Anaerobic digestion of sludge is one of the most widely used processes for biogas and energy production. Conventionally, anaerobic digesters are operated at 35 degrees C to overcome the hydrolysis rate-limiting step. However, the energy expenditure for heating anaerobic digesters may be significant. The feasibility of operating anaerobic digesters at low mesophilic temperature (20 degrees C) by combining sludge ozonation was studied. Operation of three anaerobic reactors for 350 days showed that integrating solids ozonation and anaerobic digestion at 20 degrees C led to a higher volatile suspended solids (VSS) destruction of 60% than anaerobic digestion at 35 degrees C with raw sludge. Methane production in the reactor at 20 degrees C with sludge ozonation was enhanced from 62.6 mL CH4/g VSSin to 713 mL CH4/g VSSin for the 35 degrees C digester without sludge ozonation. Energy analysis showed that the 20 degrees C-ozonated digester produced 35% more energy than the 35 degrees C digester, with a net energy balance of +174 GJ/d and +129 GJ/d, respectively. The 20 degrees C-ozonated digester had a higher Energy Sustainability Index (ESI) (2.88) than the 35 degrees C digester (2.33) suggesting a more energetically sustainable option. (C) 2017 Elsevier Ltd. All rights reserved.