Microbial electrodialysis cell for simultaneous water desalination and hydrogen gas production

Investigation of electrochemical properties, leachate purification, organic matter characteristics, and microbial diversity in a sludge treatment wetland- microbial fuel cell

Sludge treatment wetland-microbial fuel cell (STW-MFC) is a unique sludge treatment process that produces bioelectricity, but its technology is still in its infancy. This study investigated the electrochemical properties, organic matter characteristics, leachate purification, and microbial community structure of STW-MFCs as affected by electrode location. When electrodes were placed in the filler layer, the STW-MFC system presented a higher power generation capacity (maximum output power density: 0.498 W/m³; peak cell voltage: 0.879 V) and organic matter degradation efficiency. The hydrophilic fraction was the main dissolved organic carbon fraction in sludge extracellular biological organic matter (EBOM) and leachate dissolved organic matter (DOM). Aromatics were mainly concentrated in the hydrophobic acid fraction. The UV-254 content of sludge EBOM decreased mainly in the hydrophilic and transphilic acid fractions. The excitation-emission matrix analysis showed that tryptophan-like protein was more easily eliminated than tyrosine-like protein. In addition, there was a strong correlation between voltage and NH₄⁺ removal efficiency; a negative correlation between total chemical oxygen demand (TCOD), total nitrogen (TN), and total phosphorus (TP) removal efficiency, and a negative correlation between pH and TN, TP, and NH₄⁺ removal efficiencies. High-throughput sequencing showed that the system was most abundant in Thermomonas, Geothrix and Geobacter when the electrodes were placed in the filled layer, while the levels of genes for membrane transport, carbohydrate metabolism and energy metabolism functions were higher than in other systems. This work will support STW- MFC widespread implementation by illuminating the underlying mechanics of different anode positions.

Spotlighting of microbial electrodeionization cells for sustainable wastewater treatment: Application of machine learning

Microbial electrodeionization cells (MECs) have been investigated for various potential applications, including the elimination of persistent pollutants, chemical synthesis, the recovery of resources, and the development of biosensors. Nevertheless, MEC technology is still developing, and practical large-scale applications face significant obstacles. This review aims to investigate MEC implementations in sustainable wastewater treatment. Ideas and concepts of MEC technology, the setup of the electrodeionization component, the membranes of MECs, the working mechanism of MECs, and the various microorganisms used in MECs are discussed. Additionally, difficulties and prospective outcomes were discussed. The goal of this review is to support scientists and engineers in fully grasping the most recent developments in MEC technologies and applications.

Construction of microbial electrodialysis cells equipped with internal proton migration pathways: Enhancement of wastewater treatment, desalination, and hydrogen production

Microbial electrodialysis cells (MEDCs) offer simultaneous wastewater treatment, water desalination, and hydrogen production. In a conventional design of MEDCs, the overall performance is retarded by the accumulation of protons on the anode due to the integration of an anion exchange membrane (AEM). The accumulation of protons reduces the anolyte pH to become acidic, affecting the microbial viability and thus limiting the charge carrier needed for the cathodic reaction. This study has modified the conventional MEDC with an internal proton migration pathway, known as the internal proton migration pathway-MEDC (IP-MEDC). Simulation tests under abiotic conditions demonstrated that the pH changes in the anolyte and catholyte of IP-MEDC were smaller than the pH changes in the anolyte and catholyte without the proton pathways. Under biotic conditions, the performance of the IP-MEDC agreed well with the simulation test, showing a significantly higher chemical oxygen demand (COD) removal rate, desalination rate, and hydrogen production than without the migration pathway. This result is supported by the lowest charge transfer resistance shown by EIS analysis and the abundance of microbes on the bioanode through field emission scanning electron microscopy (FESEM) observation. However, hydrogen production was diminished in the second-fed batch cycle, presumably due to the active diffusion of high Cl¯ concentrations from desalination to the anode chamber, which was detrimental to microbial growth. Enlarging the anode volume by threefold improved the COD removal rate and hydrogen production rate by 1.7- and 3.4-fold, respectively, owing to the dilution effect of Cl¯ in the anode. This implied that the dilution effect satisfies both the microbial viability and conductivity. This study also suggests that the anolyte and catholyte replacement frequencies can be reduced, typically at a prolonged hydraulic retention time, thus minimizing the operating cost (e.g., solution pumping). The use of a high concentration of NaCl (35 g L^-1) in the desalination chamber and catholyte provides a condition that is close to practicality.

Prospective review on bioelectrochemical systems for wastewater treatment: Achievements, hindrances and role in sustainable environment

Bioelectrochemical systems (BESs) are a relatively new arena for producing bioelectricity, desalinating sea water, and treating industrial effluents by removing organic matter. Microbial electrochemical technologies (METs) are promising for obtaining value-added products during simultaneous remediation of pollutants from wastewater. The search for more affordable desalination technology has led to the development of microbial desalination cells (MDCs). MDC combines the operation of microbial fuel cells (MFC) with electrodialysis for water desalination and energy generation. It has received notable interest of researchers in desalination and wastewater treatment because of low energy requirement and eco-friendly nature. Firstly, this article provides a brief overview of MDC technology. Secondly, factors affecting functioning of MDC and its applications have been accentuated. Additionally, challenges and future outlook on the development of this technology have been delineated. State-of-the-art information provided in this review would expand the scope of interdisciplinary and translational research.

Effective salt removal from domestic reverse osmosis reject water in a microbial desalination cell

Microbial desalination cells (MDC) are evaluated as an environmentally friendly approach for purifying saline water by using power generated by the decomposition of organic materials in the wastewater. The present study is to evaluate the ferrocyanide-redox and biocathode approach in treating simulated saline water and subsequently recovering bio-electricity using actual domestic reverse osmosis reject water. For the desalination of simulated saline water and domestic reverse osmosis reject water, a three-chamber microbial desalination cell with graphite electrodes and anion and cation exchange membranes was constructed. When treating simulated saline water, the biocathode technique achieved a 5% improvement in salt removal and a 4.9% increase in current and power density when compared to the ferrocyanide-redox approach. When biocathode MDC was used to treat domestic reverse osmosis reject water, a maximum current and power density of 3.81 μA/cm² and 0.337 μW/cm², respectively, were recorded, as well as COD removal of 83.9% at the desalination chamber and ions reduction for Na, K, and Ca of up to 79%, 76.5%, and 72%, respectively, in a batch operation for 31 days with a stable pH (≈ 7). Thus, the study revealed a microbial desalination cell capable of recovering bioenergy and reducing salt from domestic reverse osmosis reject water with a consistent pH range.

Renewable Energy-Driven Desalination: New Trends and Future Prospects of Small Capacity Systems

New trends and future prospects for small capacity systems of Renewable Energy-driven DESalination (REDES) are reviewed and assessed in this paper over a nominal desalination capacity range of 3–1000 m3/d. A thorough literature review is reported in order to evaluate current research and developing activities. Outstanding commercial prospects in the near future are identified for two off-grid REDES technologies under development. First, wave energy converters with direct coupling to seawater desalination. Second, solar micro gas turbines with biofuel backup coupled to reverse osmosis (RO) desalination and/or zero liquid discharge water treatment. These systems, as well as mature REDES plants (namely PV/RO and wind turbines/RO), will benefit from forthcoming advances in energy efficiency in the RO process itself. The Closed Circuit RO desalination (CCROTM) concept may be a key configuration for enhancing RE-driven RO desalination. Additionally, opportunities for innovation in seawater RO desalination with variable power consumption are highlighted. On the other hand, our conclusions highlight opportunities for developing novel portable REDES systems based on solar membrane distillation with a portable linear Fresnel concentrator manufactured by SOLATOM. Additionally, the concept of portable systems could foster the commercial development of microbial desalination cells combined with solar PV energy and RO powered by tidal currents.

Fundamental understanding of microbial fuel cell technology: Recent development and challenges

The research on microbial fuel cells (MFCs) is rising tremendously but its commercialization is restricted by several microbiological, material, and economic constraints. Hence, a systematic assessment of the research articles published previously focusing on potential upcoming directions in this field is necessary. A detailed multi-perspective analysis of various techniques for enhancing the efficiency of MFC in terms of electric power production is presented in this paper. A brief discussion on the central aspects of different issues are preceded by an extensive analysis of the strategies that can be introduced to optimize power generation and reduce energy losses. Various applications of MFCs in a broad spectrum ranging from biomedical to underwater monitoring rather than electricity production and wastewater treatment are also presented followed by relevant possible case studies. Mathematical modeling is used to understand the concepts that cannot be understood experimentally. These methods relate electrode geometries to microbiological reactions occurring inside the MFC chamber, which explains the system's behavior and can be improved. Finally, directions for future research in the field of MFCs have been suggested. This article can be beneficial for engineers and researchers concerned about the challenges faced in the application of MFC.

A state-of-the-art review on microbial desalination cells

The rapid growth in population has increased the demand for potable water. Available technologies for its generation are the desalination of sea water through reverse osmosis, electrodialysis etc., which are energy and cost intensive. In this context, microbial desalination cell (MDC) presents a low-cost and sustainable option which can simultaneously treat wastewater, desalinate saline water, produce electrical energy and recover nutrients from wastewater. This review paper is focussed on presenting a detailed analysis of MDCs starting from the principle of operation, microbial community analysis, basic architecture, evolution in design, operational challenges, effect of process parameters, scale-up studies, application in multiple arenas and future prospects. After thorough review, it can be inferred that MDCs can be used as a stand-alone option or pre-treatment step for conventional desalination techniques without the application of external energy. MDCs have been used in multiple applications ranging from desalination, remediation of contaminated water, recovery of energy and nutrients from wastewater, softening of hardwater, biohydrogen production to degradation of waste engine oil. Although, MDCs have been used for multiple applications, still a number of operational challenges have been reported viz., interference of co-existing ions during desalination, membrane fouling, pH imbalance and limited potential of exoelectrogens. However, the re-circulation of anolytes with electrodialysis chamber has led to the maintenance of optimal pH for favourable microbial growth leading to improvement in the overall performance of MDCs. In future, genetic engineering may be used for improving the electrogenic activity of microbial community, next generation materials may be used as anode and cathode, varied sources of wastewater may be explored as anolytes, life cycle analysis and exergy analysis may be carried out to study the impact on environment and detailed pilot scale studies have to be carried out for assessing the feasibility of operation at large scale.

Recent trends in upgrading the performance of yeast as electrode biocatalyst in microbial fuel cells

Microbial fuel cell (MFC) is an optimistic fuel cell technology that applies microorganism's biochemical catalytic activities in consuming organic substrate and produce electricity. In the past, several researchers have reported power generation from Saccharomyces cerevisiae, but nowadays, most of the studies are centred around bacterial biofilms (prokaryotes) as anode biocatalyst. Yeast (a eukaryote) has also been applied as a biocatalyst in MFCs as they are non-pathogenic, easy to handle and tolerant to various environmental conditions. Yeast strains such as Arxula adeninvorans, Candida melibiosica, Hansenula polymorpha, Hansenula anomala, Kluyveromyces marxianus and Saccharomyces cerevisiae have been utilized in MFCs. This review summarizes the application of yeast as an anode biocatalyst together with a discussion on the mechanism of electron transfer from yeast cells to the anode and highlights the techniques applied in improving the efficiency of yeast-based MFCs. The recent challenges and benefits of utilizing yeast in MFCs have been also encapsulated in this review.

Pushing microbial desalination cells towards field application: Prevailing challenges, potential mitigation strategies, and future prospects

Microbial desalination cells (MDCs) have been experimentally proven as a versatile bioelectrochemical system (BES). They have the potential to alleviate environmental pollution, reduce water scarcity and save energy and operational costs. However, MDCs alone are inadequate to realise a complete wastewater and desalination treatment at a high-efficiency performance. The assembly of identical MDC units that hydraulically and electrically connected can improve the performance better than standalone MDCs. In the same manner, the coupling of MDCs with other BES or conventional water reclamation technology has also exhibits a promising performance. However, the scaling-up effort has been slowly progressing, leading to a lack of knowledge for guiding MDC technology into practicality. Many challenges remain unsolved and should be mitigated before MDCs can be fully implemented in real applications. Here, we aim to provide a comprehensive chronological-based review that covers technological limitations and mitigation strategies, which have been developed for standalone MDCs. We extend our discussion on how assembled, coupled and scaled-up MDCs have improved in comparison with standalone and lab-scale MDC systems. This review also outlines the prevailing challenges and potential mitigation strategies for scaling-up based on large-scale specifications and evaluates the prospects of selected MDC systems to be integrated with conventional anaerobic digestion (AD) and reverse osmosis (RO). This review offers several recommendations to promote up-scaling studies guided by the pilot scale BES and existing water reclamation technologies.

Recent progress in environmentally friendly bio-electrochemical devices for simultaneous water desalination and wastewater treatment

Bio-electrochemical systems (BESs) use electroactive micro-organisms for degrading organic materials in wastes for energy and/or chemical production. Microbial based desalination system is a cost-effective and environmentally friendly technique that can be used for water desalination with simultaneous wastewater treatment and energy harvesting. These systems can be used as a standalone technology for water desalination such as microbial desalination cell, microbial electrolysis desalination cell, or a hybrid with other desalination technology. This review summarized the recent progress in using BESs for water desalination, including microbial fuel cell-based desalination (MDC) and microbial electrolysis cell-based desalination (MEDC). The different scaling up trials to commercialize this technology, including the controlling parameters, are discussed. Moreover, the different hybrid desalination systems based on BES are summarized. Finally, the challenges facing the commercialization of the MDC systems were summarized.

Electricity Generation, Salt and Nitrogen Removal and Microbial Community in Aircathode Microbial Desalination Cell for Saline-Alkaline Soil-Washing Water Treatment

An aircathode microbial desalination cell (AMDC) was successfully started by inoculating anaerobic sludge into the anode of a microbial desalination cell and then used to study the effects of salinity on performance of AMDC and effect of treatment of coastal saline-alkaline soil-washing water. The results showed that the desalination cycle and rate gradually shorten, but salt removal gradually increased when the salinity was decreased, and the highest salt removal was 98.00 ± 0.12% at a salinity of 5 g/L. COD removal efficiency was increased with the extension of operation cycle and largest removal efficiency difference was not significant, but the average coulomb efficiency had significant differences under the condition of each salinity. This indicates that salinity conditions have significant influence on salt removal and coulomb efficiency under the combined action of osmotic pressure, electric field action, running time and microbial activity, etc. On the contrary, COD removal effect has no significant differences under the condition of inoculation of the same substrate in the anode chamber. The salt removal reached 99.13 ± 2.1% when the AMDC experiment ended under the condition of washing water of coastal saline-alkaline soil was inserted in the desalination chamber. Under the action of osmotic pressure, ion migration, nitrification and denitrification, NH4+-N and NO3−-N in the washing water of the desalination chamber were removed, and this indicates that the microbial desalination cell can be used to treatment the washing water of coastal saline-alkaline soil. The microbial community and function of the anode electrode biofilm and desalination chamber were analyzed through high-throughput sequencing, and the power generation characteristics, organics degradation and migration and transformation pathways of nitrogen of the aircathode microbial desalination cell were further explained.

Nanoscale nickel metal organic framework decorated over graphene oxide and carbon nanotubes for water remediation

In this report, highly crystalline and well-dispersed nano-sized nickel metal organic framework (MOFs) was decorated over graphene oxide (GO) and carbon nanotubes (CNTs) platforms to form hybrid nanocomposites. These as-synthesized hybrid nanocomposites were synthesized through a one-pot green solvothermal method. The prepared nanocomposites were characterized by SEM, TEM, EDS, XRD, FT-IR, Raman and TGA techniques. XRD analysis revealed the crystalline structure of the hybrid nanocomposites. Morphological and elemental studies also verified successful decoration of nickel-benzene dicarboxylate (Ni-BDC) MOFs over GO and CNT platforms. Chemical analysis collected through IR, and thermal analysis collected through TGA technique, illustrated the presence of all the components in the hybrid nanomaterials. Methylene blue (MB) was used as a model organic pollutant to analyze the adsorption capacity of the prepared nanocomposites. According to the findings, a strong interaction exists between the MB molecule and the developed adsorbents at which due to the synergistic effect, the hybrid nanocomposites show several times higher adsorption capacity compared to that of parent materials. This improvement can be due to several reasons: high surface area of the MOFs in the composites resulting from the smaller size of MOFs, presence of the pores formed between the MOFs and the platforms and different morphological characteristic of Ni-BDC MOFs in hybrid nanocomposites, compared to bare Ni-BDC MOFs. Furthermore, the isotherm and kinetic studies revealed that the adsorption of MB onto the newly prepared adsorbents could best be explained by the Langmuir and Pseudo-second order kinetic models. A regeneration study demonstrated the highly stable nature of the hybrid nanocomposites.

Neglected Effects of Inoculum Preservation on the Start-Up of Psychrophilic Bioelectrochemical Systems and Shaping Bacterial Communities at Low Temperature

Bioelectrochemical systems (BESs) are capable of simultaneous wastewater treatment and resource recovery at low temperatures. However, the direct enrichment of psychrophilic and electroactive biofilms in BESs at 4°C is difficult due to the lack of understanding in the physioecology of psychrophilic exoelectrogens. Here, we report the start-up and operation of microbial fuel cells (MFCs) at 4°C with pre-acclimated inocula at different temperatures (4°C, 10°C, 25°C, and -20°C) for 7 days and 14 days. MFCs with 7-day-pretreated inocula reached higher peak voltages than did those with 14-day-pretreated inocula. The highest power densities were obtained by MFCs with 25°C - 7-day-, 25°C - 14-day-, and 4°C - 7-day-pretreated inocula (650-700 mW/m2). In contrast, the control MFCs with untreated inocula were stable at 450 mW/m2. The power densities of MFCs with 7-day-pretreated inocula were higher than those obtained by MFCs with 14-day-pretreated inocula. The MFCs with 10°C - 7-day-pretreated inocula and the control MFCs showed higher chemical oxygen demand (COD) removal (90-91%) than other MFCs. Illumina HiSeq sequencing based on 16S rRNA gene amplicons indicated that bacterial communities of the anode biofilms were shaped by pretreated inocula at different temperatures. Compared with the control MFCs with untreated inocula, MFCs with temperature-pretreated inocula demonstrated higher microbial diversity, but did not do so with -20°C-pretreated inocula. Principal components analysis (PCA) revealed an obvious separation between the inocula pretreated at 4°C and those pretreated at 10°C, implying that bacterial community structures could be shaped by pretreated inocula at low temperatures. The pretreatment period also had a diverse impact on the abundance of exoelectrogens and non-exoelectrogens in MFCs with inocula pretreated at different temperatures. The majority of the predominant population was affiliated with Geobacter with a relative abundance of 17-70% at different pre-acclimated temperatures, suggesting that the exoelectrogenic Geobacter could be effectively enriched at 4°C even with inocula pretreated at different temperatures. This study provides a strategy that was previously neglected for fast enrichment of psychrophilic exoelectrogens in BESs at low temperatures.

Innovative operation of microbial fuel cell-based biosensor for selective monitoring of acetate during anaerobic digestion

Volatile fatty acids (VFAs) especially acetate concentration have been proved to be a sensitive and reliable indicator for many anaerobic processes such as anaerobic digestion (AD). Microbial fuel cells (MFC) have been demonstrated as a promising VFAs sensor due to simple reactor design and operating conditions among microbial electrochemical biosensors. However, the conventional MFC biosensors may fail to distinguish between VFAs and other organics as real digestates containing complex organics and microbes are fed into anode directly. In the present study, an MFC based biosensor was developed and operated in a smart way for selective acetate detection. In the biosensor, acetate ions contained in the AD sample was first fed into the cathode, and then acetic ion transferred through the membrane from the cathode to anode chamber where it was further used as the sole substrate by pre-enriched electroactive biofilm for the current generation. A linear correlation between the current density and acetate concentrations (0.5-20 mM) at varied reaction time (1-5 h) was established. Then, the interference from propionate, butyrate, isobutyrate, and glucose on the performance of the biosensor was evaluated. Furthermore, the influence of sample temperatures (37 and 55 °C) was also studied. Finally, the VFAs content in real AD effluent with this biosensor was measured. The results corresponded well with gas chromatographic measurements. This simple, and reliable biosensor could serve as a promising alternative method for acetate detection in the AD process or any other acetate-rich fluids.

Electricity generation, salinity, COD removal and anodic biofilm microbial community vary with different anode CODs in a microbial desalination cell for high-salinity mustard tuber wastewater treatment

A three-chamber microbial desalination cell (MDC) was constructed for high-salinity mustard tuber wastewater (MTWW) treatment. The effect of anode COD on electricity generation, salinity, COD removal and the anodic biofilm microbial community in MDC for the MTWW treatment was investigated. The results showed that electricity generation was better when the anode COD was 900 mg L⁻¹versus when it was 400 or 1400 mg L⁻¹. The ionic strength and conductivity of the anolyte were higher than those at 400 mg L⁻¹; thus, the ohmic internal resistance was lower. In addition, the mass transfer internal resistance was lower than that at 1400 mg L⁻¹, which made the system internal resistance the lowest; consequently, the voltage and power density were the highest. The output voltage, power density and coulombic efficiency of the 1000 Ω external resistors were 555 mV, 3.03 W m⁻³ and 26.5% ± 0.4%, respectively. Desalination was the highest when the anode COD was 400 mg L⁻¹. The lowest ionic strength and osmotic pressure of the anolyte resulted in the strongest osmosis, thereby producing the highest desalination rate; the desalination rate was 5.33 mg h⁻¹. When MDC was coupled with the dual-chamber microbial fuel cell (MFC), the desalinated MTWW could be used as the anode substrate of the MFC; its high COD could be removed continuously, and the COD removal values were 86.2% ± 2.5%, 83.0% ± 2.0% and 84.3% ± 2.4%. High-throughput sequencing analysis indicated that hydrolytic and fermentative bacteria were the core anode bacteria of MDC. The abundances of electrochemically active bacteria in the anode biofilms of the three groups were 11.78% (400 mg L⁻¹ COD), 14.06% (900 mg L⁻¹ COD) and 13.68% (1400 mg L⁻¹ COD). Therefore, the differences in anode CODs impacted the abundance of electrochemically active bacteria, which led to differences in electricity generation performances.

A three-chamber microbial desalination cell (MDC) was constructed for high-salinity mustard tuber wastewater (MTWW) treatment.

Improved Microbial Electrolysis Cell Hydrogen Production by Hybridization with a TiO2 Nanotube Array Photoanode

A microbial electrolysis cell (MEC) consumes the chemical energy of organic material producing, in turn, hydrogen. This study presents a new hybrid MEC design with improved performance. An external TiO2 nanotube (TNT) array photoanode, fabricated by anodization of Ti foil, supplies photogenerated electrons to the MEC electrical circuit, significantly improving overall performance. The photogenerated electrons help to reduce electron depletion of the bioanode, and improve the proton reduction reaction at the cathode. Under simulated AM 1.5 illumination (100 mW cm−2) the 28 mL hybrid MEC exhibits a H2 evolution rate of 1434.268 ± 114.174 mmol m−3 h−1, a current density of 0.371 ± 0.000 mA cm−2 and power density of 1415.311 ± 23.937 mW m−2, that are respectively 30.76%, 34.4%, and 26.0% higher than a MEC under dark condition.

Biological fuel cells produce bioelectricity with in-situ brackish water purification

Biological fuel cells, namely microbial desalination cells (MDCs) are a promising alternative to traditional desalination technologies, as microorganisms can convert the energy stored in wastewater directly into electricity and utilize it in situ to drive desalination, producing a high-quality reuse water. However, there are several challenges to be overcome in order to scale up from laboratory research. This study was conducted in order to better understand the performance of MDCs inoculated with marine sediments during the treatment of brackish water (5.0 g L^-1 of NaCl) under three different configurations and cycles of desalination, envisaging the future treatment of saline wastewaters with conductivities lower than 10 mS cm^-1. Results have shown that by increasing the desalination cycle three times, the efficiency of salt removal was improved by 3.4, 2.4 and 2.3 times for 1-MDC, 3-MDC, and 5-MDC, respectively. The same trend was observed for electrochemical data. Findings encourage further development of the MDC for sustainable brackish water and wastewater purification and future on-site utilization.

Bioremediation of steel plant wastewater and enhanced electricity generation in microbial desalination cell

Microbial desalination cell (MDC) is a propitious technology towards water desalination by utilizing wastewater as an energy source. In this study, a multi-chambered MDC was used to bioremediate steel plant wastewater using the same wastewater as a fuel for anodic bacteria. A pure culture of Pseudomonas putida MTCC 1194 was isolated and inoculated to remove toxic phenol. Three different inoculum conditions, namely P. putida (INC-A), a mixture of P. putida and activated sludge (INC-B), and activated sludge alone (INC-C) were employed in an anodic chamber to mainly compare the electricity generation and phenol degradation in MDCs. The study revealed the maximum phenol removal of 82 ± 2.4%, total dissolved solids (TDS) removal of 68 ± 1.5%, and power generation of 10.2 mW/m² using INC-B. The synergistic interactions between microorganisms, can enhance the toxic phenol degradation and also electricity generation in MDC for onsite wastewater application.

Efficiently "pumping out" value-added resources from wastewater by bioelectrochemical systems: A review from energy perspectives

Bioelectrochemical systems (BES) can accomplish simultaneous wastewater treatment and resource recovery via interactions between microbes and electrodes. Often deemed as "energy efficient" technologies, BES have not been well evaluated for their energy performance, such as energy production and consumption. In this work, we have conducted a review and analysis of energy balance in BES with parameters like normalized energy recovery, specific energy consumption, and net energy production. Several BES representatives based on their functions were selected for analysis, including direct electricity generation in microbial fuel cells, hydrogen production in microbial electrolysis cells, nitrogen recovery in BES, chemical production in microbial electrosynthesis cells, and desalination in microbial desalination cells. Energy performance was normalized to water volume (kWh m^-3), organic removal (kWh kg COD^-1), nitrogen recovery (kWh kg N^-1), chemical production (kWh kg^-1), or removed salt during desalination (kWh kg^-1). The key operating factors such as pumping system (recirculation/feeding pumps) and external power supply were discussed for their effects on energy performance. This is an in-depth analysis of energy performance of various BES and expected to encourage more thinking, analysis, and presentation of energy data towards appropriate research and development of BES technology for resource recovery from wastewater.

Computational and experimental analysis of organic degradation positively regulated by bioelectrochemistry in an anaerobic bioreactor system

Methane production was tested in membrane-less microbial electrolysis cells (MECs) under closed-circuit (R_CC) and open-circuit (R_OC) conditions, using glucose as a substrate, to understand the regulatory effects of bioelectrochemistry in anaerobic digestion systems. A dynamic model was built to simulate methane productions and microbial dynamics of functional populations, which were colonized in groups R_CC and R_OC during the start-up stage. The experiment results showed significantly greater methane production in R_CC than R_OC, the average methane production of R_CC was 0.131 m³/m³/d, which was 1.4 times higher than that of R_OC (0.055 m³/m³/d). The simulation results revealed that bioelectrochemistry had a significant influence on the abundance of microorganisms involved in acidogenesis and methanogenesis. The abundance of glucose-uptaking microorganisms was 87% of the total biomass in R_OC without applied voltage, which was 20% higher than that in R_CC (67%) when external voltages were applied between the anode and cathode. The abundance of hydrogenotrophic methanogens in R_CC was 6% higher than that in R_OC. The simulation results were verified through 16S rDNA high-throughput sequencing analysis. An electron balance analysis revealed that alteration of the acidogenesis type led to more acetate and hydrogen production from glucose fermentation, compared with the situation without bioelectrochemistry. An additional pathway from acetate to hydrogen was introduced by bioelectrolysis. These two factors resulted in significant enhancement of methane production in R_CC. Bioelectrolysis process directly contributed to 26% of the total methane production after the start-up stage. When the applied voltages were cut down or decreased, R_CC could maintain considerable methane productions, because the microbial communities and electron transfer pathways were already formed. Starting-up with high voltage, but operating under low voltage, could be an energy-favorable strategy for accelerating biogas production in bioelectro-anaerobic bioreactors.

Unravelling and Reconstructing the Nexus of Salinity, Electricity, and Microbial Ecology for Bioelectrochemical Desalination

Microbial desalination cells (MDCs) are an emerging concept for simultaneous water/wastewater treatment and energy recovery. The key to developing MDCs is to understand fundamental problems, such as the effects of salinity on system performance and the role of microbial community and functional dynamics. Herein, a tubular MDC was operated under a wide range of salt concentrations (0.05-4 M), and the salinity effects were comprehensively examined. The MDC generated higher current with higher salt concentrations in the desalination chamber. When fed with 4 M NaCl, the MDC achieve a current density of 300 A m^-3 (anode volume), which was one of the highest among bioelectrochemical system studies. Community analysis and electrochemical measurements suggested that electrochemically active bacteria Pseudomonas and Acinetobacter transferred electrons extracellularly via electron shuttles, and the consequent ion migration led to high anode salinities and conductivity that favored their dominance. Predictive functional dynamics and Bayesian networks implied that the taxa putatively not capable of extracellular electron transfer (e.g., Bacteroidales and Clostridiales) might indirectly contribute to bioelectrochemical desalination. By integrating the Bayesian network with logistic regression, current production was successfully predicted from taxonomic data. This study has demonstrated uncompromised system performance under high salinity and thus has highlighted the potential of MDCs as an energy-efficient technology to address water-energy challenges. The statistical modeling approach developed in this study represents a significant step toward understating microbial communities and predicting system performance in engineered biological systems.

Improved performance of the microbial electrolysis desalination and chemical-production cell with enlarged anode and high applied voltages

The aim of this study was to improve performance of the microbial electrolysis desalination and chemical-production cell (MEDCC) using enlarged anode and high applied voltages. MEDCCs with anode lengths of 9 and 48cm (i.e., the 9cm-anode MEDCC and 48cm-anode MEDCC, respectively) were tested under different voltages (1.2-3.0V). Our results demonstrated for the first time that the MEDCC could maintain high performance even under the applied voltage higher than that for water dissociation (i.e., 1.8V). Under the applied voltage of 2.5V, the maximum current density in the 48cm-anode MEDCC reached 32.8±2.6A/m², which is one of the highest current densities reported so far in the bioelectrochemical system (BES). The relative abundance of Geobacter was changed along the anode length. Our results show the great potential of the BES with enlarged anode and high applied voltages.

Engineering Shewanella oneidensis enables xylose-fed microbial fuel cell

BACKGROUND

The microbial fuel cell (MFC) is a green and sustainable technology for electricity energy harvest from biomass, in which exoelectrogens use metabolism and extracellular electron transfer pathways for the conversion of chemical energy into electricity. However, Shewanella oneidensis MR-1, one of the most well-known exoelectrogens, could not use xylose (a key pentose derived from hydrolysis of lignocellulosic biomass) for cell growth and power generation, which limited greatly its practical applications.

RESULTS

Herein, to enable S. oneidensis to directly utilize xylose as the sole carbon source for bioelectricity production in MFCs, we used synthetic biology strategies to successfully construct four genetically engineered S. oneidensis (namely XE, GE, XS, and GS) by assembling one of the xylose transporters (from Candida intermedia and Clostridium acetobutylicum) with one of intracellular xylose metabolic pathways (the isomerase pathway from Escherichia coli and the oxidoreductase pathway from Scheffersomyces stipites), respectively. We found that among these engineered S. oneidensis strains, the strain GS (i.e. harbouring Gxf1 gene encoding the xylose facilitator from C. intermedi, and XYL1, XYL2, and XKS1 genes encoding the xylose oxidoreductase pathway from S. stipites) was able to generate the highest power density, enabling a maximum electricity power density of 2.1 ± 0.1 mW/m².

CONCLUSION

To the best of our knowledge, this was the first report on the rationally designed Shewanella that could use xylose as the sole carbon source and electron donor to produce electricity. The synthetic biology strategies developed in this study could be further extended to rationally engineer other exoelectrogens for lignocellulosic biomass utilization to generate electricity power.

Bioelectrochemical systems-driven directional ion transport enables low-energy water desalination, pollutant removal, and resource recovery

Bioelectrochemical systems (BESs) are integrated water treatment technologies that generate electricity using organic matter in wastewater. In situ use of bioelectricity can direct the migration of ionic substances in a BES, thereby enabling water desalination, resource recovery, and valuable substance production. Recently, much attention has been placed on the microbial desalination cells in BESs to drive water desalination, and various configurations have optimized electricity generation and desalination performance and also coupled hydrogen production, heavy metal reduction, and other reactions. In addition, directional transport of other types of charged ions can remediate polluted groundwater, recover nutrient, and produce valuable substances. To better promote the practical application, the use of BESs as directional drivers of ionic substances requires further optimization to improve energy use efficiency and treatment efficacy. This article reviews existing researches on BES-driven directional ion transport to treat wastewater and identifies a few key factors involved in efficiency optimization.

Hydrogen production in microbial reverse-electrodialysis electrolysis cells using a substrate without buffer solution

The aim of this work was to use substrate without buffer solution in a microbial reverse-electrodialysis electrolysis cell (MREC) for hydrogen production under continuous flow condition (10 cell pairs of RED stacks, HRT=5, 7.5, and 15h). Decreasing in the HRT (increasing in the organic matter) made cell current stable and increased. Hydrogen gas was produced at a rate of 0.61m(3)-H2/m(3)-Van/d in H-MREC, with a COD removal efficiency of 81% (1.55g/L/d) and a Coulombic efficiency of 41%. This MREC system without buffer solution could successfully produce hydrogen gas at a consistent rate.

Microbial Electrochemical Monitoring of Volatile Fatty Acids during Anaerobic Digestion

Volatile fatty acid (VFA) concentration is known as an important indicator to control and optimize anaerobic digestion (AD) process. In this study, an innovative VFA biosensor was developed based on the principle of a microbial desalination cell. The correlation between current densities and VFA concentrations was first evaluated with synthetic digestate. Two linear relationships were observed between current densities and VFA levels from 1 to 30 mM (0.04 to 8.50 mA/m(2), R(2) = 0.97) and then from 30 to 200 mM (8.50 to 10.80 mA/m(2), R(2) = 0.95). The detection range was much broader than that of other existing VFA biosensors. The biosensor had no response to protein and lipid which are frequently found along with VFAs in organic waste streams from AD, suggesting the selective detection of VFAs. The current displayed different responses to VFA levels when different ionic strengths and external resistances were applied, though linear relationships were always observed. Finally, the biosensor was further explored with real AD effluents and the results did not show significance differences with those measured by GC. The simple and efficient biosensor showed promising potential for online, inexpensive, and reliable measurement of VFA levels during AD and other anaerobic processes.

Analysis of long-term performance and microbial community structure in bio-cathode microbial desalination cells

A microbial desalination cell (MDC) could desalinate salt water without energy consumption and simultaneously generate bioenergy. Compared with an abiotic cathode MDC, an aerobic bio-cathode MDC is more sustainable and is less expensive to operate. In this study, the long-term operation (5500 h) performance of a bio-cathode MDC was investigated in which the power density, Coulombic efficiency, and salt removal rate were decreased by 71, 44, and 27 %, respectively. The primary reason for the system performance decrease was biofouling on the membranes, which increased internal resistance and reduced the ionic transfer and energy conversion efficiency. Changing membranes was an effective method to recover the MDC performance. The microbial community diversity in the MDC anode was low compared with that of the reported microbial fuel cell (MFC), while the abundance of Proteobacteria was 30 % higher. The content of Planctomycetes in the cathode biofilm sample was much higher than that in biofouling on the cation exchange membrane (CEM), indicating that Planctomycetes were relevant to cathode oxygen reduction.

A comparative evaluation of different types of microbial electrolysis desalination cells for malic acid production

The aim of this study was to investigate different microbial electrolysis desalination cells for malic acid production. The systems included microbial electrolysis desalination and chemical-production cell (MEDCC), microbial electrolysis desalination cell (MEDC) with bipolar membrane and anion exchange membrane (BP-A MEDC), MEDC with bipolar membrane and cation exchange membrane (BP-C MEDC), and modified microbial desalination cell (M-MDC). The microbial electrolysis desalination cells performed differently in terms of malic acid production and energy consumption. The MEDCC performed best with the highest malic acid production rate (18.4 ± 0.6 mmol/Lh) and the lowest energy consumption (0.35 ± 0.14 kWh/kg). The best performance of MEDCC was attributable to the neutral pH condition in the anode chamber, the lowest internal resistance, and the highest Geobacter percentage of the anode biofilm population among all the reactors.

Three-dimensional graphene/Pt nanoparticle composites as freestanding anode for enhancing performance of microbial fuel cells

Microbial fuel cells (MFCs) are able to directly convert about 50 to 90% of energy from oxidation of organic matters in waste to electricity and have great potential application in broad fields such as wastewater treatment. Unfortunately, the power density of the MFCs at present is significantly lower than the theoretical value because of technical limitations including low bacteria loading capacity and difficult electron transfer between the bacteria and the electrode. We reported a three-dimensional (3D) graphene aerogel (GA) decorated with platinum nanoparticles (Pt NPs) as an efficient freestanding anode for MFCs. The 3D GA/Pt-based anode has a continuous 3D macroporous structure that is favorable for microorganism immobilization and efficient electrolyte transport. Moreover, GA scaffold is homogenously decorated with Pt NPs to further enhance extracellular charge transfer between the bacteria and the anode. The MFCs constructed with 3D GA/Pt-based anode generate a remarkable maximum power density of 1460 mW/m(2), 5.3 times higher than that based on carbon cloth (273 mW/m(2)). It deserves to be stressed that 1460 mW/m(2) obtained from the GA/Pt anode shows the superior performance among all the reported MFCs inoculated with Shewanella oneidensis MR-1. Moreover, as a demonstration of the real application, the MFC equipped with the freestanding GA/Pt anode has been successfully applied in driving timer for the first time, which opens the avenue toward the real application of the MFCs.

Bioelectrochemical recovery of waste-derived volatile fatty acids and production of hydrogen and alkali

Volatile fatty acids (VFA) are organic compounds of great importance for various industries and environmental processes. Fermentation and anaerobic digestion of organic wastes are promising alternative technologies for VFA production. However, one of the major challenges is development of sustainable downstream technologies for VFA recovery. In this study, an innovative microbial bipolar electrodialysis cell (MBEDC) was developed to meet the challenge of waste-derived VFA recovery, produce hydrogen and alkali, and potentially treat wastewater. The MBEDC was operated in fed-batch mode. At an applied voltage of 1.2 V, a VFA recovery efficiency of 98.3%, H2 of 18.4 mL and alkali production presented as pH of 12.64 were obtained using synthetic fermentation broth. The applied voltage, initial VFA concentrations and composition were affecting the VFA recovery. The energy balance revealed that net energy (5.20-6.86 kWh/kg-VFA recovered) was produced at all the applied voltages (0.8-1.4 V). The coexistence of other anionic species had no negative effect on VFA transportation. The VFA concentration was increased 2.96 times after three consecutive batches. Furthermore, the applicability of MBEDC was successfully verified with digestate. These results demonstrate for the first time the possibility of a new method for waste-derived VFA recovery and valuable products production that uses wastewater as fuel and bacteria as catalyst.

Effect of initial salt concentrations on cell performance and distribution of internal resistance in microbial desalination cells

Microbial desalination cells (MDCs) are modified microbial fuel cells (MFCs) that concurrently produce electricity and desalinate seawater, but adding a desalination compartment and an ion-exchange membrane may increase the internal resistance (Ri), which can limit the cell performance. However, the effects of a desalination chamber and initial NaCl concentrations on the internal resistances and the cell performances (i.e. Coulombic efficiency (CE), current and power density) of MDCs have yet to be thoroughly explored; thus, the cell performance and Ri distributions of MDCs having different initial concentrations and an MFC having no desalination chamber were compared. In the MDCs, the current and power density generation increased from 2.82 mA and 158.2 mW/m2 to 3.17 mA and 204.5 mW/m2 when the initial NaCl concentrations were increased from 5 to 30 g/L, as a consequence of the internal resistances decreasing from 2432.0 to 2328.4 Ω. And even though the MFC has a lower Ri than the MDCs, lower cell performances (current: 2.59 mA; power density: 141.6 mW/m2 and CE: 62.1%) were observed; there was no effect of improved junction potential in the MFC. Thus, in the MDCs, the higher internal resistances due to the addition of a desalination compartment can be offset by reducing the electrolyte resistance and improving the junction potential at higher NaCl concentrations.

Properties governing the transport of trace organic contaminants through ion-exchange membranes

Ion exchange membranes could provide a solution to the selective separation of organic and inorganic components in industrial wastewater. The phenomena governing the transport of organics through the IEM however, are not yet fully understood. Therefore, the transport of trace organic contaminants (TOrCs) as a model for a wide variety of organic compounds was studied under different conditions. It was found that in the absence of salt and external potential, the chemical equilibrium is the main driver for TOrC-transport, resulting in the transport of mainly charged TOrCs. When salt is present, the transport of TOrCs is hampered in favor of the NaCl transport, which shows a preferential interaction with the membranes due to its small size, high mobility and concentration. It is hypothesized that electrostatic interactions and electron donor/acceptor interactions are the main drivers for TOrC transport and that transport is mainly diffusion driven. This was confirmed in the experiments with different current densities, where the external potential seemed to have only a minor influence on the transport of TOrCs. It is only when the salt becomes nearly completely depleted that the TOrCs are transported as charge carriers. This shows that it is very difficult to get preferential transport of organic compounds due to the diffusive nature of their transport.

A ten liter stacked microbial desalination cell packed with mixed ion-exchange resins for secondary effluent desalination

The architecture and performance of microbial desalination cell (MDC) have been significantly improved in the past few years. However, the application of MDC is still limited in a scope of small-scale (milliliter) reactors and high-salinity-water desalination. In this study, a large-scale (>10 L) stacked MDC packed with mixed ion-exchange resins was fabricated and operated in the batch mode with a salt concentration of 0.5 g/L NaCl, a typical level of domestic wastewater. With circulation flow rate of 80 mL/min, the stacked resin-packed MDC (SR-MDC) achieved a desalination efficiency of 95.8% and a final effluent concentration of 0.02 g/L in 12 h, which is comparable with the effluent quality of reverse osmosis in terms of salinity. Moreover, the SR-MDC kept a stable desalination performance (>93%) when concentrate volume decreased from 2.4 to 0.1 L (diluate/concentrate volume ratio increased from 1:1 to 1:0.04), where only 0.875 L of nonfresh water was consumed to desalinate 1 L of saline water. In addition, the SR-MDC achieved a considerable desalination rate (95.4 mg/h), suggesting a promising application for secondary effluent desalination through deriving biochemical electricity from wastewater.

Microbial electrolysis cells turning to be versatile technology: recent advances and future challenges

Microbial electrolysis cells (MECs) are an electricity-mediated microbial bioelectrochemical technology, which is originally developed for high-efficiency biological hydrogen production from waste streams. Compared to traditional biological technologies, MECs can overcome thermodynamic limitations and achieve high-yield hydrogen production from wide range of organic matters at relatively mild conditions. This approach greatly reduces the electric energy cost for hydrogen production in contrast to direct water electrolysis. In addition to hydrogen production, MECs may also support several energetically unfavorable biological/chemical reactions. This unique advantage of MECs has led to several alternative applications such as chemicals synthesis, recalcitrant pollutants removal, resources recovery, bioelectrochemical research platform and biosensors, which have greatly broaden the application scopes of MECs. MECs are becoming a versatile platform technology and offer a new solution for emerging environmental issues related to waste streams treatment and energy and resource recovery. Different from previous reviews that mainly focus on hydrogen production, this paper provides an up-to-date review of all the new applications of MECs and their resulting performance, current challenges and prospects of future.

Bioelectrochemical desalination and electricity generation in microbial desalination cell with dewatered sludge as fuel

Microbial desalination cells (MDCs) with common liquid anodic substrate exhibit a slow startup and destructive pH drop, and abiotic cathodes have high cost and low sustainability. A biocathode MDC with dewatered sludge as fuel was developed for synergistic desalination, electricity generation and sludge stabilization. Experimental results indicated that the startup period was reduced to 3d, anodic pH was maintained between 6.6 and 7.6, and high stability was shown under long-term operation (300d). When initial NaCl concentrations were 5 and 10g/L, the desalinization rates during stable operation were 46.37±1.14% and 40.74±0.89%, respectively. The maximum power output of 3.178W/m(3) with open circuit voltage (OCV) of 1.118V was produced on 130d. After 300d, 25.71±0.15% of organic matter was removed. These results demonstrated that dewatered sludge was an appropriate anodic substrate to enhance MDC stability for desalination and electricity generation.

Photosynthetic microbial desalination cells (PMDCs) for clean energy, water and biomass production

Current microbial desalination cell (MDC) performances are evaluated with chemical catalysts such as ferricyanide, platinum catalyzed air-cathodes or aerated cathodes. All of these methods improve power generation potential in MDCs, however, they are not preferable for large scale applications due to cost, energy and environmental toxicity issues. In this study, performance of microbial desalination cells with an air cathode and an algae biocathode (Photosynthetic MDC - PMDC) were evaluated, both under passive conditions (no mechanical aeration or mixing). The results indicate that passive algae biocathodes perform better than air cathodes and enhance COD removal and utilize treated wastewater as the growth medium to obtain valuable biomass for high value bioproducts. Maximum power densities of 84 mW m(-3) (anode volume) or 151 mW m(-3) (biocathode volume) and a desalination rate of 40% were measured with 0.9 : 1 : 0.5 volumetric ratios of anode, desalination and algae biocathode chambers respectively. This first proof-of-concept study proves that the passive mechanisms can be beneficial in enhancing the sustainability of microbial desalination cells.

A comprehensive review of microbial electrochemical systems as a platform technology

Microbial electrochemical systems (MESs) use microorganisms to covert the chemical energy stored in biodegradable materials to direct electric current and chemicals. Compared to traditional treatment-focused, energy-intensive environmental technologies, this emerging technology offers a new and transformative solution for integrated waste treatment and energy and resource recovery, because it offers a flexible platform for both oxidation and reduction reaction oriented processes. All MESs share one common principle in the anode chamber, in which biodegradable substrates, such as waste materials, are oxidized and generate electrical current. In contrast, a great variety of applications have been developed by utilizing this in situ current, such as direct power generation (microbial fuel cells, MFCs), chemical production (microbial electrolysis cells, MECs; microbial electrosynthesis, MES), or water desalination (microbial desalination cells, MDCs). Different from previous reviews that either focus on one function or a specific application aspect, this article provides a comprehensive and quantitative review of all the different functions or system constructions with different acronyms developed so far from the MES platform and summarizes nearly 50 corresponding systems to date. It also provides discussions on the future development of this promising yet early-stage technology.