Titolo della tesi: Innovative Bioelectrochemical Processes for Biogas Upgrading to Biomethane
BioElectrochemical Systems (BES) represent an innovative technology that exploits the ability of electroactive bacteria to exchange electrons with a solid-state electrode. Among BES, Microbial Electrolysis Cells (MECs) are the optimal candidates to combine the treatment of waste (organic) streams with biofuels production. MECs (Figure 1) are typically constituted by two compartments separated by an ion exchange membrane: the anode and the cathode. At the MEC anode the oxidation of COD (Chemical Oxygen Demand) occurs thanks to the presence of electroactive microorganisms that can be selected from an activated sludge directly into the anode. COD oxidation produces electric current, CO2 and H+; the latter could migrate across a proton exchange membrane where electrons generated provide (in part) the reducing power required for H+ reduction into H2 at the cathode. MECs allow H2 recovery as green hydrogen when the cathode configuration is abiotic, or CH4 recovery when the cathode is inoculated with methanogens (biotic configuration) according to the following equations: 4H2 + CO2 CH4 + 2H2O (hydrogenophilic methanogenesis); CO2 + 8H++ 8e- CH4 + 2H2O (electromethanogenesis).
The main objective of the present thesis (1st part) was to study the performance of an integrated AD-MEC (Anaerobic Digestion-Microbial Electrolysis Cell) process considering the MEC as a downstream tool to combine the treatment of the digestate (liquid effluent of AD) at the MEC anode with biogas (gaseous effluent of AD) upgrading at the MEC cathode. Indeed, even if almost 20 years of scientific research is reported about MEC, still few literature studies reported its performance treating waste matrices. In the present thesis, two different waste matrices were tested: a fermentate (i.e., the liquid effluent of a a pilot-scale acidogenic fermentor) and a digestate (i.e., the liquid effluent of a full-scale anaerobic digestor). Anode biofilm demonstrated its resilience in treating different substrates and the possibility to gain H2 and CH4 starting for waste matrices was also assessed valorizing the concept of waste to energy (WTE).
In the 2nd part of the present thesis, different strategies were explored to optimize the MEC performance at both the anode and the cathode. As an example, the role of conductive particles (CMs), in particular magnetite nanoparticles (Fe3O4 NPs), at the anode of H-type bioelectrochemical cells demonstrated the ability to steer the fermentation of glucose towards specific products of interest (e.g., butyric acid). The role of Fe3O4 NPs was assessed also at the cathode of H-type cells wherein its presence in the reaction media boosted H2 and CH4 production using a selected hydrogenophilic methanogens culture. Another investigation regarded the study of different electrode cathode materials to find the optimal one in catalyzing the electrochemical Hydrogen Evolution Reaction (HER). Other experiments were carried out with the aim to better understand methanogens metabolism by the exploitation of redox mediators, which allow to discriminate between the two main methanogens metabolic pathways: hydrogenophilic and acetoclastic methanogenesis.
In conclusion, this thesis confirms the feasibility of treating waste organic matrices at the MEC anode and the possibility to simultaneously gain bio-energy at the MEC cathode. At the same time, different limiting steps to scale-up the process were faced during the thesis pathway and the investigated strategies to optimize the overall process could open a door for new research insight.