Contrasting behaviors of pre-ozonation on ceramic membrane biofouling: Early stage vs late stage

Contamination and Cleaning of Ceramic Membrane in Phosphate Slurry Dewatering and Filtration Processes

Ceramic membrane dewatering and filtration technology is effective in reducing the water content of the phosphate slurry. However, membrane fouling remains an unavoidable issue. Herein, by investigating the mechanism of membrane contamination and developing innovative cleaning solutions, we can effectively address this issue. The main fouling form of ceramic membranes was observed to be complete blockage through analysis of the fouling process at various pollution time intervals by scanning electron microscopy (SEM) and mathematical model fitting. In addition, after cleaning severely contaminated membranes with a pollution rate of approximately 90%, a cleaning agent composed of surfactants, acid-washing agents, chelating agents, and auxiliaries was developed to address the phosphate contaminants. Owing to the combined effect of the detergent components, heavily soiled ceramic membranes can achieve a high flux recovery rate of over 90% after cleaning. This work offers new insights into ceramic membrane fouling and cleaning during phosphate slurry filtration.

Ceramic membrane fouling caused by recycling biological activated carbon filter backwash water: Effective backwash with ozone micro-nano bubbles

The widespread use of ceramic membranes in wastewater recycling is still hampered by membrane fouling problems. Frequent chemical cleaning increases operating and maintenance costs. This work proposes ozone micro-nano-bubble (O3-MNB) backwash as a new backwashing method to control the ceramic membrane fouling. Activated carbon filter backwash water (ACFBW) was used as feed water for the ceramic membrane and the effect of O3-MNB backwash was compared with tap water backwash, air-micro-nano-bubble (Air-MNB) backwash and ozone water backwash. The results of the flux tests showed that the irreversible fouling resistance (RFi) for the O3-MNB backwash was only 4.8 %, 10.0 % and 23.3 % of the RFi for the tap water backwash, Air-MNB backwash and O3 water backwash, respectively. The results of the SEM and CLSM analyses demonstrated that the combination of ozone with MNB for backwashing was an effective method for the removal of viable cells and majority of proteins and polysaccharides from the surface of the ceramic membrane. However, the application of ozone also led to the release of microbial DNA, which increased its binding to Al₂O₃ on the ceramic membrane. Furthermore, the increased ozone concentration transported by the MNB could promote the generation of a large number of hydroxyl radicals (•OH) due to the effect of Al₂O₃, which potentially enhanced the oxidation of macromolecular contaminants in the pores. At the same time, the electrostatic repulsion and hydrophobic action provided by the MNB improved the efficacy of peeling off the filter cake layer when cleaning the membrane pores. Consequently, this study demonstrated the effectiveness of O3-MNB backwash in the long-term operation of ceramic membranes and provided insights into the fundamental mechanism by which this process controlled the membrane fouling.

Zirconium-embedded ceramic membrane catalyzed moderate ozonation: dual-function synergy for simultaneous control of algal odorants and membrane fouling in water treatment

The integration of ozone with ceramic membrane offers a promising approach for treating algae-laden water but presents challenges in balancing oxidation efficacy with the preservation of cell integrity. In this study, catalytic ceramic membrane system embedded with zirconium (Zr) developed achieved 58.4 % removal of 2-methylisoborneol and 68.2 % removal of geosmin through dehydration and ring opening via hydroxyl radical-mediated degradation pathways generated in situ on the membrane under optimal ozone dosage. It is worth noting that the mild ozone concentration increased the hydrophobic interaction energy between algal cells and the filter cake layer from -13.7 mJ/m2 to -0.3 mJ/m2, thereby effectively reducing the deposition of pollutants on the membrane. By controlling oxidative intensity, the ceramic membrane's reversible and irreversible resistances were decreased by 86.7 % and 80.1 %, respectively, while maintaining >95 % algal cell integrity. This study establishes a mild catalytic oxidation paradigm for ceramic membrane-based algae-laden water treatment, achieving simultaneous degradation of odorants and membrane fouling control.

The mitigation potential of synergistic quorum quenching and antibacterial properties for biofilm proliferation and membrane biofouling

Biofouling has been a persistent problem hindering the application of membranes in water treatment, and quorum quenching has been identified as an effective method for mitigating biofouling, but surface accumulation of live bacteria still induces biofilm secretion, which poses a significant challenge for sustained prevention of membrane biofouling. In this study, we utilized quercetin, a typical flavonoid with the dual functions of quorum quenching and bacterial inactivation, to evaluate its role in preventing biofilm proliferation and against biofouling. Quercetin exhibited excellent antibacterial activity against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus), and the decreased bioactivity was positively correlated with the quercetin concentration, with inhibition rates of 53.1 % and 57.4 %, respectively, at the experimental concentrations. The RT-qPCR results demonstrated that quercetin inhibited AI-2 of E. coli and AGR of S. aureus mediated quorum sensing system, and reduced the expression of genes such as adhesion, virulence, biofilm secretion, and key regulatory proteases. As a result, the bacterial growth cycle was retarded and the biomass and biofilm maturation cycles were alleviated with the synergistic effect of quorum quenching and antibacterial activity. In addition, membrane biofouling was significantly declined in the dynamic operation experiments, dead cells in the biofilm overwhelmingly dominated, and the final normalized water fluxes were increased by more than 49.9 % and 34.5 % for E. coli and S. aureus, respectively. This work demonstrates the potential for mitigating biofouling using protocols that quorum quenching and inactivate bacteria, also provides a unique and long-lasting strategy to alleviate membrane fouling.

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.

Molecular insights into complexation between protein and silica: Spectroscopic and simulation investigations

The synergistic effect of protein-silica complexation leads to exacerbated membrane fouling in the membrane desalination process, exceeding the individual impacts of silica scaling or protein fouling. However, the molecular-level dynamics of silica binding to proteins and the resulting structural changes in both proteins and silica remain poorly understood. This study investigates the complexation process between silica and proteins-negatively charged bovine serum albumin (BSA) and positively charged lysozyme (LYZ) at neutral pH-using infrared spectroscopy (IR), in situ attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), and multiple computational simulations. The findings reveal that both protein and silica structures undergo changes during the complexation process, with calcium ions in the solution significantly exacerbating these alterations. In particular, in situ ATR-FTIR combined with two-dimensional correlation spectroscopy analysis shows that BSA experiences more pronounced unfolding, providing additional binding sites for silica adsorption compared to LYZ. The adsorbed proteins promote silica polymerization from lower-polymerized to higher-polymerized species. Furthermore, molecular dynamics simulations demonstrate greater conformational variation in BSA through root-mean-square-deviation analysis and the bridging role of calcium ions via mean square displacement analysis. Molecular docking and density functional theory calculations identify the binding sites and energy of silica on proteins. In summary, this research offers a comprehensive understanding of the protein-silica complexation process, contributing to the knowledge of synergistic behaviors of inorganic scaling and organic fouling on membrane surfaces. The integrated approach used here may also be applicable for exploring other complex complexation processes in various environments.

Alleviating Ultrafiltration Membrane Fouling Caused by Effluent Organic Matter Using Pre-Ozonation: A Perspective of EEM and Molecular Weight Distribution

Wastewater reclamation has gradually become an important way to cope with the global water crisis. Ultrafiltration plays an imperative part as a safeguard for the aim but is often limited by membrane fouling. Effluent organic matter (EfOM) has been known to be a major foulant during ultrafiltration. Hence, the primary aim of this study was to investigate the effects of pre-ozonation on the membrane fouling caused by EfOM in secondary wastewater effluents. In addition, the physicochemical property changes of EfOM during pre-ozonation and the subsequent influence on membrane fouling were systemically investigated. The combined fouling model and the morphology of fouled membrane were adopted to scrutinize the fouling alleviation mechanism by pre-ozonation. It was found that membrane fouling by EfOM was dominated by hydraulically reversible fouling. In addition, an obvious fouling reduction was achieved by pre-ozonation with 1.0 mg O₃/mg DOC. The resistance results showed that the normalized hydraulically reversible resistance was reduced by ~60%. The water quality analysis indicated that ozone degraded high molecular weight organics such as microbial metabolites and aromatic protein and medium molecular weight organics (humic acid-like) into smaller fractions and formed a looser fouling layer on the membrane surface. Furthermore, pre-ozonation made the cake layer foul towards pore blocking, thereby reducing fouling. In addition, there was a little degradation in the pollutant removal performance with pre-ozonation. The DOC removal rate decreased by more than 18%, while UV₂₅₄ decreased by more than 20%.

Maintaining Antibacterial Activity against Biofouling Using a Quaternary Ammonium Membrane Coupling with Electrorepulsion

Antibacterial modification is a chemical-free method to mitigate biofouling, but surface accumulation of bacteria shields antibacterial groups and presents a significant challenge in persistently preventing membrane biofouling. Herein, a great synergistic effect of electrorepulsion and quaternary ammonium (QA) inactivation on maintaining antibacterial activity against biofouling has been investigated using an electrically conductive QA membrane (eQAM), which was fabricated by polymerization of pyrrole with QA compounds. The electrokinetic force between negatively charged Escherichia coli and cathodic eQAM prevented E. coli cells from reaching the membrane surface. More importantly, cathodic eQAM accelerated the detachment of cells from the eQAM surface, particularly for dead cells whose adhesion capacity was impaired by inactivation. The number of dead cells on the eQAM surface was declined by 81.2% while the number of live cells only decreased by 49.9%. Characterization of bacteria accumulation onto the membrane surface using an electrochemical quartz crystal microbalance revealed that the electrorepulsion accounted for the cell detachment rather than inactivation. In addition, QA inactivation mainly contributed to minimizing the cell adhesion capacity. Consequently, the membrane fouling was significantly declined, and the final normalized water flux was promoted higher than 20% with the synergistic effect of electrorepulsion and QA inactivation. This work provides a unique long-lasting strategy to mitigate membrane biofouling.

Inhibition of biofouling on reverse osmosis membrane surfaces by germicidal ultraviolet light side-emitting optical fibers

Biofouling of membrane surfaces poses significant operational challenges and costs for desalination and wastewater reuse applications. Ultraviolet (UV) light can control biofilms while reducing chemical usage and disinfection by-products, but light deliveries to membrane surfaces in spiral wound geometries has been a daunting challenge. Thin and flexible nano-enabled side-emitting optical fibers (SEOFs) are novel light delivery devices that enable disinfection or photocatalytic oxidation by radiating UV light from light-emitting diodes (LEDs). We envision SEOFs as an active membrane spacer to mitigate biofilm formation on reverse osmosis (RO) membranes. A lab-scale RO membrane apparatus equipped with SEOFs allowed comparison of UV-A (photocatalysis-enabled) versus UV-C (direct photolysis disinfection). Compared against systems without any light exposure, systems with UV-C light formed thinner-but denser-biofilms, prevented permeate flux declines due to biofouling, and maintained the highest salt rejection. Results were corroborated by in-situ optical coherence tomography and ex-situ measurements of biofilm growth on the membranes. Transcriptomic analysis showed that UV-C SEOFs down-regulated quorum sensing and surface attachment genes. In contrast, UV-A SEOFs upregulated quorum sensing, surface attachment, and oxidative stress genes, resulting in higher extracellular polymeric substances (EPS) accumulation on membrane surfaces. Overall, SEOFs that deliver a low fluence of UV-C light onto membrane surfaces are a promising non-chemical approach for mitigating biofouling formation on RO membranes.