The aim of this study was to maximise methane production from waste activated sludge (WAS) originating from extended aeration process and presenting a low methane potential (190 mL CH4.g-1 OM). WAS co-digestion with fatty residues (FR, 560 mL CH4.g-1 OM) produced during pretreatments of the effluents from wastewater treatment plants in the Lille area and fatty wastewaters (FW, around 700 mL CH4.g-1 OM) collected from restaurants was assessed by batch experiments. Moreover saponification/alkali pretreatments improved kinetics of anaerobic digestion but had a low impact on methane potential (+ 6-7%) of the mixed waste composed of 66.6% of FW, 33.3% of WAS and 0.1% of FR. As results did not depend on pH ranging from 8 to 10 (addition of 0.12 to 0.21 gKOH gOM-1 ) nor temperature ranging from 80 to 120 °C, the least severe studied pretreatment conditions (80°C and pH=8) may be selected for further studies on continuous anaerobic reactors.