The objective of this work was to investigate the effect of a low temperature pre-treatment (70 °C) on the efficiency of thermophilic anaerobic digestion of primary and secondary waste sludge. Firstly, effect of sludge pre-treatment time (9, 24, 48 and 72 h) was evaluated by the increase in volatile dissolved solids (VDSs), volatile fatty acids (VFAs) and biogas production in thermophilic batch tests. Secondly, semi-continuous process performance was studied in a lab-scale reactor (5 L) working at 55 °C and 10 days solid retention time. The 70 °C pre-treatment showed an initial solubilization effect (increasing VDS by almost 10 times after 9 h), followed by a progressive generation of VFA (from 0 to nearly 5 g L⁻¹ after 72 h). Biogas production increased up to 30% both in batch tests and in semi-continuous experiments. Our results suggest that a short period (9 h) low temperature pre-treatment should be enough to enhance methane production through thermophilic anaerobic digestion of sludge.