Excess sludge from a pilot-scale high-rate activated sludge (HRAS) system was digested anaerobically under mesophilic conditions. The pilot-scale HRAS system was initially fed with only wastewater and following that, fed with wastewater together with food waste. Impact of food waste inclusion on anaerobic digestion performance was investigated. Thereafter, a feasibility study was carried out using the performance data obtained from the experimental studies. The results showed that digestion of HRAS sludge could be a preferable option unless excellent biomass-liquid separation could be employed and mineralization of influent chemical oxygen demand (COD) could be suppressed. Additionally, co-treatment of wastewater and food waste increased the methane yield and improved the energy balance of the wastewater treatment plants (WWTPs). Thus, WWTPs can have a surplus in the energy budget. (C) 2019 Elsevier Ltd. All rights reserved.