Application of pulsed UV-irradiation and pre-coagulation to control ultrafiltration membrane fouling in the treatment of micro-polluted surface water

Spectral fusion-based machine learning classifiers for discriminating membrane breakage in multiple scenarios

Membrane breakage can lead to filtration failure, which allows harmful substances to enter the effluent, posing potential hazards to human health and the environment. This study is an innovative combination of fluorescence and ultraviolet-visible (UV-Vis) spectroscopy to identify membrane breakage. It aims to unravel more comprehensive information, improve detection sensitivity and selectivity, and enable real-time monitoring capabilities. Fluorescence and UV-Vis data are extracted through variance partitioning analysis (VPA) and integrated through a decision tree algorithm to form a superior system with enhanced discrimination capabilities. VPA improves discrimination efficiency by extracting key information from spectral data and eliminating redundancy. The decision tree algorithm, on the other hand, can process large amounts of data simultaneously. In addition, the method has a wide range of applications and can be used in various scenarios accurately. The scenarios include domestic sewage, micropollutant water, aquaculture wastewater, and secondary treated sewage. The experimental results validate the application of machine learning classifiers in membrane breakage detection with an accuracy rate of 96.8 % to 97.4 %.

Combination of magnetic field and ultraviolet for fouling control in saline wastewater distribution systems

Fouling is a significant challenge for recycling and reusing saline wastewaters for industrial, agricultural or municipal applications. In this study, we propose a novel approach of magnetic field (MaF) and ultraviolet (UV) combined application for fouling mitigation. Results showed, combination of MaF and UV (MaF-UV) significantly decreased the content of biofouling and reduced the complexity of microbial networks, compared to UV and MaF alone treatments. This was due to MaF as pretreatment effectively reduced the water turbidity, improve the influent water quality of UV disinfection and increases UV transmittance, eliminating the adverse impacts of UV scattering and shielding, hence increased the inactivation effectiveness of UV disinfection process. MaF assisted UV also reduced the abundance of UV-resistant bacteria and inhibited the risk of bacterial photoreactivation and dark repair. Meanwhile, MaF-UV drastically reduced the contents of precipitates and particulate fouling by accelerating the transformation rate of CaCO3 crystal from compact calcite to loosen hydrated amorphous CaCO3, and enhancing the flocculation process. These findings demonstrated that MaF-UV is an effective anti-fouling strategy, and provide insights for sustainable application of saline wastewaters.

Deciphering the synergistic effects of photolysis and biofiltration to actuate elimination of estrogens in natural water matrix

The presence of estrogens in water environments has raised concerns for human health and ecosystems balance. These substances possess potent estrogenic properties, causing severe disruptions in endocrine systems and leading to reproductive and developmental problems. Unfortunately, conventional treatment methods struggle to effectively remove estrogens and mitigate their effects, necessitating technological innovation. This study investigates the effectiveness of a novel sequential photolysis-granular activated carbon (GAC) sandwich biofiltration (GSBF) system in removing estrogens (E1, E2, E3, and EE2) and improving general water quality parameters. The results indicate that combining photolysis pre-treatment with GSBF consistently achieved satisfactory performance in terms of turbidity, dissolved organic carbon (DOC), UV254, and microbial reduction, with over 77.5 %, 80.2 %, 89.7 %, and 92 % reduction, respectively. Furthermore, this approach effectively controlled the growth of microbial biomass under UV irradiation, preventing excessive head loss. To assess estrogen removal, liquid chromatography-tandem mass spectrometry (LC-MS) measured their concentrations, while bioassays determined estrogenicity. The findings demonstrate that GSBF systems, with and without photolysis installation, achieved over 96.2 % removal for estrogens when the spike concentration of each targeted compound was 10 µg L-1, successfully reducing estrogenicity (EA/EA0) to levels below 0.05. Additionally, the study evaluated the impact of different thicknesses of GAC layer filling (8 cm, 16 cm, and 24 cm) and found no significant difference (p>0.05) in estrogen and estrogenicity removal among them.

Recent advances on micro/nanoplastic pollution and membrane fouling during water treatment: A review

Effluent from sewage treatment plant, as an important source of microplastics (MPs) in receiving water, has attracted extensive attention. Membrane separation process shows good microplastic removal performance in the existing tertiary water treatment process. Problematically, membrane fouling and insufficient removal of small organic molecules are still the key obstacles to its further extensive application. Dissolved organics, extracellular polymers and suspended particles in the influent are deposited on the membrane surface and internal structure, reducing the number and pore diameter of effective membrane aperture, and increasing the resistance of membrane filtration. Exploring the mechanism and approach of membrane fouling caused by micro/nanoplastics is the key to alleviate fouling and allow membranes to operate longer. In this paper, removal performance of micro/nanoplastics by current membrane filtration and the contribution to membrane fouling during water treatment are thoroughly reviewed. The coupling mechanisms between micro/nanoplastics and other pollutants and mechanism of membrane fouling caused by composite micro/nanoplastics are discussed. Additionally, on this basis, the prospect of combined process for micro/nanoplastic removal and membrane fouling prevention is also proposed and discussed, which provides a valuable reference for the preferential removal of micro/nanoplastics and development of antifouling membrane.

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.

The characters of dissolved organic matters from litter-mimic with the different humification states and their effects on drinking water treatment processes

Dissolved organic matter (DOM) is one kind of the main pollutant in surface water that will cause many problems during drinking water treatment processes. In this study, a simulated humification process of forest litter-mimic was investigated for eight weeks continuously to study the variations in chemical properties such as DOM composition, polysaccharide/protein ratio, average molecular weight, oxidation degree, hydrophilicity, etc., as well as the impact of these variations on the coagulation, ultra-/nanofiltration (UF/NF). Results showed that the removal rate of coagulation (from 67.5 % to 37.0 %) and UF (from 14.4 % to 5.8 %) decreased significantly during the humification process as a function of time, while the removal rate of NF increased from 40.0 % to 72.9 % at first, and then decreased to 47.4 %. This study gave a deep insight into the effect of DOM with different humification ages on the drinking water treatment process with the influence of seasons and vegetation around the water source, which finally aimed to improve drinking water treatment.

Establishing a high-performance anti-fouling PEI-ZIF-PAA membrane with improved Lewis acid-base interactions and hydrophilicity

Membrane fouling and the trade-off between membrane permeability and selectivity restrict the potential applications of membrane filtration for water treatment. ZIF-8 was found having great permeability and antibiofouling performance, but with issue on particle aggregation makes it difficult to achieve high ZIFs loading and fabricate a defect-free molecular sieving membrane in previous research. In this study, we formed a scalable antibiofouling surface with improved permeability and fouling resistance on a PEI-ZIF-PAA membrane using a layer-by-layer assembly technique. The synergistic effects of being sandwiched between two different polyelectrolyte layers with opposite charges endowed the ZIF nanoparticles with improved stability and scalability for membrane modification. The PEI-ZIF-PAA membrane exhibited a satisfactory water flux of 120.78 LMH, which was 46.97% higher than that of the pristine PES membrane. The normalized water flux loss was serious in the absence of ZIF-8, and the flux increased with the ZIF-8 concentration. Antifouling tests suggested that the PEI-ZIF-PAA membrane possessed good antifouling performance due to the much higher surface hydrophilicity and positive Lewis acid-base interactions with foulants. The HA rejection increased with the ZIF-8 concentration and reached a maximum of 92.1% in the presence of 1.00% (w/v) ZIF-8. The membrane regeneration was tested under physical and chemical cleaning with flux recovery rates of about 85% and 95%. XDLVO analysis showed that the total interaction energy between HA and the PEI-ZIF-8-PAA membrane was 26.45 mJ/m², and the superior antifouling performance was mainly attributed to Lewis acid-base interactions. This study indicates that ZIF-8 nanocrystals are promising materials for fabricating novel membranes for sewage treatment.

Enhanced permeate flux by air micro-nano bubbles via reducing apparent viscosity during ultrafiltration process

Micro-nano bubbles (MNBs) play important roles in the reduction of membrane fouling during membrane separation; however, such improvements are always attributed to the reduced concentration polarization on the surface of membranes and little attention has been paid on the variations of physicochemical properties of the feed caused by MNBs. In this study, the separation efficiencies of the feed containing humic acid (HA), bovine serum albumin (BSA), sodium alginate (SA) or dyes can be improved by MNBs during ultrafiltration, and the normalized fluxes can be maximally increased to 139% and 127% in the dead-end and cross-flow modes, respectively in the treatment of HA solution. We further reveal that the decreased apparent viscosity of the feed in the presence of MNBs is the key factor that enhances the normalized flux during ultrafiltration. This study gives new insight on the importance of MNBs in membrane separation and provides valuable clues for other chemical processes.

The performance of electrode ultrafiltration membrane bioreactor in treating cosmetics wastewater and its anti-fouling properties

The membrane fouling problem of the membrane bioreactor (MBR) for wastewater treatment reduces the membrane flux and the pollutants removal efficiencies, which is the major obstacle limiting its application and should be properly solved. The combination of membrane and electricity can effectively slow down the membrane fouling rate due to electric repulsion between the pollutants and the membrane. In this study, the performance and the membrane fouling features of an electrode ultrafiltration membrane bioreactor (EMBR) fed with cosmetics wastewater were compared with a conventional ultrafiltration membrane bioreactor (UMBR). The results showed the COD removal efficiency increased by 4.43% and the transmembrane pressure (TMP) reduced by 50% in the EMBR as compared with the UMBR. The specific surface areas of electrode ultrafiltration membrane and conventional ultrafiltration membrane declined by 56.9% and 78.8% after 90 days of operation, respectively. The Protein (PN), polysaccharide (PS) and humic acids (HA) in the cake layer of EMBR were only 61.27%, 78.37% and 34.85% of that of UMBR, which contributed to its loose and porous structure and thus decreased the growth rate of TMP and extended the operation cycle. Extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory calculation proved that the energy barrier between the electrode ultrafiltration membrane and the pollutants was 50% higher than that between the conventional ultrafiltration membrane and the pollutants. Therefore, the strong anti-fouling property of the electrode ultrafiltration membrane could reduce the chemicals dosage and manpower consumption for membrane cleaning and could be preferred for the treatment of cosmetics or alike wastewater containing high concentrations of surfactants and fatty acids.

Preparation of polyvinylidene fluoride composite ultrafiltration membrane for micro-polluted surface water treatment

Blending modification of graphene oxide (GO) and deposition of silver carbonate (Ag₂CO₃) on the membrane surface by suction filtration was used to prepare polyvinylidene fluoride (PVDF) composite ultrafiltration (UF) membranes (denoted as PGA membranes). The effect of this strategy on the morphology and performance of the pure PVDF membrane was investigated. Owing to an increased hydrophilicity and the formation of a more open pore, the pollution resistance and permeability of the PGA membrane were improved. The pure water flux of the PGA-3 membrane (254 LMH) was increased to more than 2-fold compared to that of the neat PVDF membrane (126 LMH). In addition, the results of antifouling experiments showed that the flux recovery rate, flux decay rate, and antibacterial performance of the PGA-3 membrane was superior to those of the other membranes synthesized in this study. Finally, after conducting multi-cycle filtration experiments with lake water, the flux and recovery rate of the PGA-3 membrane was observed to be the highest, and the water quality of the lake water filtered by the PGA-3 membrane was the best. Thus, the above results indicate that this membrane modification strategy is extraordinarily effective in improving the antifouling properties and permeability of the PVDF UF membranes in practical applications.

The suitability of titanium salts in coagulation removal of micropollutants and in alleviation of membrane fouling

Coagulation is a conventional method in water treatment. In recent decades, with the rapid development of membrane filtration, the use of coagulation is facing some new challenges. How to minimize the membrane fouling became a leading-edge topic in the study of coagulation. Here, the performances of three types of titanium coagulants were evaluated in terms of both the coagulation removal of toxic micropollutants and the alleviation of membrane fouling. Three oxysalts and two antibiotics were taken as representatives of inorganic and organic micropollutants. As compared with titanium tetrachloride (TiCl₄) and polytitanium chloride (PTC), titanium xerogel (TXC) with a higher polymerization degree showed much better performances in direct coagulation removal of oxysalts and antibiotics and in pre-coagulation for mitigating membrane fouling in both coagulation-sedimentation-ultrafiltration (CSUF) and in-line coagulation-ultrafiltration (CUF) processes. In the CSUF system, the membrane permeate flux with TXC pre-coagulation (89.5%) was much higher than those of TiCl₄ (56.1%) and PTC (57.4%). After a 5 day continuous operation, the transmembrane pressure in the CUF system with TXC coagulation was increased only to 4.9 kPa, while those of PTC and TiCl₄ were 12.2 and 18.5 kPa, respectively. The results here demonstrate that TXC is a promising coagulant for pollutant removal and membrane fouling alleviation, due to the following merits: better floc properties, weaker pH-dependence, and higher resistance to coordination with organic pollutants. The observation shed new lights on the fabrication and application of coagulants in a wide variety of scenarios.

Orbitrap mass spectrometry for the molecular characterization of water resource recovery from polluted surface water using membrane bioreactor

The increasing organic contamination of surface water hinders the conventional tap water treatment process. Membrane bioreactors (MBRs) are a promising alternative technology for recovering water from polluted surface water. In this study, the composition changes of dissolved organic matters (DOMs) in MBR and ultraviolet/ozone (UV/O₃)-MBR systems for polluted surface water treatment were investigated using Orbitrap mass spectrometry analysis with unknown screening. The intense DOM ions within a mass-to-charge ratio range of 100-500 was detected, and 2340 molecular formulae from 5743 peaks were assigned to the two systems. The most abundant components were formulae with C, H, O, N, and CHO only classes. The highest formulae decrease including CHO, CHON, CHOS, and CHONS were attributed to the bio-carrier used in both systems. Results showed that bioprocess was the main contributor in the DOM reduction, and the integration of UV/O₃ into the MBR improved the DOM composition changes. Biodegradable components with low O/C ratio in the CHO and CHON classes remarkably increased in the UV/O₃-MBR system. The integration of UV/O₃ as a polishing step in the recirculation stream of MBR system was effective in improve the DOM removal.

Synergistic fouling behaviors and mechanisms of calcium ions and polyaluminum chloride associated with alginate solution in coagulation-ultrafiltration (UF) process

Effects of calcium ions and polyaluminum chloride (PACl) on membrane fouling in coagulation-ultrafiltration (UF) process were investigated in this study. Filtration tests demonstrated three interesting filtration behaviors: 1) high specific filtration resistance (SFR) of alginate solution with low CaCl₂ or PACl addition (e. g. 3.51×10¹⁵ m·kg ^-1 under the condition of 1.5 mM CaCl₂ addition); 2) unimodal pattern of alginate SFR with PACl or CaCl₂ addition alone; 3) synergistic effects between CaCl₂ and PACl on alginate SFR. It was found that, the foulant morphological changes driven by the thermodynamic mechanisms based on Flory-Huggins lattice theory take the critical roles in these filtration behaviors. Density functional theory (DFT) calculations showed that initial coordination of Ca²⁺ and Al³⁺ ions with alginates tended to form tetrahedron geometry and geometry of coordinating three terminal carboxyl groups, respectively, which facilitated to elongate the alginate chains (without clustering the flocs) and form more stable gel, increasing SFR. Improving Ca²⁺ and Al³⁺ dosages triggered transition to other geometries for clustering polymeric network and flocculation, reducing SFR. Due to the higher binding affinity of Ca²⁺ over Al³⁺, Ca²⁺ and Al³⁺ sequentially take roles of enlarging polymeric network and clustering the coordination compounds, and then facilitate to form large size flocs and reduce SFR, causing the synergistic effects between CaCl₂ and PACl additions. The proposed thermodynamic mechanisms satisfactorily explained these interesting fouling behaviors, allowing to further optimize coagulation-UF process.

Increased E. coli bio-adsorption resistance of microfiltration membranes, using a bio-inspired approach

Cells have inherent anti-fouling properties. The mechanisms underpinning these natural properties inform the design of an anti-biosorption coating for a polyethersulfone microfiltration membrane, which includes polydopamine and chitosan layers. This tri-layered membrane is created using quick and easy synthesis method. Its ability to resist bio-adsorption and membrane extracellular polymeric substances (EPS) formation is investigated using the bacterium E. coli (ATCC 11775, 1.5 × 10⁷ CFU/mL). In addition, the proliferative bio-adsorption process is explored on the microfiltration membrane surface, using natural water under static and shaken conditions, while monitoring the bio-adsorption kinetics and EPS dynamic changes. The characterization results show that the modification by polydopamine and chitosan change the membrane surface morphology and increase its hydrophilicity. After 10 min dipping in 5 g/L chitosan solution, the pure water flux of the modified membrane is 5469 ± 30 L/(m²·h) (0.2 bar) and the contact angle decreases to 36.7 ± 1.0°, compared with 9889 ± 23 L/(m²·h) (0.2 bar) and 60.3 ± 1.5° for the unmodified polyethersulfone membrane, respectively. In proliferative bio-adsorption tests, the modified membrane is shown to decrease bio-adsorption by 0.4-2.3 orders of magnitude. However, no antimicrobial function is observed, probably due to the alkaline environment and insufficient functional amino groups. A series of linear and non-linear kinetic models is applied to fit the proliferative bio-adsorption process. The pseudo-second-order model is found to describe the proliferative bio-adsorption process best. Neither total organic carbon (TOC) nor protein is detected on the modified membrane surface. In contrast, on the unmodified PES membrane the ratios of protein/TOC (%), TOC/abundance ((μg/cm²)/CFU (log)) and protein/abundance ((μg/cm²)/CFU(log)) are 10%-16%, 0.17-0.28 and 0.02-0.04, respectively. No significant difference (p > 0.05) is found between static and shaken conditions. All these results point to improved anti-biosorption properties for water treatment applications, encouraging further studies on this membrane.

A new perspective of membrane fouling control by ultraviolet synergic ferrous iron catalytic persulfate (UV/Fe(II)/PS) as pretreatment prior to ultrafiltration

The main purpose of this study was to control membrane fouling by degrading natural organic matter, mainly based on free radicals, with a dead-end ultrafiltration system integrated with pretreatment. Four advanced oxidation processes, namely, ultraviolet (UV)/Fe(II), UV/persulfate (PS), Fe(II)/PS and UV/Fe(II)/PS, were used to pretreat raw water prior to ultrafiltration. The priority of the pretreatment effect followed the order of UV/Fe(II)/PS > Fe(II)/PS > UV/PS > UV/Fe(II). In the UV/Fe(II)/PS pretreatment (Fe(II) = 100 μM and PS = 400 μM), the removal rates of UV₂₅₄ with a UV irradiation time of 60 min reached 93.07%. Degradation experiments of free-radical probes (carbamazepine) and free-radical scavenger addition (sodium hyposulfite or tert-butanol) showed that the sulfate radical (SO₄^-) was dominant in degrading organic compounds. The specific flow rate (J/J₀) increased by 139.13% and the irreversible fouling resistance was reduced by 69.94%. The total interfacial energy of the colloid-membrane interaction decreased by 84.42% and the separation distance was shortened to ~2 nm. The release of Fe(III) from water under UV radiation and its possible conversion to Fe(II) were observed on the surface of the fouled membranes. After UV/Fe(II)/PS pretreatment, bulky and rough pollutant particles were transformed into a slew of sheet-contaminated layer with the appearance of numerous permeable holes, and the average surface roughness was reduced to 38.1 nm according to atomic-force-microscopy characterization.

MBR-UV/Cl2 system in treating polluted surface water with typical PPCP contamination

This study proposed the membrane bioreactor–ultraviolet/chlorine (MBR-UV/Cl₂) process for treating polluted surface water with pharmaceutical personal care product (PPCP) contamination. Results showed that MBR-UV/Cl₂ effectively removed the organic matters and ammonia at approximately 80% and 95%. MBR-UV/Cl₂ was used in the removal of sulfadiazine(SDZ), sulfamethoxazole(SMZ), tetracycline(TC), oxytetracycline(OTC), ciprofloxacin(CIP), ofloxacin(OFX), erythromycin(ERY), roxithromycin(ROX), ibuprofen(IBU) and, naproxen(NAX) at 12.18%, 95.61%, 50.50%, 52.97%, 33.56%, 47.71%, 87.57%, 93.38%, 93.80%, and 71.46% in which their UV/Cl₂ contribution was 12.18%, 95.61%, 29.04%, 38.14%, 25.94%, 7.20%, 80.28%, 33.79%, 73.08%, and 23.05%, respectively. The removal of 10 typical PPCPs using UV/Cl₂ obtained higher contributions than those of the MBR process, except OTC, ROX, and IBU. The UV/Cl₂ process with 3-min hydraulic retention time and chlorine concentration at 3 mg/L effectively removed the trace of PPCPs. MBR-UV/Cl₂ has the potential to be developed as an effective technology in treating polluted surface water with PPCP contamination.

Carbon doped Fe3O4 peroxidase-like nanozyme for mitigating the membrane fouling by NOM at neutral pH

Oxidation is a widely used method in drinking water treatment to mitigate the membrane fouling caused by the natural organic matters (NOM) from the surface water during ultra-filtration (UF) and nano-filtration (NF) processes, and H₂O₂ is one of the common oxidants for it. However, the oxidation capability of H₂O₂ at neutral pH is lower, compared to the acidic and alkaline conditions. In order to improve the efficiency of NOM oxidation at neutral pH, a carbon-doped Fe₃O₄ peroxidase-like nanozyme (CFPN) was synthesized in this study and used as a high-performance catalyst for H₂O₂ to generate hydroxyl radical. The oxygen-containing groups on the carbon structure of CFPN can form an acidic microenvironment, allowing H₂O₂ to produce hydroxyl radical by catalysis in neutral conditions. The results of hydrophilicity analysis, zeta potential, high-performance liquid size exclusion chromatography (HPSEC), Fourier transform infrared spectrum (FTIR) and flux indicated that the hydroxyl radical can oxidize the hydrophobic matters of humic acid (HA) into hydrophilic matters by Fenton reaction or electrophilic addition reaction, which can mitigate the fouling of NF membranes. The results of the same test for the bovine serum albumin (BSA) indicated that the hydroxyl radical can mitigate the fouling of UF membranes by degrading the tertiary and secondary structures of BSA and partly oxidizing the side chain groups. In addition, two types of surface water samples were used to verify the above mechanism, and the results indicated that the hydroxyl radical treatment at neutral pH is a new viable and effective strategy to significantly mitigate the NOM fouling of UF and NF membranes.

Tracking metal ion-induced organic membrane fouling in nanofiltration by adopting spectroscopic methods: Observations and predictions

Natural organic matter (NOM) with the size approaching to membrane pore size is commonly considered as the crucial component leading to severe pore blocking and superfluous energy consumption. Aquatic metal ions coexisting with this NOM constituent (target NOM) exert a significant influence on membrane filtration performance; however, little work elucidated their interactions and the impacts on nanofiltration (NF). Therefore, we systematically investigated this issue by titrating three environmentally-relevant metal ions (Al³⁺, Fe³⁺ and Cu²⁺) into the target NOM sample obtained by pre-filtering using NF membrane. Fast spectrophotometric techniques were employed to observe the interactive performance. Results suggested that all metal ions at their critical concentrations caused severe flux decline; Cu²⁺ at a very low concentration of 5 μM, Al³⁺ and Fe³⁺ at 20 μM. NF performance recovered when the concentrations were beyond their critical values, and was improved at excessive concentration when flocs formed. Relationship between spectroscopic characteristics and NF performance was particularly addressed. UV-vis spectrum can be expected to be useful and predictive in membrane fouling control when Al³⁺ or Fe³⁺ presented. However, fluorescence fingerprint was not likely that effective since fluorescence intensity continuously reduced with the increasing metal ion concentration, attributed to their quenching effect on NOM fluorophores.

The Fe-N-C oxidase-like nanozyme used for catalytic oxidation of NOM in surface water

The removal of natural organic matter (NOM), particularly humic substances (HS) from surface waters during drinking water treatment is necessary to avoid various water quality problems in supply, such as the formation of disinfection by-products. As an alternative to conventional processes (e.g. coagulation), and in the light of the rapidly increasing applications of nanozyme in bio-catalysis, a novel Fe-N-C oxidase-like nanozyme (FeNZ) has been prepared and used to catalyze the oxidative degradation of NOM during simple aeration. Using humic acid (HA) as a model NOM it was found that the HA removal (as TOC) was increased by a factor of 6 with a low dose (10 mg/L) of FeNZ compared to an aerated solution without FeNZ. A variety of analytical methods was used to investigate the oxygen reduction reaction, including cyclic voltammetry, electron spin resonance, and density functional theory (DFT) simulation. Based on these studies, a catalytic oxidation mechanism described as "adsorption-activation-oxidation" was proposed. The enhanced NOM removal performance of FeNZ catalytic oxidation was confirmed with samples of natural surface water in terms of organic mineralization and conversion of hydrophobic to hydrophilic components. The results show great potential for the use of oxidase-like nano catalytic materials in the field of water treatment.

Quantitative analysis of cake characteristics based on SEM imaging during coagulation-ultrafiltration process

Cake formed by flocs is a crucial factor to affect membrane fouling during coagulation-ultrafiltration process. To investigate the role of floc properties on cake, cake characteristics under various coagulant dosage conditions were calculated by scanning electron microscope (SEM) imaging. Results found that one SEM image with × 5000 magnification could accurately estimate cake porosity with relative error lower than 5.00% for all conditions, whereas more SEM images with × 10,000 magnification or × 20,000 magnification should be applied to calculate cake porosity precisely. This could be explained by different pore information of SEM images with various magnifications. Compared to single SEM image with × 10,000 magnification and × 20,000 magnification, single SEM image with × 5000 magnification contained the most comprehensive pore information and slightly overestimated pore area for pore smaller than 0.4 μm² due to lower resolution. To verify feasibility by SEM image evaluating cake characteristics, cake porosity calculated by SEM image and Carman-Kozeny equation were analyzed. The results showed that cake porosity estimated by these two methods were nearly the same, proving the feasibility of this method. Moreover, with the increase of coagulant dosage, cake porosity presented similar variation with floc average size, indicating that floc average size was likely to dominate cake porosity in this study. For pore characteristics, pore average characteristic length and pore average area were in accordance with floc fractal dimension, whereas pore fractal dimension and pore amount were consistent with floc average size. This gives specific information about the relation between floc properties and cake characteristics.

Effect of pre-oxidation on low pressure membrane (LPM) for water and wastewater treatment: A review

Low pressure membrane (LPM) filtration is a promising technology for drinking water production, wastewater reclamation as well as pretreatment for seawater desalination. However, wider implementation of LPM is restricted by their inherent drawbacks, i.e., membrane fouling and insufficient rejection for dissolved contaminants. Pretreatment of feed water is a major method to improve the performance of LPM, and pre-oxidation has gained extensive attention because it can significantly alter compositions and properties of feed water through chemical reactions. This paper attempts to systematically review efficiency and mechanisms of pre-oxidation in membrane fouling control and permeate water quality improvement. On the basis of briefly discussing major foulants and fouling mechanisms of LPM, advantages and disadvantages of pre-oxidation in mitigating organic fouling, inorganic fouling and biofouling are discussed in detail. Impacts of pre-oxidation on removal of micropollutants, bulk organic matter and inorganic pollutants are summarized, and potential by-products of different oxidants are presented. As a prerequisite for the integration of chemical oxidation with LPM filtration, compatibility of membrane with oxidants at low concentration and long exposure time are highlighted. Finally, the existing challenges and future research needs in practical application of chemical oxidation to improve performance of LPM are also discussed.

Comparative study on the pretreatment of algae-laden water by UV/persulfate, UV/chlorine, and UV/H2O2: Variation of characteristics and alleviation of ultrafiltration membrane fouling

In this study, ultraviolet based advanced oxidation processes (UV-AOPs) including UV/persulfate (UV/PS), UV/chlorine, and UV/H₂O₂ were employed to alleviate ultrafiltration membrane fouling during the treatment of algae-laden water. The results show that UV/PS pretreatment exhibited the best performance on fouling control, followed by the UV/H₂O₂ pretreatment. The fouling mitigation performance improved with the increase of oxidant dose. However, UV/chlorine pretreatment aggravated membrane fouling, and the irreversible fouling resistance increased by five times compared with that of raw water. The dissolved organic carbon (DOC) in the algae-laden solution was reduced after UV/PS pretreatment, while either UV/chlorine or UV/H₂O₂ pretreatment had little influence on the DOC of feed water. UV/PS and UV/H₂O₂ pretreatments were effective in the degradation of fluorescent compounds, thus reducing the deposition of organic matter on the membrane surface. Additionally, the decreased concentration of hydrophobic organics, algal cells, and debris in feed water after UV/PS pretreatment was also contributed to the fouling alleviation. The aggravated irreversible fouling after UV/chlorine pretreatment was probably ascribed to the increased accumulation of hydrophobic fractions in the membrane pores. Modeling result indicates that membrane fouling during the filtration of raw algae-laden water was dominated by intermediate blocking and cake filtration mechanisms. Both UV/PS and UV/H₂O₂ pretreatments transformed the combined fouling mechanism into standard blocking, while UV/chlorine pretreatment aggravated the pore blocking in the initial filtration period.

Pre-coagulation coupled with sponge-membrane filtration for organic matter removal and membrane fouling control during drinking water treatment

A new hybrid system was developed in this study for the treatment of drinking water consisting of pre-coagulation using polyaluminium chloride (PACl) and membrane filtration (MF) with sponge cubes acting as biomass carriers (P-SMF). When compared to a conventional MF (CMF) and a MF after coagulation by utilizing PACl (P-MF), better removal of nutrients, UV₂₅₄ and dissolved organic carbon (DOC) (>65%) was obtained from the P-SMF. The accumulation of biopolymers (including polysaccharides and proteins), humic substances, hydrophilic organics, and other small molecular weight (MW) organic matter in the CMF led to the most severe membrane fouling coupled with the highest pore blocking and cake resistance. Pre-coagulation was ineffective in eliminating small MW and hydrophilic organic matter. Conversely, the larger MW organics (i.e. biopolymers and humic substances), small MW organics and hydrophilic organic compounds could be removed in significantly larger quantities in the P-SMF by PACl coagulation. This was achieved via adsorption and the biodegradation by attached biomass on these sponges and by the suspended sludge. Further analyses of the microbial community indicated that the combined addition of PACl and sponges generated a high enrichment of Zoolgloea, Amaricoccus and Reyranella leading to the reduction of biopolymers, and Flexibacter and Sphingobium were linked to the degradation of humic substances. Moreover, some members of Alphaproteobacteria in the P-SMF may be responsible for the removal of low MW organics. These results suggest that the pre-coagulation process coupled with adding sponge in the MF system is a promising technology for mitigating membrane fouling.

Surface coating of UF membranes to improve antifouling properties: A comparison study between cellulose nanocrystals (CNCs) and cellulose nanofibrils (CNFs)

The inherent properties of hydrophilicity and environmental preferability of cellulose nanocrystals (CNCs) and cellulose nanofibers (CNFs) make them great candidates for application in water-treatment membranes. In this study, the antifouling properties of CNCs and CNFs, modified ultrafiltration (UF) membranes, were directly compared. A facile modification method was conducted by coating CNCs and CNFs on the surface of polyethersulfone (PES) membranes to prepare CNC-coating membranes and the CNF-coating membranes. Membrane surface morphology was characterized by atomic force microscopy (AFM), and the results showed that the CNF-coating membranes exhibited greater surface roughness than the CNC-coating membranes. Pure water flux measurements demonstrated that the flux of the CNC-coating membranes was slightly lower than that of the CNF-coating membranes. Antifouling properties were evaluated and compared for the two types of membranes by filtration of NOM foulant models, humic acid (HA) and bovine serum albumin (BSA). The results showed that the antifouling properties of the modified membranes were enhanced through the coating of either CNCs or CNFs to a control PES membrane. The CNC-coating membranes outperformed the CNF-coating membranes in alleviating both reversible fouling and irreversible fouling caused by HA and BSA. In addition, the antifouling performance of the coating membranes was enhanced with increased coating content.

Ultraviolet/persulfate (UV/PS) pretreatment of typical natural organic matter (NOM): Variation of characteristics and control of membrane fouling

The effects of ultraviolet/persulfate (UV/PS) pretreatment on ultrafiltration (UF) membrane fouling caused by typical natural organic matter (NOM) fractions including humic acid (HA), sodium alginate (SA), and bovine serum albumin (BSA) were investigated. UF membrane fouling during the filtration of different NOM fractions after UV/PS pretreatment was compared through the evaluation of normalized membrane flux decline and membrane fouling reversibility. The fouling mitigation mechanisms were investigated through the characterization of ultraviolet absorbance (UV₂₅₄), dissolved organic matter, zeta potential, particle size distribution, fluorescence excitation-emission matrix spectra, and fitness of four classic fouling models. Furthermore, the fouled membranes were characterized by Fourier-transform infrared spectroscopy and scanning electron microscopy. The results showed that UV/PS pretreatment significantly alleviated membrane fouling caused by HA, SA, and HA-SA-BSA mixture, and the fouling control performance improved at high PS doses. However, either UV alone or UV/PS pretreatment at low PS dose (10 mg/L) significantly aggravated BSA fouling with the normalized flux at the end of first filtration cycle being 8% and 15%, respectively. The increased particle size of BSA after UV/PS pretreatment was attributed to the formation of aggregates, which mainly accumulated in membrane pores and aggravated membrane fouling. Modeling results suggest that the mitigation of membrane fouling derived from SA and mixed organic fractions was primarily ascribed to the control of cake filtration, while the mitigation of HA fouling was attributed to the declined contribution of standard blocking.

Characterization of dissolved organic matter and membrane fouling in coagulation-ultrafiltration process treating micro-polluted surface water

Coagulation-ultrafiltration (C-UF) is widely used for surface water treatment. With the removal of pollutants, the characteristics of organic matter change and affect the final treatment efficiency and the development of membrane fouling. In this study, we built a dynamic C-UF set-up to carry out the treatment of micro-polluted surface water, to investigate the characteristics of dissolved organic matter from different units. The influences of poly aluminum chloride and poly dimethyldiallylammonium chloride (PDMDAAC) on removal efficiency and membrane fouling were also investigated. Results showed that the dosage of PDMDAAC evidently increased the UV₂₅₄ and dissolved organic carbon removal efficiencies, and thereby alleviated membrane fouling in the C-UF process. Most hydrophobic bases (HoB) and hydrophobic neutral fractions could be removed by coagulation. Similarly, UF was good at removing HoB compared to hydrophilic substances (HiS) and hydrophobic acid (HoA) fractions. HiS and HoA fractions with low molecule weight accumulated on the surface of the membrane, causing the increase of transmembrane pressure (TMP). Membrane fouling was mainly caused by a removable cake layer, and mechanical cleaning was an efficient way to decrease the TMP.

Effect of sulfate radical-based oxidation pretreatments for mitigating ceramic UF membrane fouling caused by algal extracellular organic matter

Algal extracellular organic matter (EOM) released from Microcystis aeruginosa can cause severe membrane fouling during algae-laden water treatment. To solve this problem, three typical sulfate radical-based advanced oxidation processes (SR-AOPs), i.e., ferrous iron/peroxymonosulfate (Fe(II)/PMS), UV/PMS and UV/Fe(II)/PMS, were employed as membrane pretreatment strategies. Their performance on mitigating EOM fouling of a ceramic UF membrane was systematically investigated and compared in the present study. The results indicated that SR-AOPs pretreatments could promote the reduction of DOC and UV₂₅₄, and the removal performance showed an apparent regularity of UV/Fe(II)/PMS > Fe(II)/PMS > UV/PMS. The pretreatments were very effective for decomposing high-MW biopolymers (>20,000 Da) into low-MW humic substances (1000-20,000 Da), thus reducing the accumulation of high-MW biopolymers on membrane surface. With respect to membrane fouling control, Fe(II)/PMS significantly mitigated both reversible and irreversible membrane fouling, whereas UV/PMS only reduced reversible fouling, and exhibited little effect on irreversible fouling. By contrast, UV/Fe(II)/PMS showed the best performance for fouling reduction due to the synergistic effect of UV and Fe(II) for PMS activation. The dominating fouling mechanism was governed by both pore blockage and cake filtration, likely due to the bimodal MW distribution of EOM, and SR-AOPs pretreatments delayed the transition from pore blockage to cake filtration. In addition, SR-AOPs prior to UF membrane were also very effective to improve the removal of micropollutants (i.e., ATZ, SMT and p-CNB). These results demonstrate the potential application of SR-AOPs as pretreatment for membrane fouling control during algae-laden water treatment.

Ultrafiltration and nanofiltration membrane fouling by natural organic matter: Mechanisms and mitigation by pre-ozonation and pH

The fouling of ultrafiltration (UF) and nanofiltration (NF) membranes during the treatment of surface waters continues to be of concern and the particular role of natural organic matter (NOM) requires further investigation. In this study the effect of pH and surface charge on membrane fouling during the treatment of samples of a representative surface water (Hyde Park recreational lake) were evaluated, together with the impact of pre-ozonation. While biopolymers in the surface water could be removed by the UF membrane, smaller molecular weight (MW) fractions of NOM were poorly removed, confirming the importance of membrane pore size. For NF membranes the removal of smaller MW fractions (800 Da-10 kDa) was less than expected from their pore size; however, nearly all of the hydrophobic, humic-type substances could be removed by the hydrophilic NF membranes for all MW distributions (greater than 90%). The results indicated the importance of the charge and hydrophilic nature of the NOM. Thus, the hydrophilic NF membrane could remove the hydrophobic organic matter, but not the hydrophilic substances. Increasing charge effects (more negative zeta potentials) with increasing solution pH were found to enhance organics removal and reduce fouling (flux decline), most likely through greater membrane surface repulsion. Pre-ozonation of the surface water increased the hydrophilic fraction and anionic charge of NOM and altered their size distributions. This resulted in a decreased fouling (less flux decline) for the UF and smaller pore NF, but a slight increase in fouling for the larger pore NF. The differences in the NF behavior are believed to relate to the relative sizes of ozonated organic fractions and the NF pores; a similar size of ozonated organic fractions and the NF pores causes significant membrane fouling.

Remediation of water and wastewater by using engineered nanomaterials: A review

Nanotechnology is currently a fast-rising socioeconomic and political knowledge-based technology owing to the unique characteristics of its engineered nanomaterials. This branch of technology is useful for water and wastewater remediation. Many scientists and researchers have been conducting different studies and experiments on the applications of engineered nanomaterials at the local to international level. This review mainly aims to provide a current overview of existing knowledge on engineered nanomaterials and their applications in water and wastewater remediation. Furthermore, the present risks and challenges of nanotechnology are examined.

Impacts of sodium hydroxide and sodium hypochlorite aging on polyvinylidene fluoride membranes fabricated with different methods

This study compared the effects of chemical aging on the polyvinylidene fluoride (PVDF) membranes fabricated with the methods of non-solvent induced phase separation (NIPS) (named NIPS-PVDF) and thermally induced phase separation (TIPS) (named TIPS-PVDF). The chemical solutions of sodium hypochlorite (NaClO) and sodium hydroxide (NaOH) were chosen at the concentration of 5000mg/L. The equivalence of 5 and 10years was respectively selected as the time of aging. The physicochemical evolutions of membrane aging are characterized on the base of morphology analysis, chemical components, permeation ability and mechanical properties. The aging of NIPS-PVDF membrane led to the elimination of surface hydrophilic additives, while NaOH focused on the dehydrofluorination process resulting in the formation of conjugated chains of polyene on the skeleton structure. The chemical components of the surface of TIPS-PVDF membrane were removed continuously during the aging processes of both NaClO and NaOH, which was caused by the saponification of surface additives and the chain scissions of skeleton structure, but without producing any obvious conjugated chains of polyene. All the aging processes led to the increase of contact angle and the decrease of mechanical properties, and the permeability was reduced first and increased later due to the enlargement of surface membrane pores and membrane block. With the influence of membrane aging, selectivity of membrane was decreased (except coliform bacteria). At the beginning of filtration, the turbidity and particle count were at relatively high levels and declined with the filtration process.

Application of Fe(II)/peroxymonosulfate for improving ultrafiltration membrane performance in surface water treatment: Comparison with coagulation and ozonation

Coagulation and ozonation have been widely used as pretreatments for ultrafiltration (UF) membrane in drinking water treatment. While beneficial, coagulation or ozonation alone is unable to both efficiently control membrane fouling and product water quality in many cases. Thus, in this study an emerging alternative of ferrous iron/peroxymonosulfate (Fe(II)/PMS), which can act as both an oxidant and a coagulant was employed prior to UF for treatment of natural surface water, and compared with conventional coagulation and ozonation. The results showed that the Fe(II)/PMS-UF system exhibited the best performance for dissolved organic carbon removal, likely due to the dual functions of coagulation and oxidation in the single process. The fluorescent and UV-absorbing organic components were more susceptible to ozonation than Fe(II)/PMS treatment. Fe(II)/PMS and ozonation pretreatments significantly increased the removal efficiency of atrazine, p-chloronitrobenzene and sulfamethazine by 12-76% and 50-94%, respectively, whereas coagulation exerted a minor influence. The Fe(II)/PMS pretreatment also showed the best performance for the reduction of both reversible and irreversible membrane fouling, and the performance was hardly affected by membrane pore size and surface hydrophobicity. In addition, the characterization of hydraulic irreversible organic foulants confirmed its effectiveness. These results demonstrate the potential advantages of applying Fe(II)/PMS as a pretreatment for UF to simultaneously control membrane fouling and improve the permeate quality.

Evolution of molecular weight and fluorescence of effluent organic matter (EfOM) during oxidation processes revealed by advanced spectrographic and chromatographic tools

Effluent organic matter (EfOM) is an emerging concern to receiving aquatic environment due to its refractory property. The degradation of EfOM in ozonation and other two advanced oxidation processes (AOPs), UV/H₂O₂ and UV/persulfate (PS), was investigated in this study. Fluorescence spectra coupled with parallel factor analysis (PARAFAC) and two-dimensional correlation gel permeation chromatography (2D-GPC) were used to track the evolution of EfOM during each oxidation process. Results showed that the degradation of EfOM indicated by dissolved organic carbon (DOC), UV₂₅₄ and fluorescence components, fitted well with pseudo-first-order kinetic model during the oxidation processes. Ozonation showed higher degradation efficiency than AOPs, while UV/PS was more effective than UV/H₂O₂ with equimolar oxidants dosage. Ozone and SO·₄^- were more reactive with terrestrial humic-like substances, while hydroxyl radical preferentially reacted with protein-like substances. Organic molecules with higher molecular weight (MW) were susceptible to ozone or radicals. Ozonation could transform higher MW (MW of 3510 and 575) organic matters into lower MW organic matters (MW of 294), while reductions of all the organics were observed in both AOPs. Due to the higher reaction rates between ozone and EfOM, ozonation maybe serve as a pre-treatment for AOPs to reduce the radical and energy consumption and improve mineralization of EfOM by AOPs. The decline in DOC, UV₂₅₄, fluorescence and reduction in oxidants increased with the increase of oxidants dosage, and linear correlations among them were found during the ozonation and AOPs.

Probing the application of a zirconium coagulant in a coagulation–ultrafiltration process: observations on organics removal and membrane fouling

The Zr coagulant offered improved turbidity and organics removal and was beneficial in alleviating membrane fouling, particularly the irreversible fouling.