**1. Introduction**

The development of humanity has led to an exponential increase in the consumption of natural resources and the generation of waste derived from anthropogenic activities. One of the most consumed resources is water. It is estimated that in the world the consumption for domestic, agricultural, and industrial activities is greater than 4000 km<sup>3</sup> of water per year [1]. As a consequence, a large amount of wastewater with a high environmental impact has been generated. According to UNESCO: "in low-income nations only 8% of domestic and industrial wastewater is treated" [2]. Wastewater treatment systems are classified into: (I) *mechanical or primary processes:* settling, screening, sedimentation; (II) *biological or secondary processes*: aerobic and anaerobic microbial metabolism of organic compounds; and (III) *physicochemical or tertiary processes*: advanced oxidations, filtrations with ionic charges, chlorination, ozonation, among others [3]. One of the most widely applied strategies for the purification of organic matter in wastewater is carried out through Wastewater Treatment Plants (WWTP), which combine primary and secondary methods [4]. The implementation of WWTPs is costly and low-income countries, whose productive activities correspond mainly to the exploitation of raw materials, and do not incorporate WWTPs into their agricultural or industrial production processes. These treatments are carried out on domestic and industrial wastewater, mainly in urban areas, and according to recent data, only an average of 20% of the wastewater produced are treated in the world before being returned to the ecosystems [5]. In recent years, research has been carried out for the non-conventional biological treatment of wastewater using bioelectrochemical systems (BES) [6–9].

The BES can be wastewater treatment systems adapted and designed based on a microbial metabolism that, through reactions of oxidation and reduction of organic matter, carry out the purification of contaminated water, producing at the same time electrical energy, biofuels, or other chemical compounds of interest [6, 8, 10]. There is a general classification of BES based on their application; (a) *microbial fuel cells* (*MFCs*): developed mainly for electricity production, but have recently been used for wastewater treatment and as biosensors for the detection of toxic chemicals; (b) *microbial electrolysis cells* (*MEC*) developed for the electrochemical production of biofuels such as hydrogen or methane, and finally, (c) *microbial electrochemical technologies* (*MET*): used to desalinate water or produce chemical compounds such as hydrogen peroxide [11]. The configuration of the BES and the origin and differentiation of microbial communities to be used in these systems depend on the composition of the substrate and the compounds of interest that can be generated from a specific cellular metabolism. The different types of BES are designed and implemented according to the objective of wastewater treatment, and some are coupled to conventional secondary treatments, such is the case of wetlands connected to MFC, in which symbiotic microorganism-plant relationships are used as a strategy of optimization in the efficiency of reduction of organic matter content [12, 13]. BES have also been implemented for the treatment of residual water or a fixed effluent, with cascade feeding or simultaneous feeding under continuous operation, in which different microbial communities have been used in each stage or system to increase the production rate energy and decontamination levels [14, 15]. Moreover, various conventional wastewater treatment technologies have been effectively combined with BES. For instance, MFCs have been coupled with stabilization ponds, while BES have been integrated into aerobic or anaerobic treatment systems specifically designed for nutrient (nitrites/nitrates, sulfates) or heavy metal (Fe, Cu, Cd, etc.) removal purposes [7, 16–23].

For the design and adaptation of BES technologies in the unconventional treatment of wastewater, it is necessary to know individually the metabolic responses and the enzymatic machinery of the microorganisms present in the system. Currently,

*Microorganisms and Microbial Communities in Bioelectrochemical Systems for Wastewater… DOI: http://dx.doi.org/10.5772/intechopen.112470*

microorganisms of different genera with the ability to degrade organic matter and with potential for application in bioelectrochemical systems have been described. Mainly bacteria and archaea have been studied, although there are reports of electrogenic activity in eukaryotes, such as yeasts [10, 24, 25]. On the other hand, in a significant amount of research, detailed descriptions of the mechanisms of mass and energy transport in the cell have been made; and in recent years, some studies have been registered that seek to adapt diverse microbial communities to BES [12, 23, 25–30]. However, in the case of microbial communities, due to the complexity of the interactions, it is difficult to establish the metabolic processes and mass and energy transport mechanisms involved in bioelectrochemical processes. This review presents a consolidation of the information on the main microorganisms used in BES, the metabolic processes they carry out for the degradation of organic matter and the use of substrates for the cogeneration of energy and the production of other metabolites of interest, and the possible interactions between the microorganisms of the microbial communities in these bioelectrochemical systems.

#### **2. Microorganisms in bioelectrochemical systems**

Microorganisms in bioelectrochemical systems are responsible for the transformations of the substances or compounds present in the organic matter or substrate, since through their metabolic processes they carry out oxidation and reduction reactions, which in turn constitute the mechanisms of energy generation and high value compounds. Its implementation for wastewater treatment and cogeneration of energy or compounds of interest depend on the interaction between the physicochemical characteristics of the substrates and the metabolic capacity associated with microbial diversity [7, 14, 19, 23, 31–36]. In the case of MFCs, the main metabolism described corresponds to the redox (electrogenic) degradation of organic matter. In MEC systems, microorganisms with a metabolism of methanogenesis and hydrogenogenesis related to the production of biofuels have been recorded [28, 37–39].

The biotechnological capacity of cells to produce energy called "electrogenic potential" was first described in 1911 by Potter, who evaluated the electrogenic potential of *Escherichia coli* and *Saccharomyces cerevisiae* [40]. Advances in molecular biology and microscopic characterization techniques have allowed the identification of multiple microorganisms and their metabolism, as well as their ability to generate energy. Some main microbial groups have been identified, with species known as model electrogenic microorganisms, which present adaptive characteristics of great interest in BES, such as the presence of some cellular structures (pili and nanotubes) and the excretion of mediating substances that act as direct and indirect electron transport mechanisms in the different bioelectrochemical systems [41–44]. The predominant microorganisms found in these systems belong to the Bacteria domain, mainly the phyla Proteobacteria, Chloroflexi and Firmicutes [13, 26, 33, 45–47]; and the genera *Geobacter* sp*., Shewanella* sp., and *Pseudomona*s sp.

#### **3. Model electrogenic microorganisms**

The Electrogenic model microorganisms have a remarkable electron transfer capacity, exhibiting physical and metabolic characteristics that allow the production of usable energy from the consumption of organic matter [10, 11, 48]. The bacterial

genus *Geobacter* sp. it has been reported as the one with the highest electricity production rates in bioelectrochemical systems [32, 43, 49, 50]. Several studies have reported this electrogenic capacity; J Chen and his collaborators established that in a double-chamber MFC fed with synthetic wastewater, in which power density values of 71.6 mW/m<sup>2</sup> and 65.4 mW/m<sup>2</sup> were achieved at the anode and the cathode, respectively; the predominant genus in the microbial community developed from activated sludge from a WWTP was *Geobacter* sp. [51]. Similarly, Shen et al. reported that in an MFC designed for the co-degradation of phenolic compounds, a production of 267.2 mW/m<sup>2</sup> was reached with the predominance of *Geobacter* sp. within the identified bacterial genera [52]. Also, Paitier et al., in a bottle-type open cathode MFC, characterized the microbial community and identified the predominance of *Geobacter* in the formation of the electrogenic biofilm on the anode during one month of operation, reaching an energy performance of approximately 50 mW/m2 . The authors reported a predominant exponential growth of *Geobacter* during the first 5 days of biofilm formation [50]. In the report by McAnulty et al., an MFC that converts methane directly into an electric current, the capacity of *Geobacter sulfurreducens* to produce electrons from acetate was evaluated. A maximum power density of 160 mW/m<sup>2</sup> was obtained [53]. On the other hand, considering different types of ESP, in a MEC dedicated to methane production, Zhao et al. evaluated the performance and interaction between *Geobacter* sp. and *Methanosaeta* sp. in the reduction of CO2 to CH4, reaching 3017.6 mL of CH4 in 51 days of operation; the authors suggest from their results that *Geobacter* develops a key metabolism as a precursor to methanogenesis in bioelectrochemical systems [41]. However, although *Geobacter* sp. is a microorganism with a great electrogenic potential, several studies indicate that it is not the only relevant microorganism in BES [11, 19, 42, 45, 52, 54–57]. For example, for the development of electrochemical technologies with unconventional materials, such as ceramic biocathodes as those made of terracotta, the predominance of fermentative bacteria has been identified, which exhibits metabolic capacities different from *Geobacter*, proving that there is an influence on the interaction between the microorganisms and materials used in BES. It has been suggested that *Geobacter* interacts more efficiently with conductive materials such as carbon felt [58].

For the bacterial genus *Pseudomonas sp*. that it is present in almost all ecosystems, the electrogenic potential associated with its metabolism has also been identified; and it has been established that it plays an important role in electron transport in BES [33, 42, 52, 59]. *P. aeruginosa* stands out as the model microorganism. However, other species within this same genus with high electrogenic capacity have also been reported. In the study by Ilamathi et al. of a single-chamber MFC, the power densities determined for a *P. aeruginosa* strain were 601 μW/m<sup>2</sup> and 323 μW/m<sup>2</sup> for a *P. fluorescens* strain [60]. Arkatkar et al. obtained a current of 0.036 mA in a doublechamber MFC inoculated with *P. aeruginosa* [42]. In turn, non-conventional MFC designs have been tested using this microorganism. For example, Zhang et al. reported a power density of 3322 mW/m<sup>2</sup> for an MFC with a cylindrical reactor design [61]. Similarly, in the comparative study by Bagchi and Behera, this microorganism was used to evaluate the influence of the manufacturing material on an MFC, reaching power density values of 0.96 W/m<sup>3</sup> for an MFC made of plastic and 0.69 W/m3 for the MFC built with ceramic materials [52]. Also, alternative strategies for the operation of bioelectrochemical systems using *P. aeruginosa* have been evaluated. For example, Qiao et al. evaluated the effect of the implementation of inhibitors such as sodium hyaluronate, on the formation of biofilm on the anode in an MFC inoculated with *P. aeruginosa*, indicating a current density of 4.8 μA/cm<sup>2</sup> [62]; Yong et al. evaluated the

#### *Microorganisms and Microbial Communities in Bioelectrochemical Systems for Wastewater… DOI: http://dx.doi.org/10.5772/intechopen.112470*

mixed aerobic-anaerobic operation of an MFC inoculated with *P. aeruginosa*, obtaining a current density of 99.8 mA/cm<sup>2</sup> [16].

Similarly, *Shewanella sp*., recognized as a facultative bacterium, has been reported as an exoelectrogenic microorganism [32, 57, 60, 63]. The presence of these microorganisms in nutrient-limited cultures has been described, a characteristic that may favor their implementation in BES [26]. This bacterial genus has been widely identified in studies with bioelectrochemical systems in which various microbial communities have been used as inoculum. In a comparative study carried out by Hassan et al., in a double-chamber MFC, they compared microbial communities in petrochemical and domestic wastewater, reaching current densities of 156 mA/m<sup>2</sup> and 123 mA/m2 , respectively. The authors reported the active presence of *Shewanella* sp. in the cells in which wastewater of domestic origin was used [64]. In particular, the *Shewanella oneidensis* MR-1 strain has been identified as an electrogenic reference, as reported by Wang et al., in their study of a non-conventional laminar flow MFC fed with synthetic wastewater and glycerol, in which a potential for 16.05 mW [65]. In another work by Wang et al., the mutualistic interaction between *Shewanella oneidensis* and *Escherichia coli* in a double-chamber MFC fed with a synthetic substrate, a current density of 2.0 μA/cm<sup>2</sup> was achieved [44]. Likewise, Hirose et al. reported a power density of approximately 250 mW/m<sup>2</sup> in a single-chamber electrolysis cell fed with a synthetic substrate, electrogenic activity that was attributed to the metabolic response of *S. oneidensis* MR-1. As stated by the authors, this microorganism possesses molecular mechanism that enables it to detect electrode potentials and effectively regulate its catabolic pathways, so this capability allows the microorganism to adapt and optimize its metabolic activities in response to the electrical conditions presented by the electrodes [66]. This molecular mechanism provides valuable insights into the intricate processes underlying the bioelectrochemical interactions and further enhances our understanding of the microbial behavior within the context of electrochemical systems.

On the other hand, another variety of electrogenic bacteria has been described in BES, and some genera are distributed depending on the origin of the wastewater or even depending on the differentiation of the microbial communities through the operation time of the system, within the frequently described bacterial genera, *Clostridium* sp. [4, 28, 49, 67, 68]. As an illustration, the *Clostridium beijerinckii* strain was subjected to evaluation in both a H2 fermentation reactor and an open cathode MFC, with the feedstock being port drainage sediment, in this study yielded impressive results, including the production of 104 mmol/L of H2, 5 mmol/L of acetate, 33 mmol/L of butyrate, 3 mmol/L of lactate, and 1 mmol/L of ethanol; additionally, the power density achieved was recorded at 1.2 W/m2 [28]. These findings demonstrate the potential of this strain and highlight the feasibility of harnessing its capabilities for efficient and sustainable bioenergy production. Another bacterial genus studied has been *Desulfovibrio* sp.; that in a double-chamber MFC it reached an electrical production of 185 mW/m<sup>2</sup> and in a MEC, where it was the predominant bacterial genus, a hydrogen production of 0.28 m<sup>3</sup> -H2/m<sup>3</sup> -d was reached [26]. Also, reports have been made of the electrogenic activity of *Klebsiella* sp. in a doublechamber MFC, and from finding in a symbiotic relationship of *Klebsiella variicola* and *P. aeruginosa*, a power density of 14.78 W/m3 was reached [69]. Additionally, the presence of *Acinetobacter* sp., *Bacillus* sp.,*Thiobacillus* sp., *Desulfomonas* sp., among other genera, has been reported. Some of the archaea studied according to their electrogenic potential are *P. furiosus* with an electrical production, in terms of power density, of 225 mW/m2 . The current density produced by *Ferroglobus placidus* and

*Geoglobus ahangari* has been reported to be 680 mA/m2 and 570 mA/m2 , respectively. The study of some archaea in BES has also been carried out in mixed communities including bacteria, where volume power density values between 2138 mW/m<sup>3</sup> and 6924 mW/m3 have been recorded [4, 11, 53, 62]. Recently, it has been reported that some genera of archaea, such as *Methanosaeta* and *Methanosarcina*, have symbiotic interactions with other electrogenic microorganisms, such as *Geobacter* sp. [4] which generates a particular interest in its presence and functionality in the microbial interactions of ecosystems in BES.

The microbial community present in bioelectrochemical systems is not exclusively constituted by electrogenic microorganisms, they coexist with a great variety of microorganisms that through their metabolic processes and enzymatic machinery carry out the degradation of complex substrates such as polymers, carbohydrates, proteins, hydrocarbons, fats and oils, among others; contributing to obtaining of precursor substances for electrogenic metabolism, which significantly influence energy productivity in BES [15, 22, 49, 64, 70–75]. The presence of different microorganisms in bioelectrochemical systems indicates the important ecological role they play in nature, as well as suggesting their participation in bioanode formation and electron transport in bioelectrochemical systems [47, 68, 76, 77]. However, a considerable amount of BES studies focusses primarily on the design and configuration of devices developed as MFCs or MECs, and research on microorganisms and their interactions is relatively minor. The study of microbial communities can be considered more complex due to the interactions that occur between microorganisms when they are active in BES. In these systems, ecological successions or interdependent mixed metabolisms could occur between electrogenic microorganisms and those that do not present electrogenic activity. Knowledge of the symbiotic relationships of microbial communities in BES would allow the adaptation of this technology for the treatment of a wide variety of substrates and environments, including wastewater from multiple industries. **Table 1** presents a list of electrogenic microorganisms reported by various authors, which were evaluated in the different BES using multiple substrates.


*Microorganisms and Microbial Communities in Bioelectrochemical Systems for Wastewater… DOI: http://dx.doi.org/10.5772/intechopen.112470*



*Microorganisms and Microbial Communities in Bioelectrochemical Systems for Wastewater… DOI: http://dx.doi.org/10.5772/intechopen.112470*


**Table 1.**

*Summary of electrogenic microorganisms reported.*

## **4. Electron transport mechanisms in electrogenic microorganisms**

The energy production in the BES is directly related to the chemical transformations of the organic matter present in the substrates fed in these systems [19, 75, 88, 89]. In the chemical reactions of oxidation and reduction, the mechanisms of electron transport (ET) are essential to ensure that the substances that are oxidized give up or transfer electrons to the substances that are reduced, that is, substances that gain electrons and process that is used in BES to achieve power generation [10, 35, 41, 46]. **Figure 1** shows a representation of the flow of electrons and protons resulting from cellular metabolism present in a bioelectrochemical system. Direct and indirect ET mechanisms have been identified and used for the general characterization and classification of electrogenic microorganisms [6, 11, 18]. Direct electron transport (DET) mechanisms refer to the physical contact between electrogenic microorganisms and the electrode, and direct contact between the outer membrane of the bacteria and the anode surface [24]. The indirect electron transport (IET) mechanism, also known as mediated electron transport (MET), occurs due to the presence of mediating substances that facilitate the flow of electrons toward the surface of the anode electrode [33, 35, 57]. These substances are produced by some microorganisms in their cellular metabolism, as occurs in the oxidation of compounds and the generation of fermentative products, such as hydrogen [22, 90, 91]; or they can be added to the systems to

#### **Figure 1.**

*Representation of the flow of electrons and protons resulting from cellular metabolism present in a BES (MFC double chamber).*

facilitate the electron transport process [25]. The development of direct or indirect mechanisms for electron transport in BES depends on the enzymatic machinery of the electrogenic microorganisms used and on the experimental conditions such as the composition and concentration of substances of the substrate used in the operation of the bioelectrochemical system [42]. The indirect electron transport mechanism presents a lower efficiency in the production of electricity in the BES [6].

In electrogenic microorganisms such as *Geobacter* sp. and *Shewanella* sp., mainly DET mechanisms have been identified, associated with the formation of membrane-extending structures known as pili [26, 42]. The pili are short membrane extensions that establish direct contact with the outside of the cell or other microorganisms for the exchange, mainly, of ions, nutrients, and genetic material [10, 41, 52, 72, 75, 92]. **Figure 2** shows a schematic representation of the direct mechanism of electron transfer through a pili of *Geobacter* sp. The formation of these membrane extensions has been described as a mechanism of ecological adaptation of cells to the environment. Different studies have established that the genetic machinery for the appearance of pili is based on the expression of cytochrome C (OmcS) [85, 86]. Similar to *Geobacter* sp., the bacterial genera *Shewanella* sp. and *Rhodoferax* sp. possess protein complexes of type C cytochromes [8, 93–95].

On the other hand, for *Shewanella* sp. the formation of nanotubes that allow the direct transport of electrons between the cell and its environment has been reported [96, 97]; and also, indirect transport mechanisms have been described with the presence of the group of quinones and quinol in the cytoplasmic membrane, which transfer electrons to extracellular acceptors such as minerals containing Fe (III), mediating substances that finally carry out the transport of electrons toward the electrodes of the BES [60]. **Figure 3** shows a schematic representation of both electron transport mechanisms for *Shewanella* sp.

Bacteria such as *Pseudomonas* sp. are included in microorganisms with indirect electron exchange mechanisms, as they excrete biomolecules to facilitate the transfer or transport of electrons [32, 33, 92]. For the bacteria *Pseudomonas* sp., and *Lactococcus*

**Figure 2.** *Schematic representation of DET mechanism of* Geobacter *sp. (pili and cytC).*

*Microorganisms and Microbial Communities in Bioelectrochemical Systems for Wastewater… DOI: http://dx.doi.org/10.5772/intechopen.112470*

sp. the presence of phenazine-type pigments (cyclic hydrocarbons with the presence of two N groups), riboflavins and quinones, produced by secondary metabolic pathways, has been identified. Some examples of these pigments are pyocyanin and 2 amino-3-carboxy-1,4-naphthoquinone (ACNQ), mediating substances that carry out the electron transport process in BES systems [35, 52]. The metabolic performance of *P. aeruginosa* depends, to a large extent, on the presence of phenazines, since these substances have been reported to be involved in virulence, signaling, iron metabolism, and electron transport [35]. The phenazine of greatest interest in the study of bioelectrochemical systems is pyocyanin (PYO) due to its participation in the transfer of electrons at high catalytic reaction rates and low overpotential [61, 96, 98]. **Figure 4** shows a schematic representation of the indirect electron transfer mechanism for *Pseudomonas* sp.

### **5. Microbial metabolism in bioelectrochemical systems**

Some main metabolic routes that electrogenic microorganisms carry out to achieve the degradation of compounds present in the substrates in BES have been identified [66, 92, 99]. In general terms, it can be considered that the oxidation of organic matter is carried out by microorganisms for cell growth and maintenance. In this process, ions associated with the energy transfer mechanisms are generated, as described in **Figure 5**, and correspond respectively to the anode and cathode reactions that take place in the BES [48, 89]. The metabolism and electrogenic activity of microorganisms on controlled substrates, especially synthetic substrates with acetate (CH3COOd) or simple sugars as carbon source, has been extensively investigated [6, 9, 18, 20, 49, 85]. Different authors have evaluated the electrogenic capacity of the so-called model microorganisms from these substrates to establish efficiencies in electron transfer processes and to identify the reaction stoichiometry present in the metabolism of microbial consortia [4, 64, 84, 100, 101]. However, the diversity in the origin of the wastewater or substrates used in the BES generate significant variations in the electrogenic potential of the system due to the composition of the substances present in these substrates; for example, it has been established that in domestic wastewater predominates oxidizable organic matter [9, 55, 80, 102]. In industrial wastewater, the composition depends on the productive activity from which it comes; and components such as fats, dyes, petroleum derivatives and a large number of recalcitrant contaminants have been identified [14, 17, 30, 56, 73, 75, 89, 103]. In substrates of agricultural origin, a variety of complex carbon compounds and nitrogen compounds derived from pesticides and fertilizers have been found [47, 89].

$$2\text{CH}\_3\text{COO}^- + 2\text{O}\_2 \rightarrow 2\text{HCO}\_3^- + H^+ \left(\Delta \text{G} = -847, 6\text{ kJ/mol}\right) \left(E^\circ = 0, 805\text{ V vs SHE}\right) \tag{1}$$

The usable energy in bioelectrochemical systems depends on organic compounds that can be biochemically transformed through redox reactions, giving rise to different substances and metabolic intermediates. Eq. (1) presents a simplified reaction of the transformation of a simple substrate such as acetate, which is achieved through the electrogenic metabolism of microbial communities in a BES [47, 93]. However, in a complex substrate such as domestic or industrial wastewater, in which the presence of multiple compounds has been identified, there are many more intermediate reactions, as a product of the metabolic activity of the microorganisms involved in the oxidation

**Figure 5.** *Schematic representation of IET through phenazines (PYO) in* Pseudomonas *sp.*

*Microorganisms and Microbial Communities in Bioelectrochemical Systems for Wastewater… DOI: http://dx.doi.org/10.5772/intechopen.112470*

and reduction processes. For example, for MEC-type systems used for the production of methane or hydrogen, a series of complementary reactions known as methanogenesis and hydrogenogenesis occur [28, 37, 39, 41, 53, 70]. The reactions present some variations, since what is intended in these BES is the production of biofuels, so the oxidation of organic matter is carried out until obtaining methane gas (CH4) or until obtaining biohydrogen (H2) [7, 11]. In this type of system, there is generally a symbiotic relationship of anaerobic microorganisms, especially bacteria and archaea, which degrade organic matter and produce biofuels with an interdependent metabolism; that is, the microorganisms in the present community mutually benefit from the metabolic intermediates and by-products produced by the successive substrate transformation reactions [11, 25, 53, 84]. **Figure 6** presents the stoichiometry of redox and methanogenesis of organic compounds in a BES using CO2 as carbon source [47].

Nutrients derived from nitrogen and sulfur from wastewater represent a particular interest for bioremediation processes due to their high impact on ecosystems. These compounds have been described as the main cause of the effects of eutrophication and pollution of natural water sources [19, 22, 71, 104]. The use of BES for the purification of nitrites, nitrates, and sulfates in wastewater has been shown to be a promising strategy to reduce the degree of pollution and co-generation of energy. An example is the study carried out by Mahmoudi and collaborators in which the performance of a bioelectrochemical reactor with two chambers (anaerobic and aerobic) was evaluated, in which a current generation of 0.841 mA and an ammonium and COD removal of 54.6% and 87.2% were observed. The authors concluded that the COD/N ratio is one of the parameters with the greatest influence on the nitrification process to control the growth of autotrophic and heterotrophic bacteria [7]. In some cases, the wastewater may contain other nutrients derived from sulfur, so a series of stoichiometric adaptations of electrogenic metabolism for sulfate reduction have been proposed [89]. Simplified metabolic reactions for nitrates and sulfates in BES are presented in **Figure 7**.

In nature, there are ecological successions and symbiotic relationships between microorganisms in which different metabolisms are expressed for the assimilation of complex nutrients and pollutants [23, 46, 84]. The expression of specific metabolic pathways could represent a strategy for the optimization of bioelectrochemical systems, since the electrogenic metabolism of a diverse microbial community would

**Figure 6.** *Metabolic reactions of methanogenesis in BES.*

**Figure 7.** *Metabolic redox reactions of nitrites/nitrates (N) and sulfates (S) in BES.*

allow an increase in the efficiency and scope of these systems for the treatment of different contaminated environmental matrices, which are constituted by recalcitrant organic and inorganic compounds [17, 48, 64, 72, 74, 75, 105].
