In order to improve anaerobic digestibility of sewage sludge, ozone pretreatment was carried out on the concentrated excess sludge, and continuous mesophilic digestion was operated using concentrated primary sludge together as a substrate. The untreated system and the ozone treated system were operated at a hydraulic retention time (HRT) of 25 and 15 days. Under HRT of 25 days, the specific biogas production (SBP) was 514 NmL/g-VS for untreated system and 658 NmL/ g-VS for ozone treated system, an increase of 28%. Under HRT of 15 days, the SBP was 473 NmL/ g-VS for untreated system and 589 NmL/g-VS for ozone treated system, an increase of 25%. In addition, the water content of dewatered sludge decreased in the ozone treated system.

　Based on the experimental results, consumed or produced energy during running on a full-scale was estimated. Digested sludge was assumed to be incinerated after dewatering. The energy evaluation was performed both on the energy of produced biogas itself and the case of biogas power generation system. The overall energy balance of the ozone treated system was positive compared to the untreated system even in the biogas power generation system, showing the effectiveness of the ozone system.