AbstractThe influence of different pre‐treatments was studied in order to observe the effects of temperature, pH and treatment time on Waste Activated Sludge (WAS) solubilization, and anaerobic digestion of pre‐treated sludge. Results showed that thermo‐chemical pre‐treatments were the most efficient on Chemical Oxygen Demand (COD) solubilization, which could reach 83% at 170 °C with pH = 12. Yet, increase in COD solubilization in thermo‐chemical pre‐treatment was not linked to an increase in soluble Volatile Solids (VS) as optimal conditions were 170 °C, and 130 °C with pH = 10, for this criterion. So, temperature was found to be the most influential parameter on COD and VS solubilization. Biodegradability batch anaerobic tests confirmed results obtained on WAS solubilization, that is to say that 170 °C and 130 °C with pH = 10 were optimal conditions, with respectively 45% and 21% of anaerobic digestion enhancement. Thus these two conditions were chosen for sludge treatment before continuous anaerobic digestion. Results, after stabilization have shown a better efficiency of 170 °C compared with 130 °C with pH = 10 pre‐treatment, since after anaerobic digestion it led to 71% of COD degradation and 59% of Total Solids (TS) degradation, with an improvement of 54% in biogas production. The main differences between those two pre‐treatments could be due to the pre‐treatments themselves more than to an effect on anaerobic digestion, because the first one led to a partial loss of WAS COD (near 17% of initial COD) and the second one to an increase in TS due to addition of base. Copyright © 2004 Society of Chemical Industry