Anaerobic digestion (AD) of sewage sludge is commonly employed for sludge stabilization and energy generation at wastewater treatment plants. Of pretreatment techniques used to accelerate rate-limiting hydrolysis step in AD, economically feasible ozonation is prevalent and permits sludge reduction via oxidation. The stability of AD process during ozonation is crucial for successful energy recovery and needs optimal dosage that encourages methanogenic consortium efficient in methane production. This study, therefore, aimed at investigating the impact of 0.03, 0.06 and 0.09 g O3/g TSS ozone doses on the productivity of mesophilic semi-batch lab-scale 5 L AD operated at a hydraulic retention time of 15 d with organic loading rates of 1.45–1.80 g VS/L.d and on its methanogenic consortium. Measurements of biogas, methane and quantification of methanogenic consortium were done by water displacement, gas chromatography, and fluorescence in situ hybridization techniques, respectively. Although all applied ozone doses improved sludge characteristics, the highest average daily removal efficiencies with 35% COD, 32% TS, 42% VS, 60% TSS and 69% VSS were achieved through ozonation with 0.06 g O3/g TSS ozone dose. The same ozone dose also yielded 48% more methane in the biogas recovered and this biomethanation was correlated to the abundance of acetotrophic Methanosaeta species.