Bioelectricity production from food waste leachate using microbial fuel cells: effect of NaCl and pH

Salinity levels influence treatment performance and the activity of electroactive microorganisms in a microbial fuel cell system for wastewater treatment

There is growing interest in developing effective treatment technologies to mitigate the environmental impact of saline wastewater while also potentially recovering valuable resources from it. However, it remains largely unknown how different salinity levels impact treatment performance, energy generation, and the diversity and composition of electroactive microorganisms in MFCs treating real effluents such as urban wastewater. This study explores the impact of three salinity levels (3.5, 7, and 15 g/L NaCl) on current production, organic removal rates, and bacterial community dynamics in a continuous-flow microbial fuel cell (MFC) fed with urban wastewater. Using metagenomics and metatranscriptomics, we explored variations in the abundance and expression of extracellular electron transfer (EET) genes and those involved in other general metabolisms. We found that low salinity (3.5 g/L NaCl) enhanced both current production and organic removal efficiency compared to higher salinity levels. This improvement was linked to an increased abundance and activity of electroactive microorganisms, particularly taxa within the Ignavibacteria class, which possess genes coding for outer membrane cytochromes and porin cytochromes. Additionally, salinity influenced general metabolic genes and microbial community composition, with higher salinity levels limiting bacterial growth and diversity. This research provides valuable insights into the interplay between salinity stress and microbial adaptation, contributing to the optimization of MFC technologies for enhanced environmental and bioengineering applications.

Inoculum selection and hydraulic retention time impacts in a microbial fuel cell treating saline wastewater

Microbial fuel cell (MFC) technology has received increased interest as a suitable approach for treating wastewater while producing electricity. However, there remains a lack of studies investigating the impact of inoculum type and hydraulic retention time (HRT) on the efficiency of MFCs in treating industrial saline wastewater. The effect of three different inocula (activated sludge from a fish-canning industry and two domestic wastewater treatment plants, WWTPs) on electrochemical and physicochemical parameters and the anodic microbiome of a two-chambered continuous-flow MFC was studied. For each inoculum, three different HRTs were tested (1 day, 3 days, and 6 days). The inoculum from the fish canning industry significantly increased voltage production (with a maximum value of 802 mV), power density (with a maximum value of 78 mW m-2), coulombic efficiency (with a maximum value of 19.3%), and organic removal rate (ORR) compared to the inocula from domestic WWTPs. This effect was linked to greater absolute and relative abundances of electroactive microorganisms (e.g., Geobacter, Desulfovibrio, and Rhodobacter) and predicted electron transfer genes in the anode microbiome likely due to better adaption to salinity conditions. The ORR and current production were also enhanced at shorter HRTs (1 day vs. 3 and 6 days) across all inocula. This effect was related to a greater abundance and diversity of bacterial communities at HRT of 1 day compared to longer HRTs. Our findings have important bioengineering implications and can help improve the performance of MFCs treating saline effluents such as those from the seafood industry. KEY POINTS: • Inoculum type and HRT impact organic matter removal and current production. • Changes in bioenergy generation were linked to the electroactive anodic microbiome. • Shorter HRT favored increases in the performance of the MFC.

Nano-pumice derived from pumice mine waste as a low-cost electrode catalyst for microbial fuel cell treating edible vegetable oil refinery wastewater for bioenergy generation and reuse

This study aimed to assess nano-pumice (NP) from pumice mining waste as a local, cost-effective anode catalyst in microbial fuel cells (MFCs) for treating edible vegetable oil refinery wastewater (EVORW) and generating bioenergy. Pumice mining waste was converted into nano in three stages: crushing up to ≤3 cm, reducing the size of the previous step particles to 150 μm and converting the previous step particles to <100 nm. Nano-pumice prepared was coated on the carbon cloth (CC) to increase anode surface area of MFC. Two MFCs were utilized, with MFC-1 serving as a control and MFC-2 incorporating a CC electrode coated with nano-pumice. The surface morphology, elemental and chemical composition, and textural characterization of CC, pumice, NP, and CC coated with NP were analyzed using FE-SEM, EDX, XRF, and BET techniques. MFC-2 achieved a maximum power density of 30±4W/m³ at a current density of 55±5A/m³. The MFC-1 reached a maximum power density of 18±4W/m³ at a current density of 35±6A/m³. In MFC-2, the EVORW treatment achieved maximum removals of COD (94 ± 2 %), NH4 +-N (85 ± 4 %), TP (76 ± 5 %), SO4 2- (68 ± 6 %), TSS (81 ± 2 %), and TDS (73 ± 1 %). MFC-1 achieved removal efficiencies of 66 ± 3 % for COD, 57 ± 6 % for NH4 +-N, 48 ± 3 % for TP, 45 ± 3 % for SO4 2-, 65 ± 3 % for TSS, and 61 ± 1 % for TDS. MFC-2 power density rose significantly, reaching 61 ± 3 % (1.6 times) higher than MFC-1and it also demonstrated a superior ability to improve raw wastewater quality compared to MFC-1. The MFC with the CC/NP anode exhibited both excellent power production and high COD removal efficiency, making nano-pumice a suitable anode catalyst for MFC applications.

Fulvic acid more facilitated the soil electron transfer than humic acid

Humus substances (HSs) participate in extracellular electron transfer (EET), which is unclear in heterogeneous soil. Here, a microbial electrochemical system (MES) was constructed to determine the effect of HSs, including humic acid, humin and fulvic acid, on soil electron transfer. The results showed that fulvic acid led to the optimal electron transfer efficiency in soil, as evidenced by the highest accumulated charges and removal of total petroleum hydrocarbons after 140 days, with increases of 161% and 30%, respectively, compared with those of the control. However, the performance of MES with the addition of humic acid and humin was comparable to that of the control. Fulvic acid amendment enhanced the carboxyl content and oxidative state of dissolved organic matter, endowing a better electron transfer capacity. Additionally, the presence of fulvic acid induced an increase in the abundance of electroactive bacteria and organic degraders, extracellular polymeric substances and functional enzymes such as cytochrome c and NADH synthesis, and the expression of m tr C gene, which is responsible for EET enhancement in soil. Overall, this study reveals the mechanism by which HSs stimulate soil electron transfer at the physicochemical and biological levels and provides basic support for the application of bioelectrochemical technology in soil.

Food waste to resource recovery: a way of green advocacy

Due to the massive growth in population and urbanization, there has been a huge increase in the volume of food waste globally. The Food and Agriculture Organization (FAO) has estimated that around one-third of all food produced each year is wasted. Food waste leads to the emission of greenhouse gas and depletion of the soil fertility. Nevertheless, it has immense potential for the recovery of high-value energy, fuel, and other resources. This review summarizes the latest advances in resource recovery from food waste by using technologies that include food waste-mediated microbial fuel cell (MFC) for bioenergy production. In addition to this, utilization of food waste for the production of bioplastic, biogas, bioethanol, and fertilizer has been also discussed in detail. Competitive benefits and accompanying difficulties of these technologies have also been highlighted. Furthermore, future approaches for more efficient use of food waste for the recovery of valuable resources have been also offered from an interdisciplinary perspective.

A comprehensive review of microbial fuel cells considering materials, methods, structures, and microorganisms

Microbial fuel cells (MFCs) are promising for generating renewable energy from organic matter and efficient wastewater treatment. Ensuring their practical viability requires meticulous optimization and precise design. Among the critical components of MFCs, the membrane separator plays a pivotal role in segregating the anode and cathode chambers. Recent investigations have shed light on the potential benefits of membrane-less MFCs in enhancing power generation. However, it is crucial to recognize that such configurations can adversely impact the electrocatalytic activity of anode microorganisms due to increased substrate and oxygen penetration, leading to decreased coulombic efficiency. Therefore, when selecting a membrane for MFCs, it is essential to consider key factors such as internal resistance, substrate loss, biofouling, and oxygen diffusion. Addressing these considerations carefully allows researchers to advance the performance and efficiency of MFCs, facilitating their practical application in sustainable energy production and wastewater treatment. Accelerated substrate penetration could also lead to cathode clogging and bacterial inactivation, reducing the MFC's efficiency. Overall, the design and optimization of MFCs, including the selection and use of membranes, are vital for their practical application in renewable energy generation and wastewater treatment. Further research is necessary to overcome the challenges of MFCs without a membrane and to develop improved membrane materials for MFCs. This review article aims to compile comprehensive information about all constituents of the microbial fuel cell, providing practical insights for researchers examining various variables in microbial fuel cell research.

Combining Geobacter spp. Dominated Biofilms and Anaerobic Digestion Effluents─The Effect of Effluent Composition and Electrode Potential on Biofilm Activity and Stability

The combination of anaerobic digestion (AD) and microbial electrochemical technologies (METs) offers different opportunities to increase the efficiency and sustainability of AD processes. However, methanogenic archaea and/or particles may partially hinder combining MET and AD processes. Furthermore, it is unclear if the applied anode potential affects the activity and efficiency of electroactive microorganisms in AD-MET combinations as it is described for more controlled experimental conditions. In this study, we confirm that 6-week-old Geobacter spp. dominated biofilms are by far more active and stable in AD-effluents than 3-week-old Geobacter spp. dominated biofilms. Furthermore, we show that the biofilms are twice as active at -0.2 V compared to 0.4 V, even under challenging conditions occurring in AD-MET systems. Paired-end amplicon sequencing at the DNA level using 16S-rRNA and mcrA gene shows that hydrogenotrophic methanogens incorporate into biofilms immersed in AD-effluent without any negative effect on biofilm stability and electrochemical activity.

Recent progress in microbial fuel cells using substrates from diverse sources

Increasing untreated environmental outputs from industry and the rising human population have increased the burden of wastewater and other waste streams on the environment. The most prevalent wastewater treatment methods include the activated sludge process, which requires aeration and is, therefore, energy and cost-intensive. The current trend towards a circular economy facilitates the recovery of waste materials as a resource. Along with the amount, the complexity of wastewater is increasing day by day. Therefore, wastewater treatment processes must be transformed into cost-effective and sustainable methods. Microbial fuel cells (MFCs) use electroactive microbes to extract chemical energy from waste organic molecules to generate electricity via waste treatment. This review focuses use of MFCs as an energy converter using wastewater from various sources. The different substrate sources that are evaluated include industrial, agricultural, domestic, and pharmaceutical types. The article also highlights the effect of operational parameters such as organic load, pH, current, and concentration on the MFC output. The article also covers MFC functioning with respect to the substrate, and the associated performance parameters, such as power generation and wastewater treatment matrices, are given. The review also illustrates the success stories of various MFC configurations. We emphasize the significant measures required to fill in the gaps related to the effect of substrate type on different MFC configurations, identification of microbes for use as biocatalysts, and development of biocathodes for the further improvement of the system. Finally, we shortlisted the best performing substrates based on the maximum current and power, Coulombic efficiency, and chemical oxygen demand removal upon the treatment of substrates in MFCs. This information will guide industries that wish to use MFC technology to treat generated effluent from various processes.

Organic Waste Substrates for Bioenergy Production via Microbial Fuel Cells: A Key Point Review

High-energy consumption globally has raised questions about the low environmentally friendly and high-cost processes used until now for energy production. Microbial fuel cells (MFCs) may support alternative more economically and environmentally favorable ways of bioenergy production based on their advantage of using waste. MFCs work as bio-electrochemical devices that consume organic substrates in order for the electrogenic bacteria and/or enzyme cultures to produce electricity and simultaneously lower the environmental hazardous value of waste such as COD. The utilization of organic waste as fuels in MFCs has opened a new research path for testing a variety of by-products from several industry sectors. This review presents several organic waste substrates that can be employed as fuels in MFCs for bioenergy generation and the effect of their usage on power density, COD (chemical oxygen demand) removal, and Coulombic efficiency enhancement. Moreover, a demonstration and comparison of the different types of mixed waste regarding their efficiency for energy generation via MFCs are presented. Future perspectives for manufacturing and cost analysis plans can support scale-up processes fulfilling waste-treatment efficiency and energy-output densities.

Recent advancements in microbial fuel cells: A review on its electron transfer mechanisms, microbial community, types of substrates and design for bio-electrochemical treatment

The development in urbanization, growth in industrialization and deficiency in crude oil wealth has made to focus more for the renewable and also sustainable spotless energy resources. In the past two decades, the concepts of microbial fuel cell have caught more considerations among the scientific societies for the probability of converting, organic waste materials into bio-energy using microorganisms catalyzed anode, and enzymatic/microbial/abiotic/biotic cathode electro-chemical reactions. The added benefit with MFCs technology for waste water treatment is numerous bio-centered processes are available such as sulfate removal, denitrification, nitrification, removal of chemical oxygen demand and biological oxygen demand and heavy metals removal can be performed in the same MFC designed systems. The various factors intricate in MFC concepts in the direction of bioenergy production consists of maximum coulombic efficiency, power density and also the rate of removal of chemical oxygen demand which calculates the efficacy of the MFC unit. Even though the efficacy of MFCs in bioenergy production was initially quietly low, therefore to overcome these issues few modifications are incorporated in design and components of the MFC units, thereby functioning of the MFC unit have improvised the rate of bioenergy production to a substantial level by this means empowering application of MFC technology in numerous sectors including carbon capture, bio-hydrogen production, bioremediation, biosensors, desalination, and wastewater treatment. The present article reviews about the microbial community, types of substrates and information about the several designs of MFCs in an endeavor to get the better of practical difficulties of the MFC technology.

Butyrate production and purification by combining dry fermentation of food waste with a microbial fuel cell

This study developed and evaluated a high-purity butyrate producing bioprocess from food waste by combining dry fermentation (DF) with a microbial fuel cell (MFC). Acclimatization of a DF reactor with an enrichment culture resulted in high food waste degradation (VS removed, %) and butyrate production. A high VS degradation of 81%, butyrate concentration of up to 24 gCOD_butyrate/L and butyrate yields of 497 gCOD_butyrate/kg VS_added was obtained in the DF reactor. As a result, butyrate comprised 83% of all short chain fatty acids (SCFA) in the DF broth. Acetate (10%) and propionate (7%) comprised the rest of the SCFA. The butyrate composition was further purified by feeding the DF broth to a multi-electrode MFC enriched with anode respiring bacteria (ARB) such as Geobacter sp. (>55%). The ARB in the MFC removed acetate and propionate while purified butyrate was recovered in the MFC effluent. Butyrate purity in the MFC effluent reached as high as 99% at hydraulic retention time of 72 h. Along with butyrate purification, the MFC produced electric power in a range of 0.1-0.6 Wh/gCODbutyrate_recovered (or 0.01-7.85 kWh/ton of food waste), demonstrating that MFCs can be an energy-positive butyrate purification bioprocess.

Agricultural Waste and Wastewater as Feedstock for Bioelectricity Generation Using Microbial Fuel Cells: Recent Advances

In recent years, there has been a significant accumulation of waste in the environment, and it is expected that this accumulation may increase in the years to come. Waste disposal has massive effects on the environment and can cause serious environmental problems. Thus, the development of a waste treatment system is of major importance. Agro-industrial wastewater and waste residues are mainly rich in organic substances, lignocellulose, hemicellulose, lignin, and they have a relatively high amount of energy. As a result, an effective agro-waste treatment system has several benefits, including energy recovery and waste stabilization. To reduce the impact of the consumption of fossil energy sources on our planet, the exploitation of renewable sources has been relaunched. All over the world, efforts have been made to recover energy from agricultural waste, considering global energy security as the final goal. To attain this objective, several technologies and recovery methods have been developed in recent years. The microbial fuel cell (MFC) is one of them. This review describes the power generation using various types of agro-industrial wastewaters and agricultural residues utilizing MFC. It also highlights the techno-economics and lifecycle assessment of MFC, its commercialization, along with challenges.

Benefits of Age-Improved Resistance of Mature Electroactive Biofilm Anodes in Anaerobic Digestion

Anaerobic digestion (AD) and microbial electrochemical technologies (MET) can be combined in manifold ways. Recent studies show negative influences of AD effluents on the performance of pre-grown Geobacter spp.-dominated biofilm anodes. In this study, it was investigated how such biofilm anodes are affected by AD effluents. Therefore, experiments using AD effluents in different concentrations (0-100%) in combination with biofilms of different ages were performed. Furthermore, the activity of methanogens was inhibited and minimized by application of 2-bromoethanesulfonate (2-BES) and microfiltration, respectively. Biofilms pre-grown for 5 weeks show higher resistance against AD effluents compared to biofilms pre-grown for only 3 weeks. Nevertheless, adaptation of biofilms to AD effluents was not successful. Biofilm activity in terms of coulombic efficiency and maximum current density (j_max) dropped by factor 32.2 ± 3.2 and 38.9 ± 8.4, respectively. The application of 2-BES and microfiltration had positive effects on the biofilm activity. The results support the assumption that methanogens or further compounds not studied here, for example, protozoans, which may have been inhibited or removed by 2-BES application or microfiltration, have an immediate influence on the stability of Geobacter spp.-dominated biofilms and may limit their practical application in AD environments.

Socio-Economic and Environmental Impacts of Biomass Valorisation: A Strategic Drive for Sustainable Bioeconomy

In the late twentieth century, the only cost-effective opportunity for waste removal cost at least several thousand dollars, but nowadays, a lot of improvement has occurred. The biomass and waste generation problems attracted concerned authorities to identify and provide environmentally friendly sustainable solutions that possess environmental and economic benefits. The present study emphasises the valorisation of biomass and waste produced by domestic and industrial sectors. Therefore, substantial research is ongoing to replace the traditional treatment methods that potentially acquire less detrimental effects. Synthetic biology can be a unique platform that invites all the relevant characters for designing and assembling an efficient program that could be useful to handle the increasing threat for human beings. In the future, these engineered methods will not only revolutionise our lives but practically lead us to get cheaper biofuels, producing bioenergy, pharmaceutics, and various biochemicals. The bioaugmentation approach concomitant with microbial fuel cells (MFC) is an example that is used to produce electricity from municipal waste, which is directly associated with the loading of waste. Beyond the traditional opportunities, herein, we have spotlighted the new advances in pertinent technology closely related to production and reduction approaches. Various integrated modern techniques and aspects related to the industrial sector are also discussed with suitable examples, including green energy and other industrially relevant products. However, many problems persist in present-day technology that requires essential efforts to handle thoroughly because significant valorisation of biomass and waste involves integrated methods for timely detection, classification, and separation. We reviewed and proposed the anticipated dispensation methods to overcome the growing stream of biomass and waste at a distinct and organisational scale.

Substrate salinity: A critical factor regulating the performance of microbial fuel cells, a review

Substrate salinity is a critical factor influencing microbial fuel cells (MFCs) performance and various studies have suggested that increasing substrate salinity first improves MFC performance. However, a further increase in salinity that exceeds the salinity tolerance of exoelectrogens shows negative effects because of inhibited bacterial activity and increased activation losses. In this review, electricity generation and contaminant removal from saline substrates using MFCs are summarized, and results show different optimal salinities for obtaining maximum performance. Then, electroactive bacteria capable of tolerating saline environments and strategies for improving salinity tolerance are discussed. In addition to ohmic resistance and bacterial activity, membrane resistance and catalyst performance will also be affected by substrate salinity, all of which jointly contribute the final overall MFC performance. Therefore, the combined effect of salinity is analyzed to illustrate how the MFC performance changes with increasing salinity. Finally, the challenges and perspectives of MFCs operated in saline environments are discussed.

The effect of additional salinity on performance of a phosphate buffer saline buffered three-electrode bioelectrochemical system inoculated with wastewater

In bioelectrochemical system (BES), phosphate buffer saline (PBS) is usually used to achieve a suitable pH condition, which also increases electrolyte salinity. A series of factors that change with salinity will affect BES performance. To simplify the scenario, a three-electrode BES is used to investigate how additional salinity affects the performance of a 50 mM PBS-buffered BES. Results demonstrated that current production decreased with increasing salinity and the dominant exoelectrogens were not inhibited with the addition of 200 mM NaCl. The distribution of system resistance was analyzed by electrochemical impedance spectroscopy. Compared to the decreased solution and biofilm resistance, the increased interfacial resistance that accounted for up to 97.8% of total resistance was the dominant reason for the decreased current production with the increasing additional salinity. The effects of additional salinity on acetate degradation and columbic efficiency were also analyzed.

Performance evaluation of microbial fuel cell for landfill leachate treatment: Research updates and synergistic effects of hybrid systems

Over half of century, sanitary landfill was and is still the most economical treatment strategy for solid waste disposal, but the environmental risks associated with the leachate have brought attention of scientists for its proper treatment to avoid surface and ground water deterioration. Most of the treatment technologies are energy-negative and cost intensive processes, which are unable to meet current environmental regulations. There are continuous demands of alternatives concomitant with positive energy and high effluent quality. Microbial fuel cells (MFCs) were launched in the last two decades as a potential treatment technology with bioelectricity generation accompanied with simultaneous carbon and nutrient removal. This study reviews capability and mechanisms of carbon, nitrogen and phosphorous removal from landfill leachate through MFC technology, as well as summarizes and discusses the recent advances of standalone and hybrid MFCs performances in landfill leachate (LFL) treatment. Recent improvements and synergetic effect of hybrid MFC technology upon the increasing of power densities, organic and nutrient removal, and future challenges were discussed in details.

Treatment of anaerobically digested effluent from kitchen waste using combined processes of anaerobic digestion-complete nitritation-ANAMMOX based on reflux dilution

In this study, an anaerobically digested effluent from kitchen waste with high concentrations of chemical oxygen demand (COD) and ammonia nitrogen was treated using combined processes of anaerobic digestion (AD), complete nitritation (CN), and anaerobic ammonium oxidation (ANAMMOX). The COD and nitrogen removal efficiency of each treatment unit were investigated. The feasibility of using the final treatment effluent to dilute the original wastewater was also discussed. Findings showed that as a pretreatment step, AD resulted in the decline in biodegradability and increase in NH 4 + - N concentration. CN was successfully and stably achieved for 106 days with an average nitritation rate of 95% by maintaining the dissolved oxygen at 2-3 mg/L and hydraulic retention time of 24 hr under 30 ± 1°C. High NH 4 + - N and NO 2 - - N . removal efficiencies of over 88% and 96% were attained in the following ANAMMOX reactor. The reflux of ANAMMOX-treated effluent for the dilution of raw wastewater or an influent of CN and ANAMMOX ensured the stable operation of the combined system. PRACTITIONER POINTS: Anaerobic digestion effluent of kitchen waste had low COD/ NH 4 + - N ratio and poor biodegradability. Stable and efficient nitritation was realized by controlling DO, HRT and TEMP. High NH 4 + - N and NO 2 - -N removal efficiency were obtained by ANAMMOX process. Average nitrogen removal rate of 0.94 kg N/m³ /day were obtained by ANAMMOX. Reflux dilution with the effluent guaranteed the system's successful operation.

Alkaliphiles: The Versatile Tools in Biotechnology

The extreme environments within the biosphere are inhabited by organisms known as extremophiles. Lately, these organisms are attracting a great deal of interest from researchers and industrialists. The motive behind this attraction is mainly related to the desire for new and efficient products of biotechnological importance and human curiosity of understanding nature. Organisms living in common "human-friendly" environments have served humanity for a very long time, and this has led to exhaustion of the low-hanging "fruits," a phenomenon witnessed by the diminishing rate of new discoveries. For example, acquiring novel products such as drugs from the traditional sources has become difficult and expensive. Such challenges together with the basic research interest have brought the exploration of previously neglected or unknown groups of organisms. Extremophiles are among these groups which have been brought to focus and garnering a growing importance in biotechnology. In the last few decades, numerous extremophiles and their products have got their ways into industrial, agricultural, environmental, pharmaceutical, and other biotechnological applications.Alkaliphiles, organisms which thrive optimally at or above pH 9, are one of the most important classes of extremophiles. To flourish in their extreme habitats, alkaliphiles evolved impressive structural and functional adaptations. The high pH adaptation gave unique biocatalysts that are operationally stable at elevated pH and several other novel products with immense biotechnological application potential. Advances in the cultivation techniques, success in gene cloning and expression, metabolic engineering, metagenomics, and other related techniques are significantly contributing to expand the application horizon of these remarkable organisms of the 'bizarre' world. Studies have shown the enormous potential of alkaliphiles in numerous biotechnological applications. Although it seems just the beginning, some fantastic strides are already made in tapping this potential. This work tries to review some of the prominent applications of alkaliphiles by focusing such as on their enzymes, metabolites, exopolysaccharides, and biosurfactants. Moreover, the chapter strives to assesses the whole-cell applications of alkaliphiles including in biomining, food and feed supplementation, bioconstruction, microbial fuel cell, biofuel production, and bioremediation.

Multiple anodic chambers sharing an algal raceway pond to establish a photosynthetic microbial fuel cell stack: Voltage boosting accompany wastewater treatment

Photosynthetic microbial fuel cells (PMFCs) allow renewable energy production from wastewater. However, system scale-up is still a major challenge hindering the use of PMFCs for practical applications. Herein, a PMFC stack, which consisted of multiple anodic chambers installed in an algal raceway pond (ARP), was established to recovery energy from anaerobically digested effluent with the assistance of a prototypical capacitor circuit. The highest voltage output of the stack reached 1.4 V with four PMFC units and four capacitors. The system can produce stable voltages through controlling charging and discharging frequencies and the voltage output remained stable around 0.60 V when the time interval decreased to 2 s. During long-term operation, the highest power density of the stack with capacitors reached 2.34 W/m³, which was 77% higher than that without capacitors (1.32 W/m³). About 98% of the ammonium in the anolyte was removed, resulting from the ammonium migration effect. The dynamics of bacterial community compositions were not greatly influenced by the capacitor circuit, and the stack with capacitors had a more stable bacterial community compared to the stack without capacitors. The variations in bacterial community composition following power density changes indicated that members of the Clostridia and Betaproteobacteria were related to power generation. Bacteria affiliated to Bacteroidetes were inhibited when power density was high, though their numbers were enriched at the end of the process. This study promotes a practical method for developing the PMFC technology into real-world applications, and furthermore reveals the main bacteria that play vital roles in power generation by analysing the anodic bacterial community during the whole process.

Electricity production enhancement in a constructed wetland-microbial fuel cell system for treating saline wastewater

The use of constructed wetlands in combination with microbial fuel cells (CW-MFC) to treat saline wastewater may enhance electricity production by increasing the ionic strength, reducing internal resistance and stimulating microbes to accelerate electron transfer. In this study, salinity did not significantly inhibit the removal of TP and COD, but TN and NH₄⁺-N removal efficiencies during saline wastewater treatment (ST) were significantly lower than during non-saline wastewater treatment (NT). However, salinity significantly increased the power density (16.4 mW m^-2 in ST and 3.9 mW m^-2 in NT, a 4-fold enhancement) by increasing the electron transfer rate and reducing internal resistance (140.29 Ω in ST and 415.21 Ω in NT). The peptides in extracellular polymeric substances (EPS) acted as electron shuttles to promote the migration of electrons and protons in ST. From start-up to stable operation, though the microorganisms in ST were reduced in diversity relative to NT, the proportion of electrochemically active bacteria (EAB), such as Ochrobactrum, significantly increased (p < 0.05) and gradually predominated in the microbial community.

Utilization of a Silicone Rubber Membrane for Passive Oxygen Supply in a Microbial Fuel Cell Treating Carbon and Nitrogen from Synthetic Coke-Oven Wastewater

This study firstly introduced a silicone rubber membrane (SRM) into microbial fuel cell (MFC) for passive oxygen supply to simultaneously remove phenol and nitrogen from synthetic coke-oven wastewater diluted with seawater. Passive oxygen transport with biofilm on the membrane was improved by ~ 18-fold in comparison with the one without a biofilm. In addition, although the oxygen supply was passive, nitrification accounted for 34% of those aeration conditions. It was also found that silicone rubber membrane can control NO₂^--N and/or NO₃^--N production. A dual-chamber MFC treating the synthetic coke-oven wastewater achieved a maximum power density of 54 mW m^-2 with a coulombic efficiency of 2.7%. We conclude that silicone rubber membrane is effective for sustainable coke-oven wastewater treatment in MFCs.

Microbial fuel cells as an alternative energy source: current status

Microbial fuel cell (MFC) technology is an emerging area for alternative renewable energy generation and it offers additional opportunities for environmental bioremediation. Recent scientific studies have focused on MFC reactor design as well as reactor operations to increase energy output. The advancement in alternative MFC models and their performance in recent years reflect the interests of scientific community to exploit this technology for wider practical applications and environmental benefit. This is reflected in the diversity of the substrates available for use in MFCs at an economically viable level. This review provides an overview of the commonly used MFC designs and materials along with the basic operating parameters that have been developed in recent years. Still, many limitations and challenges exist for MFC development that needs to be further addressed to make them economically feasible for general use. These include continued improvements in fuel cell design and efficiency as well scale-up with economically practical applications tailored to local needs.

Using a tubular photosynthetic microbial fuel cell to treat anaerobically digested effluent from kitchen waste: Mechanisms of organics and ammonium removal

Anaerobically digested effluent from kitchen waste (ADE-KW) was used herein as the substrate of a tubular photosynthetic microbial fuel cell (PMFC) for power production, and also, after being diluted, as a medium for cultivation of algae in the cathodic chamber. Adding 3 mg/L phosphorus to the catholyte could efficiently enhance the algal growth and the PMFC performance. About 0.94 g/L algal biomass and 0.57 kWh/m³-ADE-KW bioelectricity were obtained from the PMFC. Soluble microbial byproduct-like material and aromatic proteins were the dominant organics in the ADE-KW, which were readily degradable in the system. About 79% of the 1550 mg/L ammonium in the anolyte transferred to the catholyte through the cation exchange membrane. The ammonium was removed mainly as electron acceptors at the cathode after being oxidized by oxygen, whereas algal assimilation only account for about 14.6% of the overall nitrogen.

Microbial fuel cells in saline and hypersaline environments: Advancements, challenges and future perspectives

This review is aimed to report the possibility to utilize microbial fuel cells for the treatment of saline and hypersaline solutions. An introduction to the issues related with the biological treatment of saline and hypersaline wastewater is reported, discussing the limitation that characterizes classical aerobic and anaerobic digestions. The microbial fuel cell (MFC) technology, and the possibility to be applied in the presence of high salinity, is discussed before reviewing the most recent advancements in the development of MFCs operating in saline and hypersaline conditions, with their different and interesting applications. Specifically, the research performed in the last 5years will be the main focus of this review. Finally, the future perspectives for this technology, together with the most urgent research needs, are presented.

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.

Enrichment of extremophilic exoelectrogens in microbial electrolysis cells using Red Sea brine pools as inocula

Applying microbial electrochemical technologies for the treatment of highly saline or thermophilic solutions is challenging due to the lack of proper inocula to enrich for efficient exoelectrogens. Brine pools from three different locations (Valdivia, Atlantis II and Kebrit) in the Red Sea were investigated as potential inocula sources for enriching exoelectrogens in microbial electrolysis cells (MECs) under thermophilic (70°C) and hypersaline (25% salinity) conditions. Of these, only the Valdivia brine pool produced high and consistent current 6.8±2.1A/m²-anode in MECs operated at a set anode potential of +0.2V vs. Ag/AgCl (+0.405V vs. standard hydrogen electrode). These results show that exoelectrogens are present in these extreme environments and can be used to startup MEC under thermophilic and hypersaline conditions. Bacteroides was enriched on the anode of the Valdivia MEC, but it was not detected in the open circuit voltage reactor seeded with the Valdivia brine pool.

The effect of algae species on the bioelectricity and biodiesel generation through open-air cathode microbial fuel cell with kitchen waste anaerobically digested effluent as substrate

Five strains algae (Golenkinia sp. SDEC-16, Chlorella vulgaris, Selenastrum capricornutum, Scenedesmus SDEC-8 and Scenedesmus SDEC-13) were screened as an effective way to promote recover electricity from MFC for kitchen waste anaerobically digested effluent (KWADE) treatment. The highest OCV, power density, biomass concentration and total lipid content were obtained with Golenkinia sp. SDEC-16 as the co-inoculum, which were 170mV, 6255mWm(-3), 325mgL(-1) and 38%, respectively. Characteristics of the organics in KWADE were analyzed, and the result showed that the hydrophilic and acidic fractions were more readily degraded, compared to the neutral fractions during the operation. Maximum COD and TN removal efficiency were 43.59% and 37.39% when inoculated with Golenkinia sp. SDEC-16, which were roughly 3.22 and 3.04 times higher than that of S. capricornutum. This study demonstrated that Golenkinia sp. SDEC-16 was a promising species for bioelectricity generation, lipid production and KWADE treatment.

Harvest and utilization of chemical energy in wastes by microbial fuel cells

Energy generated from wastes by using MFC technology could be effectively stored and utilized for real-world applications.

Influence of ionic conductivity in bioelectricity production from saline domestic sewage sludge in microbial fuel cells

This study aimed at manipulating ionic conductivity (EC) to harvest the maximum electrical energy from seawater-based domestic wastewater sewage sludge (SWS), unique to only a few cities, through microbial fuel cell (MFC). SWS has never been investigated as a MFC substrate before, and thus the influence of high in-situ EC on the energy recovery was unknown. In this study, the EC of the SWS was reduced through mixing it with fresh water-based domestic wastewater sewage sludge (FWS) or diluted 50% using deionized water while FWS and SWS were individually served as reference treatments. SWS:FWS mix (1:1) exhibited a maximum Coulombic efficiency of 28.6±0.5% at a COD removal of 59±3% while the peak power density was 20-fold higher than FWS. The improved performance was due to the lower ohmic internal resistance (36.8±4.2Ω) and optimal conductivity (12.8±0.2mScm(-1)). Therefore, dilution with FWS could enhance energy recovery from SWS.

Possibilities for extremophilic microorganisms in microbial electrochemical systems

Microbial electrochemical systems exploit the metabolism of microorganisms to generate electrical energy or a useful product. In the past couple of decades, the application of microbial electrochemical systems has increased from the use of wastewaters to produce electricity to a versatile technology that can use numerous sources for the extraction of electrons on the one hand, while on the other hand these electrons can be used to serve an ever increasing number of functions. Extremophilic microorganisms grow in environments that are hostile to most forms of life and their utilization in microbial electrochemical systems has opened new possibilities to oxidize substrates in the anode and produce novel products in the cathode. For example, extremophiles can be used to oxidize sulfur compounds in acidic pH to remediate wastewaters, generate electrical energy from marine sediment microbial fuel cells at low temperatures, desalinate wastewaters and act as biosensors of low amounts of organic carbon. In this review, we will discuss the recent advances that have been made in using microbial catalysts under extreme conditions and show possible new routes that extremophilic microorganisms open for microbial electrochemical systems.