Life cycle assessment was used to assess environmental impacts and a sensitivity analysis of 17 key technological parameters to identify the main environmental hotspots associated with the performance of a sewage sludge treatment. The scenario takes into account primary and secondary sludge and includes gravity and centrifugal thickening, anaerobic digestion with cogeneration of heat and electricity from biogas, mechanical dewatering and spreading on agricultural fields. All the relevant inputs (electricity, heat, reagents, infrastructure, fuel, transport) and outputs (emissions to air, water and soil) are accounted for in the assessment, including the sludge return liquors from thickening and dewatering processes. This study focuses on climate change, terrestrial acidification, freshwater eutrophication, human toxicity and ionizing radiation impact categories assessed using the ReCiPe E v1.08/Europe baseline method. The results showed that the most sensitive parameters are the biodegradation rate of volatile solids and the nitrogen mineralization rate during anaerobic digestion, phosphorus and nitrogen capture rates during the thickening and dewatering processes, and the consumption of FeCl3 for the treatment of sludge before dewatering. The results suggest that the environmental performances of the sewage sludge treatment line could be enhanced by increasing the production of biogas (by increasing the biodegradation rate of volatile solids), by decreasing the consumption of FeCl3 used for sludge treatment, and by finding alternative treatments for sludge return liquors.