Municipal wastewater treatment plants (WWTPs) are energy-intensive and thus cost-intensive facilities; therefore, it is crucial to increase energy production directly at the WWTP. Enhancement of biogas production by addition of external substrates is one of the solutions to increase energy self-sufficiency of the WWTPs with an additional benefit of cutting down the greenhouse gas emission. The main aim of the work was to investigate full utilization of the capacity of full-scale digesters at the municipal WWTP by addition of poultry industry waste. At first, laboratory trials were conducted in order to identify the most suitable dose for co-digestion with primary and waste activated sludge and finally, based on the achieved laboratory results, full-scale trials were carried out directly at the municipal WWTP. Poultry industrial waste yielded between 0.39 and 0.88 m(3) of methane per kg of volatile solids during laboratory trials, depending on the added concentration. During full-scale investigation yield of 0.81 m(3)/kg VS was achieved. Enhanced biogas production improved WWTP energy self-sufficiency bringing closer to the aim of increasing the share of self-produced energy up to 80%. (C) 2015 Elsevier Ltd. All rights reserved.