Effects of anaerobic pre-treatment were evaluated on the dewatered-sewage sludge from a municipal wastewater treatment plant in order to improve its biodegradability through anaerobic digestion. The pre-treatment was conducted in laboratory scale at 25, 50 and 70 degrees C for an incubation time of two days. As a reference, sludge sample was also autoclaved at 121 degrees C for 20 min to determine the thermal effect to the subsequent sludge digestion. Characteristics of dewatered-sludge such as viscosity, pH and soluble chemical oxygen demand (SCOD) were affected by the pre-treatment. A higher SCOD after the pretreatment did not necessarily imply an increase in methane yield, although initial biodegradability rate was improved. In fact, a 'great' improvement in SCOD concentration (up to 27%) was translated in only 8% increase in the methane yield (298 +/- 9 and 276 +/- 6 Nml CH(4) gVS(added)(-1) for pre-treated and untreated samples, respectively). Increasing the anaerobic pre-treatment time from 12 h to 2 days at 50 degrees C led to an 11% improvement in methane yield. Methane content in biogas increased from an average of 65-69% for the pre-treated and untreated substrates, respectively. Volatile solids (VS) reduction increased from 42% to 51%. The overall digestion time was not affected by the pre-treatment but 90% of methane was produced in the first 12 days of incubation for 50 degrees C pre-treated samples whereas it took 2-5 days more for 25, 70 degrees C pre-treated and untreated sludge samples. In this study, thermophilic digestion was also found to be a better option in terms of faster digestion and higher VS-reduction, but it showed lower methane yield as compared to mesophilic digestion, i.e. 9% and 11% increment in methane yields for thermophilic and mesophilic digestions, respectively. (C) 2008 Elsevier Ltd. All rights reserved.