Bioelectrochemical systems: an outlook for practical applications

Improving Scalability of copper recovery in saline microbial fuel cells with microtubular polypyrrole-based cathodic electrocatalysts

Microbial fuel cells (MFC) are emerging energy-efficient systems for copper (Cu) electrowinning from waste streams by coupling it with anodic oxidation of organics in wastewater. However, there is a lack of research examining scalable electrocatalysts for Cu electrowinning at low cathodic overpotentials in highly saline catholytes often found in e-waste leachates. The challenge of developing resilient anodic biofilms that withstand the antagonistic effects of ions migrating from catholytes in saline MFC also needs to be addressed. In this study, polypyrrole (PPy) cathodic electrocatalysts were developed and coupled with a robust halophilic anodic biofilm in MFC to improve the kinetics of Cu electrowinning from acidic chloride-based catholytes. Electrochemical characterisation of these cathodes revealed shuttling of electrons by redox-active PPy via the formation of intermediate Cu+-complexes as an energy-efficient pathway for producing metallic Cu. High power densities ranging from 0.63 ± 0.17 to 0.73 ± 0.05 W m-2 were achieved with undoped-PPy and phytic acid doped-PPy cathodes with simultaneous recovery of ∼97% Cu. These electrocatalysts also exhibited low charge transfer resistance (3-8 mΩ m2) that met the requisites for scalable cathodes in MFC. However, a decrease in the efficiency of PPy cathodes was observed over 5 d due to competing reactions at their interfaces, including re-oxidation of deposited Cu and cathodic corrosion, with further studies suggested to enhance their corrosion resistance. Nonetheless, integrating PPy electrocatalysts for Cu electrowinning in saline MFC has advanced its outlooks as an energy-efficient downstream process for urban mining of Cu from e-waste.

Polyethylene glycol hydrogel coatings for protection of electroactive bacteria against chemical shocks

Loss of bioelectrochemical activity in low resource environments or from chemical toxin exposure is a significant limitation in microbial electrochemical cells (MxCs), necessitating the development of materials that can stabilize and protect electroactive biofilms. Here, polyethylene glycol (PEG) hydrogels were designed as protective coatings over anodic biofilms, and the effect of the hydrogel coatings on biofilm viability under oligotrophic conditions and ammonia-N (NH4+-N) shocks was investigated. Hydrogel deposition occurred through polymerization of PEG divinyl sulfone and PEG tetrathiol precursor molecules, generating crosslinked PEG coatings with long-term hydrolytic stability between pH values of 3 and 10. Simultaneous monitoring of coated and uncoated electrodes co-located within the same MxC anode chamber confirmed that the hydrogel did not compromise biofilm viability, while the coated anode sustained nearly a 4 × higher current density (0.44 A/m2) compared to the uncoated anode (0.12 A/m2) under oligotrophic conditions. Chemical interactions between NH4+-N and PEG hydrogels revealed that the hydrogels provided a diffusive barrier to NH4+-N transport. This enabled PEG-coated biofilms to generate higher current densities during NH4+-N shocks and faster recovery afterwards. These results indicate that PEG-based coatings can expand the non-ideal chemical environments that electroactive biofilms can reliably operate in.

Scaling up of dual-chamber microbial electrochemical systems - An appraisal using systems design approach

Impetus to minimise the energy and carbon footprints of evolving wastewater resource recovery facilities has promoted the development of microbial electrochemical systems (MES) as an emerging energy-neutral and sustainable platform technology. Using separators in dual-chamber MES to isolate anodic and cathodic environments creates endless opportunities for its myriad applications. Nevertheless, the high internal resistance and the complex interdependencies among various system factors have challenged its scale-up. This critical review employed a systems approach to examine the complex interdependencies and practical issues surrounding the implementation and scalability of dual-chamber MES, where the anodic and cathodic reactions are mutually appraised to improve the overall system efficiency. The robustness and stability of anodic biofilms in large-volume MES is dependent on its inoculum source, antecedent history and enrichment strategies. The composition and anode-respiring activity of these biofilms are modulated by the anolyte composition, while their performance demands a delicate balance between the electrode size, macrostructure and the availability of substrates, buffers and nutrients when using real wastewater as anolyte. Additionally, the catholyte governed the reduction environment and associated energy consumption of MES with scalable electrocatalysts needed to enhance the sluggish reaction kinetics for energy-efficient resource recovery. A comprehensive assessment of the dual-chamber reactor configuration revealed that the tubular, spiral-wound, or plug-in modular MES configurations are suitable for pilot-scale, where it could be designed more effectively using efficient electrode macrostructure, suitable membranes and bespoke strategies for continuous operation to maximise their performance. It is anticipated that the critical and analytical understanding gained through this review will support the continuous development and scaling-up of dual-chamber MES for prospective energy-neutral treatment of wastewater and simultaneous circular management of highly relevant environmental resources.

Application of biochar on soil bioelectrochemical remediation: behind roles, progress, and potential

Bioelectrochemical systems (BESs) that combine electrochemistry with biological methods have gained attention in the remediation of polluted environments, including wastewater, sludge, sediments, and soils. The most attractive advantage of BESs is that the solid electrode is used as an inexhaustible electron acceptor or donor, and biocurrent directly converted from organics can afford the reaction energy of contaminant breakdown, crossing the internal energy barrier of endothermic degradation, which achieves a continuous biodegradation process without the simultaneous use of exogenetic chemicals and bioelectricity recovery. However, soil BESs are hindered by expensive electrode materials, difficult pollutant and electron transfer, low microbial competitive activity, and biocompatibility in contamination remediation. Fortunately, introducing biochar into soil BESs could reveal a high potential in addressing these BES inadequacies. The characteristics of biochar, e.g., conductivity, transferability, high specific surface area, high porosity, large functional groups, and biocompatibility, can improve the performance of soil BESs. In fact, biochar not only carries electrons but also transfers nutrients, pollutants, and even bacteria by facilitating transmission in the bioelectric field of BESs. Consequently, the abilities of biochar make for better functionality of BESs. This review collates information on the roles, application, and progress of biochar in soil BESs, and future prospects are given. It is beneficial for environmental researchers and engineers to extend BES application in environmental remediation and to assist the progress of carbon sequestration and emission reduction based on the inertia of biochar and the blocking of electron flow to form methane.

Interspecies ecological competition rejuvenates decayed Geobacter electroactive biofilm

Bioelectrochemical systems (BESs) exploit electroactive biofilms (EABs) for promising applications in biosensing, wastewater treatment, energy production, and chemical biosynthesis. However, during the operation of BESs, EABs inevitably decay. Seeking approaches to rejuvenate decayed EABs is critical for the sustainability and practical application of BESs. Prophage induction has been recognized as the primary reason for EAB decay. Herein, we report that introducing a competitive species of Geobacter uraniireducens suspended prophage induction in Geobacter sulfurreducens and thereby rejuvenated the decayed G. sulfurreducens EAB. The transcriptomic profile of G. sulfurreducens demonstrated that the addition of G. uraniireducens significantly affected the expression of metabolism- and stress response system-related genes and in particular suppressed the induction of phage-related genes. Mechanistic analyses revealed that interspecies ecological competition exerted by G. uraniireducens suppressed prophage induction. Our findings not only reveal a novel strategy to rejuvenate decayed EABs, which is significant for the sustainability of BESs, but also provide new knowledge for understanding phage-host interactions from an ecological perspective, with implications for developing therapies to defend against phage attack.

A review on recent environmental electrochemistry approaches for the consolidation of a circular economy model

Availability of raw materials in the chemical industry is related to the selection of the chemical processes in which they are used as well as to the efficiency, cost, and eventual evolution to more competitive dynamics of transformation technologies. In general terms however, any chemically transforming technology starts with the extraction, purification, design, manufacture, use, and disposal of materials. It is important to create a new paradigm towards green chemistry, sustainability, and circular economy in the chemical sciences that help to better employ, reuse, and recycle the materials used in every aspect of modern life. Electrochemistry is a growing field of knowledge that can help with these issues to reduce solid waste and the impact of chemical processes on the environment. Several electrochemical studies in the last decades have benefited the recovery of important chemical compounds and elements through electrodeposition, electrowinning, electrocoagulation, electrodialysis, and other processes. The use of living organisms and microorganisms using an electrochemical perspective (known as bioelectrochemistry), is also calling attention to "mining", through plants and microorganisms, essential chemical elements. New process design or the optimization of the current technologies is a major necessity to enhance production and minimize the use of raw materials along with less generation of wastes and secondary by-products. In this context, this contribution aims to show an up-to-date scenario of both environmental electrochemical and bioelectrochemical processes for the extraction, use, recovery and recycling of materials in a circular economy model.

Microbial electrochemical system: an emerging technology for remediation of polycyclic aromatic hydrocarbons from soil and sediments

Worldwide industrialization and other human activities have led to a frightening stage of release of hazardous, highly persistent, toxic, insoluble, strongly adsorbed to the soil and high molecular weight ubiquitous polycyclic aromatic hydrocarbons (PAHs) in soils and sediments. The various conventional remediation methods are being used to remediate PAHs with certain drawbacks. Time taking process, high expenditure, excessive quantities of sludge generation, and various chemical requirements do not only make these methods outdated but produce yet much resistant and toxic intermediate metabolites. These disadvantages may be overcome by using a microbial electrochemical system (MES), a booming technology in the field of bioremediation. MES is a green remediation approach that is regulated by electrochemically active microorganisms at the electrode in the system. The key advantage of the system over the conventional methods is it does not involve any additional chemicals, takes less time, and generates minimal sludge or waste during the remediation of PAHs in soils. However, a comprehensive review of the MES towards bioremediation of PAHs adsorbed in soil and sediment is still lacking. Therefore, the present review intended to summarize the recent information on PAHs bioremediation, application, risks, benefits, and challenges based on sediment microbial fuel cell and microbial fuel cell to remediate mount-up industrial sludge, soil, and sediment rich in PAHs. Additionally, bio-electrochemically active microbes, mechanisms, and future perspectives of MES have been discussed.

Understanding the impacts of intermittent electro field on the bioelectrochemical aniline degradation system: Performance, microbial community and functional enzyme

On account of the lack of a sustainable electron donor source and the inhibitory effect of aniline on denitrogenation make it tough to achieve simultaneous removal of aniline and nitrogen. Herein, the strategy of adjusting electric field mode was applied to the electro-enhanced sequential batch reactors (E-SBRs: R1 (continuous ON), R2 (2 h-ON/2 h-OFF), R3 (12 h-ON/12 h-OFF), R4 (in the aerobic phase ON), R5 (in the anoxic phase ON)) to treat aniline wastewater. Aniline removal rate reached approximately 99% in the five systems. Decreasing electrical stimulation interval from 12 to 2 h significantly improved the electron utilization efficiency for aniline degradation and nitrogen metabolism. The total nitrogen removal was achieved from 70.31% to 75.63%. Meanwhile, the hydrogenotrophic denitrifiers of Hydrogenophaga, Thauera, and Rhodospirillales, enriched in reactors of minor electrical stimulation interval. Accordingly, the expression of functional enzyme related to electron transport was incremental with the proper electrical stimulation frequency.

Bioelectrochemical extraction of ammonium from low-strength wastewater with concomitant generation of high-purity hydrogen

Bioelectrochemical systems (BES) are emerging environmental biotechnology for recovering ammonia from waste streams. It has been tested extensively for treating ammonium-rich wastewater. This study examined the suitability of BES to facilitate carbon removal and ammonium extraction from a low ammonium liquor (3.7 mM) that mimics municipal wastewater, and concomitant production of high-purity hydrogen gas, which could potentially be harnessed as a fuel or internally recycled for ammonia stripping. Results showed that a two-chamber cation exchange membrane-equipped BES enabled a high hydrogen yield (22.8 m3 H2 m-3 d-1; > 98% cathodic efficiency) and chemical oxygen demand (COD) removal (80%; 2.43 kg COD m-3 d-1 at a hydraulic retention time of 4.4 h). However, for the treatment of wastewater, the system demanded high energy (2.3 kWh kg COD-1; 152 kWh kg-1 N removed) and base for pH adjustment. The technology may be more suitable for recovering ammonium from wastewaters with molar ammonium to BOD ratio closer to the desired stoichiometric ratio of four, and for waste streams containing sufficient alkalinity or pH-buffering capacity, eliminating the need for dosing cation-bearing alkali.

Scale-up of the bioelectrochemical system: Strategic perspectives and normalization of performance indices

Electrochemists and ecological engineers find environmental bioelectrochemistry appealing; however, there is a big gap between expectations and actual progress in bioelectrochemical system (BES). Implementing such technology opens new opportunities for novel electrochemical reactions for resource recovery and effective wastewater treatment. Loopholes of BES exist in its scaling-up applications, and numerous attempts toward practical applications (200, 1000, and 1500 L) are key successive indicators toward its commercialization. This review emphasized the critical rethinking of standardization of performance indices i.e. current generation (A/m²), net energy recovery (kWh/kg·COD), product/resource yield (mM), and economic feasibility ($/kWh) to make fair comparison with the existing treatment system. Therefore, directional perspectives, including modularity, energy-cost balance, energy and resource recovery, have been proposed for the sustainable market of BES. The current state of the art and up-gradation in resource recovery and contaminant removal warrants a systematic rethinking of functional worth and niches of BES for practical applications.

Tannery wastewater treatment coupled with bioenergy production in upflow microbial fuel cell under saline condition

Tannery industry handling huge amount of leather materials release immense amount of saline organic content wastewater. The present research was focused on the treatment of tannery industrial wastewater in UMFC (upflow microbial fuel cell) under saline condition (4%). The UMFC reactor was operated at different organic load (OL) such as 0.6, 1.2, 1.8 and 2.4 gCOD/L respectively. Total chemical oxygen demand (TCOD) removal at 0.6, 1.2, 1.8 gCOD/L was 87 ± 1.2%, 91 ± 1.2% and 93 ± 1.8% respectively. Soluble chemical oxygen demand (SCOD) removal in UMFC at 0.6, 1.2, 1.8 gCOD/L was 85 ± 0.6%, 88 ± 1.2% and 91 ± 1.8% respectively. Total suspended solids (TSS) removal was 49%, 78%, 81% at 0.6, 1.2, 1.8 gCOD/L OL in UMFC. Further, raise in OL to 2.4 gCOD/L showed decrease in TCOD, SCOD (80% and 72%) and TSS (60%) removal. Maximal power production of 854 mV with corresponding PD (power density) of 462 mW/m² and CD (current density) of 523 mA/m² was registered at 1.8 gCOD/L OL in UMFC. Increase in OL to 2.4 gCOD/L revealed decline in energy production to 810 mV with PD (385 mW/m²) and CD (438 mA/m²) in UMFC. Maximal bioaccumulation of chromium (95%) was recorded at 1.8 gCOD/L OL. Among different OL used 1.8 gCOD/L OL was optimum for the treatment of tannery wastewater and energy production. Bacterial community analysis in anode of UMFC revealed the dominance of promising electrogenic halophilic strains such as Ochrobactrum, Marinobacter, Rhodococcus and Bacillus in all the OL. Thus, the research clearly revealed the efficacy of halophilic consortium to treat the saline tannery wastewater coupled with bioenergy production in UMFC.

Catalysing electrowinning of copper from E-waste: A critical review

Smart technologies and digitalisation have increased the consumption of scarce metals that threaten the sustainability of intricated industries. Additionally, the growing streams of waste electrical and electronic equipment (e-waste) are significant hazards to public health and the environment. Thus, there is an escalating need to recover metals from e-waste for sustainable management of metal resources. Hydrometallurgical processing of e-waste, involving copper (Cu) leaching and its subsequent recovery from pregnant leach solution (PLS) via electrowinning, has emerged as an efficient strategy to close the recycling loop. Electrowinning from PLS demonstrated higher Cu recovery efficiency and operational feasibility with a lower reagent use and lesser waste generation. Nevertheless, multiple issues challenged its practical implementation, including selective recovery of Cu from PLS containing mixed metals and high energy consumption. This review (1) identifies the factors affecting Cu electrowinning from PLS; (2) evaluates the composition of lixiviants influencing Cu electrowinning; (3) appraises various catalysts developed for enhancing Cu electrodeposition; and (4) reviews coupled systems that minimised process energy consumption. From the literature review, electrocatalysts are prospective candidates for improving Cu electrowinning as they reduced the cathodic reduction overpotentials, enhanced surface reaction kinetics and increased current efficiency. Other catalysts, including bioelectrocatalysts and photoelectrocatalysts, are applicable for dilute electrolytes with further investigations required to validate their feasibility. The coupled systems, including slurry electrolysis, bioelectrochemical systems and coupled redox fuel cells, minimise process energy requirements by systematically coupling the cathodic reduction reaction with suitable anodic oxidation reactions having thermodynamically low overpotentials. Among these systems, slurry electrolysis utilising a single-step processing of e-waste is feasible for commericalisation though operational challenges must be addressed to improve its sustainability. The other systems require further studies to improve their scalability. It provides an important direction for energy-efficient Cu electrowinning from PLS, ultimately promoting a circular economy for the scarce metal resources.

In vivo characterization of electroactive biofilms inside porous electrodes with MR Imaging

Identifying the limiting processes of electroactive biofilms is key to improve the performance of bioelectrochemical systems (BES). For modelling and developing BES, spatial information of transport phenomena and biofilm distribution are required and can be determined by Magnetic Resonance Imaging (MRI) in vivo, in situ and in operando even inside opaque porous electrodes. A custom bioelectrochemical cell was designed that allows MRI measurements with a spatial resolution of 50 μm inside a 500 μm thick porous carbon electrode. The MRI data showed that only a fraction of the electrode pore space is colonized by the Shewanella oneidensis MR-1 biofilm. The maximum biofilm density was observed inside the porous electrode close to the electrode-medium interface. Inside the biofilm, mass transport by diffusion is lowered down to 45% compared to the bulk growth medium. The presented data and the methods can be used for detailed models of bioelectrochemical systems and for the design of improved electrode structures.

The use of magnetic resonance imaging can contribute to a better understanding of limiting processes occurring in electroactive biofilms especially inside opaque porous electrodes.

Prevention and removal of membrane and separator biofouling in bioelectrochemical systems - a comprehensive review

Bioelectrochemical systems (BESs) have made significant progress in recent years in all aspects of their technology. BESs usually work with a membrane or a separator, which is one of their most critical components affecting performance. Quite often, biofilm from either the anolyte or catholyte forms on the membrane, which can negatively affect its performance. In critical cases, the long-term power performance observed for microbial fuel cells (MFCs) has dropped by over 90%. Surface modification and composite material approaches as well as chemical and physical cleaning techniques involving surfactants, acids, hydroxides, and ultrasounds have been successfully implemented to combat biofilm formation. Surface modifications produced up to 6–7 times higher power performance in the long-term, whereas regeneration strategies resulted in up to 100% recovery of original performance. Further studies include tools such as fluid dynamics-based design and plasma cleaning. The biofouling area is still underexplored in the field of bioelectrochemistry and requires systematic improvement. Therefore, this review summarizes the most recent knowledge with the aim of helping the research and engineering community select the best strategy and discuss further perspectives for combating the undesirable biofilm.

The Rational Design of a Financially Viable Microbial Electrolysis Cell for Domestic Wastewater Treatment

Microbial electrolysis cells (MECs) are yet to achieve commercial viability. Organic removal rates (ORR) and capital costs dictate an MEC’s financial competitiveness against activated sludge treatments. We used numerical methods to investigate the impact of acetate concentration and the distance between opposing anodes’ surfaces (anode interstices width) on MEC cost-performance. Numerical predictions were calibrated against laboratory observations using an evolutionary algorithm. Anode interstices width had a non-linear impact on ORR and therefore allowable cost. MECs could be financially competitive if anode interstices widths are carefully controlled (2.5 mm), material costs kept low (£5–10/m2-anode), and wastewater pre-treated, using hydrolysis to consistently achieve influent acetate concentrations >100 mg-COD/l.

Electrified bioreactors: the next power-up for biometallurgical wastewater treatment

Over the past decades, biological treatment of metallurgical wastewaters has become commonplace. Passive systems require intensive land use due to their slow treatment rates, do not recover embedded resources and are poorly controllable. Active systems however require the addition of chemicals, increasing operational costs and possibly negatively affecting safety and the environment. Electrification of biological systems can reduce the use of chemicals, operational costs, surface footprint and environmental impact when compared to passive and active technologies whilst increasing the recovery of resources and the extraction of products. Electrification of low rate applications has resulted in the development of bioelectrochemical systems (BES), but electrification of high rate systems has been lagging behind due to the limited mass transfer, electron transfer and biomass density in BES. We postulate that for high rate applications, the electrification of bioreactors, for example, through the use of electrolyzers, may herald a new generation of electrified biological systems (EBS). In this review, we evaluate the latest trends in the field of biometallurgical and microbial‐electrochemical wastewater treatment and discuss the advantages and challenges of these existing treatment technologies. We advocate for future research to focus on the development of electrified bioreactors, exploring the boundaries and limitations of these systems, and their validity upon treating industrial wastewaters.

Recent Developments in Microbial Electrolysis Cell-Based Biohydrogen Production Utilizing Wastewater as a Feedstock

Carbon constraints, as well as the growing hazard of greenhouse gas emissions, have accelerated research into all possible renewable energy and fuel sources. Microbial electrolysis cells (MECs), a novel technology able to convert soluble organic matter into energy such as hydrogen gas, represent the most recent breakthrough. While research into energy recovery from wastewater using microbial electrolysis cells is fascinating and a carbon-neutral technology that is still mostly limited to lab-scale applications, much more work on improving the function of microbial electrolysis cells would be required to expand their use in many of these applications. The present limiting issues for effective scaling up of the manufacturing process include the high manufacturing costs of microbial electrolysis cells, their high internal resistance and methanogenesis, and membrane/cathode biofouling. This paper examines the evolution of microbial electrolysis cell technology in terms of hydrogen yield, operational aspects that impact total hydrogen output in optimization studies, and important information on the efficiency of the processes. Moreover, life-cycle assessment of MEC technology in comparison to other technologies has been discussed. According to the results, MEC is at technology readiness level (TRL) 5, which means that it is ready for industrial development, and, according to the techno-economics, it may be commercialized soon due to its carbon-neutral qualities.

Microbial Fuel Cell for Energy Production, Nutrient Removal and Recovery from Wastewater: A Review

The world is facing serious threats from the depletion of non-renewable energy resources, freshwater shortages and food scarcity. As the world population grows, the demand for fresh water, energy, and food will increase, and the need for treating and recycling wastewater will rise. In the past decade, wastewater has been recognized as a resource as it primarily consists of water, energy-latent organics and nutrients. Microbial fuel cells (MFC) have attracted considerable attention due to their versatility in their applications in wastewater treatment, power generation, toxic pollutant removal, environmental monitoring sensors, and more. This article provides a review of MFC technologies applied to the removal and/or recovery of nutrients (such as P and N), organics (COD), and bioenergy (as electricity) from various wastewaters. This review aims to provide the current perspective on MFCs, focusing on the recent advancements in the areas of nutrient removal and/or recovery with simultaneous power generation.

Shewanella oneidensis MR-1 as a bacterial platform for electro-biotechnology

The genus Shewanella comprises over 70 species of heterotrophic bacteria with versatile respiratory capacities. Some of these bacteria are known to be pathogens of fishes and animals, while many are non-pathogens considered to play important roles in the global carbon cycle. A representative strain is Shewanella oneidensis MR-1 that has been intensively studied for its ability to respire diverse electron acceptors, such as oxygen, nitrate, sulfur compounds, metals, and organics. In addition, studies have been focused on its ability as an electrochemically active bacterium that is capable of discharging electrons to and receiving electrons from electrodes in bioelectrochemical systems (BESs) for balancing intracellular redox states. This ability is expected to be applied to electro-fermentation (EF) for producing value-added chemicals that conventional fermentation technologies are difficult to produce efficiently. Researchers are also attempting to utilize its electrochemical ability for controlling gene expression, for which electro-genetics (EG) has been coined. Here we review fundamental knowledge on this bacterium and discuss future directions of studies on its applications to electro-biotechnology (EB).

Microbial fuel cells: a comprehensive review for beginners

Microbial fuel cells (MFCs) have shown immense potential as a one-stop solution for three major sustainability issues confronting the world today-energy security, global warming and wastewater management. MFCs represent a cross-disciplinary platform for research at the confluence of the natural and engineering sciences. The diversity of variables influencing performance of MFCs has garnered research interest across varied scientific disciplines since the beginning of this century. The increasing number of research publications has made it necessary to keep track of work being carried out by research groups across the globe and consolidate significant findings on a regular basis. Review articles are often the nodal points for beginners who are required to undertake an exploratory survey of literature to identify a suitable research problem. This 'review of reviews' is a ready-reckoner that directs readers to relevant reviews and research articles reporting significant developments in MFC research in the last two decades. The article also highlights the areas needing research attention which when addressed could take this technology a few more steps closer to practical implementation.

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.

Dissecting the Structural and Conductive Functions of Nanowires in Geobacter sulfurreducens Electroactive Biofilms

Conductive nanowires are thought to contribute to long-range electron transfer (LET) in Geobacter sulfurreducens anode biofilms. Three types of nanowires have been identified: pili, OmcS, and OmcZ. Previous studies highlighted their conductive function in anode biofilms, yet a structural function also has to be considered. We present here a comprehensive analysis of the function of nanowires in LET by inhibiting the expression of each nanowire. Meanwhile, flagella with poor conductivity were expressed to recover the structural function but not the conductive function of nanowires in the corresponding nanowire mutant strain. The results demonstrated that pili played a structural but not a conductive function in supporting biofilm formation. In contrast, the OmcS nanowire played a conductive but not a structural function in facilitating electron transfer in the biofilm. The OmcZ nanowire played both a structural and a conductive function to contribute to current generation. Expression of the poorly conductive flagellum was shown to enhance biofilm formation, subsequently increasing current generation. These data support a model in which multiheme cytochromes facilitate long-distance electron transfer in G. sulfurreducens biofilms. Our findings also suggest that the formation of a thicker biofilm, which contributed to a higher current generation by G. sulfurreducens, was confined by the biofilm formation deficiency, and this has applications in microbial electrochemical systems. IMPORTANCE The low power generation of microbial fuel cells limits their utility. Many factors can affect power generation, including inefficient electron transfer in the anode biofilm. Thus, understanding the mechanism(s) of electron transfer provides a pathway for increasing the power density of microbial fuel cells. Geobacter sulfurreducens was shown to form a thick biofilm on the anode. Cells far away from the anode reduce the anode through long-range electron transfer. Based on their conductive properties, three types of nanowires have been hypothesized to directly facilitate long-range electron transfer: pili, OmcS, and OmcZ nanowires. However, their structural contributions to electron transfer in anode biofilm have not been elucidated. Based on studies of mutants lacking one or more of these facilitators, our results support a cytochrome-mediated electron transfer process in Geobacter biofilms and highlight the structural contribution of nanowires in anode biofilm formation, which contributes to biofilm formation and current generation, thereby providing a strategy to increase current generation.

Microbial Electrolysis Cells for Decentralised Wastewater Treatment: The Next Steps

Traditional wastewater treatment methods have become aged and inefficient, meaning alternative methods are essential to protect the environment and ensure water and energy security worldwide. The use of microbial electrolysis cells (MEC) for wastewater treatment provides an innovative alternative, working towards circular wastewater treatment for energy production. This study evaluates the factors hindering industrial adoption of this technology and proposes the next steps for further research and development. Existing pilot-scale investigations are studied to critically assess the main limitations, focusing on the electrode material, feedstock, system design and inoculation and what steps need to be taken for industrial adoption of the technology. It was found that high strength influents lead to an increase in energy production, improving economic viability; however, large variations in waste streams indicated that a homogenous solution to wastewater treatment is unlikely with changes to the MEC system specific to different waste streams. The current capital cost of implementing MECs is high and reducing the cost of the electrodes should be a priority. Previous pilot-scale studies have predominantly used carbon-based materials. Significant reductions in relative performance are observed when electrodes increase in size. Inoculation time was found to be a significant barrier to quick operational performance. Economic analysis of the technology indicated that MECs offer an attractive option for wastewater treatment, namely greater energy production and improved treatment efficiency. However, a significant reduction in capital cost is necessary to make this economically viable. MEC based systems should offer improvements in system reliability, reduced downtime, improved treatment rates and improved energy return. Discussion of the merits of H2 or CH4 production indicates that an initial focus on methane production could provide a stepping-stone in the adoption of this technology while the hydrogen market matures.

Influence of Nickel molybdate nanocatalyst for enhancing biohydrogen production in microbial electrolysis cell utilizing sugar industrial effluent

Biohydrogen production in Microbial Electrolysis Cell (MEC) had inspired the researchers to overcome the challenges associated towards sustainability. Despite microbial community and various substrates, economical cathode catalyst development is most significant factor for enhancing hydrogen production in the MEC. Hence, in this study, the performance of MEC was investigated with a sugar industry effluent (COD 4200 ± 20 mg/L) with graphite anode and modified Nickel foam (NF) cathode. Nickel molybdate (NiMoO₄) coated NF achieved a higher hydrogen production rate 0.12 ± 0.01 L.L^-1D^-1 as compared to control under favorable conditions. Electrochemical characterizations demonstrated that the improved catalytic activity of novel nanocatalyst with lower impedance favoring faster hydrogen evolution kinetics. The MEC with the novel catalyst performed with 58.2% coloumbic efficiency, 20.36% cathodic hydrogen recovery, 11.96% overall hydrogen recovery and 54.38% COD removal efficiency for a 250 mL substrate during 5 days' batch cycle. Hence, the potentiality of modified cathode was established with the real time industrial effluent highlighting the waste to wealth bio-electrochemical technology.

Electrifying biotrickling filters for the treatment of aquaponics wastewater

This work aimed to study the electrification of biotrickling filters by means of Microbial electrochemical technologies (MET) to develop an easy-to-assemble and easy-to-use MET for nitrogen removal without external aeration nor addition of chemicals. Four different designs were tested. The highest ammonium and nitrate removal rates (94 gN·m^-3·d^-1 and 43 gN·m^-3·d^-1, respectively) were reached by combining an aerobic zone with an electrified anoxic zone. The standards of effluent quality suitable for hydroponics were met at low energy cost (8.3 × 10^-2 kWh·gN^-1). Electrified biotrickling filters are a promising alternative for aquaponics and a potential treatment for organic carbon-deficient ammonium-contaminated waters.

A bioelectrochemical-system-based trickling filter reactor for wastewater treatment

A bioelectrochemical system (BES)-based trickling filter (TF) reactor was utilized for wastewater treatment. At a COD load of 1.0 g-COD/L/day, effluent chemical oxygen demand (COD) and total nitrogen (TN) were 115 and 108 mg/L, respectively, which were allowed for discharge. Superior performance was achieved at 0.5 g-COD/L/day with a circulation rate of 8 L/h, and both COD and TN removal were >98%. Coulombic efficiency was 11% at 1.0 g-COD/L/day and at most 16% at 0.5 g-COD/L/day. COD removal decreased when the BES was removed, demonstrating that BES improved COD removal capability. In anodic biofilms, exoelectrogenic, facultative, nitrifying, and sulfate-reducing bacteria could coexist. Geobacter for current generation grew inside the biofilm, and bacteria in the middle and outer layers consumed oxygen and degraded organic matter and nitrogen. This BES-based TF reactor may be used for efficient and cost-effective COD and TN removal at high loads without excess sludge removal.

Technologies for the recovery of nutrients, water and energy from human urine: A review

The global demand for a constant supply of fertilizer is increasing with the booming of the population. Nowadays more focus is given to the recovery and reuse of the nutrients rather than synthesis of the fertilizer from chemicals. Human urine is the best available resource for the primary macronutrients (Nitrogen, Phosphorus and Potassium) for the fertilizer as it contains 10-12 g/L nitrogen, 0.1-0.5 g/L phosphorous and 1.0-2.0 g/L potassium. For the recovery of these nutrients from human urine, various technologies are available which requires source separation and treatment. . In this review, a wide range of the technologies for the treatment of source-separated human urine are covered and discussed in detail. This review has categorized the technologies based on the recovery of nutrients, energy, and water from human urine. Among the various technologies available, Bio-electrochemical technologies are environmental friendly and recovers energy along with the nutrients. Forward Osmosis is the best available technology for the water recovery and for concentrating the nutrients in urine, without or minimal consumption of energy. However, experimental work in this technology is at its prior stage. A single technology is still not sufficient to recover nutrients, water and energy. Therefore, integration of two or more technologies seems essential.

Glucose-Oxygen Biofuel Cell with Biotic and Abiotic Catalysts: Experimental Research and Mathematical Modeling

The demand for alternative sources of clean, sustainable, and renewable energy has been a focus of research around the world for the past few decades. Microbial/enzymatic biofuel cells are one of the popular technologies for generating electricity from organic substrates. Currently, one of the promising fuel options is based on glucose due to its multiple advantages: high energy intensity, environmental friendliness, low cost, etc. The effectiveness of biofuel cells is largely determined by the activity of biocatalytic systems applied to accelerate electrode reactions. For this work with aerobic granular sludge as a basis, a nitrogen-fixing community of microorganisms has been selected. The microorganisms were immobilized on a carbon material (graphite foam, carbon nanotubes). The bioanode was developed from a selected biological material. A membraneless biofuel cell glucose/oxygen, with abiotic metal catalysts and biocatalysts based on a microorganism community and enzymes, has been developed. Using methods of laboratory electrochemical studies and mathematical modeling, the physicochemical phenomena and processes occurring in the cell has been studied. The mathematical model includes equations for the kinetics of electrochemical reactions and the growth of microbiological population, the material balance of the components, and charge balance. The results of calculations of the distribution of component concentrations over the thickness of the active layer and over time are presented. The data obtained from the model calculations correspond to the experimental ones. Optimization for fuel concentration has been carried out.

A review on the contribution of electron flow in electroactive wetlands: Electricity generation and enhanced wastewater treatment

In less than a decade, bioelectrochemical systems/microbial fuel cell integrated constructed wetlands (electroactive wetlands) have gained a considerable amount of attention due to enhanced wastewater treatment and electricity generation. The enhancement in treatment has majorly emanated from the electron transfer or flow, particularly in anaerobic regions. However, the chemistry associated with electron transfer is complex to understand in electroactive wetlands. The electroactive wetlands accommodate diverse microbial community in which each microbe set their own potential to further participate in electron transfer. The conductive materials/electrodes in electroactive wetlands also contain some potential, due to which, several conflicts occur between microbes and electrode, and results in inadequate electron transfer or involvement of some other reaction mechanisms. Still, there is a considerable research gap in understanding of electron transfer between electrode-anode and cathode in electroactive wetlands. Additionally, the interaction of microbes with the electrodes and understanding of mass transfer is also essential to further understand the electron recovery. This review mainly deals with the electron transfer mechanism and its role in pollutant removal and electricity generation in electroactive wetlands.

Urine in Bioelectrochemical Systems: An Overall Review

In recent years, human urine has been successfully used as an electrolyte and organic substrate in bioelectrochemical systems (BESs) mainly due of its unique properties. Urine contains organic compounds that can be utilised as a fuel for energy recovery in microbial fuel cells (MFCs) and it has high nutrient concentrations including nitrogen and phosphorous that can be concentrated and recovered in microbial electrosynthesis cells and microbial concentration cells. Moreover, human urine has high solution conductivity, which reduces the ohmic losses of these systems, improving BES output. This review describes the most recent advances in BESs utilising urine. Properties of neat human urine used in state-of-the-art MFCs are described from basic to pilot-scale and real implementation. Utilisation of urine in other bioelectrochemical systems for nutrient recovery is also discussed including proofs of concept to scale up systems.

Community Structure Analyses of Anodic Biofilms in a Bioelectrochemical System Combined with an Aerobic Reactor

Bioelectrochemical system (BES)-based reactors have a limited range of use, especially in aerobic conditions, because these systems usually produce current from exoelectrogenic bacteria that are strictly anaerobic. However, some mixed cultures of bacteria in aerobic reactors can form surface biofilms that may produce anaerobic conditions suitable for exoelectrogenic bacteria to thrive. In this study, we combined a BES with an aerobic trickling filter (TF) reactor for wastewater treatment and found that the BES-TF setup could produce electricity with a coulombic efficiency of up to 15% from artificial wastewater, even under aerobic conditions. The microbial communities within biofilms formed at the anodes of BES-TF reactors were investigated using high throughput 16S rRNA gene sequencing. Efficiency of reduction in chemical oxygen demand and total nitrogen content of wastewater using this system was >97%. Bacterial community analysis showed that exoelectrogenic bacteria belonging to the genera Geobacter and Desulfuromonas were dominant within the biofilm coating the anode, whereas aerobic bacteria from the family Rhodocyclaceae were abundant on the surface of the biofilm. Based on our observations, we suggest that BES-TF reactors with biofilms containing aerobic bacteria and anaerobic exoelectrogenic bacteria on the anodes can function in aerobic environments.

NanoSIMS imaging reveals metabolic stratification within current-producing biofilms

Electricity-producing bacteria are potential power sources, fermentation platforms, and desalination systems, if current densities could be increased. These organisms form conductive biofilms on electrodes, allowing new cell layers to contribute to current production until a limit is reached, but the biological underpinning of this limit is not well-understood. We investigated the limitation behind this phenomenon using stable isotope probing and nanoscale secondary ion mass spectrometry, showing active cells are restricted to layers closest to the electrode. This metabolic observation fundamentally changes our understanding of electron flow and cell growth within current-producing biofilms and provides constraints on the physical structure of natural communities reliant on this process for growth. We predict improvements in biofilm conductivity will yield higher current-producing systems.

Metal-reducing bacteria direct electrons to their outer surfaces, where insoluble metal oxides or electrodes act as terminal electron acceptors, generating electrical current from anaerobic respiration. Geobacter sulfurreducens is a commonly enriched electricity-producing organism, forming thick conductive biofilms that magnify total activity by supporting respiration of cells not in direct contact with electrodes. Hypotheses explaining why these biofilms fail to produce higher current densities suggest inhibition by formation of pH, nutrient, or redox potential gradients; but these explanations are often contradictory, and a lack of direct measurements of cellular growth within biofilms prevents discrimination between these models. To address this fundamental question, we measured the anabolic activity of G. sulfurreducens biofilms using stable isotope probing coupled to nanoscale secondary ion mass spectrometry (nanoSIMS). Our results demonstrate that the most active cells are at the anode surface, and that this activity decreases with distance, reaching a minimum 10 µm from the electrode. Cells nearest the electrode continue to grow at their maximum rate in weeks-old biofilms 80-µm-thick, indicating nutrient or buffer diffusion into the biofilm is not rate-limiting. This pattern, where highest activity occurs at the electrode and declines with each cell layer, is present in thin biofilms (<5 µm) and fully grown biofilms (>20 µm), and at different anode redox potentials. These results suggest a growth penalty is associated with respiring insoluble electron acceptors at micron distances, which has important implications for improving microbial electrochemical devices as well as our understanding of syntrophic associations harnessing the phenomenon of microbial conductivity.

Functional group surface modifications for enhancing the formation and performance of exoelectrogenic biofilms on the anode of a bioelectrochemical system

Various new energy technologies have been developed to reduce reliance on fossil fuels. The bioelectrochemical system (BES), an integrated microbial-electrochemical energy conversion process, is projected to be a sustainable and environmentally friendly energy technology. However, low power density is still one of the main limiting factors restricting the practical application of BESs. To enhance power output, functional group modification on anode surfaces has been primarily developed to improve the bioelectrochemical performances of BESs in terms of startup, power density, chemical oxygen demand (COD) removal and coulombic efficiency (CE). This modification could change the anode surface characteristics: roughness, hydrophobicity, biocompatibility, chemical bonding and electrochemically active surface area. This will facilitate bacterial adhesion, biofilm formation and extracellular electron transfer (EET). Additionally, some antibacterial functional groups are applied on air cathodes in order to suppress aerobic biofilms and enhance cathodic oxygen reduction reactions (ORRs). Various modification strategies such as: soaking, heat treatment and plasma modification have been reported to introduce functional groups typically as O-, N- and S-containing groups. In this review, the effects of anode functional groups on electroactive bacteria through the whole biofilm formation process are summarized. In addition, the application of those modification technologies to improve bioelectricity generation, resource recovery, bioelectrochemical analysis and the production of value-added chemicals and biofuels is also discussed. Accordingly, this review aims to help scientists select the most appropriate functional groups and up-to-date methods to improve biofilm formation.

Remediation of Potential Toxic Elements from Wastes and Soils: Analysis and Energy Prospects

The aim of this study is to evaluate the application of the main hazardous waste management techniques in mining operations and in dumping sites being conscious of the inter-linkages and inter-compartment of the contaminated soils and sediments. For this purpose, a systematic review of the literature on the reduction or elimination of different potential toxic elements was carried out, focusing on As, Cd and Hg as main current contaminant agents. Selected techniques are feasible according to several European countries’ directives, especially in Spain. In the case of arsenic, we verified that there exists a main line that is based on the use of iron minerals and its derivatives. It is important to determine its speciation since As (III) is more toxic and mobile than As (V). For cadmium (II), we observed a certain predominance of the use of biotic techniques, compared to a variety of others. Finally, in mercury case, treatments include a phytoremediation technique using Limnocharis flava and the use of a new natural adsorbent: a modified nanobiocomposite hydrogel. The use of biological treatments is increasingly being studied because they are environmentally friendly, efficient and highly viable in both process and energy terms. The study of techniques for the removal of potential toxic elements should be performed with a focus on the simultaneous removal of several metals, since in nature they do not appear in isolation. Moreover, we found that energy analysis constitutes a limiting factor in relation to the feasibility of these techniques.

Evaluating a multi-panel air cathode through electrochemical and biotic tests

To scale up microbial fuel cells (MFCs), larger cathodes need to be developed that can use air directly, rather than dissolved oxygen, and have good electrochemical performance. A new type of cathode design was examined here that uses a "window-pane" approach with fifteen smaller cathodes welded to a single conductive metal sheet to maintain good electrical conductivity across the cathode with an increase in total area. Abiotic electrochemical tests were conducted to evaluate the impact of the cathode size (exposed areas of 7 cm², 33 cm², and 6200 cm²) on performance for all cathodes having the same active catalyst material. Increasing the size of the exposed area of the electrodes to the electrolyte from 7 cm² to 33 cm² (a single cathode panel) decreased the cathode potential by 5%, and a further increase in size to 6200 cm² using the multi-panel cathode reduced the electrode potential by 55% (at 0.6 A m^-2), in a 50 mM phosphate buffer solution (PBS). In 85 L MFC tests with the largest cathode using wastewater as a fuel, the maximum power density based on polarization data was 0.083 ± 0.006 W m^-2 using 22 brush anodes to fully cover the cathode, and 0.061 ± 0.003 W m^-2 with 8 brush anodes (40% of cathode projected area) compared to 0.304 ± 0.009 W m^-2 obtained in the 28 mL MFC. Recovering power from large MFCs will therefore be challenging, but several approaches identified in this study can be pursued to maintain performance when increasing the size of the electrodes.

Prospects in bioelectrochemical technologies for wastewater treatment

Bioelectrochemical technologies are emerging as innovative solutions for waste treatment, offering flexible platforms for both oxidation and reduction reaction processes. A great variety of applications have been developed by utilizing the energy produced in bioelectrochemical systems, such as direct electric power generation, chemical production or water desalination. This manuscript provides a literature review on the prospects in bioelectrochemical technologies for wastewater treatment, including organic, nutrients and metals removal, production of chemical compounds and desalination. The challenges and perspectives for scale-up were discussed. A technological strategy to improve the process monitoring and control based on big data platforms is also presented. To translate the viability of wastewater treatment based on bioelectrochemical technologies into commercial application, it is necessary to exploit interdisciplinary areas by combining the water/wastewater sector, energy and data analytics technologies.

In situ Biofilm Quantification in Bioelectrochemical Systems by using Optical Coherence Tomography

Detailed studies of microbial growth in bioelectrochemical systems (BESs) are required for their suitable design and operation. Here, we report the use of optical coherence tomography (OCT) as a tool for in situ and noninvasive quantification of biofilm growth on electrodes (bioanodes). An experimental platform is designed and described in which transparent electrodes are used to allow real-time, 3D biofilm imaging. The accuracy and precision of the developed method is assessed by relating the OCT results to well-established standards for biofilm quantification (chemical oxygen demand (COD) and total N content) and show high correspondence to these standards. Biofilm thickness observed by OCT ranged between 3 and 90 μm for experimental durations ranging from 1 to 24 days. This translated to growth yields between 38 and 42 mgCODbiomass  gCODacetate -1 at an anode potential of -0.35 V versus Ag/AgCl. Time-lapse observations of an experimental run performed in duplicate show high reproducibility in obtained microbial growth yield by the developed method. As such, we identify OCT as a powerful tool for conducting in-depth characterizations of microbial growth dynamics in BESs. Additionally, the presented platform allows concomitant application of this method with various optical and electrochemical techniques.

Dual-Function Electrocatalytic and Macroporous Hollow-Fiber Cathode for Converting Waste Streams to Valuable Resources Using Microbial Electrochemical Systems

Dual-function electrocatalytic and macroporous hollow-fiber cathodes are recently proposed as promising advanced material for maximizing the conversion of waste streams such as wastewater and waste CO₂ to valuable resources (e.g., clean freshwater, energy, value-added chemicals) in microbial electrochemical systems. The first part of this progress report reviews recent developments in this type of cathode architecture for the simultaneous recovery of clean freshwater and energy from wastewater. Critical insights are provided on suitable materials for fabricating these cathodes, as well as addressing some challenges in the fabrication process with proposed strategies to overcome them. The second and complementary part of the progress report highlights how the unique features of this cathode architecture can solve one of the intrinsic bottlenecks (gas-liquid mass transfer limitation) in the application of microbial electrochemical systems for CO₂ reduction to value-added products. Strategies to further improve the availability of CO₂ to microbial catalysts on the cathode are proposed. The importance of understanding microbe-cathode interactions, as well as electron transfer mechanisms at the cathode-cell and cell-cell interface to better design dual-function macroporous hollow-fiber cathodes, is critically discussed with insights on how the choice of material is important in facilitating direct electron transfer versus mediated electron transfer.

Predictive modelling of chromium removal using multiple linear and nonlinear regression with special emphasis on operating parameters of bioelectrochemical reactor

Bioelectrochemical system (BES) is a novel, self-sustaining metal removal technology functioning on the utilization of chemical energy of organic matter with the help of microorganisms. Experimental trials of two chambered BES reactor were conducted with varying substrate concentration using sodium acetate (500 mg/L to 2000 mg/L COD) and different initial chromium concentration (Cr_i) (10-100 mg/L) at different cathode pH (pH 1-7). In the current study mathematical models based on multiple linear regression (MLR) and non-linear regression (NLR) approach were developed using laboratory experimental data for determining chromium removal efficiency (CRE) in the cathode chamber of BES. Substrate concentration, rate of substrate consumption, Cr_i, pH, temperature and hydraulic retention time (HRT) were the operating process parameters of the reactor considered for development of the proposed models. MLR showed a better correlation coefficient (0.972) as compared to NLR (0.952). Validation of the models using t-test analysis revealed unbiasedness of both the models, with t critical value (2.04) greater than t-calculated values for MLR (-0.708) and NLR (-0.86). The root-mean-square error (RMSE) for MLR and NLR were 5.06 % and 7.45 %, respectively. Comparison between both models suggested MLR to be best suited model for predicting the chromium removal behavior using the BES technology to specify a set of operating conditions for BES. Modelling the behavior of CRE will be helpful for scale up of BES technology at industrial level.

Mitigating external and internal cathode fouling using a polymer bonded separator in microbial fuel cells

Microbial fuel cell (MFC) cathodes rapidly foul when treating domestic wastewater, substantially reducing power production over time. Here a wipe separator was chemically bonded to an activated carbon air cathode using polyvinylidene fluoride (PVDF) to mitigate cathode fouling and extend cathode performance over time. MFCs with separator-bonded cathodes produced a maximum power density of 190 ± 30 mW m^-2 after 2 months of operation using domestic wastewater, which was ∼220% higher than controls (60 ± 50 mW m^-2) with separators that were not chemically bonded to the cathode. Less biomass (protein) was measured on the bonded separator surface than the non-bonded separator, indicating chemical bonding reduced external bio-fouling. Salt precipitation that contributed to internal fouling was also reduced using separator-bonded cathodes. Overall, the separator-bonded cathodes showed better performance over time by mitigating both external bio-fouling and internal salt fouling.

Prototype of a scaled-up microbial fuel cell for copper recovery

BACKGROUND

Bioelectrochemical systems (BESs) enable recovery of electrical energy through oxidation of a wide range of substrates at an anode and simultaneous recovery of metals at a cathode. Scale-up of BESs from the laboratory to pilot scale is a challenging step in the development of the process, and there are only a few successful experiences to build on. This paper presents a prototype BES for the recovery of copper.

RESULTS

The cell design presented here had removable electrodes, similar to those in electroplating baths. The anode and cathode in this design could be replaced independently. The prototype bioelectrochemical cell consisted of an 835 cm² bioanode fed with acetate, and a 700 cm² cathode fed with copper. A current density of 1.2 A/^-2 was achieved with 48 mW m^-2 of power production. The contribution of each component (anode, electrolytes, cathode and membrane) was evaluated through the analysis of the internal resistance distribution. This revealed that major losses occurred at the anode, and that the design with removable electrodes results in higher internal resistance compared with other systems. To further assess the practical applicability of BES for copper recovery, an economic evaluation was performed.

CONCLUSION

Analysis shows that the internal resistance of several lab-scale BESs is already sufficiently low to make the system economic, while the internal resistance for scaled-up systems still needs to be improved considerably to become economically applicable.© 2017 The Authors. Journal of Chemical Technology & Biotechnology published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.