The effect of thermal pre-treatment on the microbial populations balance and biogas production was studied during anaerobic digestion of waste activated sludge (WAS) coming from urban (US: urban sludge) and industrial (IS: industrial sludge) wastewater treatment plants (WWTP). The highest biogas yields of 0.42 l/g volatile solid (VS) removed and 0.37 l/g VS removed were obtained with urban and industrial sludge pre-treated at 120 degrees C, respectively. Fluorescent in situ hybridization (FISH) was used to quantify the major Bacteria and Archaea groups. Compared to control trails without pretreatment, Archaea content increased from 34% to 86% and from 46% to 83% for pretreated IS and US, respectively. In fact, the thermal pre-treatment of WAS enhanced the growth of hydrogen-using methanogens (HUMs), which consume rapidly the H-2 generated to allow the acetogenesis. Therefore, the stable and better performance of digesters was observed involving the balance and syntrophic associations between the different microbial populations. (C) 2016 Elsevier Ltd. All rights reserved.