Sewage-treatment plants generate sludge which needs to be neutralized for legal, esthetical, and practical reasons. Economical and ecological concerns indicate that sludge, as long as it is possible, should be returned to the natural environment. The article presents the process of autothermal thermophilic aerobic digestion (ATAD) as a measure to produce biomass, which can be naturally utilized. The description of theoretical principles of the process in question is accompanied by presentation of an ATAD installation which has been functioning since 2009 at the municipal sewage-treatment plant in Olecko. Results of microbiological and chemical tests carried out in the years 2010–2012 have been analyzed. Examinations embraced sludge which was the final product of the aforementioned sewage-treatment plant.