Stable Zr-Based Metal-Organic Framework Nanoporous Membrane for Efficient Desalination of Hypersaline Water.

Treatment of hypersaline waters is a critical environmental challenge. Pervaporation (PV) desalination is a promising technique to address this challenge, but current PV membranes still suffer from challenging issues such as low flux and insufficient stability. Herein, we propose in situ nanoseeding followed by a secondary growth strategy to fabricate a high-quality stable metal-organic framework (MOF) thin membrane (UiO-66) for high-performance pervaporation desalination of hypersaline waters. To address the issue of membrane quality, a TiO2 nano-interlayer was introduced on coarse mullite substrates to favor the growth of a UiO-66 nanoseed layer, on which a well-intergrown UiO-66 selective membrane layer with thickness as low as 1 µm was finally produced via subsequent secondary growth. The PV separation performance for hypersaline waters was systematically investigated at different salt concentrations, feed temperatures, and long-term operation in different extreme chemical environments. Besides having nearly complete rejection (99.9%), the UiO-66 membrane exhibited high flux (37.4 L·m-2·h-1) for hypersaline waters, outperforming current existing zeolite and MOF membranes. The membrane also demonstrated superior long-term operational stability under various harsh environments (hypersaline, hot, and acidic/alkaline feed water) and mild fouling behavior. The rational design proposed in this study is not only applicable for the development of a high-quality UiO-66 membrane enabling harsh hypersaline water treatment but can also be potentially extended to other next-generation nanoporous MOF membranes for more environmental applications.

Cross-linked Graphene Oxide Framework Membranes with Robust Nano-Channels for Enhanced Sieving Ability.

It remains challenging for graphene oxide (GO) membranes to achieve highly efficient performance and sufficient stability for aqueous molecule/ion precise separations. Herein, a molecular-level rational structure design protocol was proposed to develop ceramic-based graphene oxide framework (GOF) membranes with significantly enhanced sieving performance and stability for efficient removal of salts and micropollutants. Via a molecular cross-linking strategy, interlayered nanochannels between GO nanosheets can be rationally designed, featuring precisely tailorable channel size, promising surface chemistries and remarkably robust stability suitable for aqueous separation. Due to a significantly decreased nanochannel size, cross-linking of TU (thiourea) molecule significantly improved monovalent salt rejection (95.6% for NaCl), outperforming existing state-of-the-art GO-based, commercial organic nanofiltration and emerging two-dimensional MoS2 membranes, while moderately decreasing water permeability. In comparison, the GOF membranes cross-linked with MPD (m-phenylenediamine) exhibited a simultaneous increase in permeability and rejection for both salts and micropollutants (21.0% and 53.3% enhancement for chloramphenicol (CAP) solution), breaking their conventional trade-off issue. Cross-linking mechanism indicates that more robust nanochannels were formed by stronger covalent bonds via dehydration condensation between amine (TU/MPD) and carboxyl groups (GO), and nucleophilic addition between amine (TU/MPD) and epoxy groups (GO). Molecule/ion separation mechanism involved size sieving (steric hindrance), electrostatic interaction, Donnan effect, and partial dehydration effect. This work provides a novel protocol for rationally designing size and surface chemistry of highly robust GO nanochannels at a subnanometer level to construct water-stable functional GOF membranes with enhanced sieving performance for water treatment applications.

Engineering a Nanocomposite Interlayer for a Novel Ceramic-Based Forward Osmosis Membrane with Enhanced Performance.

Rational design of a high-performance defect-free polyamide (PA) layer on a robust ceramic substrate is challenging for forward osmosis (FO) water treatment applications. In this study, we first demonstrated a robust ceramic-based thin-film composite (TFC) FO membrane by engineering a novel nanocomposite interlayer of titanium dioxide and carbon nanotube (TiO2/CNT). The structural morphologies and properties were systematically characterized for different substrates (without interlayer, with TiO2 interlayer, or with TiO2/CNT interlayer) and the corresponding ceramic-based TFC-FO membranes. Introduction of low roughness nanocomposite interlayers with decreased pore size created an interface with improved surface characteristics, favoring the formation of a defect-free nanovoid-containing PA layer with high cross-linking degree. The resulting ceramic-based FO membrane had a water permeability of approximately 2 L/(m2 h bar) and a NaCl rejection of 98%, showing simultaneous enhancements in both compared to the control membrane without an interlayer. Mechanism analysis indicates that such a special nanocomposite interlayer not only provided more active sites for the formation of a thinner defect-free nanovoid-containing PA layer without penetration into substrate but also acted as a highly porous three-dimensional network structure for rapid water transport. This work provides a novel protocol for rational design and fabrication of a high-performance multilayered inorganic FO membrane as well as extended applications in water treatment with enhanced performance.

Flexible Superhydrophobic Metal-Based Carbon Nanotube Membrane for Electrochemically Enhanced Water Treatment.

Treatment of highly saline wastewaters via conventional technology is a key challenging issue, which calls for efficient desalination membranes featuring high flux and rejection, low fouling, and excellent stability. Herein, we report a high-strength and flexible electro-conductive stainless steel-carbon nanotube (SS-CNT) membrane, exhibiting significantly enhanced anticorrosion and antifouling ability via a microelectrical field-coupling strategy during membrane distillation. The membrane substrates exhibited excellent mechanical strength (244.2 ± 9.8 MPa) and ductility, thereby overcoming the critical bottleneck of brittleness of traditional inorganic membranes. By employing a simple surface activation followed by self-catalyzed chemical vapor deposition, CNT was grown in situ on SS substrates via a tip-growth mechanism to finally form robust superhydrophobic SS-CNT membrane. To address the challenging issues of significant corrosion and fouling, using a negative polarization microelectrical field-coupling strategy, simultaneously enhanced antifouling and anticorrosion performance was realized for treatment of organic high salinity waters while exhibiting stable high flux and rejection via an electrostatic repulsion and electron supply mechanism. This application-oriented rational design protocol can be potentially used to extend toward high performance composite membranes derived from other electro-conductive metal substrates functionally decorated with CNT network and to other applications in water treatment.

Optimized anti-biofouling performance of bactericides/cellulose nanocrystals composites modified PVDF ultrafiltration membrane for micro-polluted source water purification.

The covalently functionalized cellulose nanocrystal (CNC) composites were synthesized by bonding common bactericides, such as dodecyl dimethyl benzyl ammonium chloride (DDBAC), ZnO and graphene oxide (GO) nanosheets, onto the CNC's surface. Then, the DDBAC/CNC, ZnO/CNC and GO/CNC nanocomposites modified polyvinylidene fluoride (PVDF) ultrafiltration membranes were fabricated by a simple one-step non-solvent induced phase separation (NIPS) process. The resultant hybrid membranes possessed porous and rough surfaces with more finger-like macropores that even extended through the entire cross-section. The hydrophilicity, permeability, antibacterial and antifouling performance and mechanism of the hybrid ultrafiltration membranes were evaluated and compared in detail, aiming at screening a superior hybrid membrane for practical application in micro-polluted source water purification. Among these newly-developed hybrid membranes, GO/CNC/PVDF exhibited an enhanced perm-selectivity with a water flux of 230 L/(m2 h bar) and humic acid rejection of 92%, the improved antibacterial activity (bacteriostasis rate of 93%) and antifouling performance (flux recovery rate (FRR) of >90%) being due to the optimized pore structure, higher surface roughness, incremental hydrophilicity and electronegativity. A lower biofouling level after three weeks' filtration of the actual micro-polluted source water further demonstrated that embedding the hydrophilic and antibacterial GO/CNC nanocomposite into the polymer matrix is an effective strategy to improve membrane anti-biofouling ability.

A bio-electrochemical membrane system for more sustainable wastewater treatment with MnO2/PANI modified stainless steel cathode and photosynthetic provision of dissolved oxygen by algae.

A competitive sewage treatment technology should meet the standard of water quality requirement and accomplish recovery of potential energy. This study presents such a new system, with coupled membrane bioreactor-microbial fuel cell features, which can not only treat wastewater, but also recovers energy from wastewater by electricity generation, and form a new resource by photosynthesis while providing the dissolved oxygen by algae. Specifically, in the system, the MnO2/polyaniline is used to modify the stainless steel mesh and to function well as system membrane and cathode, with satisfactory filtration and catalysis performance. The system enables continuous wastewater treatment with stable pollutant removal and electricity generation. Under the membrane flux of 119.4 Lm-2 h-1, a maximum power density of 1.2 W m-3 can be achieved, the algae multiply 6.1 times, and satisfactory wastewater treatment effect is achieved.

Highly effective permeability and antifouling performances of polypropylene non-woven fabric membranes modified with graphene oxide by inkjet printing and immersion coating methods.

In the current study, graphene oxide (GO)-modified polypropylene non-woven fabric (PP-NWF) membranes were prepared via inkjet printing and immersion coating methods. Scanning electron microscopy, Fourier transform infrared spectroscopy, contact angle measurements, pure water permeation (JPWP) and protein adsorption were tested to evaluate the impact of the GO nanosheet on the characteristics and performance of modified PP-NWF membranes. The results showed that the exfoliated GO nanosheets uniformly deposited on the membrane surface and firmly embedded into the interlaced fibers, resulting in the improvement of membrane hydrophilicity, permeability and antifouling properties comparing with original PP-NWF membranes. The GO-printed and GO-coated membranes had 113 and 188% higher fluxes, and 70.95 and 75.74% lower protein adsorptions than the original PP-NWF membranes, respectively. After cross-linked treatment, ultrasound processing was conducted to evaluate the stability of the modified PP-NWF membranes. The results demonstrated that there was almost no decrease in permeation after ultrasonic treatment indicating that the cross-linking treatment could enhance the immobilization of the GO nanosheets on and into the modified membranes.

Cost and efficiency perspectives of ceramic membranes for water treatment.

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

Robust zirconia ceramic membrane with exceptional performance for purifying nano-emulsion oily wastewater.

While membrane-based oil-water separation has been widely explored, using conventional membranes to treat oily wastewaters remains practically challenging especially when such wastewaters contain more stable nano-sized oil droplets and are of high oil content, and harsh chemical conditions. Herein, we report a novel protocol of efficiently separating both synthetic and real oil nano-emulsions via specially designed robust zirconia membranes. The best-performing zirconia membrane, fabricated at low sintering temperature, has relatively uniform sub-100 nm pores and is underwater superoleophobic. Such zirconia membranes possess not only outstanding separation performance under long-term operation but robust structural stability at harsh conditions. At different cross-flow velocities, a combined model of intermediate pore blocking and cake filtration dominated membrane fouling behavior. Specifically, at high pH value (especially > pH(IEP)), membrane fouling was effectively mitigated due to a dominant role of electrostatic repulsion interaction at membrane-oil interface. Compared with conventional and commercial ceramic membranes, our zirconia membrane is the first reported in literature that can effectively reject nano-sized oil droplets (∼18 nm) with over 99% rejection. Moreover, the zirconia membrane has also been challenged with real degreasing wastewater with very high oil content (∼4284 mg L-1) and pH (∼12.4) and delivered consistently high separation performance over many operation cycles.

Recent development of pressure retarded osmosis membranes for water and energy sustainability: A critical review.

With the goal of zero-liquid discharge and green energy harvest, extraction of abundant green energy from saline water via pressure retarded osmosis (PRO) technology is a promising but challenging issue for water treatment technologies to achieve water and energy sustainability. Development of high performance PRO membranes has received increased concerns yet still under controversy in practical applications. In this review, a comprehensive and up-to-date discussion of some key historical developments is first introduced covering the major advances of PRO engineering applications and novel membranes especially made in recent years. Then the critical performance indicators of PRO membranes including water flux and power density are briefly discussed. Subsequently, sufficient discussion on four performance limiting factors in PRO membrane and process is presented including concentration polarization, reverse solute diffusion, membrane fouling and mechanical stability. To fully address these issues, an updated insight is provided into recent major progresses on advanced fabrication and modification techniques of novel PRO membranes featuring enhanced performance with different configurations and materials, which are also reviewed in detail based on the viewpoint of design rationales. Afterwards, antifouling strategies and engineering applications are critically introduced. Finally, conclusions and future perspective of PRO membrane for practical operation are briefly discussed.

Integration of nitrification and denitrification by combining anoxic and aerobic conditions in a membrane bioreactor.

A membrane bioreactor (MBR) was developed to achieve nitrogen removal by combining nitrification and denitrification conditions in one reactor. The activated sludge was alternated between aerobic and anoxic conditions using peristaltic pump. The biomass concentration and floc morphological properties were observed to be similar in anoxic and aerobic compartments. However, the homogeneous properties of the activated sludge did not lead to the failure of oxygen gradient formation in the reactor. Due to the position of the air diffuser, an anoxic compartment at the bottom and an aerobic compartment in the upper part of the reactor were formed after 40 days. The average total nitrogen (TN) removal efficiency was then observed to increase to 77%. The microbial characterization using polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) analysis, as well as the specific nitrogen utilization rate measurements, indicated that the nitrogen removal in the reactor occurred via nitrification and denitrification processes.

Spinel-based ceramic membranes coupling solid sludge recycling with oily wastewater treatment.

Highly efficient and economic treatment of wastewater sludges and wastewaters in one way is a challenging issue in the water treatment field. Herein we present a waste-to-resource strategy for rational fabrication of low-cost ceramic membranes, which simultaneously addresses the treatment of heavy metal-laden sludges and the separation of oil-in-water (O/W) emulsions. A thermal conversion mechanism is proposed for complicated reactions between simulated nickel-laden wastewater sludge and bauxite mineral. In addition to full stabilization and recycling of heavy metal wastewater sludges, rational tailoring of ceramic membrane structures can also be realized to achieve high water flux and favorable mechanical and surface properties. With rational structure design, the tailored spinel-based ceramic membranes exhibited high rejection and high flux (7473 LMH·bar-1) simultaneously for separation of oily wastewater, outperforming other reported state-of-the-art ceramic membranes. The membrane fouling mechanism revealed the dominance of cake layer formation at low cross flow velocities, while a combined model of cake layer formation and pore blocking dominated membrane fouling at high cross-flow velocities. The proposed strategy can be potentially extended toward design of functional ceramic membranes derived from other heavy metal wastewater sludges and for other water treatment applications.

The development of a novel hybrid aerating membrane-anaerobic baffled reactor for the simultaneous nitrogen and organic carbon removal from wastewater.

A novel hybrid aerating membrane-anaerobic baffled reactor (HMABR), based on the installation of aerating membrane into an anaerobic baffled reactor (ABR), to achieve simultaneous removal of nitrogenous and carbonaceous organic pollutants was developed in this study. The results demonstrated that after the installation of membrane module, total VFA and COD concentration in the HMABR effluent were decreased by 68.1 and 59.5% respectively, with increased nitrogenous pollutant remove efficiency by 83.5%, at influent COD concentration of 1600 mg/L and NH(4)(+)-N concentration of 80 mg/L. Fluorescence in situ hybridization (FISH) results of the aerating membrane biofilm showed that the biofilm stratification for the spatial profiles of ammonia-oxidizing bacteria, nitrite-oxidizing bacteria, aerobic heterotrophic bacteria, and denitrifying bacteria. The potential usage of HMABR widens the usage of aerobic-anaerobic combination technology for industrial wastewater treatment.

Recent advances in membrane bioreactors (MBRs): membrane fouling and membrane material.

Membrane bioreactors (MBRs) have been actively employed for municipal and industrial wastewater treatments. So far, membrane fouling and the high cost of membranes are main obstacles for wider application of MBRs. Over the past few years, considerable investigations have been performed to understand MBR fouling in detail and to develop high-flux or low-cost membranes. This review attempted to address the recent and current developments in MBRs on the basis of reported literature in order to provide more detailed information about MBRs. In this paper, the fouling behaviour, fouling factors and fouling control strategies were discussed. Recent developments in membrane materials including low-cost filters, membrane modification and dynamic membranes were also reviewed. Lastly, the future trends in membrane fouling research and membrane material development in the coming years were addressed.

Mechanism of calcium mitigating membrane fouling in submerged membrane bioreactors.

Two parallel membrane bioreactors (MBRs) were operated under different calcium dosages (168.5, 27 mg/L) to gain a better understanding of the mechanism of retarding membrane fouling by adding calcium. The results showed that the particle size of sludge flocs increased and the particle size distribution tended to be narrow at the optimum dosage (168.5 mg/L). Calcium was effective in decreasing loosely bound extracellular polymeric substances (LB-EPS) in microbial flocs and soluble microbial products (SMP) in the supernatant at the dosage of 168.5 mg/L by strengthening the neutralization and bridging of EPS with flocs. Furthermore, the amount of CODs and CODc decreased in both the mixed liquor and the fouling cake layer on the membrane surface. In order to compare the filtration characteristics of cake layers from the MBRs with the two calcium dosages, the specific cake resistance and the compressibility coefficient were measured. The specific cake resistance from the MBR with optimum dosage (168.5 mg/L) was distinctly lower than that with low dosage (27 mg/L). The compressibility coefficient of the cake layers under two dosages were respectively attained as 0.65, 0.91. Scanning electron microscopy (SEM) and three-dimensional confocal scanning laser microscope analysis (CLSM) images were utilized to observe the gel layer directly.

Long-term storage and subsequent reactivation of aerobic granules.

This study investigated a seven month storage and the subsequent reactivation of aerobic granules. The granule size and structure integrity were remained during storage, whereas some cavities and pleats appeared on the surface and further deteriorated the settleability. Along with the reactivation, the physical characteristics and microbial activities of aerobic granules were gradually improved. Activities of heterotrophs and nitrifiers can be fully recovered within 16days and 11days, respectively. Nitrifiers decayed slower during storage and reinstated rapider during reactivation than heterotrophs. In fresh aerobic granules, the dominated ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) were Nitrosomonas and Nitrospira, respectively. During storage, the initially dominated populations decayed rapider than the initially less dominated ones. Extracellular polymeric substances (EPS) significantly decreased within the first month, and then gradually accumulated during the last six months storage. Accumulation of EPS was an effective strategy for maintaining structural integrity of aerobic granules during long-term storage.

The integration of methanogenesis with shortcut nitrification and denitrification in a combined UASB with MBR.

A combined system consisting of an up-flow anaerobic sludge blanket (UASB) and an aerobic membrane bioreactor (MBR) was operated at 28-30 degrees C and pH 7.8-8.1 for the treatment of low-strength synthetic wastewater enriched with organic carbon and NH4Cl. The MBR slurry was recirculated into the UASB with a ratio of 50-800%. It was found that nitrite was able to accumulate steadily during the nitrification step in the MBR at a low TOC/NH4+-N ratio. The mixed liquid containing NOX(-)-N in the MBR was recirculated to the UASB, and denitrification rather than methanogenesis became the preferred pathway. Whereas, the less carbon requirement for denitrification via nitrite rather than nitrate allowed methanogenesis to proceed simultaneously in the same reactor. The combination of membrane filtration and partial nitrification in the MBR with simultaneous denitrification and methanogenesis in the UASB could stably reach 98% TOC removal and 48.1-82.8% TN removal with recirculation ratio increasing from 50% to 800%.

Characterization of functional microbial community in a membrane-aerated biofilm reactor operated for completely autotrophic nitrogen removal.

The 16S rDNA-based molecular technique was applied to investigate the functional microbial community of a membrane-aerated biofilm (MAB) that was used for completely autotrophic nitrogen removal over nitrite (CANON). The relationships among two kinds of key bacteria responsible for CANON: aerobic ammonia-oxidizing bacteria (AOB) and Anammox bacteria, and their possible distributions in the MAB were discussed based on the microbial community analysis. FISH analysis showed the existence of two visible active layers in experimental MAB. One is the partial nitrifying layer located in the region of oxygen-rich membrane-biofilm interface, dominated by NSO190-positive AOB. The other is the Anammox active layer located in the region of anoxic liquid-biofilm interface, dominated by PLA46 and AMX820-positive Anammox microorganisms. As a result of this study, the AOB as well as Anammox bacteria were present and active in experimental MABR, and the cooperation between AOB and Anammox bacteria was considered to be responsible for CANON.

Enhancement of nitroaromatic compounds anaerobic biotransformation using a novel immobilized redox mediator prepared by electropolymerization.

Functionalized polypyrrole (PPy) composites were prepared by incorporation of a model redox mediator, anthraquinonedisulphonate (AQDS), as doping anion during the electropolymerization of pyrrole (Py) monomer on active carbon felt (ACF) electrode. Then, the resulting composite, ACF/PPy/AQDS as a novel immobilized redox mediator for catalyzing anaerobic biotransformation of the model nitroaromatic compounds (NACs), such as nitrobenzene (NB), 2,4- and 2,6-dinitrotoluene (DNT), were investigated in detail. The results showed that ACF/PPy/AQDS exhibited good catalytic activity and stability, and its addition effectively accelerated the NACs anaerobic reduction to the corresponding amino compounds. In order to estimate the relationship between community dynamics and the function of immobilized redox mediator, a combined method based on fingerprints (ribosomal intergenic spacer analysis, RISA) and 16S rRNA gene sequencing was used. The results indicated that the existence of ACF/PPy/AQDS made the potent AQDS-reducing bacteria keeping predominant in the catalytic systems. Based on the results above, it can be concluded that this novel immobilized redox mediator is feasible and potentially useful to enhance NACs anaerobic reduction.

Effects of COD/N ratio and DO concentration on simultaneous nitrification and denitrification in an airlift internal circulation membrane bioreactor.

The effects of chemical oxygen demand and nitrogen (COD/N) ratio and dissolved oxygen concentration (DO) on simultaneous nitrification and denitrification (SND) were investigated using an airlift internal circulation membrane bioreactor (AIC-MBR) with synthetic wastewater. The results showed that the COD efficiencies were consistently greater than 90% regardless of changes in the COD/N ratio. At the COD/N ratio of 4.77 and 10.04, the system nitrogen removal efficiency became higher than 70%. However, the nitrogen removal efficiency decreased to less than 50%, as the COD/N ratio shifted to 15.11. When the operating DO concentration was maintained at 1.0 mg/L in AIC-MBR, a satisfying SND was achieved. Either low or high DO concentration could restrain SND.