Biogas is a renewable energy produced through anaerobic digestion, which means that organic matter is degraded by microorganisms under anaerobic conditions. The produced biogas can then be used for cogeneration, electricity, heat or upgraded to vehicle gas. Eskilstuna Energy & Environment AB has four digesters at Ekeby water sewage treatment plant, Eskilstuna. One of the biogas reactor (RK4) is equipped with a jet-mixing system while the remaining digester uses top-mixing. The mixing system is an important parameter to achieve optimum operating conditions for the process regarding gas production and degree of digestion. This study aims to evaluate the two mixing systems, jet-mixing and top- mixing. The design of the reactors is identical and they are assumed to be fed with the same amount of substrate. By collection, analysis and compiling of process- and operational data such as gas production, degree of digestion and energy consumption for the two different systems their performance has been compared to each other. Different operational conditions considering the energy consumption for the jet-mixing system has been done theoretical to try to reduce the energy consumption. The result shows that both rectors operates under stable process conditions, although the result indicates that the gas production for the reactor installed with jet-mixing system is greater than the system with top-mixing system. Hopefully, the result of the study could be used of Ekeby water sewage treatment plant as an indication of how the system could be run. Keywords: Biogas, mixing, mixing system, anaerobic digestion, gas production, energy consumption, degree of digestion, Ekeby water sewage plant. Nyckelord: Biogas, omrörning, omrörningssystem, anaerob rötning, gasproduktion, energiförbrukning, utrörningsgrad, Ekeby reningsverk.