**6. Microbial communities in bioelectrochemical systems**

Numerous studies report BES in which a great diversity of consortiums and microbial communities are used for the treatment of domestic and industrial wastewater for the degradation or reduction of polluting compounds such as organic waste, dyes, pesticides, phenolic compounds and petrochemicals; as well as for the production of metabolites with high added value such as volatile fatty acids, organic acids, biofuels, among others [8, 41, 50, 64, 80, 89, 106–108]. In some cases, the presence of "model" microorganisms is indicative of electrogenic activity. As reported by Liang et al., there is a presence of *Geobacter* sp. and *Pseudomonas* sp. in a 1000-L reactor, in which a production of 7.68 W/m2 was obtained with synthetic wastewater and 3.64 W/m<sup>2</sup> with domestic wastewater [87]. However, the diversity of the microbial communities used in bioelectrochemical systems depends fundamentally on the origin or nature of the inoculum or the substrate used.

In the treatment of domestic wastewater in these systems, the presence of *Paracoccus denitrificans* and *Clostridium* sp., denitrifying bacteria with the capacity to metabolize sucrose, glucose, starch, lactic acid, and other sugars, has been reported [80]. Also, the genera *Enterobacter* sp. involved in the electrochemical reduction of oxygen and *Escherichia* sp. involved in the reduction of extracellular iron [36]. Urine has been evaluated as domestic waste for its treatment in BES, achieving a degradation of antibiotics such as ampicillin greater than 99%. In these investigations, the most abundant bacterial phyla identified were Proteobacteria, Firmicutes, Bacteroidetes, Chloroflexi, Actinobacteria. The most abundant bacterial genera were *Pseudomonas* sp., *Anaerolineae* sp. known for their cellulolytic capacity and their cellular adhesiveness; and the genera *Nitrospira* sp. and *Methyloversatilis* sp. associated with the

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

metabolism of nitrogenous compounds (NH4/NO3) [76]. Xiao et al. reported the presence of the bacterial genera *Desulfovibrio*, *Lysinibacillus*, *Clostridium* and *Lachnoclostridium* in an MFC directly inoculated with wastewater, achieving greater efficiency in energy production of 543.75 mW/m<sup>2</sup> and a removal of nutrients such as total nitrogen with 76.15%, ammoniacal nitrogen with 83.23%, and a COD reduction of 75.59%, compared to that achieved with a defined inoculum or consortium. The authors indicate a higher microbial diversity in the MFC operated with wastewater [103].

In the case of industrial wastewater, bioelectrochemical treatments of wastewater from the food industry have been reported in the presence of *Geobacter* sp. and *Petrimonas* sp. known for their action to oxidize or ferment substrates such as brewery waste [56], or the metabolic activity of *Pseudomonas* sp. to degrade azo-type dyes derived from the textile industry or even for the production of pigments [78]. Some archaea of the order Methanomicrobiales and Methanosarcinales have also been identified in wastewater from the meat industry that could establish syntrophic interactions with acetate-oxidizing microorganisms and also develop exoelectrogenic activity [4]. In an investigation published by Marshal et al., in a BES with an inoculum from brewery wastewater, there were described the genera *Acetobacterium* sp., *Sulfurospirillum* sp. and *Desulfovibrio* sp. participating in functions such as fixation of carbon in the electrode, indication of a microaerophilic ecosystem and in the possible expression of protein hydrogenases, formate dehydrogenase and proton-to-hydrogen reducing cytochromes [109]. In a study describing the electrogenic behavior of controlled microbial consortia, strains of *Methanococcus maripaludis* and *Acetobacterium woodii* were used as model microorganisms for the hydrogenotrophic synthesis of methane and acetate; electromethanogenesis rates of 0.1–0.14 <sup>μ</sup>mol cm<sup>2</sup> <sup>h</sup><sup>1</sup> at 400 mV (vs SHE) and 0.6–0.9 <sup>μ</sup>mol cm<sup>2</sup> <sup>h</sup><sup>1</sup> at 500 mV were observed and acetate formation was observed at rates of 0.21–0.23 <sup>μ</sup>mol cm<sup>2</sup> <sup>h</sup><sup>1</sup> at 400 mV and 0.57–0.74 <sup>μ</sup>mol cm<sup>2</sup> <sup>h</sup><sup>1</sup> at 500 mV, respectively [84]. Some microorganisms such as *Desulfovibrio* sp. and *Thiobacillus* sp., *Azospirillum* sp. and *Mycobacterium* sp. have the ability to metabolize nutrients such as SO4, PO4, and NO3/NO2, and transform them into simple molecules and gases with less impact on the environment [19, 30, 74, 97]. Some cyanobacteria, photosynthetic microorganisms, have also been related to the production of intermediate compounds such as acetate and lactate, and it is suggested that they could be used for the denitrification of organic matter and co-generation of energy through exoelectrogens in BES [13].

On the other hand, the characterization of microbial communities in bioelectrochemical systems has been subject to studies evaluating the performance of BES that are subjected to different operational conditions. For example, Koók et al. reported that the predominance of the genera *Geobacter* sp., *Azospirillum* sp., *Castellaniella* sp., *Pandoraea* sp.,*Treponema* sp., *Clostridium* sp. varies in relation to the external resistance used in an MFC. The authors report a higher yield, but a lower stability of the system when a dynamic system of external resistances is used, which is attributed to the acidification of the biofilm. These results suggest the adaptive evolution of the microbial community in response to the operational conditions of the BES [110]. In a mixed system known as MFC-CW that has been described as an MFC coupled to a constructed wetland, Tao et al. reported that the most abundant phyla were Proteobacteria, Cyanobacteria, Bacteroidetes, Acidobacteria, Chloroflexi, and Nitrospirae, in the case of the predominant genera *Geobacter* sp., *Desulfobulbus* sp., *Bacillus* sp., and *Geothrix* sp. In this mixed system, a nitrate reduction greater than 90% and a power density of 6.09 mW/m<sup>2</sup> were achieved when xylose was used as

carbon source; in comparison, only a 10% nitrate reduction and a power density of 2.91 mW/m2 were achieved when cellulose was used [13]. In another study by Ge et al., using an MFC-CW, reduction values of COD, NO3-N, total inorganic nitrogen, and total phosphorus of 71.9, 70.1, 63.2, and 91.2%, respectively, were reported; and current and power densities of 47.77 mA/m<sup>2</sup> and 6.74 mW/m<sup>2</sup> , respectively [19]. The predominant electrogenic microorganisms found were the Proteobacteria, Actinobacteria, Acidobacteria, Bacteroidetes, and Chloroflexi phyla. The genera *Geobacter* sp. and *Dechloromonas* sp. associated with autotrophic denitrification and phosphate accumulation; the genus *Thiobacillus* sp. related to pyrite-driven autotrophic denitrification and the genera *Desulfovibrio* sp., *Desulfomicrobium* sp., *Desulfobulbus* sp., and *Desulfuromonas* sp. identified as sulfate-reducing bacteria for the oxidation of organic compounds such as biofilm-detached biomass and residual organic pollutants [19]. For the treatment of organosulfur compounds, Elzinga et al. used a type H MFC, and determined an abundance of the families *Halomonadaceae*, *Clostridiaceae*, *Eubacteriaceae*, and *Clostridiaceae*. The coulombic efficiency associated with this system in relation to different compounds was 1.7% for methanethiol, 3.4% for ethanethiol, and 5.0% for propanethiol and dimethyl disulfide [22]. Research in microbial ecology for BES has increased in the recent years. However, there are few reports focused directly on the study of native or autochthonous microbial communities in BES. Examples include the work of Revelo et al., who evaluated the effect of the inoculum in an MFC with anodic and cathodic chambers separated by a salt bridge. Anaerobic sludge from a solid waste treatment plant and sulfidogenic sludge and wastewater from artisanal tanneries were evaluated. The researchers reported reductions of 97.9% in organic matter content and 86.3% in chromium content, as well as a coulombic efficiency of 0.92% [111]. In a recent study, Agudelo-Escobar et al. reported the electrogenic capacity of native microorganisms in complex communities originating from wastewater from the coffee agroindustry, in an open cathode MFC with an unconventional design, achieving a reduction in organic matter of up to 70% and a power density of 21.16 W/m3 . The authors reported the presence of the genera *Clostridium* sp., *Cohnella* sp., *Aneurinibacillus* sp., *Candida* sp., *Gluconacetobacter* sp., *Clostridium* sp., *Acetobacter* sp., *Bacillus* sp., *Weissella* sp., *Leucononostoc* sp., and *Lactobacillus* sp., microorganisms known mainly in anaerobic or microaerophilic ecosystems, associated with the reduction of organic compounds and that they suggest are related to the electrochemical performance of the system [112]. Do et al. indicate that the bacterial community at the anode is mainly affected by the type of substrates used and ultimately this influences current generation [113]. It has been shown that current may be sensitive not only to COD concentrations, but also to the presence of aerobic and electrogenic bacteria, as well as nutrient ratios in water samples [114]. The success of the implementation of a BES system for the treatment of wastewater, the cogeneration of energy and the generation of high value-added products, will be conditioned by the type and origin of the substrate used and by the choice of microorganism or microbial community, which will be responsible for carrying out the metabolic transformation and use of organic matter in the system. **Figure 8** proposes a consolidated scheme that groups the main components identified in the wastewater with the intermediate metabolites and final products that can be obtained through the main metabolic activities identified in the electrogenic microorganisms in the BES.

What is proposed is a route to identify the best bioelectrochemical alternative. To do this, the purpose of the implementation must be established, that is, to identify whether the main objective is bioremediation or treatment of a specific wastewater or the obtaining of biofuels, energy generation, or obtaining high value-added

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

**Figure 8.**

*Unified scheme unveiling wastewater transformation by electrogenic microorganisms.*

metabolites. The origin and composition of the wastewater must be identified, to establish what type of substances will be present in the substrate and thus choose the adapted microbial community with the metabolic capacity to transform that type of substrate. For example, for the treatment of domestic wastewater, the use of an MFC or a combined system such as the MFC-CW could be implemented. For industrial wastewater, depending on their origin, MECs or METs could be implemented coupled to gas capture systems, in which the use of microbial communities with metabolic capacity for the successive transformation of organic substrates, until reaching the production of biofuels such as H2 and CH4.

### **7. Perspectives of electroactive microorganisms**

Microorganisms and different microbial communities play a fundamental role in oxidation-reduction processes for the transformation of organic matter, energy production, and obtaining compounds of interest in bioelectrochemical systems. Some main metabolic pathways have been identified that are directly associated with the degradation of organic matter for the recovery of electrical energy or the obtaining of biofuels for some specific microorganisms. However, in diverse biological systems, such as microbial communities, in which there are ecological successions and different metabolic reactions that are essential to achieve the transformation of complex organic and inorganic compounds, as well as the transformation of highly polluting substances such as heavy metals, pesticides, and other contaminants; the combination of metabolic pathways that; they do not necessarily obey bioelectrochemical processes; have not been clearly established. Although knowledge of the microbial communities responsible for the biochemical functioning of BES has been progressively deepening in relation to technological advances in molecular biology and genetic

engineering, the population and ecological study of microbial communities is constantly updated [45, 54, 59, 113], and research directly focused on the ecological study of complex microbial communities in bioelectrochemical systems is scarce.

Scientific advances have made it possible to identify which microorganisms are present in these systems and the possible metabolic mechanisms that favor their ability to transform organic matter and electrochemical performance. However, research focuses mainly on the design, configuration, and electrochemical performance of these systems. The detailed study of the changes in the diversity and abundance of microbial communities in BES provide fundamental information to determine the role that each microorganism plays in the microbial community and establish their participation in the oxidation-reduction processes of organic matter, the energy production, the electron transport mechanism, and the formation of biofilms and their interaction with the substrate and other microorganisms in BES systems; there is fundamental information for the implementation of bioelectrochemical systems as biotechnological alternatives for wastewater treatment and co-generation of clean energy. As a perspective, it is necessary to deepen the investigation of the biological interactions of microbial communities to analyze the electrochemical potential with application in the optimization of bioelectrochemical systems. For example, for the implementation of BES as biotechnological tools such as biosensors based on microbial fuel cells known as economic *in situ* sensing technologies of wastewater quality [113, 114].

There is evidence of microorganisms' capability to generate electricity and biofuels from complex organic matters such as cellulose, pectin, pesticides, dyes. While many studies delve into the treatment of domestic wastewater, industrial wastewater, particularly of agricultural origin, could be considered as unconventional substrates for biotechnological processes like BES. Agro-industrial wastewater consists mainly of organic matter and nutrients derived from both plant sources (biomass) and the addition of manure and fertilizers to crops. This composition makes them substrates with high potential for microbial metabolism.

Access to wastewater treatment systems in rural and agricultural areas is primarily limited by the difficulty of reaching these locations. Moreover, the extensive cultivation areas and their geographical separation make it challenging to aggregate the waste. The use of bioelectrochemical systems as an alternative for treating agroindustrial waste shows promise as a sustainable strategy that allows for the implementation of local treatment systems in agricultural zones. Considering the properties and composition of agro-industrial wastewater, as well as the adaptability of electrogenic microorganisms to different environmental conditions and substrates, supports the applicability of these systems in rural areas.

It is necessary to delve deeper into the study and characterization of native microbial communities in agro-industrial wastewater responsible for energy production through the oxidation-reduction of organic matter in bioelectrochemical systems. These communities offer several advantages, such as their adaptation to the composition, their development under ambient conditions, and their stable ecological relationships in these substrates. Compared to the introduction of exogenous microorganisms and microbial consortia into the treatment matrix, native microbial communities do not require adaptation phases to achieve electrogenic metabolism.

In-depth exploration of biostimulus and bioaugmentation techniques holds great promise in the optimization of wastewater bioelectrochemical treatment processes. These techniques involve adapting and enhancing the microorganisms present in the system, leading to a range of benefits in terms of electrogenic potential and overall efficiency of BES [9]. Through biostimulus techniques, the electrogenic activity and

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

performance of the microorganisms can be stimulated and optimized providing suitable growth factors, adjusting environmental parameters, or employing specific additives that enhance microbial activity and promote electron transfer processes. Considering bioaugmentation, it is proposed to evaluate the introduction of selected microorganisms or microbial consortia into the existing native microbial community of the BES. These added microorganisms could improve specific capabilities, such as enhanced electron transfer abilities or the ability to degrade specific complex organic or remove inorganic compounds. By introducing these specialized microorganisms, the electrogenic potential of the system can be enhanced, leading to improved performance in terms of electricity generation or pollutant removal.

Both biostimulus and bioaugmentation techniques offer exciting opportunities for optimizing wastewater bioelectrochemical treatment processes. Through the adaptation and enhancement of microorganisms, these strategies can unlock the full potential of BES, improving their electrogenic performance and overall efficiency. However, further research is needed to understand the specific mechanisms and conditions that lead to optimal results, as well as to ensure the long-term stability and sustainability of these approaches in real-world applications.

#### **Conflict of interest**

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

#### **Notes/thanks/other declarations**

All authors listed have made a substantial, direct, and intellectual contribution to the work, and approved it for publication.

#### **Author details**

Lina María Agudelo-Escobar\* and Santiago Erazo Cabrera Research Group Biotransformación, School of Microbiology, University of Antioquia, Colombia

\*Address all correspondence to: lina.agudelo@udea.edu.co

© 2023 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
