Harvesting Biofuels with Microbial Electrochemical Technologies (METs): State of the Art and Future Challenges

Microorganisms and microbial technologies have accompanied mankind throughout its history. The growing demand for energy and increasing waste production require combined eco-friendly smart solutions. Microbial technologies have been involved in the waste treatment process since the beginning of industrialization. However, their natural capacity to recover nutrients and energy has not been fully exploited in recent decades. Indeed, energy recovery by biogas production and nutrient recovery from wastewater treatment plants and anaerobic digestors have considerably increased their capacity. An emerging biotechnology field for environmental applications is represented by microbial electrochemical technologies, which are based on using so-called electroactive microorganisms, which can exchange electrons with conductive materials under particular environmental conditions. If the electrodic material is adopted as the final acceptor of electrons produced by oxidizing metabolism, the bio-electrochemical interphase is named bioanode. On the other hand, if microorganisms use the electrode as a source of reducing power, it is called a biocathode. Microbial electrochemical technologies are promising for their ability to harvest energy from waste organic materials reducing the energy costs of producing high-value products such as biofuels and chemicals. Despite the possibility of directly converting the organic waste into electricity in microbial fuel cells (MFCs), in a microbial electrolysis cell (MEC) several products can be obtained by applying an external electric voltage to the cell. MECs can produce hydrogen in the cathodic compartment of the cell while treating wastewater on the anodic side; moreover, by choosing optimal operating conditions, additional processes like biogas upgrading into biomethane can be performed simultaneously. Even if many environmental applications have been successfully investigated in METs, those technologies are going through the upscaling process. To avoid the technological valley of death, the systems must continue improving. This chapter, reports state of the art and the future challenges of the most recent advances in biofuel generation through MECs. An overview of the microbiology of METs and the metagenomic approaches to characterize their microbial communities are included. Moreover, particular attention will be addressed to highlight the most recent scale-up attempt of MECs devoted to hydrogen and/or methane generation.