inorganic membrane fouling simultaneously. This study may provide a new strategy for decentralized potable water treatment, especially in a single household or community.

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.

Micro-polluted water resources treatment by PVDF-TiO2 membrane combined with Fe2+/sodium dithionite (DTN)/O2 pre-oxidation process

Modifying PVDF membrane by blending hydrophilic nano TiO₂ has been highly concerning, but its practical application is not well investigated. In this study, PVDF-TiO₂ membrane was employed in two modes to treat micro-polluted raw water for the first time, direct membrane filtration and pre-oxidation assists membrane filtration. At two filtration modes, the PVDF-TiO₂ membrane had comparable rejection capability to the unmodified PVDF membrane, as the removal of permanganate index (COD_Mn) was 0.26-0.72 mg/L, UV₂₅₄ was 0.0070-0.0618 cm^-1, turbidity was 1.60-4.49 NTU, and the total number of colonies was 360-23,780 CFU/mL. As for raw water treatment, using Fe²⁺/sodium dithionite (DTN)/O₂ system as the pre-oxidation process to assist the filtration of the PVDF-TiO₂ membrane was feasible. After optimization, the applicable conditions of the Fe²⁺/DTN/O₂ process were DTN dosage at 100 mg/L and a C_Fe/C_DTN of 1:4. As a result, the effluent qualities of the PVDF-TiO₂ membrane significantly improved. It was investigated that atrazine (ATZ), COD_Mn, UV_254, and turbidity reduced, which was realized by the synergic effects of the pre-oxidation by free radicals and flocculation by iron. Pre-oxidation of the Fe²⁺/DTN/O₂ process could also enhance the permeability of the PVDF-TiO₂ membrane from 53.6 to 58.0 L/(m²·h), nearly two times the PVDF membrane. Besides, the practical fouling of the PVDF-TiO₂ membrane was stably alleviated by the reduced R_t, R_re, and R_ir, mainly due to constraining the internal pore fouling effectively.

Impacts of Calcium Addition on Humic Acid Fouling and the Related Mechanism in Ultrafiltration Process for Water Treatment

Humic acid (HA) is a major natural organic pollutant widely coexisting with calcium ions (Ca²⁺) in natural water and wastewater bodies, and the coagulation-ultrafiltration process is the most typical solution for surface water treatment. However, little is known about the influences of Ca²⁺ on HA fouling in the ultrafiltration process. This study explored the roles of Ca²⁺ addition in HA fouling and the potential of Ca²⁺ addition for fouling mitigation in the coagulation-ultrafiltration process. It was found that the filtration flux of HA solution rose when Ca²⁺ concentration increased from 0 to 5.0 mM, corresponding to the reduction of the hydraulic filtration resistance. However, the proportion and contribution of each resistance component in the total hydraulic filtration resistance have different variation trends with Ca²⁺ concentration. An increase in Ca²⁺ addition (0 to 5.0 mM) weakened the role of internal blocking resistance (9.02% to 4.81%) and concentration polarization resistance (50.73% to 32.17%) in the total hydraulic resistance but enhanced membrane surface deposit resistance (33.93% to 44.32%). A series of characterizations and thermodynamic analyses consistently suggest that the enlarged particle size caused by the Ca²⁺ bridging effect was the main reason for the decreased filtration resistance of the HA solution. This work revealed the impacts of Ca²⁺ on HA fouling and demonstrated the feasibility to mitigate fouling by adding Ca²⁺ in the ultrafiltration process to treat HA pollutants.

Effect of vacuum ultraviolet/ozone pretreatment on alleviation of ultrafiltration membrane fouling caused by algal extracellular and intracellular organic matter

Algal blooms in source water can cause algal organic matter (AOM)-related membrane fouling in drinking water treatment. Herein, the effects of vacuum ultraviolet/ozone (VUV/O₃) pretreatment on alleviating ultrafiltration membrane fouling caused by AOM, including extracellular organic matter (EOM) and intracellular organic matter (IOM), were investigated systematically. Compared to its sub-processes (UV/O₃, O₃, VUV, and UV), VUV/O₃ pretreatment showed the best performance on AOM removal and membrane fouling mitigation. After VUV/O₃ pretreatment, the DOC of EOM and IOM in feed decreased by 51.1% and 26.7%, respectively, and fluorescence components and UV₂₅₄ of EOM and IOM in feed decreased obviously. Hence, the final specific fluxes of the membranes increased significantly under the impacts of VUV/O₃, and VUV/O₃ achieved 89.5% and 97.2% mitigation of reversible fouling caused by EOM and IOM, respectively. VUV/O₃ pretreatment also reduced the foulants on membrane surface and surface roughness. Moreover, under the effects of reactive oxygen species oxidation, VUV photolysis, and direct O₃ oxidation, VUV/O₃ decreased organic load and changed the molecular weight distribution, hydrophilicity, and interaction-free energy of AOM, thus mitigating membrane fouling. Furthermore, the effects of O₃ dosage and molecular weight cut-off of ultrafiltration membrane on membrane fouling mitigation by VUV/O₃ were also investigated. All results highlighted that VUV/O₃ pretreatment had huge potential in mitigating AOM-induced membrane fouling.

Effects of UV/Fe(II)/sulfite pre-treatment on NOM-enhanced Ca2+ scaling during nanofiltration treatment: Fouling mitigation, mechanisms, and correlation analysis of membrane resistance

This study was aimed to evaluate the effects of a pre-treatment involving sulfite (S(IV)) synergistically activated by ultraviolet (UV)/Fe(II) on natural organic matter (NOM)-enhanced Ca²⁺ scaling during nanofiltration treatment. Based on the variations in the physicochemical properties and correlation analyses of irreversible resistance, the intrinsic fouling mechanisms were revealed from two aspects: bulk crystallization (interaction between NOM and inorganic ions) and surface crystallization (morphology of surface crystallization and a change in the Ca²⁺ concentration in the scaling layer). Furthermore, the degradation contribution rates of different free radicals during the UV/Fe(II)/S(IV) (UFS) treatment process were evaluated. During the reactions in the UFS, three free radicals (SO·-4, OH^·- and e- aq) were generated, and in-situ Fe(III) was formed in-situ. The carboxyl groups of the NOM were attacked by the free radicals, resulting in decreased of carboxyl concentration and density. In addition, the bond between Ca²⁺ and NOM weakened, and hydrophobic (HPO) substances were mineralized. However, the Fe(III) formed in-situ was active and electropositive, competing with Ca²⁺ for the complexation active sites on the NOM. The synergy effect of bulk crystallization and surface crystallization led to a significant decrease in the particle size of feed solution. The crystal size and roughness of membrane surface also decreased, which was conducive to reducing the membrane irreversible resistance. Correlation analysis revealed that the HPO ratio, carboxyl density and particle size (> 100 nm) ratio were effective characterization parameters for predicting irreversible resistance. This study not only provides guidance for alleviating membrane fouling caused by NOM-enhanced Ca²⁺ scaling during the nanofiltration process, but also presents the rationality of irreversible resistance during nanofiltration process and various indicators with strong linear correlation.

Advanced oxidation processes and selection of industrial water source: A new sight from natural organic matter

Natural organic matter (NOM) refers to the dissolved organic matter in natural water that can pass through 0.45 μm filter membrane. As a pivotal role in the surface water body, it has a significant effect on the efficiency of AOPs. In this study, Excitation emission matrix - parallel factor (EEM-PARAFAC) analysis is used to elucidate the changes of NOM fluorescence peaks after electrochemical oxidation process, two-dimensional correlation spectroscopy (2D-FTIR-COS) and Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) are utilized to clarify the molecular characteristics of NOM in surface water and the effects of electrochemical oxidation on NOM molecules. The results indicate that parts of NOM molecules are mineralized into simple compounds and precursors of refractory organic matters produced by some NOM molecules after AOPs. It is concluded that the precursors of these refractory organic matters may belong to terrestrial humus (C2). Therefore, for the purpose of avoiding more refractory organic pollutants produced by NOM which can reduce the performance of AOPs in the water treatment process, factories should choose water sources with less humus as industrial water supply, or degrade humus by physical or chemical methods before industrial water supply.

Layered metal oxides loaded ceramic membrane activating peroxymonosulfate for mitigation of NOM membrane fouling

Catalytic membrane can achieve sieving separation and advanced oxidation simultaneously, which can improve the effluent water quality while reducing membrane fouling. In this study, the catalytic membranes (M²⁺Al@AM) were fabricated by loading different binary layered metal oxides (M²⁺Al-LMO: MnAl-LMO, CuAl-LMO and CoAl-LMO) on alumina ceramic substrate membranes (AM) via vacuum filtration followed by calcination process. The performance of the catalytic membranes was investigated by filtering actual surface water. It was found that the presence of peroxymonosulfate (PMS) could mitigate membrane fouling effectively, as evidenced by the increase of normalized flux from 0.28 to 0.62 in CoAl@AM/PMS system, from 0.25 to 0.52 in CuAl@AM/PMS system, and from 0.22 to 0.31 in MnAl@AM/PMS system, respectively. Correspondingly, the CoAl@AM exhibited the highest removal for UV₂₅₄, TOC and fluorescent components in the surface water, followed by CuAl@AM and MnAl@AM. Quenching effect of phenol and furfuryl alcohol proposed the surface-bound radicals and singlet oxygen were the major reactive oxygen species in the M²⁺Al@AM/PMS systems. Interface free energy calculations confirmed the in-situ PMS activation could enhance the repulsive interactions between NOM and the membranes, thus mitigating membrane fouling. This work provides an original but simple strategy for catalytic ceramic membrane preparation and new insights into the mechanism of membrane fouling mitigation in catalytic membrane system.

Dissecting the structure-property relationship of ceramic membrane with asymmetric multilayer structures for maximizing permselectivity

Robust ceramic membranes presented attractive features of easy cleaning and excellent stability compared to polymeric membranes. Nevertheless, their inherent relationships between the membrane microstructures and separation properties are not completely clear. In this work, we established a quantitative structure-property model using α-Al₂O₃ membrane on account of the theory of filtrated cake to predict the effects of membrane structure-controlled factors (i.e., α-Al₂O₃ particle size and layer thickness) on separation performances (i.e., solute rejection and water permeance). The simulation results show that membrane pore size mainly depends upon α-Al₂O₃ particle size rather than the layer thickness. When the microstructure of top layer in a double-layer asymmetric ceramic membrane is fixed, there exists optimum particle size and layer thickness that constitute the support layer to achieve maximum water permeance. For a triple-layer ceramic membrane, a similar matching relationship exists between top layer and intermediate layer, indicating that the intermediate layer has a vital role in determining water permeance. While the bottom layer has little effect on overall separation property. Finally, the upper-bound tradeoff relationship between permeance and selectivity is further established for the α-Al₂O₃ membrane. This study reveals the structure-property relationship of ceramic membrane and provides insights into performance enhancement.

Cost and efficiency perspectives of ceramic membranes for water treatment

More robust ceramic membranes with tailorable structures and functions are increasingly employed for water treatment, particularly in some harsh applications for their ultra-long service lifespan due to their high mechanical, structural, chemical and thermal stability and anti-fouling properties. Decreasing cost and enhancing efficiency are two key but quite challenging application-oriented issues for broader and larger-scale engineering application of current ceramic membranes, and are required to make ceramic membranes a highly efficient and economic water treatment technique. In this review, we critically discuss these two significant concerns of both cost and efficiency for water treatment ceramic membranes, focusing on an overview of various advanced strategies and mechanism insights. A brief up-to-date discussion is first introduced about recent developments of ceramic membranes covering the major advances of novel membranes and applications. Then some promising strategies for decreasing the cost of ceramic membranes are discussed, including membrane material cost and processing cost. To fully address the issue of moderate efficiency with single separation function, valuable and considerable insights are provided into recent major progress and mechanism understandings in application with other unit processes, such as advanced oxidation and electrochemistry techniques, to significantly enhance treatment efficiency. Subsequently, a review of recent ceramic membrane applications emphasizing harsh operating environments is presented, such as oil-water separation, saline water, refractory organic and emerging contaminant wastewater treatment. Finally, engineering application, conclusions, and future perspectives of ceramic membrane for water treatment applications are critically discussed offering new insight based on understanding the issues of cost and efficiency.

Mechanism study on the effect of peracetic acid (PAA), UV/PAA and ultrasonic/PAA oxidation on ultrafiltration performance during algae-laden water treatment

In this work, peracetic acid (PAA), ultraviolet (UV)/PAA and ultrasonic (US)/PAA pre-oxidation were applied to enhance ultrafiltration (UF) performance during algae-laden water treatment. The results showed that 10 mg/L PAA, exhibiting an optimal performance with membrane fouling resistance reduced by 76.26%. Low dosage of UV/PAA can effectively control fouling by enhancing the degradation of dissolved organics. Though more radicals were generated with the increasing dosage of PAA during the UV/PAA process, flux deterioration was occurred when PAA dosage over 10 mg/L, owing to a negative correlation between fouling resistance and algal integrity loss. Compared with UV, US exhibited a worse activation effect on PAA with less reactive radicals produced. Even worse, US can stimulate the stress metabolism of algal cells with slightly integrity loss, which then resulted in an exacerbation of permeate quality. Fouling mechanism analysis revealed that the delay formation of cake layer with membrane fouling alleviation mainly through efficient degradation of macromolecular organics. The investigation of synergistic and individual effect of EOM degradation, algae rupture and IOM release on the filtration performance revealed that EOM degradation was the primary mechanism for fouling control while algae rupture rather than IOM release was crucial for membrane fouling aggravation. This indicates that moderate oxidation, with property of high organic degradation and low cell rupture, was the working principal and objectives for algae-laden water treatment. Additionally, it was found that the ·OH radicals produced during UV/PAA process can efficiently degrade representative odors. In general, pretreatments of PAA and low dosages of UV/PAA showed promising prospects in improving the UF performance of algae-laden water and treating algal secretions.

A sacrificial protective layer as fouling control strategy for nanofiltration in water treatment

High-performance nanofiltration (NF) membrane with super antifouling capability as well as reusability is highly desired in water treatment. A new antifouling strategy by a coating-decoating-recoating cycle was investigated for effective removal of fouling and restoring the original membrane performance. The functional membrane surface was fabricated by in-situ coating a 'green' and biodegradable carboxymethyl chitosan (CMCS) layer as physical barrier. The CMCS layer can be decoated and re-coated by simple procedures. Results showed that (i) the CMCS layer enhanced surface hydrophilicity, surface smoothness and fouling resistance of NF membrane, (ii) both the unfouled and fouled CMCS layer were easily decoated by the strong acid solution, (iii) the CMCS layer was easily re-coated by facile recoating and (iv) the water flux recovery ratio of membrane with coating layer was maintained more than 88.8% during fouling testing by natural organic matter (NOM) after four sequential cycles of coating, decoating and recoating process. The re-coated membrane exhibited stable, improved membrane operational and antifouling performance. The coating-decoating-recoating approach is proven to be low-cost and eco-friendly strategy for NOM fouling control on NF membrane in water treatment applications.

Degradation of methyl orange by pyrite activated persulfate oxidation: mechanism, pathway and influences of water substrates

Degradation mechanism of methyl orange (MO), a typical azo dye, with pyrite (FeS₂) activated persulfate (PS) was explored. The results showed that when the initial concentration of MO was 0.1 mM, FeS₂ was 1.6 g/L and PS was 1.0 mM, the removal rate of MO could reach 92.9% in 150 min, and the removal rate of total organic carbon could reach 14.1%. In addition, both pH ≤ 2 and pH ≥ 10 could have an inhibitory effect in the FeS₂/PS system. Furthermore, Cl^- and low concentrations of HCO^-₃ had little effect on the degradation of MO with FeS₂/PS. However, H₂PO^-₄ and high concentrations of HCO^-₃ could inhibit the degradation of MO in the system. Besides, MO in river water and tap water were not degraded in FeS₂/PS system, but acidification (pH = 4) would greatly promote the degradation. In addition, the removal rate of MO with FeS₂/PS could still reach about 90% after five cycles of FeS₂. Furthermore, the intermediates and possible degradation pathways were speculated by LC-MS, and the degradation mechanism of MO by FeS₂/PS was that the cycle of Fe(III)/Fe(II) could continuously activate persulfate to produce SO₄^•-. The results could provide technical support for azo dye degradation in the FeS₂/PS system.

Kinetics and Transformations of Diverse Dissolved Organic Matter Fractions with Sulfate Radicals

Dissolved organic matter (DOM) scavenges sulfate radicals (SO₄^•-), and SO₄^•--induced DOM transformations influence disinfection byproduct (DBP) formation when chlorination follows advanced oxidation processes (AOPs) used for pollutant destruction during water and wastewater treatment. Competition kinetics experiments and transient kinetics experiments were conducted in the presence of 19 DOM fractions. Second-order reaction rate constants for DOM reactions with SO₄^•- (k_DOM,SO4^•-) ranged from (6.38 ± 0.53) × 10⁶ M^-1 s^-1 to (3.68 ± 0.34) × 10⁷ M_C^-1 s^-1. k_DOM,SO4^•- correlated with specific absorbance at 254 nm (SUVA₂₅₄) (R² = 0.78) or total antioxidant capacity (R² = 0.78), suggesting that DOM with more aromatics and antioxidative moieties reacted faster with SO₄^•-. SO₄^•- exposure activated DBP precursors and increased carbonaceous DBP (C-DBP) yields (e.g., trichloromethane, chloral hydrate, and 1,1,1-trichloropropanone) in humic acid and fulvic acid DOM fractions despite the great reduction in their organic carbon, chromophores, and fluorophores. Conversely, SO₄^•--induced reactions reduced nitrogenous DBP yields (e.g., dichloroacetonitrile and trichloronitromethane) in wastewater effluent organic matter and algal organic matter without forming more C-DBP precursors. DBP formation as a function of SO₄^•- exposure (concentration × time) provides guidance on optimization strategies for SO₄^•--based AOPs in realistic water matrices.

Activation of persulfate by humic substances: Stoichiometry and changes in the optical properties of the humic substances

Persulfate activation through electron transfer from humic substances (HS) was investigated. Persulfate consumption in the presence of standard HS and HS model compounds linearly correlated with the phenol contents of the HS. Redox-active carbonyl groups such as aromatic ketones and quinone also contributed to persulfate consumption by donating electrons while being reduced. Phenols activated persulfate through direct electron transfer from the phenolate forms but reduced ketones activated persulfate through reactions between their organic radicals and persulfate. Persulfate was activated more by terrestrially derived aquatic HS containing large numbers of phenol groups than by other species, and this caused more benzene oxidation to occur in the presence of terrestrially derived aquatic HS than in the presence of other species. Larger amounts of sulfate radicals were scavenged by soil-derived HS than other types of HS because soil-derived HS were composed of larger molecules than other types of HS. The fluorescence regional integration volume for HS reacted with persulfate linearly correlated with persulfate consumption. Decreases in the fluorescence regional integration value could be used to predict persulfate activation through electron transfer from HS to persulfate if the electron-donating capacity cannot be determined. Persulfate activation by HS is expected to be stoichiometrically more advantageous than conventional persulfate-Fe²⁺ processes when treating an aquifer containing large amounts of electron-rich HS.

Study on the Control of Membrane Fouling by Pulse Function Feed and CFD Simulation Verification

A complex-function fluid controller placed in front of a membrane module was used to control the velocity change with feed fluid and reduce membrane fouling. Using humic acid as the simulated pollutant, the effects of the square wave function, sine function, reciprocal function, and power function feeding on the membrane flux were investigated. For sine function feeding, the membrane-specific flux was the largest and was maintained above 0.85 under the intermittent frequency of 9 s. Compared with the final membrane-specific flux with steady-flow feeding of 0.55, functional feeding could significantly reduce membrane fouling. SEM results showed that sine feeding led to slight contamination on the membrane surface. Furthermore, the Computational Fluid Dynamics (CFD) simulation results showed that the shear force of sine function feeding was about three times that of the steady flow (6 × 10⁵ N). Compared with steady feeding, functional feeding could significantly improve the shear force on the membrane surface and reduce membrane fouling.

Exploring the fate of dissolved organic matter at the molecular level in the reactive electrochemical ceramic membrane system using fluorescence spectroscopy and FT-ICR MS

This research evaluated the performance of reactive electrochemical ceramic membrane (REM) in treating secondary effluent and investigated the fate of dissolved organic matter (DOM) at the molecular level. The role of adsorption, electrosorption, and oxidation in DOM removal was comprehensively elucidated based on fluorescence spectroscopy and high-resolution mass spectrometry (FT-ICR MS). Among the fluorescence components (C1-C3) in secondary effluent, microbial humic-like C2 showed fewer adsorption on the REM surface without applying an electrical potential. The electrosorption helped an enhanced uptake of all DOM components and transformed them onto the electrode surface. The fluorescence components and all three fractions (hydrophilic, transphilic, and hydrophobic) were rapidly degraded, and finished water with stable DOM was obtained. The leading degradation phenomena were the change of the unsaturated compounds to the aliphatic and transformation of large-sized molecules to medium and small-sized ones. Above 70% of the compounds in the secondary effluent acted as precursors, which were mineralized/degraded and transformed products were found on the REM surface and in the finished water. The compounds containing sulfur (CHOS) were easily and preferably degraded/mineralized, followed by the compounds containing nitrogen (CHON) and CHO. The oxidation of DOM led to the extensive formation of organo-chlorinated compounds, which contributed above 80% in products. Overall, the combination of fluorescence spectroscopy and FT-ICR MS provided unique behavior of DOM in the secondary effluent toward electro-oxidation in the REM system. These findings could help explore the potential of REM for different water matrices to project the possible composition of DOM in the finished water.

Use of an ultraviolet light-activated persulfate process to degrade humic substances: effects of wavelength and persulfate dose

Natural organic matter (NOM), commonly found in surface and ground waters, form disinfection by-products in drinking water. Generally, advanced oxidation processes (AOPs) featuring hydrogen peroxide are used to treat water; however, sulfate radical recently has been used to treat recalcitrant organics, because it is associated with a higher oxidation potential and more effective removal than hydroxyl radicals. Hence, in this research, we evaluated persulfate oxidation efficiency in terms of reductions in humic substance levels and investigated the degradation mechanism. The results showed that ultraviolet-activated persulfate effectively treated humic substances compared with hydrogen peroxide and direct irradiation. Treatment was dose and wavelength dependent; higher persulfate concentrations or shorter UV wavelengths were more effective for treating humic substances as high concentration sulfate radicals were created. The degradation mechanism was similar to that of hydrogen peroxide. Aromatic and chromophore components were more susceptible to degradation than were lower molecular weight components, being initially decomposed into the latter, reducing UV₂₅₄ absorbance and the SUVA₂₅₄. Lower molecular weight materials were eventually degraded to end products: NPOC levels fell. And we also treated the inflow of a drinking water treatment plant with persulfate, and humic substances were effectively removed.

Effects of electroflocculation/oxidation pretreatment on the fouling characteristics of ultrafiltration membranes

In order to reduce the membrane pollution of ultrafiltration caused by natural organic matter and improve the treatment efficiency, electroflocculation/oxidation is used as the premembrane treatment method. The membrane specific flux attenuation characteristics was compared and analyzed under the conditions of direct ultrafiltration and electroflocculation/oxidation-ultrafiltration. Combined with the analysis of the reversibility of membrane fouling, the mechanism of electroflocculation/oxidation pretreatment to alleviate ultrafiltration membrane fouling was evaluated, and the membrane pore clogging model was used to fit the fouling law. The results show that, in the continuously fed filtration experiment, the electroflocculation/oxidation process involved in the pretreatment and the direct ultrafiltration membrane filtration decreased the ultrafiltration membrane flux to 79.1% and 28.5%, respectively. The reversible resistance generated by ultrafiltration and electroflocculation/oxidation-ultrafiltration processes accounted for 37.70% and 62.26% of their total pollution resistance, whereas the irreversible resistance generated accounted for 47.30% and 12.40%, respectively. Meanwhile, the direct correlation between the the flux dropped and complete clogging became less than that of the ultrafiltration process. The pretreatment significantly strengthened irreversible fouling resistance of the membrane pores. The membrane permeation flux was significantly increased after the electroflocculation/oxidation pretreatment.

The nitrogen-doped multi-walled carbon nanotubes modified membrane activated peroxymonosulfate for enhanced degradation of organics and membrane fouling mitigation in natural waters treatment

The synthesized catalyst nitrogen-doped multi-walled carbon nanotubes (N-MWCNTs) were introduced into membrane technology for peroxymonosulfate (PMS) activation. The enhanced permeability of the N-MWCNTs-modified membrane might be attributed to the increase in hydrophilicity and membrane porosity. The catalytic degradation and membrane filtration performance for the N-MWCNTs-modified membrane/PMS system in treating different types of natural waters were evaluated. The removal of phenol by the N-MWCNTs-modified membrane was 83.67% in 2 min, which was greater than the phenol removal by the virgin membrane (3.39%) and N-MWCNT powder (41.42%), respectively. Moreover, the resultant membrane coupled with PMS activation exhibited outstanding removal effects on the fluorescent organics in the secondary effluent and Songhua River water. The combination effectively reduced the total membrane fouling caused by the secondary effluent, Songhua River water, and three typical model organics by 28.19-61.98%. Electron paramagnetic resonance and classical quenching tests presented that the active species (SO4·-, ·OH, and 1O2) and other non-radical processes generated by N-MWCNTs activated PMS decreased the foulants deposition on the membrane surface. Meanwhile, the membrane interception accelerated the aggregation of pollutants and PMS towards the membrane surface through applied pressure, facilitating their mass transfer to the N-MWCNTs surface for the catalysis exerted more effectively. This study demonstrated the potential application of the coupling of N-MWCNTs catalytic oxidation and the UF, which offers a promising prospect to improve the permeate quality and simultaneously overcome the membrane fouling barriers.

Key Points of Advanced Oxidation Processes (AOPs) for Wastewater, Organic Pollutants and Pharmaceutical Waste Treatment: A Mini Review

Advanced oxidation procedures (AOPs) refer to a variety of technical procedures that produce OH radicals to sufficiently oxidize wastewater, organic pollutant streams, and toxic effluents from industrial, hospital, pharmaceutical and municipal wastes. Through the implementation of such procedures, the (post) treatment of such waste effluents leads to products that are more susceptible to bioremediation, are less toxic and possess less pollutant load. The basic mechanism produces free OH radicals and other reactive species such as superoxide anions, hydrogen peroxide, etc. A basic classification of AOPs is presented in this short review, analyzing the processes of UV/H2O2, Fenton and photo-Fenton, ozone-based (O3) processes, photocatalysis and sonolysis from chemical and equipment points of view to clarify the nature of the reactive species in each AOP and their advantages. Finally, combined AOP implementations are favored through the literature as an efficient solution in addressing the issue of global environmental waste management.