At ambient temperature (25 degrees C), bioelectrochemical anaerobic digestion of sewage sludge was investigated with a hydraulic retention time (HRT) of 10 days and compared to that at a mesophilic condition (35 degrees C). The methane production and methane content in the biogas at ambient temperature were 612.8 mL/L.d and 73.3%, respectively, which were not significantly lower than that of the mesophilic condition. Additionally, the VS removal was 54.5% which was similar for both temperature conditions. However, for a HRT of 20 days, the bioelectrochemical anaerobic digestion at ambient temperature became more stable, and the VS removal improved up to 65.0%. For the HRT of 10 days, the net energy production at ambient temperature was about 168 kJ/L.d, which was similar to the mesophilic condition; however, the apparent energy efficiency at ambient temperature was 249.2% which was significantly higher than 197.7% at the mesophilic condition. The bioelectrochemical anaerobic digestion that can save the thermal energy input at ambient temperature is recommended for the treatment of organic waste including sewage sludge in moderate and cold climate regions. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.