Microbial Fuel Cell Systems for Wastewater Treatment and Energy Generation from Organic Carbon and Nitrogen: Fundamentals, Optimization and Novel Processes

Multiple types of wastewater contain abundant nutrients and chemical energy within organic and inorganic compounds, as well as reusable water. Thus, based on a circular economy perspective, wastewater treatment plants could be designed as biorefineries and potential energy producers, reducing the demand for fossil fuels and, consequently, CO2 emissions. The microbial fuel cell (MFC) is a bioelectrochemical system aligned with this systematic perspective. This novel technology, relying on interactions between electrochemically active bacteria and solid-state electrodes (anode and cathode), can treat wastewater and directly convert biodegradable compounds into electricity. MFCs are mostly considered for converting organic carbon into electricity, but recent findings demonstrated their ability to convert nitrogen compounds as well. New possibilities include the oxidation of ammonia nitrogen on the anode coupled with current generation and utilization of oxidized nitrogen (nitrite and nitrate) as electron acceptors on the cathode. This is particularly important for industrial and agro-industrial effluents, which commonly presents high concentration of organic carbon and nitrogen species. Despite its remarkable advantages, large-scale implementation of MFC technology is still limited by relatively low power densities. In this regard, the proper selection of electrode materials can optimize the MFC performance in terms of power output. Tridimensional carbonaceous electrodes offer unique properties regarding morphology, surface, adhesion of the biofilm, electron transfer, and oxidation of the substrate. Furthermore, the capacitive properties of high specific surface electrodes can be utilized in charge and discharge cycles to increase power output by combining faradaic and non-faradaic currents. Operating parameters such as temperature and resistance of the external circuit can affect the level of the current generation by modulating the activity and predominance of electroactive bacteria in the biofilm of MFCs. In this chapter, MFC systems are reviewed. Fundamentals of the process occurring within MFCs are presented, and main contributions in the field of MFC focused on carbon and nitrogen conversions, tridimensional electrodes, and capacitive properties are discussed. Then, relevant insights towards their potential applications in energy generation from wastewater are presented.