Wastewater treatment plants (WWTPs) aim to increase energy independence by intensifying biogas production. The study involved Life Cycle Assessment (LCA) comparing sewage sludge (SS) anaerobic co-digestion (AcD) with the organic fraction of municipal solid waste (OFMSW) at WWTP against SS anaerobic mono-digestion (AD). The LCA was based on the authors' research showing that methane production related to wet mass in AcD was higher than in AD by 86.4, 225.8, and 354.3 % for SS:OFMSW mixing ratios of 75:25, 50:50, and 25:75, respectively. The LCA was conducted for 1 kWh of biogas energy produced at a WWTP and included two subsystems: energy production (AD/AcD and CHP) and associated digestate management (drying, incineration with energy reuse, residual landfilling). CML-IA baseline 2013 and Recipe 2016 Midpoint (H) LCIA methodologies indicated the environmental impacts on abiotic depletion - fossil fuels (fossil resource scarcity), global warming, human toxicity (non-carcinogenic), freshwater, and marine aquatic ecotoxicity. AD of SS in all categories was characterized by the highest impact. The impact decreased with an increase in the OFMSW rate. Both methodologies showed a significant impact of AD and AcD on global warming, which indicated the need for the determination of the carbon footprint (CF) of 1kWh biogas energy production (gCO kWh) at WWTP, using IPCC 2021 GWP100 (incl. CO uptake). The CF was the highest for biogas energy production in SS AD (1509 gCO kWh) and it decreased with the increase of OFMSW content in AcD (872-481 gCO kWh). This was also observed for the management of associated digestate. CF changed from 1508 gCO kWh for SS AD to 396 gCO kWh for SS: OFMSW mixing ratio of 25: 75 in AcD. The environmental burdens associated with biogas energy production and digestate management were equally significant. Finally, the formula for economic and ecological comparison of SS AD and AcD with OFMSW was proposed.