**2. Bioelectrochemical systems: What they are?**

vice versa) using microbes as catalysts [1]. Although the ability of certain bacteria to generate electrical current was first described more than 100 years ago [2], it was not until the beginning of the present century that this phenomenon started to draw real interest form scientists and engineers. During the last 15 years, the progress made in the fields of bioelectrochemistry and BES has allowed to take the leap from the laboratory to the pilot scale [3, 4] so that com-

Initial research efforts were focused on exploring the possibilities that BES offered for the treatment and energy valorization (as electric power or hydrogen) of diverse waste streams [5, 6]. To date, the range of applications has broadened dramatically, extending to diverse fields such as desalination [7], bioremediation of contaminated water and soils [8], nutrients recovery [9], or the synthesis of valuable chemicals [10], among many others. Moreover, the versatility and multifaceted nature of BES open the way for applications that lay far beyond bio-based industrial processes. Indeed, when BES are operated in electrolytic mode (see Section 2 in the present chapter), they demand a certain amount of electrical energy, part of which ends up stored in chemical products (hydrogen, methane, etc.). This feature opens the way to using BES as an alternative technology for storing excess electrical power within electrical grids [11]. Thus, BES would offer new storage opportunities, especially in decentralized smart grids, providing a nexus with the waste management systems and offering alternative

This chapter explores some of the most relevant opportunities that BES offer for energy valorization of waste streams (**Figure 1**). It begins by briefly describing the principles of operation

mercial development seems to be at hand.

128 Energy Systems and Environment

waste and energy management strategies.

**Figure 1.** Scope of applications of BES for energy valorization of wastes.

BES can be understood as electrochemical systems in which at least one of the electrode reactions (anodic and/or cathodic) is biologically catalyzed [14]. They share with traditional electrochemical systems the key feature of being operationally reversible, i.e., they can be run as galvanic cells (the redox reactions are spontaneous) or as electrolytic cells (the redox reactions are non-spontaneous and require a certain amount of electrical energy to proceed). The first BES prototypes operated in galvanic mode were termed as microbial fuel cells (MFC), and when they were operated in electrolytic mode, they were usually referred to as microbial electrolysis cells (MEC). Although this terminology has been somehow transcended as a result the increasing number of BES typologies and architectures that have appeared during the past decade [15], it remains still useful as it mirrors the two basic modes of operation in electrochemical systems. **Figure 2** shows a schematic representation of the principle of operation of BES systems. For more detailed information about the basic principles of BES, the reader is referred to [15].

**Figure 2.** Schematic representation of the principle of operation of a BES. "Ox." stands for oxidized compound, while "red." stands for reduced compound.
