Electro-Fermentation in Aid of Bioenergy and Biopolymers

: The soaring levels of industrialization and rapid progress towards urbanization across the world have elevated the demand for energy besides generating a massive amount of waste. The latter is responsible for poisoning the ecosystem in an exponential manner, owing to the hazardous and toxic chemicals released by them. In the past few decades, there has been a paradigm shift from “waste to wealth”, keeping the value of high organic content available in the wastes of biological origin. The most practiced processes are that of anaerobic digestion, leading to the production of methane. However; such bioconversion has limited net energy yields. Industrial fermentation targeting value-added bioproducts such as—H 2 , butanediols; polyhydroxyalkanoates, citric acid, vitamins, enzymes, etc. from biowastes/lignocellulosic substrates have been planned to ﬂourish in a multi-step process or as a “Bioreﬁnery”. Electro-fermentation (EF) is one such technology that has attracted much interest due to its ability to boost the microbial metabolism through extracellular electron transfer during fermentation. It has been studied on various acetogens and methanogens, where the enhancement in the biogas yield reached up to 2-fold. EF holds the potential to be used with complex organic materials, leading to the biosynthesis of value-added products at an industrial scale.


Introduction
Due to rapid industrialization and rising population, the energy crisis and pollution have become the foremost problems that humankind will be facing in the future [1,2]. The greenhouse effect in the atmosphere is increasing day by day due to the rapid usage of fossil fuels that discharge CO 2 , the major greenhouse gas that damages the environment [3]. The hastily intensifying global demand for the limited fossil fuel reserves and the need to control greenhouse gas effects due to CO 2 emission into the atmosphere resulting from their usage are driving the research for sustainable carbon-free energy carriers [4,5]. In this scenario, many research efforts are currently being dedicated to the search, development, and optimization of eco-friendly and sustainable energy resources to replace fossil fuels. In parallel, novel fermentation technologies, such as the electro-fermentation (EF), is being developed to augment and enhance the potential of conventional fermentation methods [6]. EF is a unique process that deals with the electrochemical control over microbial metabolism. In other words, it is a type of reactor where the biological and electrochemical process occurs at the same time to produce electricity and other bioproducts. The electrodes used during the process can work as In this context, use of waste biomass in a biorefinery could be a potential alternative. Waste generated from agriculture, non-edible or other food wastes, effluents of biochemical industries, etc. are potential sources of raw materials [6]. A large quantity of food wastes is generated annually from food supply chain to agriculture and industrial processing systems. The quantum of such wastes is expected to grow rapidly in the coming 25 years [19,20]. Managing such huge quantities of wastes has become a global problem [21,22]. Since they mostly consist of cellulose, lignin, lipids, and proteins, it is imperative to exploit them for producing value-added products.
It thereby gives endless opportunities and offers a win-win situation by the fact that these wastes, including municipal solid wastes go to landfills, leading to groundwater pollution, greenhouse gas emission, and other harmful effects in the vicinity [21,23]. Uses of waste biomass have been shown to produce H 2 , CH 4 , volatile fatty acids (VFA), PHAs, ethanol, and many other bioproducts. Various stochastic models having tremendous industrial potential have been proposed that can be utilized in tandem on a large scale [24]. MFC based systems have also been exploited to treat food and agriculture wastes and extract valuable chemicals along with electricity generation [6]. Fermented canteen wastes have been found to generate 530 mA/m 3 of power density with a concomitant reduction in chemical oxygen demand (COD) of up to 44% [25]. The rising attention in this area of research has opened intriguing possibilities for the mankind. Figure 1 shows the number of papers published between 1991 and 2017 for the keywords 'bioelectrochemical system', 'microbial fuel cell', and 'microbial electrolysis cell' according to the Scopus database [26]. This indicates that the research interest in the EF-based systems for the generation of other value-added products has increased sharply in the past few years. An enhancement of >2. 5

Waste Biomass: Veiled Opportunity
The growing urbanization is posing a serious impact on natural resources and environment. The dependency on fossil fuel based energy sources leads to the global problem of pollution and climate change [16,17]. The quest to find alternative resources for petroleum-based products (especially for transportation) has been the major impetus to produce bioenergy from renewable resources [18]. In this context, use of waste biomass in a biorefinery could be a potential alternative. Waste generated from agriculture, non-edible or other food wastes, effluents of biochemical industries, etc. are potential sources of raw materials [6]. A large quantity of food wastes is generated annually from food supply chain to agriculture and industrial processing systems. The quantum of such wastes is expected to grow rapidly in the coming 25 years [19,20]. Managing such huge quantities of wastes has become a global problem [21,22]. Since they mostly consist of cellulose, lignin, lipids, and proteins, it is imperative to exploit them for producing value-added products.
It thereby gives endless opportunities and offers a win-win situation by the fact that these wastes, including municipal solid wastes go to landfills, leading to groundwater pollution, greenhouse gas emission, and other harmful effects in the vicinity [21,23]. Uses of waste biomass have been shown to produce H2, CH4, volatile fatty acids (VFA), PHAs, ethanol, and many other bioproducts. Various stochastic models having tremendous industrial potential have been proposed that can be utilized in tandem on a large scale [24]. MFC based systems have also been exploited to treat food and agriculture wastes and extract valuable chemicals along with electricity generation [6]. Fermented canteen wastes have been found to generate 530 mA/m 3 of power density with a concomitant reduction in chemical oxygen demand (COD) of up to 44% [25]. The rising attention in this area of research has opened intriguing possibilities for the mankind. Figure 1 shows the number of papers published between 1991 and 2017 for the keywords 'bioelectrochemical system', 'microbial fuel cell', and 'microbial electrolysis cell' according to the Scopus database [26]. This indicates that the research interest in the EF-based systems for the generation of other value-added products has increased sharply in the past few years. An enhancement of >2.

Anaerobic Digestion and Its Limitations
Anaerobic digestion is considered as a well-established technology for biogas/biomethane (bio-CH 4 ) production. The final product of anaerobic digestion consists of bio-CH 4 (60% to 80%) and CO 2 (20% to 40%), whose comparative quantities chiefly depend on the type of substrate used. The anaerobic digestion is a multistep biological process (hydrolysis, acidogenesis, acetogenesis, Energies 2018, 11, 343 4 of 20 and methanogenesis) mediated by a specific group of microbes, which works together under a strict anaerobic environment [27]. The optimum temperature for the anaerobic digestion process varies between 35 and 55 • C. However, the balance between energy input and total biogas yield is a key factor to determine the choice of anaerobic digester operating temperature. Its successful operation is depended on the acclimatization of mixed cultures, pH fluctuations, organic loads, retention time, etc. Controlling so many parameters including the variations in the substrate composition makes the process vulnerable for continuous operations and therefore requires consistent observation.

Microbial Electro-Fermentation: An Alternative Method of Fermentation
Electrofermentation is the effective electron capturing process using specialized electrodes during microbial fermentation. It utilizes a low-cost platform with self-sustaining anodifilic microbes to oxidize organic and inorganic acids usually present in waste materials [6]. At its core, a cascade of redox reaction occurs to convert chemical energy into electricity or other products, wherein specific membranes separate the cathode and anode chambers [28][29][30]. It is the common principle for almost all the EF based reactors. Its dimension and application get a wider reach by the fact that reduction based reaction can take place in the cathode chamber. In the anode compartment, electrochemically active bacteria utilize organic matter and generate electrons and protons. These protons transferred via proton exchange membrane and reach the cathode. Meanwhile, electrons were captured by the anode electrode and transferred them via an electrical circuit to the cathode. In the cathode compartment, electrons and protons are reduced with oxygen molecule, which acts as a terminal electron acceptor. Depending upon the number of compartments and operation type, EF can be categorized into single chambered, double-chambered, tubular, stacked, baffled, up-flow system, etc. [24,[31][32][33][34][35][36][37]. On the other hand, with different modifications, EFs can further be categorized into bioelectrochemical treatment system (BET), microbial desalination system (MDS), microbial electrolysis cell (MEC), and bioelectrochemical system (BES) extending its utility [24,[38][39][40][41]. During EF, specialized microbes are exploited based on their electrochemical activities and can be grouped into electrochemically active bacteria, exoelectrogen, electricigen, or anode-respiring bacteria. This implies that microbes are able to transfer electrons outside the cell membranes in a different manner. Microbes belonging to the exoelectrogen category are able to produce electricity at anode through electron generation, apparently through their ability of metal-reduction. The case is the opposite with endoelectrogen; it consumes electric current at cathode through oxidation of metals [38,42]. They can either directly use membrane proteins/structures, pili, filaments, cytochromes, or secrete mediators for electron transfer; viz. Geobacter sulfurreducens uses pili as conductive wires for inter-cellular conduction of electrons. On the other hand, Shewanella spp. possesses both conductive filaments as well as mediators such as riboflavin. On the other hand, ferredoxin was found to work as mediators in bacteria belonging to Clostridia (Clostridium pasteurianum, C. ljungdahlii, and C. aceticum), Sporomusa ovata, Methanococcus maripaludis, etc. Interestingly, three different possible mediators-formate dehydrogenase, hydrogenase, and rubredoxin-have been found in sulfate-reducing bacteria (Desulfovibrio sp.) [38]. Theoretically, any biodegradable material can be transformed into energy and organic compounds [7]. The most widely exploited waste substrates include industrial wastewater effluents, waste biomass, inorganic substrates like ammonia, acid mine drainage, etc. [43]. However, co-operation among microbes are required to achieve the target. Thus, usually mixed cultures are used for this purpose [44]. The first sets of microbes are usually having high hydrolytic abilities to degrade complex wastes into simple substrates. The latter can be utilized by the second set of the electrochemically active bacterial population that can oxidize them in the anode chamber. Cathode based EFs differ from the one described above as for reduction of oxidized compounds customized systems must be used, which may or may not require external electric supply. Figure 2 shows the various mechanisms for electron transfer from electrodes to microorganisms. Energies 2018, 11, x FOR PEER REVIEW 5 of 20 described above as for reduction of oxidized compounds customized systems must be used, which may or may not require external electric supply. Figure 2 shows the various mechanisms for electron transfer from electrodes to microorganisms. Four general variants of EFs have been shown. The basic one is commonly known as MFC, which is used for direct generation of electricity from organic compounds. If MFC is supplied with external current to reduce the cathode potential, it becomes a MEC, wherein H2 and other products are generated in the cathode chamber [45]. If the MEC type system is used to remove contaminants (chlorinated compounds, uranium, etc.) from the solution, it is termed as microbial remediation cell (MRC), whereas it is known as microbial electrosynthesis (MES) if value-added chemicals are produced through microbial catalysis at the cathode (Table 1) [46][47][48][49]. An additional separating chamber can also be used between anode and cathode, which uses internal potential for desalination of water. The latter is known as microbial desalination cell (MDC) and is unique since it does not require external electricity to reduce the cathode potential [50][51][52][53][54][55][56][57]. There are many other variants available, which are customized for specific purposes but share the common principle [40]. Four general variants of EFs have been shown. The basic one is commonly known as MFC, which is used for direct generation of electricity from organic compounds. If MFC is supplied with external current to reduce the cathode potential, it becomes a MEC, wherein H 2 and other products are generated in the cathode chamber [45]. If the MEC type system is used to remove contaminants (chlorinated compounds, uranium, etc.) from the solution, it is termed as microbial remediation cell (MRC), whereas it is known as microbial electrosynthesis (MES) if value-added chemicals are produced through microbial catalysis at the cathode (Table 1) [46][47][48][49]. An additional separating chamber can also be used between anode and cathode, which uses internal potential for desalination of water. The latter is known as microbial desalination cell (MDC) and is unique since it does not require external electricity to reduce the cathode potential [50][51][52][53][54][55][56][57]. There are many other variants available, which are customized for specific purposes but share the common principle [40].

Hydrogen
Hydrogen is considered as the clean source of energy and has the potential to replace fossil fuel based energy sources [60,70]. It has attracted considerable global interest in the past few decades; however, its large-scale production through biological routes is limited due to expensive feed material and process costs [71]. The theoretical yield 4 mol/mol can be achieved using dark fermentation that further limits the up-scaling at larger scale [60]. However, it was found that integrating such process using anaerobic MEC could enhance the H 2 yield up to 4-folds. The common metabolism during dark fermentation through acetate or butyrate follows the following reaction: This limits the achievable H 2 yield to 2 mol/mol and 4 mol/mol with butyrate and acetate as an end-product, respectively [60]. Integrating this process with an MEC using Geobacter, Shewanella, Pseudomonas, etc. leads to produce H 2 from protons and electrons produced by microbes as shown below: Acetate is the simplest substrate that produces H 2 in MEC systems (Table 1). In addition, hydrogen-producing bioreactor effluents comprise a great quantity of intermediate metabolites (VFA), which can be considered as an alternative to the synthetic substrate for hydrogen production in MECs [24]. In this way, MECs can be effectively integrated with a hydrogen producing bioreactor to enhance the overall H 2 yield, which makes the process economically more viable. Moreover, effluents from CH 4 producing bioreactor can also be used as potential feedstock in MECs for H 2 production. In MEC systems, H 2 evolution at cathode using acetate and electrogenic bacteria (E An ∼ = 0.2 V) is insufficient at the cathode (E cell = −0.414 V, pH = 7). It was observed that producing H 2 on the cathode at MEC requires very low voltage as compared to direct electrolysis of water. In addition, the use of the complete anaerobic condition increases the coulombic efficiency of the process. By this approach, a maximum yield of 11 mol H 2 /mol glucose was obtained at the rate of 1 m 3 H 2 /day/m 3 in the reactor. A novel system for simultaneous H 2 and struvite (MgNH 4 PO 4 ·6H 2 O) generation was studied on the principle that struvite crystallization occurs at the cathode during a bioelectrochemical reaction in a single chamber MEC and was termed as MESC [14].
Hydrogen generation rate can be significantly improved by eliminating membranes from the dual chamber MEC. However, it also led to the consumption of H 2 by methanogens to produce CH 4 . During MEC operation, methanogens compete directly with exoelectrogenic biocatalyst for substrate and also consume H 2 , which adversely affect the overall process efficiency [2,72]. In addition, purity of H 2 generation in the MECs is also greatly influenced by hydrogenotrophic methanogens ( Table 2). The addition of methanogen inhibitors or exposing the reactor to the aerobic condition may help to a certain extent, but it remains a major hurdle in an H 2 generation by MECs [64]. Another limitation of this process is the use of expensive metal catalysts at the cathode.
In this context, a single-chambered MEC was used with acidogenic effluent (VFA rich) of an H 2 production process to recover more H 2 by applying 0.6 V (external) to the system with a concomitant utilization of 68% VFAs [73]. A wide variety of electrode materials (copper, platinum, graphite, etc.) were used as a potential electrode material in MECs for H 2 production. However, electrically highly conductive electrode materials could be more effective in MECs for efficient H 2 production [61,62,74].

Biogas
Biomethane is usually produced by methanogens using acetate, formate, and hydrogen obtained from pure compounds or various biowastes as substrates in an anaerobic reactor [87,88]. The first route (acetate substrate) is operated by acetoclastic methanogens while the second route (using H 2 substrate) is by hydrogenotrophic methanogenesis [89]. Factors affecting biogas production include pH, hydraulic retention time, substrate load, temperature, C/N ratio, acclimation period, and substrate composition [47,90]. For any given experimental setup controlling two or more such parameters is easy on a lab scale, while on a large scale controlling parameters, particularly the feed composition is difficult to maintain over a long period of time. Recently, an agricultural biogas plant was used to demonstrate that optimized feed composition based on a detailed database and the linear program model could be helpful in achieving consistency in running similar biogas plants [90]. However, it was emphasized that optimization of such feedstock is promising but limited by the high organic contents contributed by maize and grass silage that governs the majority of the biogas produced. Any transient decrease in the feedstock may raise the energy crops proportion within the substrates, leading to higher biogas evolution. The requirement of land to grow energy crops is comparatively less than food-crops; therefore, a systematic economic evaluation of any given agricultural biogas plant is desirable [90,91]. Besides the above-mentioned major factors, the changes in microbial communities during the operation cycle have been studied. Acclimatization of the mixed cultures (defined or undefined) along with other co-feeding strategies has shown potential in enhancing the consistency and performance [89,92]. In contrast, MEC based EF could provide various advantages over the conventional biogas production. During this process, methanogens directly accept electrons (from cathode) and are reported to produce CH 4 largely from H 2 gas generated at the cathode and partly through acetate conversion [7]. Therefore, about 86% enriched CH 4 production was observed compared to conventional anaerobic digestion in addition to the consumption of CO 2 during the process [75]. The thermodynamically plausible process of CH 4 generation through CO 2 reduction at 0.169 V, pH 7 had also been proposed: In contrast to anaerobic digestion, it was found that, in the MEC, most of the CH 4 produced during the process are derived from H 2 . However, the source of CH 4 in MEC (acetate or H 2 ) is yet to be clearly understood. Methanogenic conversion of H 2 to CH 4 leads to specific thermodynamic loss of energy (~15%). Thus, it was suggested that linking MECs to anaerobic digestion process may become a viable option for bioenergy [93]. Organic loading, long reaction time, ammonia levels and higher substrate concentrations are the key to enhanced methane production. Up to 40% enrichment of acetoclastic methanogen population was observed during an anaerobic digestion-MEC, which was proposed to efficiently degrade the organic material and convert VFA to CH 4 [85,94]. Methanobacterium palustre was among the first few organisms studied for the CH 4 generation with an abiotic anode and biocathode [47]. In this process, biotic carbon cathode favors CH 4 generation while plain cathode leads to H 2 production (Table 2). Here, current seems to play a major role. At a supply of <0.5 V, H 2 level decreases while CH 4 concentration increases [72,95]. On the other hand, when the applied voltage was >0.7 V, H 2 levels were restored [95]. Here, carbonate concentration and medium pH shows an important role in shifting the metabolism from H 2 (↓carbonate and pH) to CH 4 (↑carbonate and pH) [89]. It was demonstrated that electromethanogenesis can use electricity generated through renewable resources into biofuel with simultaneous capturing of carbon dioxide. Since CH 4 generation is quite unavoidable in the H 2 producing MEC, the purification of H 2 becomes quite expensive leading to higher production cost. Thus, production of CH 4 becomes feasible through electromethanogenesis. Additionally, it is easy to store, compress, and transport. Nowadays, the research focus is more on developing membrane-less MECs. A two-chambered MEC present loss of concentration due to the accumulation of [H + ] or [OH − ]. Production of methane through MEC provides the additional advantage of sludge loss and alleviating aeration cost [93]. It was also demonstrated that, in a single chamber MEC, CH 4 production is indeed linked with the current generation and H 2 production instead of acetoclastic methanogenesis [89]. The work of Clauwaert et al. showed that the CH 4 generation increased up to 0.41 mol/mol acetate when two-chamber MEC was used [86]. Later, they developed a membrane-less MEC using graphite granule electrodes that helped in producing CH 4 as the main product under MEC operated with the carbonate-limited acidified medium [96]. However, limitation of methane solubility at different temperature and pH conditions remains similar to that of conventional fermentation. Progress is underway to exploit biowastes for CH 4 generation through the bioelectrochemical system. In this context, garbage slurry, food wastes, and wastewater have been explored as a substrate for biogas production [94,97,98]. MEC works well on low-strength wastewater and must be supported by anaerobic digesters when the feed has very high-strength wastewater or organic load. One must understand that MEC based technology (developed to date) may not potentially work as a standalone process and thus do not have the desired capacity to replace existing methodology, rather it will require the support of other methods to achieve higher efficiency. The integrative bioprocesses may prove helpful in the near future to treat wastes and wastewater while producing bioenergy in a sustainable manner.

1,3-Propanediol
1,3-Propanediol (PDO) is an industrially important chemical that is used in cosmetics, resins, solvents, polymers, etc. Its biological production is usually carried out using glycerol as feed. In recent years, the flourishing biodiesel industry resulted in unmanageable quantities of glycerol rich effluents [99]. In addition, glycerol has high electron content (4.7 moles e − /mol of carbon). There are plenty of microbes possessing the ability to bioconvert glycerol into PDO. The recent advancements in cathodic EFs have led to enhanced PDO yield with the following advantages: (i) direct electron dissipation into PDO synthesis; (ii) selection of bacterial population adapted to reduced condition; and (iii) enhancing PDO generation as a sole NADH dissipating pathway [79]. Few studies have been performed on mixed culture glycerol EF using the strong negative potential of −1.14 V to achieve PDO production of 0.50 mol/mol glycerol, which is 2-fold higher than batch fermentation [80,81]. In another study where the cathodic current was supplied, the PDO production was found to be positively influenced by the electron input leading to a yield of 0.51 mol/mol glycerol. In a fed-batch system equipped with −1.34 V at the cathode, the yield could reach 0.46 mol/mol glycerol. Thus, the EF strategy clearly out beats the conventional fermentation by applying minimal external current supply. However, the small amount of H 2 produced due to water at cathode makes it difficult to assess the predominant effect of EF. In addition, while working with Geobacter sulfurreducens, it was observed that the fermentation pattern, as well as the microbial population could be influenced by EFs. A similar observation was made with C. pasteurianum that showed 21% shift in NADH-consumption to produce PDO using glycerol as substrate [79]. Such study increases the possibilities of efficient production of electron dense compounds using small quantities of electricity.

Alcohols
The global demand for alcohols especially ethanol as a potent energy alternative continues to surge [100]. Alcohols have traditionally been produced through various fermentation strategies that usually require 48-90 h to achieve maximum concentration. With the advancements in strain improvement programs, high gravity ethanol fermentation (ethanol tolerance of 10-12%) can be achieved with an initial sugar feed of >180 g/L. Recently, MFC was modified to BES, which can use the reduction mechanism at the cathode to be used for electrogenesis as well as producing valuable end products such as acetate, aldehydes, ethanol, etc. [6]. The EF strategy had been adopted to govern the glucose consumption in Saccharomyces cerevisiae. This process involved an external supply Energies 2018, 11, 343 12 of 20 of −1.5 V to the reactor containing S. cerevisiae and 120 g/L of glucose. The strain grew faster reportedly due to the electrons supplied through the cathode, which leads to achieving 53 g/L ethanol yield after 50 h of incubation [101]. Similarly, in another experiment electrochemical-electrostatic technique in a transistor-like device was used for short-term ethanol production by S. cerevisiae [102]. On the other hand, Zymomonas mobilis is a well-known organism for industrial ethanol production that can ferment sugars through the Enter-Doudoroff pathway and tolerate higher ethanol concentrations. Under electrochemical fermentation equipped with the cathode (DC −3 V) and air anode, Z. mobilis showed higher biomass, as well as 50% increased ethanol production [103]. In contrast, electrostatic fermentation could enhance the ethanol yield up to 2-fold with a concomitant decline (only 24 h of fermentation) in the operation period. In such process, complete fermentation of glucose was observed by S. cerevisiae leading to 14% v/v ethanol in 20 h of incubation without the requirement of external electricity [100]. Besides yeasts, engineered Shewanella oneidensis (a known exoelectrogen) was used for oxidization of glycerol into ethanol through deposition of reducing equivalents at the anode. Similarly, glucose was successfully converted to more reduced products like ethanol and butanol by Clostridium acetobutylicum ( Table 2). Another bacterium, C. thermocellum, was shown to display a considerable shift in the concentration of metabolites when an external potential of −1.5 V was provided. This leads to 61% enhancement in the overall ethanol yield, increasing the potential negatively influences the process [101]. Similarly, C. pasteurianum under electrochemical system displayed a shift in the metabolic pathway for enhanced production of butanol [79]. Besides glucose, other substrates such as acetate, glycerol, CO 2 , butyrate, etc. have also been used for alcohol production [66,79]. A wild-type strain of C. pasteurianum was able to accept electrons from cathode during heterotrophic growth on glucose as well as glycerol leading to NADH-consuming compounds such as butanol and PDO [79]. Another species, Clostridium ljungdahlii was also found to convert CO 2 into acetate while ethanol, methane, and hydrogen were also produced in minor quantities [82]. Reduction of acetate to ethanol can also be achieved using methyl viologen (MV) mediator in the cathode chamber. The role of mediator was to inhibit side reactions (i.e., methanogenesis) and thus enhance the ethanol production. A similar set-up can be utilized for butanol production using butyrate as a substrate and MV as a mediator [93]. An engineered strain of Ralstonia eutropha was found to reduce CO 2 using H 2 obtained from water splitting (from the bioelectrochemical system) to produce up to 0.216 g/L of isopropanol [104]. CO 2 fixation at the cathode mimics the natural photosynthesis and holds a great potential in future [79]: nCO 2 + (6 n + 2) (e −1 + H + ) → C n H 2n+2 + 2nH 2 O.
If a good redox link is established among electrode and the bacteria, the EFs become advantageous over conventional fermentation by (i) enhanced process control; (ii) lower biomass yields; and (iii) efficient electron distribution. However, for enhanced ethanol production in an EF non-mediated reduction at cathode must be developed, possibly by growing microbes at the cathode [93]. The competition among traditional and advanced biofuels will affect their large-scale production since they depend largely upon the limited land resources, crops (food/non-food), and lignocellulosic wastes. An EF strategy can contribute both to a more efficient energy production and livestock feed (indirectly to food supply) with the use of the byproducts of bioethanol production [18]. An integrative process was also developed combining bioethanol production along with MEC to produce hydrogen with concurrent degradation of phenolic compounds and its derivatives (furfurals) from the by-products of bioethanol (Figure 3). It was estimated that one ton of wheat straw can generate up to 190 kg of ethanol, 23 kg of hydrogen, and 140 kg of lignin [105].
The yield would depend on the cellulose and hemicellulose content of the wastes being used. It was proposed that such a process may lead to around 83% energy recovery with a gross value of 200 Euro/ton of wheat straw (including lignin).

Polyhydroxyalkanoates
PHAs are the only polyesters of biological origin that has promising features to replace fossil fuel-based plastics. In nature, PHAs are produced by bacteria in the form of lipid granules under nutrition imbalance conditions and acts as a reserve carbon source [106][107][108]. Thermomechanical properties of this polymer are quite comparable to synthetic plastics with an added advantage that it is biodegradable and biocompatible in nature. However, large-scale PHA production is still uneconomical and requires more research effort towards reducing the production cost by employing cheap carbon sources as feed and using green solvents for extraction. It had been emphasized to produce value added side-products during the PHA production such that the overall production cost gets minimized. Some of the reported examples include PHA-H2, PHA-carotenoids, PHA-rhamnolipids, etc. [108][109][110]. With the growing advancements in BES, it was realized that biocathode can be used for enhanced electrogenesis with concomitant production of PHA at the cathode [83]. Here, using synthetic water supplemented with glucose, the electrogenic activity, and PHA accumulation was recorded to be 512 mV and 19% w/w, respectively. Enriched mixed cultures from an anaerobic digester and the activated sludge unit were used at anode and cathode, respectively. Under microaerophilic environment, PHA accumulating organisms may act as a terminal electron acceptor at the cathode. Such strategies have multiple advantages of producing electricity, PHA, and wastewater treatment in a single fermentative reaction [83]. However, much research is required to improve the PHA yield in such processes, since at low PHA yield the extraction cost may increase.

Integrative Approach of Utilizing EF in a Biorefinery Prospective
Lignocellulosic biomass has emerged as the most promising substrates for bioenergy generation, provided efficient pretreatments methods are available. Such wastes, e.g., corn stover and straw usually consist of 30-40% cellulose, 20-30% hemicellulose, and 15-20% lignin [105]. Various individual sugars present in the lignocellulosic materials (glucose, cellobiose, xylose, arabinose, etc.) have been used for fermentative H2, CH4, PHA, ethanol, biodiesel, etc. through different strategies [105,109]. When MEC was linked with wheat straw hydrolysate after ethanol production, it was found to remove toxic compounds (furfurals) and successfully converted the residual sugars into hydrogen (13-24 g/kg straw). During this process, 96% of hemicellulose, 100% VFAs, and 50% of the phenolic compounds were utilized. This may allow water recirculation in biorefinery and alleviate the use of fresh water [105]. Co-production of various biofuels and organic

Polyhydroxyalkanoates
PHAs are the only polyesters of biological origin that has promising features to replace fossil fuel-based plastics. In nature, PHAs are produced by bacteria in the form of lipid granules under nutrition imbalance conditions and acts as a reserve carbon source [106][107][108]. Thermomechanical properties of this polymer are quite comparable to synthetic plastics with an added advantage that it is biodegradable and biocompatible in nature. However, large-scale PHA production is still uneconomical and requires more research effort towards reducing the production cost by employing cheap carbon sources as feed and using green solvents for extraction. It had been emphasized to produce value added side-products during the PHA production such that the overall production cost gets minimized. Some of the reported examples include PHA-H 2 , PHA-carotenoids, PHA-rhamnolipids, etc. [108][109][110]. With the growing advancements in BES, it was realized that biocathode can be used for enhanced electrogenesis with concomitant production of PHA at the cathode [83]. Here, using synthetic water supplemented with glucose, the electrogenic activity, and PHA accumulation was recorded to be 512 mV and 19% w/w, respectively. Enriched mixed cultures from an anaerobic digester and the activated sludge unit were used at anode and cathode, respectively. Under microaerophilic environment, PHA accumulating organisms may act as a terminal electron acceptor at the cathode. Such strategies have multiple advantages of producing electricity, PHA, and wastewater treatment in a single fermentative reaction [83]. However, much research is required to improve the PHA yield in such processes, since at low PHA yield the extraction cost may increase.

Integrative Approach of Utilizing EF in a Biorefinery Prospective
Lignocellulosic biomass has emerged as the most promising substrates for bioenergy generation, provided efficient pretreatments methods are available. Such wastes, e.g., corn stover and straw usually consist of 30-40% cellulose, 20-30% hemicellulose, and 15-20% lignin [105]. Various individual sugars present in the lignocellulosic materials (glucose, cellobiose, xylose, arabinose, etc.) have been used for fermentative H 2 , CH 4 , PHA, ethanol, biodiesel, etc. through different strategies [105,109]. When MEC was linked with wheat straw hydrolysate after ethanol production, it was found to remove toxic compounds (furfurals) and successfully converted the residual sugars into hydrogen (13-24 g/kg straw). During this process, 96% of hemicellulose, 100% VFAs, and 50% of the phenolic compounds were utilized. This may allow water recirculation in biorefinery and alleviate the use of fresh water [105]. Co-production of various biofuels and organic compounds using wastes and CO 2 as direct substrates indicates the promising future with MFC based technologies [58,[76][77][78]84]. Similarly, an electrodialysis system was used to extract the organic acids produced during dark fermentation of glucose for H 2 production by Escherichia coli. The extracted organics were later used as a potential nitrogen free carbon source for photo-fermentative H 2 production by Rhodobacter sphaeroides [111]. Later, this process was successfully demonstrated on food waste hydrolysates [112]. Such extractive fermentation is anticipated to sustain indefinitely; however, membrane fouling, the requirement of constant ionic composition, pH imbalance, etc. are the major hurdles. MECs were also integrated with reverse electrodialysis called MRECs, which showed natural H 2 generation due to the merging of the impulse from organic oxidation at the anode and energy of salinity gradient; in addition, the salt solutions can be constantly regenerated with waste heat (~38-40 • C) [59,63]. In a recent effort, solid state bio-electrofermentation system (SBES) was developed for the integrated production of electricity, H 2 , and ethanol on the basis of self-induced potential [6]. Here, food waste substrates were directly converted to multiple products with the help of anaerobic enriched cultures of acidogenic bacteria as biocatalyst in the SBES [6]. It has been emphasized that, for successful waste management, an intellectually designed BES could successfully convert actual organic wastes through integrated solid-state fermentation making the whole process sustainable.

Limitations and Challenges for Electro-Fermentations
The progress in EFs has advanced in recent years, both in terms of higher power output and increased efficiency [65,[67][68][69]. However, a consensus is yet to be developed on the mechanism of electron transfer across the membrane and microbial surface. In an electrochemical cell, electrolytes are a major component that provides a path for proton to cross from anode to cathode chamber. Most commonly used electrolytes include Nafion, etc. In addition, the use of expensive separating membrane hinders the large-scale applications. Other major issues are low power densities, pH variations, ohmic voltage loss (due to ionic and electronic resistance), low transport across membranes, overpotential (due to mass transport resistance), oxidant and proton flux towards the bio-cathode, bacterial growth on membranes, turnover per cells, and lastly a uniform standard data reporting system (for worldwide acceptance and fast work on technological realization). Some possible alternatives are being explored to overcome this limitation. For example, Nafion can be replaced by soil/clay based electrolytes. In addition, mediators used during BES should be regenerated after the process, i.e., it must not be consumed in the reaction [66]. Call and Logan have developed a single chamber EF system, thereby leaving the necessity to use costly separating membranes [72]. The advantage of microbial electrosynthesis is the possible on-site use of current for bioproduction and its independence from agricultural lands. However, all of these practices are being explored only in lab scale and are yet to be tested in large scale. The actual large-scale implementation would require EFs to be used in stacks, but such configuration is highly vulnerable to cell reversal [93]. In an EF system, the power generation (density) decreases with increase in anode surface area. Thus, maintaining low internal resistance with an increase in the electroactive biomass, separation of the anode from the cathode, and the reactor design are the three major challenges in scaling-up the EF based process [15,58,93]. Overall, all these limitations are believed to increase the reactor design costs for long-term operation. Biocatalysis at both electrodes and its integration with other biorefinery models are some of the potential solutions. It has been estimated that to become commercially competitive and viable over anaerobic digestion, performance of the EF based system must exceed 500 W/m 3 . Recent advancements and the research efforts being incurred in bioelectrofermentation holds the potential success of this technology in near future.

Future Perspectives
With the recent developments in EF technology, harvesting electricity using the waste of various origins seems an attractive option. The advances in microbial electrosynthesis have provided a myriad option to produce and/or extract useful chemicals from pure substrates as well as biowastes. Nevertheless, few critical issues such as pH, high ohmic resistance, and overpotential are yet to be studied carefully for its successful implementation. Exploration of unique materials and microbes is of utmost importance for superior performance and attractive operating cost. In addition, integrating these processes with other biodegradation methods may lead to its successful utilization in developing waste biorefinery models for sustainable bioeconomy.