**7. Conclusions**

The term bioelectrochemical systems (BES) encompass a group of relatively novel technologies which hold a great potential for energy valorization of a wide variety of waste streams. Perhaps the clearest niche of application of BES lays in the field of wastewater treatment where this technology could help to improve the energy efficiency of the process by converting the energy content of the organic matter present in the wastewater into either electrical energy or a fuel gas. In fact, most of the pilot scale experiences developed to date have been designed for these purposes. Moreover, operational versatility of BES might bring additional opportunities for wastewater valorization, as they also allow to recover valuable chemicals and nutrients such as ammonium or phosphorus. Here, it is important to highlight that BES can perform this at a reduced energy and economic costs compared to more conventional technologies used in fertilizers industries.

Gaseous CO<sup>2</sup> -rich waste streams represent another suitable raw material for BES. Here, BES might provide a means for converting CO2 into valuable organic chemicals such as methane or short chain fatty acids. This can potentially provide a cost-effective and environmentally friendly method for limiting CO2 emissions into the atmosphere.

In short, BES can be seen as a group of technologies capable of valorizing a wide range of liquid to gaseous waste streams. In most cases, the operation of BES requires large amounts of electrical energy, most of which ends up stored in chemical energy (methane, hydrogen, etc.) that can be readily converted back into electrical energy when required by using well-established technologies (fuel cells, cogeneration, etc.). This feature would enable BES to operate as electrical regulation system which would bring further commercial opportunities for these technologies.
