In this study, a combined sludge digestion process consisting of a mesophilic anaerobic digester (MAD) and a thermophilic aerobic digester (TAD) was developed. The effects of recirculation on the combined sludge digestion process were investigated. The full name 필요 (VSS) removal rate increased from 76% to 97% when internal recirculation was applied and remained stable until the end of the experiment. In contrast, the concentration of full name 필요 (TSS) increased slightly because of the accumulation of inert materials in the reactors. The methane production of the combined process showed the highest level of 260 mL/L/day in phase two and decreased to 170 mL/L/day as the strength of the pre-treatment decreased. At phase five, the influent and recirculated sludge were supplied without pre-treatment. During this phase, methane production rate decreased from 170 mL/L/day to 40 mL/L/day, and rapid accumulation of TSS and VSS occurred. However, the thermophilic aerobic digester was stable, and VSS accumulation was not observed. This indicates that the sludge removal ability of the combined process can be stably maintained without sludge pretreatment. Collectively, results of this experiment indicate that pre-treatment of influent sludge and internal recirculation of treated sludge from the TAD to the combined process are necessary for effective energy production and improved sludge removal in the combined process.