This Master's Thesis focuses on the first steps in the simulation and optimization of a new bio-electrical reactor used for producing biomethane from organic waste waters, which may come from: sewage, cheese factories, breweries, distilleries. The work deals with the initial development of a CFD model for a biomethane-producing electrochemical reactor, and its validation with respect to experimental data. The model will be later adopted to layout and predict the performance of cylindrical cells for different anode and cathode geometries, positioning, and morphology. Outputs of interest include current density, methane production rate, liquid-gas separation, and utilization factor of organic contaminants. This work belongs to the broader research objective of realizing a novel bioelectrical reactor to produce highly upgraded biogas, with methane content over 90%, from wastewaters and other CO2 sources, ideally ready for gas grid injection.