Biofouling by ultra-low pressure filtration of surface water: The paramount role of initial available biopolymers

Enhancing performance in gravity-driven membrane systems through pre-coating with aluminum-based flocs: Mechanism and energy saving analysis

The gravity-driven membrane (GDM) system is an energy-efficient and environmentally sustainable water purification process; however, after prolonged operation, its membrane flux remains relatively low, making it necessary to adopt effective strategies for improving system performance. In this study, the effects of hydrostatic pressure (60, 100, 200 mbar) and pre-coating with aluminum-based flocs (ABF) on GDM flux and organic matter removal were investigated, and the regulatory mechanisms of the bio-cake layer were explored through interactions between morphological structure, composition and microbes. The results showed that the stable flux of the GDM-ABF system at a hydrostatic pressure of 60 mbar was almost equal to that at 100 mbar, and it outperformed higher hydrostatic pressure in organic matter removal, resulting in a more porous bio-cake layer structure. GDM-ABF system at 60 mbar achieved 38.51% energy saving compared to that at 100 mbar. Increased hydrostatic pressure led to a denser biofouling layer and higher EPS concentrations, whereas pre-coating reduced the EPS concentration and resulted in a looser biofouling layer. Hydrostatic stress and pre-coating determined membrane fouling by regulating microbial communities and key metabolites. Increasing hydrostatic pressure down-regulated arginine and proline metabolism and aggravated membrane fouling, while pre-coating ABF up-regulated arginine and proline metabolism, down-regulated galactose metabolism, and alleviated the membrane fouling. Hydrostatic stress and pre-coating altered the abundance of keystone species involved in extracellular polymeric substances (EPS) formation within the bio-cake layer. Pre-coating with ABF at low hydrostatic pressure can achieve stable flux and effective water purification in GDM systems, similar to high hydrostatic pressure conditions, with the added benefits of being more environmentally friendly and low-carbon. This study proposes a strategy to balance flux and energy consumption in GDM systems, providing theoretical and technical support for the efficient application of GDM technology in membrane water treatment processes.

Typical Heterotrophic and Autotrophic Nitrogen Removal Process Coupled with Membrane Bioreactor: Comparison of Fouling Behavior and Characterization

There is limited research on the relationship between membrane fouling and microbial metabolites in the nitrogen removal process coupled with membrane bioreactors (MBRs). In this study, we compared anoxic-oxic (AO) and partial nitritation-anammox (PNA), which were selected as representative heterotrophic and autotrophic biological nitrogen removal-coupled MBR processes for their fouling behavior. At the same nitrogen loading rate of 100 mg/L and mixed liquor suspended solids (MLSS) concentration of 4000 mg/L, PNA-MBR exhibited more severe membrane fouling compared to AO-MBR, as evidenced by monitoring changes in transmembrane pressure (TMP). In the autotrophic nitrogen removal process, without added organic carbon, the supernatant of PNA-MBR had higher concentrations of protein, polysaccharides, and low-molecular-weight humic substances, leading to a rapid flux decline. Extracellular polymeric substances (EPS) extracted from suspended sludge and cake sludge in PNA-MBR also contributed to more severe membrane fouling than in AO-MBR. The EPS subfractions of PNA-MBR exhibited looser secondary structures in protein and stronger surface hydrophobicity, particularly in the cake sludge, which contained higher contents of humic substances with lower molecular weights. The higher abundances of Candidatus Brocadia and Chloroflexi in PNA-MBR could lead to the production of more hydrophobic organics and humic substances. Hydrophobic metabolism products as well as anammox bacteria were deposited on the hydrophobic membrane surface and formed serious fouling. Therefore, hydrophilic membrane modification is more urgently needed to mitigate membrane fouling when running PNA-MBR than AO-MBR.

The effect of temperature on fouling in anaerobic membrane bioreactor: SMP- and EPS-membrane interactions

Biofouling is the main challenge in the operation of anaerobic membrane bioreactors (AnMBRs). Biofouling strongly depends on temperature; therefore, we hypothesize that the interactions and viscoelastic properties of soluble microbial products (SMP) and extracellular polymeric substances (EPS) vary with temperature, consequently influencing membrane permeability. This study compares the performance of an AnMBR operated at a similar permeate flux at two temperatures. The transmembrane pressure (TMP) rose rapidly after 5 ± 2 days at 25 °C but only after 18 ± 2 days at 35 °C, although the reactor's biological performance was similar at both temperatures, in terms of the efficiency of dissolved organic carbon removal and biogas composition, which were obtained by changing the hydraulic retention time. Using confocal laser scanning microscopy (CLSM), a higher biofilm amount was detected at 25 °C than at 35 °C, while quartz crystal microbalance with dissipation (QCM-D) showed a more adhesive, but less viscous and elastic EPS layer. In situ optical coherence tomography (OCT) of an ultra-filtration membrane, fed with the mixed liquor suspended solids (MLSS) at the two temperatures, revealed that while a higher rate of TMP increase was obtained at 25 °C, the attachment of biomass from MLSS was markedly less. Increased EPS adhesion to the membrane can accelerate TMP increase during the operation of both the AnMBR and the OCT filtration cell. EPS's reduced viscoelasticity at 25 °C suggests reduced floc integrity and possible increased EPS penetration into the membrane pores. Analysis of the structures of the microbial communities constituting the AnMBR flocs and membrane biofilms reveals temperature's effects on microbial richness, diversity, and abundance, which likely influence the observed EPS properties and consequent AnMBR fouling.

Purifying surface water contaminated with azo dyes using nanofiltration: Interactions between dyes and dissolved organic matter

In this research, the interactions of two azo dyes, Methyl Orange (MO) and Eriochrome Black T (EBT), with dissolved organic matter (DOM) in surface water were studied, emphasizing their removal using nano-filtration membranes (NF-270 and NF-90). High-Performance Size Exclusion Chromatography (HPSEC) findings indicated that the dyes' molecular weight in deionized (DI) water ranged from 500 to 15k Dalton (Da), adjusting peak intensities with Jingmi River (JM) water Beijing. Notably, when dyes were diluted in JM water, ultraviolet (UV533 & 466, and UV254), together with total organic carbon (TOC) parameters, revealed color removal rates of 99.49% (EBT), 94.2% (MO), 87.6% DOM removal, and 86% TOC removal for NF-90. The NF-90 membrane demonstrated a 75% flux decline for 50 mL permeate volume due to its finer pore structure and higher rejection effectiveness. In contrast, the NF-270 membrane showed a 60% decline in flux under the same conditions. Attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR) analysis of dye-treated membranes in JM water revealed that the NF-270 showed a CC bond peak at 1660 cm-1 across various samples, while analyzing NF-90, the peaks at 1400 cm-1, 1040 cm-1, 750 cm-1, and 620 cm-1 disappeared for composite sample removal. The hydrophobicity of each membrane is measured by the contact angle (CA), which identified that initial CAs for NF-270 and NF-90 were 460 and 700, respectively, that were rapidly declined but stabilized after a few seconds of processing. Overall, this investigation shows that azo dyes interact with DOM in surface waters and enhance the removal efficiency of NF membranes.

Excessive Ozonation Stress Triggers Severe Membrane Biofilm Accumulation and Fouling

The established benefits of ozone on microbial pathogen inactivation, natural organic matter degradation, and inorganic/organic contaminant oxidation have favored its application in drinking water treatment. However, viable bacteria are still present after the ozonation of raw water, bringing a potential risk to membrane filtration systems in terms of biofilm accumulation and fouling. In this study, we shed light on the role of the specific ozone dose (0.5 mg-O3/mg-C) in biofilm accumulation during long-term membrane ultrafiltration. Results demonstrated that ozonation transformed the molecular structure of influent dissolved organic matter (DOM), producing fractions that were highly bioavailable at a specific ozone dose of 0.5, which was inferred to be a turning point. With the increase of the specific ozone dose, the biofilm microbial consortium was substantially shifted, demonstrating a decrease in richness and diversity. Unexpectedly, the opportunistic pathogen Legionella was stimulated and occurred in approximately 40% relative abundance at the higher specific ozone dose of 1. Accordingly, the membrane filtration system with a specific ozone dose of 0.5 presented a lower biofilm thickness, a weaker fluorescence intensity, smaller concentrations of polysaccharides and proteins, and a lower Raman activity, leading to a lower hydraulic resistance, compared to that with a specific ozone dose of 1. Our findings highlight the interaction mechanism between molecular-level DOM composition, biofilm microbial consortium, and membrane filtration performance, which provides an in-depth understanding of the impact of ozonation on biofilm accumulation.

Less pressure contributes to gravity-driven membrane ultrafiltration with greater performance: Enhanced driving efficiency and reduced disinfection by-products formation potential

Gravity-driven membrane (GDM) systems have been well developed previously; however, impacts of driving (i.e., transmembrane) pressure on their performance received little attention, which may influence GDM performance. In this study, we evaluated 4 GDM systems via altering the transmembrane pressure from 50 mbar to 150 mbar with 2 groups, treating surface water in Beijing, China. Results showed that less driving pressure was more favorable. Specifically, compared to groups (150 mbar), groups under a pressure of 50 mbar were found to have greater normalized permeability and lower total resistance. During the whole operation period, the quality of effluents was gradually improved. For example, the removal efficiency of UV254 was significantly improved; particularly, under low driving pressure, the removal efficiency of UV254 in PES GDM system increased by 11.91%, as compared to the corresponding system under high driving pressure. This observation was consistent with the reduction on disinfection by-products (DBPs) formation potential; groups under 50 mbar achieved better DBPs potential control, indicating the advantages of lower driving pressure. Biofilms were analyzed and responsible for these differences, and distinct distributions of bacteria communities of two GDM systems under 50 and 150 mbar may be responsible for various humic-like substances removal efficiency. Overall, GDM systems under less pressure should be considered and expected to provide suggestions on the design of GDM systems in real applications.

How to identify biofouling species in marine and freshwater

The identification and management of biofouling remain pressing challenges in marine and freshwater ecosystems, with significant implications for environmental sustainability and industrial operations. This comprehensive review synthesizes the current state-of-the-art in biofouling identification technologies, examining eight prominent methodologies: Microscopy Examination, Molecular Biology, Remote Sensing, Community Involvement, Ecological Methods, Artificial Intelligence, Chemical Analysis, and Macro Photography. Each method is evaluated for its respective advantages and disadvantages, considering factors such as precision, scalability, cost, and data quality. Furthermore, the review identifies current obstacles that inhibit the optimal utilization of these technologies, ranging from technical limitations and high operational costs to issues of data inconsistency and subjectivity. Finally, the review posits a future outlook, advocating for the development of integrated, standardized systems that amalgamate the strengths of individual approaches. Such advancement will pave the way for more effective and sustainable strategies for biofouling identification and management.

Enhanced permeation performance of biofiltration-facilitated gravity-driven membrane (GDM) systems by in-situ application of UV and VUV: Comprehensive insights from thermodynamic and multi-omics perspectives

Biofouling is a major challenge limiting the practical application of biofiltration-facilitated gravity-driven membrane (GDM) systems in drinking water treatment. In this study, ultraviolet irradiation, including ultraviolet (UV) and vacuum ultraviolet (VUV) irradiation, was used for in-situ purification of membrane tanks to control membrane biofouling. After using UV and VUV, the permeate flux increased significantly by 26.1% and 78.3%, respectively, which was mainly due to the decreased cake layer resistance (Rc). The permeability of the biofouling layer improved after UV and VUV application, as evidenced by the increased surface porosity and decreased thickness. The contents of loosely bound extracellular proteins (LB-PN) and tightly bound extracellular proteins (TB-PN) in the biofouling layer were reduced after UV and VUV irradiation. The decreased LB-PN and TB-PN improved the interfacial free energy between the fouling itself and between the fouling and the membrane, which contributed to the reduction of interfacial cohesion and adhesion, resulting in a looser and thinner biofouling layer and a cleaner membrane. The concentration of protein-like material in the membrane tank decreased after UV and VUV irradiation, significantly altering the bacterial community structure on the membrane surface (Mantel's r > 0.7, p < 0.05). The changes in the metabolic state were responsible for the differences in the LB-PN and TB-PN contents. The inhibition of "Alanine, aspartate and glutamate metabolism" and "Glycine, serine and threonine metabolism" reduced amino acid biosynthesis, which restricted the secretion of LB-PN and TB-PN. Critical genera in the Proteobacteria phylum, such as Hirschia, Rhodobacter, Nordella, Candidatus_Berkiella, and Limnohabitans, were involved in metabolite transformation. Overall, the in-situ application of UV and VUV can be an effective alternative strategy to mitigate membrane biofouling, which would facilitate the practical application of biofiltration-facilitated GDM systems in drinking water treatment.

Biofilm Formation and Biofouling Development on Different Ultrafiltration Membranes by Natural Anaerobes from an Anaerobic Membrane Bioreactor

Biofouling in anaerobic membrane bioreactors (AnMBRs) has not been studied widely. Moreover, the effect of membrane surface properties on biofilm formation beyond initial deposition is controversial. We investigated biofouling with polyvinyldifluoride, polyacrylonitrile, and zwitterion-modified polyethersulfone ultrafiltration membranes having different properties during 72 h filtration using natural anaerobes isolated from AnMBR and analyzed biofilm characteristics by physicochemical and molecular techniques. A decrease in membrane performance was positively correlated with biofilm formation on polyvinyldifluoride and polyacrylonitrile membranes, and as expected, physical cleaning effectively mitigated biofilm on hydrophilic and low-roughness membranes. Surprisingly, while the biofilm on the hydrophilic and low-surface roughness zwitterion-modified membrane was significantly impaired, the impact on transmembrane pressure was the highest. This was ascribed to the formation of a soft compressible thin biofilm with high hydraulic resistance, and internal clogging and pore blocking due to high pore-size distribution. Anaerobe community analysis demonstrated some selection between the bulk and biofilm anaerobes and differences in the relative abundance of the dominant anaerobes among the membranes. However, correlation analyses revealed that all membrane properties studied affected microbial communities' composition, highlighting the system's complexity. Overall, our findings indicate that the membrane properties can affect biofilm formation and the anaerobic microbial population but not necessarily alleviate biofouling.

Controlling the hydraulic resistance of membrane biofilms by engineering biofilm physical structure

The application of membrane technology for water treatment and reuse is hampered by the development of a microbial biofilm. Biofilm growth in micro-and ultrafiltration (MF/UF) membrane modules, on both the membrane surface and feed spacer, can form a secondary membrane and exert resistance to permeation and crossflow, increasing energy demand and decreasing permeate quantity and quality. In recent years, exhaustive efforts were made to understand the chemical, structural and hydraulic characteristics of membrane biofilms. In this review, we critically assess which specific structural features of membrane biofilms exert resistance to forced water passage in MF/UF membranes systems applied to water and wastewater treatment, and how biofilm physical structure can be engineered by process operation to impose less hydraulic resistance ("below-the-pain threshold"). Counter-intuitively, biofilms with greater thickness do not always cause a higher hydraulic resistance than thinner biofilms. Dense biofilms, however, had consistently higher hydraulic resistances compared to less dense biofilms. The mechanism by which density exerts hydraulic resistance is reported in the literature to be dependant on the biofilms' internal packing structure and EPS chemical composition (e.g., porosity, polymer concentration). Current reports of internal porosity in membrane biofilms are not supported by adequate experimental evidence or by a reliable methodology, limiting a unified understanding of biofilm internal structure. Identifying the dependency of hydraulic resistance on biofilm density invites efforts to control the hydraulic resistance of membrane biofilms by engineering internal biofilm structure. Regulation of biofilm internal structure is possible by alteration of key determinants such as feed water nutrient composition/concentration, hydraulic shear stress and resistance and can engineer biofilm structural development to decrease density and therein hydraulic resistance. Future efforts should seek to determine the extent to which the concept of "biofilm engineering" can be extended to other biofilm parameters such as mechanical stability and the implication for biofilm control/removal in engineered water systems (e.g., pipelines and/or, cooling towers) susceptible to biofouling.