In the present study, the sludge was pretreated with microwave irradiation and low-temperature thermal method, both conducted under the same temperature range (30–100°C). Microwave pretreatment was found to be superior over the thermal treatment with respect to sludge solubilization and biogas production. Taking into account the specific energy demand of solubilization, the sludge pre-treated at 60–70°C by microwaves of 900W was chosen for further experiments in continuous mode, which was more energetically sustainable compared to lower value (700W) and thermal treatment. Continuous biogas reactor experiments indicated that pre-treated sludge (microwave irradiation: 900W, temperature: 60–70°C) gave 35% more methane, compared to untreated sludge. Moreover, the results of this study clearly demonstrated that microwave pretreated sludge showed better degree of sanitation.