Research worldwide focuses on energy from renewable sources. The goal is to completely use existing resources with the lowest possible environmental impact. Therefore, the production of electricity and heat from biogas is established over 100 years. This work investigates the losses of biogas and its climate relevance at the anaerobic mesophilic sludge stabilization. By conducting measurements with different methods on a big scale plant with 260.000 population equivalents, potential places of losses are detected at first and investigated on qualitative and quantitative aspects afterwards. These losses and emissions arise from gas bubbles carried with the sludge across the outlet of the digestion tank, in the stabilized sludge solved gas, and the rest gas potential still stored in the stabilized sludge. In addition, other losses happen during the digestion and gas utilization process. For example, through leakages in the septic tanks, incomplete combustion in the combined heat and power unit and flare (methane slip). These sources of losses and the resulting emissions of greenhouse gases are investigated in context of optimal gas production out of the treated excess and primary sludge and the co-processed co-substrate. The focus is on methane, which is not only used as a source of energy, but rather its greenhouse potential is 25 times higher than those of carbon dioxide.