Fed-batch ex-situ biomethanation lab tests: Preliminary technical assessments

Given the goal of achieving carbon neutrality by 2050 and the widespread adoption of renewable energy sources, it is crucial to examine technologies that reduce environmental impact while maximizing efficiency. The aim of this work is to develop a fed-batch continuous-stirred tank reactor at laboratory-scale wherein a mesophilic ex-situ biomethanation process occurs. This process utilizes a feedstock comprising digestate from a wastewater treatment plant and a gaseous mixture of H2/CO2 in a 4:1 ratio. The work is structured into four parts. The first part addresses the current status of renewable energy sources in the European Union and the directives related to their utilization, including the Renewable Energy Directive. This is followed by a description of the technologies used in Power-to-Gas (PtG) plants, starting from hydrogen generated from renewable electrolysis and carbon dioxide derived from other processes. Special attention is given to ex-situ biomethanation, focusing on operational principles, various existing configurations, and efficiencies reported in scientific literature. The second part concerns the experimental section, where tests conducted on the laboratory-scale Continuous Stirred-Tank Reactor (CSTR) are described, with particular focus on parameters such as Methane Evolution Rate (MER) and methane content, which are useful for evaluating the system’s operational efficiency. The obtained results include pH values, concentrations of Volatile Fatty Acids (VFAs), macronutrients and biomass present inside the reactor. The methodologies employed in the various analyses are reported in the appendices in the fourth part. The third part presents the conclusions drawn from the conducted tests, focusing on the efficiency of the system’s operation, also in comparison with systems reported in the literature. In particular, it is observed that the system is able to achieve high MER values, up to 8.57 L CH4/L/d, but accompanied by hydrogen content values not sufficiently low to allow direct injection into the natural gas grid, according to the limits set by technical regulations. Reducing the partial pressure of carbon dioxide and hydrogen could be an option to address this issue. Furthermore, it emerges that the analysed system is thermally self-sufficient. This aspect is important for evaluating its implementation also on real scale in various contexts, such as within a wastewater treatment plant.