Electrically conductive membranes for anti-biofouling in membrane distillation with two novel operation modes: Capacitor mode and resistor mode

Elucidating biofouling development and succession in membrane distillation using treated effluent

Biofouling in membrane distillation (MD) has several repercussions, including reduced efficiency of the MD process and limiting membrane life. Additionally, the evaluation of MD biofouling using treated effluents from wastewater treatment plants remains an unexplored area. Thus, biofouling formation and development in a long term MD process (15 days) using treated effluent from a wastewater treatment plant was explored in this study. The results revealed that flux decline occurred in four phases: i) initial decline (0-1 d), ii) gradual decline (1-5 d), iii) progressive decline (5-10 d), and iv) rapid decline (10-15 d). Liquid Chromatography-Organic Carbon Detection (LC-OCD) analysis demonstrated that the treated effluent contained humic-like substances, which deposited on the membrane surface in phase 1. Whereas biopolymers development on the membrane surface in phase 2 and 3 was linked to biofouling. Microbial community analysis revealed that the initial colonisers were predominantly thermophilic bacteria, which were different from the microbial community of the treated effluent. The biofilm-forming bacteria included Schlegelella, Meiothermus, and Vulcaniibacterium. These microorganisms proliferate and release excessive extracellular polymeric substances (EPS), leading to the development of mature biofilm on membrane surface. This helped in the deposition of organics and inorganics from the bulk feed, which led to microbial community succession in phase 4 with the emergence of the Kallotenue genus. The results suggested that organic substances and microbial communities on membrane surface at different stages in a long-term MD process had a significant influence on MD performance for high-quality wastewater reuse.

Advancements in Nanoenabled Membrane Distillation for a Sustainable Water-Energy-Environment Nexus

The emergence of nano innovations in membrane distillation (MD) has garnered increasing scientific interest. This enables the exploration of state-of-the-art nano-enabled MD membranes with desirable properties, which significantly improve the efficiency and reliability of the MD process and open up opportunities for achieving a sustainable water-energy-environment (WEE) nexus. This comprehensive review provides broad coverage and in-depth analysis of recent innovations in nano-enabled MD membranes, focusing on their role in achieving desirable properties, such as strong liquid-repellence, high resistance to scaling, fouling, and wetting, as well as efficient self-heating and self-cleaning functionalities. The recent developments in nano-enhanced photothermal-catalytic applications for water-energy co-generation within a single MD system are also discussed. Furthermore, the bottlenecks are identified that impede the scale-up of nanoenhanced MD membranes and a future roadmap is proposed for their sustainable commercialiation. This holistic overview is expected to inspire future research and development efforts to fully harness the potential of nano-enabled MD membranes to achieve sustainable integration of water, energy, and the environment.

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.

Applications of electrically conductive membranes in water treatment via membrane distillation: Joule heating, membrane fouling/scaling/wetting mitigation and monitoring

Membrane distillation (MD) is a thermally driven separation process that is driven by phase change. The core of this technology is the hydrophobic microporous membrane that prevents mass transfer of the liquid while allowing the vapor phase to pass through the membrane's pores. Currently, MD is challenged by its high energy consumption and membrane degradation due to fouling, scaling and wetting. The use of electrically conductive membranes (ECMs) is a promising alternative method to overcome these challenges by inducing localized Joule heating, as well as mitigating and monitoring membrane fouling/scaling/wetting. The objective of this review is to consolidate recent advances in ECMs from the standpoint of conductive materials, membrane fabrication methodologies, and applications in MD processes. First, the mechanisms of ECMs-based MD processes are reviewed. Then the current trends in conductive materials and membrane fabrication methods are discussed. Thereafter, a comprehensive review of ECMs in MD applications is presented in terms of the different processes using Joule heating and various works related to membrane fouling, scaling, and wetting control and monitoring. Key insights in terms of energy consumption, economic viability and scalability are furnished to provide readers with a holistic perspective of the ECMs potential to achieve better performances and higher efficiencies in MD. Finally, we illustrate our perspectives on the innovative methods to address current challenges and provide insights for advancing new ECMs designs. Overall, this review sums up the current status of ECMs, looking at the wide range of conductive materials and array of fabrication methods used thus far, and putting into perspective strategies to deliver a more competitive ECMs-based MD process in water treatment.

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.

Design and In Situ Validation of Low-Cost and Easy to Apply Anti-Biofouling Techniques for Oceanographic Continuous Monitoring with Optical Instruments

Biofouling is the major factor that limits long-term monitoring studies with automated optical instruments. Protection of the sensing areas, surfaces, and structural housing of the sensors must be considered to deliver reliable data without the need for cleaning or maintenance. In this work, we present the design and field validation of different techniques for biofouling protection based on different housing materials, biocides, and transparent coatings. Six optical turbidity probes were built using polylactic acid (PLA), acrylonitrile butadiene styrene (ABS), PLA with copper filament, ABS coated with PDMS, ABS coated with epoxy and ABS assembled with a system for in situ chlorine production. The probes were deployed in the sea for 48 days and their anti-biofouling efficiency was evaluated using the results of the field experiment, visual inspections, and calibration signal loss after the tests. The PLA and ABS were used as samplers without fouling protection. The probe with chlorine production outperformed the other techniques, providing reliable data during the in situ experiment. The copper probe had lower performance but still retarded the biological growth. The techniques based on transparent coatings, epoxy, and PDMS did not prevent biofilm formation and suffered mostly from micro-biofouling.

Co3 O4 Nanowires Capable of Discharging Low Voltage Electricity Showing Potent Antibacterial Activity for Treatment of Bacterial Skin Infection

Overuse of antibiotics has led to multidrug resistance in bacteria, posing a tremendous challenge to the healthcare system. There is an urgent need to explore unconventional strategies to overcome this issue. Herein, for the first time, we report a capacitive Co₃ O₄ nanowire (NW) electrode coated on flexible carbon cloth, which is capable of eliminating bacteria while discharging, for the treatment of skin infection. Benefiting from the unique NW-like morphology, the Co₃ O₄ NW electrode with increased active sites and enhanced capacitive property exhibits a prominent antibacterial effect against both Gram-positive and Gram-negative bacteria after charging at a low voltage of 2 V for 30 min. Furthermore, the electrode is demonstrated to be recharged for multiple antibacterial treatment cycles without significant change of antibacterial activity, allowing for practical use in a non-clinical setting. More importantly, this Co₃ O₄ NW electrode is capable of damaging bacterial cell membrane and inducing the accumulation of intracellular reactive oxygen species without impairing viability of skin keratinocytes. In a mouse model of bacterial skin infection, the Co₃ O₄ electrode shows significant therapeutic efficacy by eradicating colonized bacteria, thus accelerating the healing process of infected wounds. This nanostructured capacitive electrode provides an antibiotic-free, rechargeable, and wearable approach to treat bacterial skin infection.

Effects of different Ca2+ behavior patterns in the electric field on membrane fouling formation and removal of trace organic compounds

The effects of Ca²⁺ on membrane fouling and trace organic compounds (TrOCs) removal in an electric field-assisted microfiltration system were investigated in the presence of Na⁺ alone for comparison. In the electric field, negatively charged bovine serum albumin (BSA) migrated towards the anode far away from the membrane surface, resulting in a 42.9% transmembrane pressure (TMP) reduction in the presence of Na⁺ at 1.5 V. In contrast, because of the stronger charge shielding of Ca²⁺, the electrophoretic migration of BSA was limited and led to a neglectable effect of the electric field (1.5 V) on membrane fouling. However, under 3 V applied voltage, the synergistic effects of electrochemical oxidation and bridging interaction between Ca²⁺ and BSA promoted the formation of denser settleable flocs and a thinner porous cake layer, which alleviated membrane fouling with a 64.5% decrease in TMP and nearly 100% BSA removal. The TrOCs elimination increased with voltage and reached 29.4%-80.4% at 3 V. The electric field could prolong the contact between TrOCs and strong oxidants generated on the anode, which enhanced the TrOCs removal. However, a stronger charge shielding ability of Ca²⁺ weakened the electric field force and thus lowered the TrOCs removal.

Relative Importance of Stochastic Assembly Process of Membrane Biofilm Increased as Biofilm Aged

The relative importance of different ecological processes controlling biofilm community assembly over time on membranes with different surface characteristics has never been investigated in membrane bioreactors (MBRs). In this study, five ultrafiltration hollow-fiber membranes - having identical nominal pore size (0.1μm) but different hydrophobic or hydrophilic surface characteristics - were operated simultaneously in the same MBR tank with a constant flux of 10 liters per square meter per hour (LMH). In parallel, membrane modules operated without permeate flux (0 LMH) were submerged in the same MBR tank, to investigate the passive microbial adsorption onto different hydrophobic or hydrophilic membranes. Samples from the membrane biofilm were collected after 1, 10, 20, and 30days of continuous filtration. The membrane biofilm microbiome were investigated using 16S rRNA gene amplicon sequencing from DNA and cDNA samples. Similar beta diversity trends were observed for both DNA- and cDNA-based analyses. Beta diversity analyses revealed that the nature of the membrane surface (i.e., hydrophobic vs. hydrophilic) did not seem to have an effect in shaping the bacterial community, and a similar biofilm microbiome evolved for all types of membranes. Similarly, membrane modules operated with and without permeate flux did not significantly influence alpha and beta diversity of the membrane biofilm. Nevertheless, different-aged membrane biofilm samples exhibited significant differences. Proteobacteria was the most dominant phylum in early-stage membrane biofilm after 1 and 10days of filtration. Subsequently, the relative reads abundance of the phyla Bacteroidetes and Firmicutes increased within the membrane biofilm communities after 20 and 30days of filtration, possibly due to successional steps that lead to the formation of a relatively aged biofilm. Our findings indicate distinct membrane biofilm assembly patterns with different-aged biofilm. Ecological null model analyses revealed that the assembly of early-stage biofilm community developed after 1 and 10days of filtration was mainly governed by homogenous selection. As the biofilm aged (days 20 and 30), stochastic processes (e.g., ecological drift) started to become important in shaping the assembly of biofilm community.

Dead-end membrane distillation with localized interfacial heating for sustainable and energy-efficient desalination

Membrane distillation (MD) has the high potential to circumvent conventional desalination limitations in treating highly saline brines. However, the performance of MD is limited by its low thermal efficiencyand temperature polarization (TP) effect. Consequently, the driving force decreases when heat loss increases.In this study, we propose to minimize TP through localized heating where the thin feed channel was heated uniformly at the membrane-liquid interface without changing the properties of the membrane.This concept was further improved by implementing a new dead-end MD configuration. Investigated for the first time,this configuration eliminated circulation heat losses, which cannot be realized in conventional MD due to a rapid temperature stratification. In addition, the accumulation of foulants on the membrane surface was successfully controlled by intermittent flushing. 3-Dimensional conjugate heat transfer modeling revealedmore uniform heat transfer and temperature gradient across the membrane due to the increased feed water temperature over a larger membrane area. The increase of water vapor flux (45%) and the reduction of heat lossobserved in the new dead-end concept led to a decrease of the specific energy consumption by 57%, corresponding to a gain output ratio increase of about 132 %, compared to a conventional bulk heating, while preserving membrane integrity. A conjugate heat transfer model was deployed in ANSYS-Fluent framework to elucidate on the mechanism of flux enhancement associated with the proposed technique. This study provides a framework for future sustainable MD developmentby maintaining a stable vapor flux while minimizing energy consumption.

Recent Progress in One- and Two-Dimensional Nanomaterial-Based Electro-Responsive Membranes: Versatile and Smart Applications from Fouling Mitigation to Tuning Mass Transport

Membrane technologies are playing an ever-important role in the field of water treatment since water reuse and desalination were put in place as alternative water resources to alleviate the global water crisis. Recently, membranes are becoming more versatile and powerful with upgraded electroconductive capabilities, owing to the development of novel materials (e.g., carbon nanotubes and graphene) with dual properties for assembling into membranes and exerting electrochemical activities. Novel nanomaterial-based electrically responsive membranes have been employed with promising results for mitigating membrane fouling, enhancing membrane separation performance and self-cleaning ability, controlling membrane wettability, etc. In this article, recent progress in novel-nanomaterial-based electrically responsive membranes for application in the field of water purification are provided. Thereafter, several critical drawbacks and future outlooks are discussed.