The effect of co-digesting sludge with bio-waste was investigated using an experimental apparatus set for reproducing the operating conditions of a full-scale digester in an existing wastewater treatment plant of 90,000 PE (population equivalent). An increase in the organic loading rate from 1.46 kgVS/m(3) day to 2.1 kgVS/m(3) day obtained by introducing 40 kg of biowaste per m(3) of sludge in the digester caused an increase in the specific methane generation from 90 NL/kgVS to 435 NL/kgVS. These results were used to assess the energetic potential of digesters in eight existing wastewater treatment plants operating in an Italian province with 28,000 PE to 90,000 PE. Results showed that these facilities were able to co digest globally about 2900 tonnes per year of bio-waste and to generate about 3400 MWh/year of electricity. (C) 2016 Elsevier Ltd. All rights reserved.