Electric energy production from food waste: Microbial fuel cells versus anaerobic digestion

New insights in food security and environmental sustainability through waste food management

Food waste has been identified as one of the major factors that constitute numerous anthropogenic activities, especially in developing countries. There is a growing problem with food waste that affects every part of the waste management system, from collection to disposal; finding long-term solutions necessitates involving all participants in the food supply chain, from farmers and manufacturers to distributors and consumers. In addition to food waste management, maintaining food sustainability and security globally is crucial so that every individual, household, and nation can always get food. "End hunger, achieve food security and enhanced nutrition, and promote sustainable agriculture" are among the main challenges of global sustainable development (SDG) goal 2. Therefore, sustainable food waste management technology is needed. Recent attention has been focused on global food loss and waste. One-third of food produced for human use is wasted every year. Source reduction (i.e., limiting food losses and waste) and contemporary treatment technologies appear to be the most promising strategy for converting food waste into safe, nutritious, value-added feed products and achieving sustainability. Food waste is also employed in industrial processes for the production of biofuels or biopolymers. Biofuels mitigate the detrimental effects of fossil fuels. Identifying crop-producing zones, bioenergy cultivars, and management practices will enhance the natural environment and sustainable biochemical process. Traditional food waste reduction strategies are ineffective in lowering GHG emissions and food waste treatment. The main contribution of this study is an inventory of the theoretical and practical methods of prevention and minimization of food waste and losses. It identifies the trade-offs for food safety, sustainability, and security. Moreover, it investigates the impact of COVID-19 on food waste behavior.

Use of Pineapple Waste as Fuel in Microbial Fuel Cell for the Generation of Bioelectricity

The excessive use of fossil sources for the generation of electrical energy and the increase in different organic wastes have caused great damage to the environment; these problems have promoted new ways of generating electricity in an eco-friendly manner using organic waste. In this sense, this research uses single-chamber microbial fuel cells with zinc and copper as electrodes and pineapple waste as fuel (substrate). Current and voltage peaks of 4.95667 ± 0.54775 mA and 0.99 ± 0.03 V were generated on days 16 and 20, respectively, with the substrate operating at an acid pH of 5.21 ± 0.18 and an electrical conductivity of 145.16 ± 9.86 mS/cm at two degrees Brix. Thus, it was also found that the internal resistance of the cells was 865.845 ± 4.726 Ω, and a maximum power density of 513.99 ± 6.54 mW/m2 was generated at a current density of 6.123 A/m2, and the final FTIR spectrum showed a clear decrease in the initial transmittance peaks. Finally, from the biofilm formed on the anodic electrode, it was possible to molecularly identify the yeast Wickerhamomyces anomalus with 99.82% accuracy. In this way, this research provides a method that companies exporting and importing this fruit may use to generate electrical energy from its waste.

Bio-Electrochemical Performance of a Ceramic Microbial Fuel Cell Treating Kitchen Waste Leachate: Effect of Organic Loading Rate and Anode Electrode Surface Area

Performance evaluation of a ceramic microbial fuel cell (CMFC) by varying organic strength, hydraulic retention time (HRT) and anode electrode surface area (AESA) to treat leachate generated from acidogenesis of kitchen waste (KW) was studied by the central composite design of experiment. The increase in organic loading rate (OLR) positively affected power density (PD) while negatively influencing organic removal and coulombic efficiency (CE). This behavior is possible due to substrate inhibition and the coercive effect of low HRT, i.e., substrate washout, biofilm abrasion, and reduced contact period, while at high HRT, the volatile fatty acid (VFA) degradation improved. Since acetic acid is the final product of long-chain VFAs degradation, a pseudo consumption order for VFAs was obtained: butyric > propionic > acetic. The AESA aided organics removal and PD but had a negligible effect on CE. According to ANOVA, the COD removal was linearly modeled, while PD and CE were quadratic. The validation runs (VR) proved efficient as the highest COD removal was for VR2 (83.7 ± 3.6%), while maximum PD and CE values obtained were 0.224 ± 0.02 W/m3 and 2.62 ± 0.33%, respectively, for VR3, supported by the lower anode potential.

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.

Exploring necessity to pre-treat organic fraction of waste prior to use in an earthen MFC modified with bentonite

In this study, the addition of bentonite at different proportions as clay minerals and various thicknesses (4, 5, and 6 mm) of ceramic membranes were evaluated for proton and oxygen mass transfer coefficients. Bentonite (20% and 4 mm) was found to be optimum and was then employed to assess earthen microbial fuel cell (EMFC) performance for different substrates (kitchen waste (KW) slurry and leachate) under batch mode. Both substrates were added in different concentrations of chemical oxygen demand (COD), i.e., 18, 15.2, 12.5, 9.7, and 6.9 g/L to EMFCs. The EMFC achieved superior organic removals for leachate (>96%). Intriguingly, the volatile fatty acids (VFAs) generation and consumption were different for each substrate. Each system expressed affinity towards acetic acid, but limited VFAs (acetic and propionic) were generated by KW while leachate generated acetic, propionic, and butyric. The leachate concentration having COD of 15.2 g/L produced the highest power density of 586.5 ± 38.8 mW/m³, while for KW, only 41.5 mW/m³ (6.9 g/L of COD for KW) was obtained. The study consolidates the need for an intermediate step to pre-treat the organic fraction of waste before its use for resource recovery. Bentonite was found as an effective clay mineral for manufacturing ceramic membranes.

Increase in Electrical Parameters Using Sucrose in Tomato Waste

The use of organic waste as fuel for energy generation will reduce the great environmental problems currently caused by the consumption of fossil sources, giving agribusiness companies a profitable way to use their waste. In this research, tomato waste with different percentages of sucrose (0-target, 5, 10, and 20%) was used in microbial fuel cells manufactured on a laboratory scale with zinc and copper electrodes, managing to generate maximum peaks of voltage and a current of 1.08 V and 6.67 mA in the cell with 20% sucrose, in which it was observed that the optimum operating pH was 5.29, while the MFC with 0% (target) sucrose generated 0.91 V and 3.12 A on day 13 with a similar pH, even though all the cells worked in an acidic pH. Likewise, the cell with 20% sucrose had the lowest internal resistance (0.148541 ± 0.012361 KΩ) and the highest power density (224.77 mW/cm2) at a current density of 4.43 mA/cm2, while the MFC with 0% sucrose generated 160.52 mW/cm2 and 4.38 mA/cm2 of power density and current density, respectively, with an internal resistance of 0.34116 ± 0.2914 KΩ. In this sense, the FTIR (Fourier-transform infrared spectroscopy) of all the substrates used showed a high content of phenolic compounds and carboxylate acids. Finally, the MFCs were connected in a series and managed to generate a voltage of 3.43 V, enough to light an LED (green). These results give great hope to companies and society that, in the near future, this technology can be taken to a larger scale.

Effect of hydrothermal pretreatment on the degrease performance and liquid substances transformation of kitchen waste

Hydrothermal treatment (HT) is a pragmatic approach for pretreatment of kitchen waste (KW). This work investigated the effect of hydrothermal pretreatment (HTP) on the deoiling, desalting and liquid substances transformation of KW. The orthogonal test method was used to study the effects of three factors at five levels, including solid to liquid ratio (A_1-5), heating time (B_1-5) and hydrothermal temperature (C_1-5). The results indicated that the floatable oil content was improved significantly after HTP. The highest floatable oil content was 84.54 mL/kg at the hydrothermal condition of 1/1.5, 20 min and 100 °C, which was 2.42 times higher than the control. The maximum desalination ratio (92.66%) was at A₅B₁C₅ (1/2.5, 5 min, 100 °C), which was 4.48 times higher than control group (No.0) (20.67%). The VFAs concentration was the highest (11441.05 mg/kg) at 1/2.5, 5 min and 100 °C, which increased by 711.03% compared to the No.0 (1410.78 mg/kg). In addition, the maximum TOC value was obtained at 53530.84 mg/kg. After HTP, the acetic acid and butyric acid concentrations of the liquid phase increased, while the ethanol concentration decreased. The contents of T,NH₄⁺-N and organic nitrogen in the liquid phase of the HTP system increased, while NO₃^--N remained at a low level (4.96-20.48 mg/kg). The range and variance analysis showed that the temperature had the greatest effect on the deoiling and the liquid substances transformation of KW among these three factors, followed by solid to liquid ratio and heating time. Based on the orthogonal experiment, the optimal parameters for KW deoiling were A₃ (1/1.5), B₄ (25 min) and C₅ (100 °C). This work provided a reference for the KW deoiling and hence improve the efficient utilization of KW.

Anaerobic Digestion, Codigestion of Food Waste, and Chicken Dung: Correlation of Kinetic Parameters with Digester Performance and On-Farm Electrical Energy Generation Potential

Valorization of agro-food waste through anaerobic digestion (AD) is gaining prominence as alternative method of waste minimization and renewable energy production. The aim of this study was to identify the key parameters for digester performance subjected to kinetic study and semicontinuous operation. Biochemical methane potential (BMP) tests were conducted in two different operating conditions: without mixing (WM) and continuous mixing (CM). Three different substrates, including food waste (FW), chicken dung (CD), and codigestion of FW and CD (FWCD) were used. Further kinetic evaluation was performed to identify mixing’s effect on kinetic parameters and correlation of the kinetic parameters with digester performance (volatile solid removal (VS%) and specific methane production (SMP)). The four models applied were: modified Gompertz, logistic, first-order, and Monod. It was found that the CM mode revealed higher values of Rm and k as compared to the WM mode, and the trend was consistently observed in the modified Gompertz model. Nonetheless, the logistic model demonstrated good correlation of kinetic parameters with VS% and SMP. In the continuous systems, the optimum OLR was recorded at 4, 5, and 7 g VS/L/d for FW, CD, and FWCD respectively. Therefore, it was deduced that codigestion significantly improved digester performance. Electrical energy generation at the laboratory scale was 0.002, 0.003, and 0.006 kWh for the FW, CD, and FWCD substrates, respectively. Thus, projected electrical energy generation at the on-farm scale was 372 kWh, 382 kWh, and 518 kWh per day, respectively. Hence, the output could be used as a precursor for large-scale digester-system optimization.

Microbial fuel cells for bioelectricity production from waste as sustainable prospect of future energy sector

Microbial fuel cell (MFC) is lauded for its potentials to solve both energy crisis and environmental pollution. Technologically, it offers the capability to harness electricity from the chemical energy stored in the organic substrate with no intermediate steps, thereby minimizes the entropic loss due to the inter-conversion of energy. The sciences underneath such MFCs include the electron and proton generation from the metabolic decomposition of the substrate by microbes at the anode, followed by the shuttling of these charges to cathode for electricity generation. While its promising prospects were mutually evinced in the past investigations, the upscaling of MFC in sustaining global energy demands and waste treatments is yet to be put into practice. In this context, the current review summarizes the important knowledge and applications of MFCs, concurrently identifies the technological bottlenecks that restricted its vast implementation. In addition, economic analysis was also performed to provide multiangle perspectives to readers. Succinctly, MFCs are mainly hindered by the slow metabolic kinetics, sluggish transfer of charged particles, and low economic competitiveness when compared to conventional technologies. From these hindering factors, insightful strategies for improved practicality of MFCs were formulated, with potential future research direction being identified too. With proper planning, we are delighted to see the industrialization of MFCs in the near future, which would benefit the entire human race with cleaner energy and the environment.

A multi-perspective review on microbial electrochemical technologies for food waste valorization

The worldwide generation of food waste (FW) has been increasing enormously due to the growing food industry and population. However, FW contains a large amount of biodegradable organics that can be converted to clean energy, which can potentially minimize the utilization of fossil fuels. Conventional biowaste valorization technologies, such as anaerobic digestion and composting, have been adopted for FW management for recovering useful biogas and compost. However, they are often limited by high capital and operation costs, low recovery efficiency, slow process kinetics, and system instability. On the other hand, microbial electrochemical technologies (METs) have been highly promising for efficiently harvesting bioenergy and high value-added products from FW. Hence, this article critically reviews up-to-date studies on applying various METs regarding their value-added products recovery efficiencies from FW. Moreover, this review lists existing challenges, ways to optimize the system performance and provides perspectives on future research needs.

Bioelectricity generation using long-term operated biocathode: RFLP based microbial diversity analysis

In the present work, power generation and substrate removal efficiencies of long-term operated microbial fuel cells, containing abiotic cathodes and biocathodes, were evaluated for 220 days. Among the two microbial fuel cell (MFC) types, the one containing biocathode showed higher power density (54 mW/m2), current density (122 mA/m2) coulombic efficiency (33%), and substrate removal efficiency (94%) than the abiotic cathode containing MFC. Voltammetric analysis also witnessed higher and sustainable electron discharge for the MFC with biocathode, when compared with the abiotic cathode MFC. Over the tested period, both MFC have shown a cell voltage drop, after 150 and 165, days, for the MFC with biocathode and abiotic cathodes, respectively. Polymerase chain reaction (PCR) based restriction fragment length polymorphism (RFLP) analysis identified 281 clones. Bacteria belonging to Acinetobacter, Acidovorax, Pseudomonas and Burkholderia were observed in the abiotic cathode MFC. Bacteria belonging to Geobacter, Cupriavidus and Acidobacteria were observed in the biocathode MFC. Almost similar types of archaea (Methanosarcinales, Methanolinea, Nitrososphaera and Methanomicrobiales) were observed in both MFCs.

Energetics, electron uptake mechanisms and limitations of electroautotrophs growing on biocathodes - A review

Electroautotrophs are microorganisms that can take the electrons needed for energy generation, CO₂ fixation and other metabolic reactions from a polarized electrode. They have been the focus of intense research for its application in wastewater treatment, bioelectrosynthetic processes and hydrogen generation. As a general trend, current densities produced by the electron uptake of these microorganisms are low, limiting their applicability at large scale. In this work, the electron uptake mechanisms that may operate in electroautotrophs are reviewed, aiming at finding possible causes for this low performance. Biomass yields, growth rates and electron uptake rates observed when these microorganisms use chemical electron donors are compared with those typically obtained with electrodes, to explore limitations and advantages inherent to the electroautotrophic metabolism. Also, the factors affecting biofilm development are analysed to show how interfacial interactions condition bacterial adhesion, biofilm growth and electrons uptake. Finally, possible strategies to overcome these limitations are described.

Application of rotten rice as a substrate for bacterial species to generate energy and the removal of toxic metals from wastewater through microbial fuel cells

Microbial fuel cells (MFCs) are the efficient and sustainable approach for the removal of toxic metals and generate energy concurrently. This article highlighted the effective use of rotten rice as an organic source for bacterial species to generate electricity and decrease the metal concentrations from wastewater. The obtained results were corresponding to the unique MFCs operation where the 510 mV voltage was produced within 14-day operation with 1000 Ω external resistance. The maximum power density and current density were found to be 2.9 mW/m2 and 168.42 mA/m2 with 363.6 Ω internal resistance. Similarly, the maximum metal removal efficiency was found to be 82.2% (Cd), 95.71% (Pb), 96.13% (Cr), 89.50% (Ni), 89.82 (Co), 99.50% (Ag), and 99.88% (Cu). In the biological test, it was found that Lysinibacillus strains, Chryseobacterium strains, Escherichia strains, Bacillus strains are responsible for energy generation and metal removal. Furthermore, a multiparameter optimization revealed that MFCs are the best approach for a natural environment with no special requirements. Lastly, the working mechanism of MFCs and future recommendations are enclosed.

Microbial profiles associated improving bioelectricity generation from sludge fermentation liquid via microbial fuel cells with adding fruit waste extracts

This first-attempt study illustrated the microbial cooperative interactions related to bioelectricity generation from the mixture of sludge fermentation liquid (SFL) and fruit waste extracts (FWEs) via microbial fuel cells (MFCs). The optimal output voltages of 0.65 V for SFL-MFCs, 0.51 V for FWEs-MFCs and 0.75 V for mixture-MFCs associated with bioelectricity conversion efficiencies of 1.061, 0.718 and 1.391 kWh/kg COD were reached, respectively. FWEs addition for substrates C/N ratio optimization contributed considerably to increase SFL-fed MFCs performance via triggering a higher microbial diversity, larger relatively abundance of functional genes and microbial synergistic interactions with genera enrichment of Clostridium, Alicycliphilus, Thermomonas, Geobacter, Paludibaculum, Pseudomonas, Taibaiella and Comamonas. Furthermore, a conceptual illustration of co-locating scenario of wastewater treatment plant(s), waste sludge in situ acidogenic fermentation, fruit waste collection/crushing station and MFC plant was proposed for the first time, which provided new thinking for future waste sludge treatment toward maximizing solid reduction and power recovery.

A Review of Urban Green and Blue Infrastructure from the Perspective of Food-Energy-Water Nexus

Small scale urban green-blue infrastructure (indicated as GBI hereafter) comprises huge underexploited areas for urban development and planning. This review article aims to highlight the relevance and knowledge gaps regarding GBI from the perspective of the food–energy–water (FEW) nexus, these being key resources for the survival of human communities. In particular, this review was focused on publications on urban ecosystem services (positive effects) and dis-services (negative effects) associated with different GBI typologies. The review proved that GBI can contribute environmentally, socially, and economically to FEW security and urban sustainability. Yet, such positive effects must be considered against ecosystem dis-services tradeoffs, including urban food production, commonly connected with heavy water and energy consumption, specifically under dry climate conditions, and sometimes related to an excessive use of manure, pesticides, or fertilizers. These conditions could pose either a risk to water quality and local insect survival or serve enhanced mosquito breeding because of irrigation. Up to now, the review evidenced that few nexus modeling techniques have been discussed in terms of their benefits, drawbacks, and applications. Guidance is provided on the choice of an adequate modeling approach. Water, energy, and food are intrinsically associated physically. However, depending on their management, their tradeoffs are often increased. There is a need to minimize these tradeoffs and to build up synergies between food, energy, and water using a holistic approach. This is why the FEW nexus approach offers good insights to address the relation between three important individual resource components of sustainability.

Valorisation and emerging perspective of biomass based waste-to-energy technologies and their socio-environmental impact: A review

The economic developments around the globe resulted in the increased demand of energy, which overburdened the supply chain sources of energy. Fossil fuel reserves are exploited to meet the high demand of energy and their combustion is becoming the main source of environmental pollution. So there is dire need to find safe, renewable and sustainable energy resources. Waste to energy (WtE) may be viewed as a possible alternate source of energy, which is economically and environmentally sustainable. Municipal solid waste (MSW) is a major contributor to the development of renewable energy and sustainable environment. At present the scarcity of renewable energy resources and disposal of MSW is a challenging problem for the developing countries, which has generated a wide ranging socioeconomic and environmental problems. This situation stimulates the researchers to develop alternatives for converting WtE under a variety of scenarios. Herein, the present scenario in developing the WtE technologies such as, thermal conversion methods (Incineration, Gasification, Pyrolysis, Torrefaction), Plasma technology, Biochemical methods, Chemical and Mechanical methods, Bio-electrochemical process, Mechanical biological treatment (MBT), Photo-biological processes for efficacious energy recovery and the challenges confronted by developing and developed countries. In this review, a framework for the evaluation of WtE technologies has been presented for the ease of researchers working in the field. Furthermore, this review concluded that WtE is a potential renewable energy source that will partially satisfy the demand for energy and ensure an efficient MSW management to overcome the environmental pollution.

Food waste valorization: Energy production using novel integrated systems

Management of food waste (FW) is a global challenge due to increasing population and economic activities. Presently, landfill and incineration are the keyways of FW management, while economical and environmental sustainability have been an issue. Therefore, the biological processes have been investigated for resource and energy recovery from FW. However, these biological approaches have certain drawbacks and cannot be a complete solution for FW management. Therefore, this review aims to offer a detailed and complete analysis of current available technologies to achieve environmental and economical sustainability. In this context, zero solid waste discharge for resource and energy recovery has been put into view. Corresponding to which several innovative technologies using integrated biological methods for resource and energy recovery from FW have been elucidated.

Linking microbial mechanism with bioelectricity production in sludge matrix-fed microbial fuel cells: Freezing/thawing liquid versus fermentation liquor

This first-attempt study elucidated the microbial mechanism associated with bioelectricity output in microbial fuel cells (MFCs) fed with sludge matrices of freezing/thawing (F/T) liquid versus fermentation liquor, while a novel schematic elucidation for exploring cooperative interactions in anodic microbial consortia of MFCs supplied with such two feeds toward electrogenesis was put forward. Moreover, the F/T liquid cultivated main genera of Azospira, Povalibacter, Thauera, Terrimonas, Alicycliphilus, Dokdonella and Simplicispira for dual organics degradation and electrogenesis with power density of 0.152 mW/m² and electrogenesis efficiency of 1.152 kWh/kg COD, while the fermentation liquor fostered higher diversity and medium evenness with the enrichment of Phenylobacterium, Cellulomonas, Edaphobacter, Burkholderia, Clostridium, Sphingomonas, Leifsonia and Microbacterium in anodic biofilm and causing larger power density of 0.182 mW/m² and 1.418 kWh/kg COD-electrogenesis efficiency. Comparative analysis results indicated that the anodic fermentative bacteria exert considerable influence on concurrent organics degradation and electricity production through the synergistic interactions with exoelectrogens toward stable running of MFCs. Besides, the higher anodic microbial diversity, relatively middling community evenness and larger abundance of functional genes associated with electrogenesis together played contributive roles on more power generation through MFCs for treating WAS matrix. This study was conducive to bring about some new microbial mechanism understanding on maximizing bioenergy recovery via MFCs in future sludge management.

Waste valorization using solid-phase microbial fuel cells (SMFCs): Recent trends and status

This review article discusses the use of solid waste processed in solid-phase microbial fuel cells (SMFCs) as a source of electrical energy. Microbial Fuel Cells (MFCs) are typically operated in the liquid phase because the ion transfer process is efficient in liquid media. Nevertheless, some researchers have considered the potential for MFCs in solid phases (particularly for treating solid waste). This has promise if several important factors are optimized, such as the type and amount of substrate, microorganism community, system configuration, and type and number of electrodes, which increases the amount of electricity generated. The critical factor that affects the SMFC performance is the efficiency of electron and proton transfer through solid media. However, this limitation may be overcome by electrode system enhancements and regular substrate mixing. The integration of SMFCs with other conventional solid waste treatments could be used to produce sustainable green energy. Although SMFCs produce relatively small amounts of energy compared with other waste-to-energy treatments, SMFCs are still promising to achieve zero-emission treatment. Therefore, this article addresses the challenges and fills the gaps in SMFC research and development.

Bioelectrochemical chlorate reduction by Dechloromonas agitata CKB

Chlorate has been described as an emerging pollutant that compromises water sources. In this study, bioelectrochemical reactors (BERs) using Dechloromonas agitata CKB, were evaluated as a sustainable alternative for chlorate removal. BERs were operated under flow-recirculation and batch modes with an applied cell-voltage of 0.44 V over a resistance of 1 kΩ. Results show chlorate removal up to 607.288 mg/L. After 115 days, scanning electron microscopy showed biofilm development over the electrodes, and electrochemical impedance spectroscopy confirmed the biocatalytic effect of CKB. The theoretical chlorate bioreduction potential (ε° = 0.792 V) was proven, and a kinetic study indicated that 6 electrons were involved in the reduction mechanism. Finally, a hypothetical bioelectrochemical mechanism for chlorate reduction in a BER was proposed. This research expands upon current knowledge of novel electrochemically active microorganisms and widens the scope of BER applications for chlorate removal.

Microbial Structure and Energy Generation in Microbial Fuel Cells Powered with Waste Anaerobic Digestate

Development of economical and environment-friendly Microbial Fuel Cells (MFCs) technology should be associated with waste management. However, current knowledge regarding microbiological bases of electricity production from complex waste substrates is insufficient. In the following study, microbial composition and electricity generation were investigated in MFCs powered with waste volatile fatty acids (VFAs) from anaerobic digestion of primary sludge. Two anode sizes were tested, resulting in organic loading rates (OLRs) of 69.12 and 36.21 mg chemical oxygen demand (COD)/(g MLSS∙d) in MFC1 and MFC2, respectively. Time of MFC operation affected the microbial structure and the use of waste VFAs promoted microbial diversity. High abundance of Deftia sp. and Methanobacterium sp. characterized start-up period in MFCs. During stable operation, higher OLR in MFC1 favored growth of exoelectrogens from Rhodopseudomonas sp. (13.2%) resulting in a higher and more stable electricity production in comparison with MFC2. At a lower OLR in MFC2, the percentage of exoelectrogens in biomass decreased, while the abundance of genera Leucobacter, Frigoribacterium and Phenylobacterium increased. In turn, this efficiently decomposed complex organic substances, favoring high and stable COD removal (over 85%). Independent of the anode size, Clostridium sp. and exoelectrogens belonging to genera Desulfobulbus and Acinetobacter were abundant in MFCs powered with waste VFAs.

Insights into redox mediators-resource harvest/application with power production from waste activated sludge through freezing/thawing-assisted anaerobic acidogenesis coupling microbial fuel cells

This first-attempted study demonstrated endogenous redox-mediators harvest/application from waste activated sludge (WAS) through freezing/thawing (F/T) pretreatment-enhanced anaerobic acidogenesis coupled with microbial fuel cells (MFCs). A total of 2.57 kWh electricity was produced from per kg soluble chemical oxygen demand (SCOD) via MFCs just in 2 d with about 90% organics removal, which contained 1.152 kWh/kg COD from F/T liquid together with 1.418 kWh/kg COD from fermentation liquid. The fermentation liquor-MFCs fostered higher anodic biodiversity and more power output as compared with the F/T liquid-MFCs. Essentially, the completely endogenous redox mediators-like substances with relatively high redox activities could be retained after MFC electrogenesis from F/T liquid and played electron shuttle-roles sufficiently in enlarging bio-energy production of MFCs, which seemed to be an effective option for harvesting endogenous redox mediators from sludge. This study might inspire progressive thinking toward aims of high-efficiency of resource recycle/bioenergy production from WAS.

A novel combination of enzymatic hydrolysis and microbial fuel cell for electricity production from bakery waste

This work developed a novel two-stage bioprocess for electricity generation from bakery waste (BW). In the first stage, commercial glucoamylase was utilized to hydrolyze the BW to produce soluble BW hydrolysate. It was found that 100 g BW could be converted to 32 g hydrolysis solid and 760 mL BW hydrolysate. The highest glucose production of 21.9 g/L could be achieved within 5 h. In the second stage, the soluble BW hydrolysate was utilized as feedstock for electricity generation in microbial fuel cell (MFC). The maximum voltage of 0.386 V was obtained. The power density reached a peak value of 29.96 mW/m² when the external resistance was 1090 Ω. It could be potentially utilized to transform high-starch containing raw materials into biofuels production which could reduce the cost of biofuels production effectively for industrial application.

Adding Zero-Valent Iron to Enhance Electricity Generation during MFC Start-Up

The low power generation efficiency of microbial fuel cells (MFCs) is always a barrier to further development. An attempt to enhance the start-up and electricity generation of MFCs was investigated by adding different doses of zero-valent iron into anaerobic anode chambers in this study. The results showed that the voltage (289.6 mV) of A2 with 0.5 g of zero-valent iron added was higher than the reference reactor (197.1 mV) without dosing zero-valent iron (A4). The maximum power density of 27.3 mW/m2 was obtained in A2. CV analysis demonstrated that A2 possessed a higher oxidation-reduction potential, hence showing a stronger oxidizing property. Meanwhile, electrochemical impedance analysis (EIS) also manifested that values of RCT of carbon felts with zero-valent iron supplemented (0.01-0.03 Ω) were generally lower. What is more, SEM images further proved and illustrated that A2 had compact and dense meshes with a hierarchical structure rather than a relatively looser biofilm in the other reactors. High-throughput sequencing analysis also indicated that zero-valent iron increased the abundance of some functional microbial communities, such as Acinetobacter, Ignavibacteriales, Shewanella, etc.

Unraveling interactive characteristics of microbial community associated with bioelectric energy production in sludge fermentation fluid-fed microbial fuel cells

This first-attempt study deciphered the interactive characteristics of anodophilic microbial community-associated bioelectricity production in waste activated sludge (WAS) fermentation fluid-fed microbial fuel cells (MFCs). A novel schematic elucidation for illustrating synergistic interactions in anodic microbial consortia towards electrogenesis was proposed. Moreover, the specific genera of Pseudomonas, Desulfovibrio, Phyllobacterium, Desulfuromonas, Chelatococcus and Aminivibrio were dominant in anodic biofilms, leading to an electrogenesis efficiency of 1.254 kWh/kg COD and peak power density of 0.182 W/m² (at feeding level of 1.20 g COD/L). It was apparently higher than those MFCs fed with glucose/acetate. The fermentative species contributed positively in reorganizing microbial community structure in anodic biofilms, positively relating to electrogenesis via interactions with exoeletrogens in MFCs. Finally, a more electrogenesis was positively associated to larger anodic microbial diversity, relatively medium anodic community evenness, together with higher abundance of functional genes related to electrogenesis in functional species in MFCs fed with WAS fermentation fluid.

Impact of cleaning procedures on restoring cathode performance for microbial fuel cells treating domestic wastewater

Degradation of cathode performance over time is one of the major drawbacks in applications of microbial fuel cells (MFCs) for wastewater treatment. Over a two month period the resistance of air cathodes (R_Ct) with a polyvinylidene fluoride (PVDF) diffusion layer increased of 111% from 70 ± 10 mΩ m² to 148 ± 32 mΩ m². Soaking the cathodes in hydrochloric acid (100 mM HCl) restored cathode performance to R_Ct = 74 ± 17 mΩ m². Steam, ethanol, or sodium hydroxide treatment produced only a small change in performance, and slightly increased R_Ct. With a polytetrafluoroethylene (PTFE) diffusion layer on the cathodes, R_Ct increased from 54 ± 14 mΩ m² to 342 ± 142 mΩ m² after two months of operation. The acid concentration was critical for effectiveness in cleaning, as HCl (100 mM) decreased R_Ct to 28 ± 8 mΩ m². A lower concentration of HCl (<1 mM) showed no improvement, and vinegar (5% acetic acid) produced 48 ± 4 mΩ m².

Turning food waste to energy and resources towards a great environmental and economic sustainability: An innovative integrated biological approach

Food waste (FW) management is a global conundrum because of the rapid population growth and growing economic activity. Currently, incineration and landfill are still the main means for FW management, while their environmental sustainability and economic viability have been in question. Recently, the biological processes including anaerobic digestion, aerobic composting, bioethanol fermentation, feed fermentation etc. have attracted increasing interest with the aims for energy and resource recovery from FW. However, these biological approaches have inherent drawbacks, and cannot provide a comprehensive solution for future FW management. Therefore, this review attempts to offer a critical and holistic analysis of current biotechnologies for FW management with the focus on the challenges and solutions forward. The biological approaches towards future FW management should be able to achieve both environmental sustainability and economic viability. In this instance, the concept of zero solid discharge-driven resource recovery has thus been put forward. According to which, several innovative biological processes for FW management are further elucidated with critical analysis on their engineering feasibility and environmental sustainability. It turns out that is an urgent need for turning current single task-orientated bioprocess to an integrated biological process with multiple tasks of concurrent recovery of water, resource and energy together with zero-solid discharge.

Insights into microbial community profiles associated with electric energy production in microbial fuel cells fed with food waste hydrolysate

Insights of microbial community profiles associated with electric energy production in microbial fuel cells (MFCs) fed with food waste hydrolysate (FWH) were investigated in this study. High power density of 0.173 W/m² was obtained from FWH which was produced from food waste after the pretreatment with fungal mash at an influent COD concentration of 1.2 g/L. The main genera in the MFCs fed with FWH were found to be Rummeliibacillus, Burkholderia, Enterococcus and Clostridium in anodic biofilms, leading to an electrogenesis efficiency of 0.977 kWh/kg COD higher than those obtained in MFCs with single carbon source feed. The key members in the anodic community responsible for electrogenesis were conceptually identified with their metabolic interactions in MFCs fed with FWH. It appeared that the syntrophic cooperation of fermentative species with exoelectrogens played an essential role in the generation of electric energy via specific microbes in anodic biofilm. The power produced from FWH was positively associated with microbial diversity, intermediate community evenness and abundance of functional genes for bioelectrogenesis.

An integrated approach for waste activated sludge management towards electric energy production/resource reuse

This study developed an integrated approach for electric energy harvest/resource reuse from waste activated sludge (WAS) pretreated by enzymolysis based on anaerobic fermentation and microbial fuel cells (MFCs). WAS solubilization by the 3-h enzymatic pretreatment (a blend of hydrolytic enzymes caused over 5300 mg/L soluble COD release) prompted volatile fatty acid (VFA) production with 3580 mg COD/L after 10-d fermentation. After solid-liquid separation, fermentation liquid with high VFA content was fed into MFCs for electric energy production, while solid residues were used for making building materials (such as blended cements). Results showed that the electricity conversion efficiency of fermentation liquid (VFA) reached 1.254 kW h/kg COD with over 90% organics removal and solid residues could be consumed potentially as qualified substitutes for producing cements. As such, this study may provide some new thinking on future WAS management towards electricity harvest/resource reuse with zero secondary wastes discharge.