The purpose of this thesis is to research the decomposition of leftover cooking oil and biomethane potential. To determine the anaerobic digestion we first used batch device Automatic Methane Potential Test System (AMPTS II). The methane production was significantly different when we used just the leftover cooking oil compared to controlled reactor. The latter was dosed the primary sludge and we created the same conditions as they have in the central cleaning plant Domžale - Kamnik. To be able to compare it to the real reactor we monitored the function of the anaerobic reactor in CCP Domžale – Kamnik. We wanted to confirm the results obtained from the batch mode also on the bigger, almost 200L semi-continuous pilot reactor.