With the objective of maximising methane production from waste-activated sludge (WAS), codigestion with fatty wastewater (FW) collected from restaurants was studied and the application of both a 170 degrees C thermal pretreatment and thermo-alkaline/saponification to mixed WAS/FW waste was evaluated. Batch anaerobic digestion tests showed that recalcitrant compounds were formed during the 170 degrees C pretreatment of fatty wastewater and thermo-alkaline pretreatments (80 degrees C or 120 degrees C, pH 8, 9 or 10) led to an increased initial methane production rate and to a very slight impact on the methane potential of the mixed waste (+4-7%). Semi-continuous anaerobic digesters were fed over 4 months with WAS, two mixed substrates WAS/FW 90/10 and WAS/FW 60/40, and with WAS/FW 90/10 pretreated at softer conditions (80 degrees C and pH = 8, 0.14 g(KOH) g(VS)(-1)). This pretreatment led to a significant increase in the methane production of semi-continuous reactors (+58%). Finally, this study showed the feasibility of the codigestion of waste activated sludge with highly concentrated fatty wastewater (40% in volume, 49% in VS and 73% in COD, equivalent to 7 g L-1 of lipids). It resulted in specific methane production of 362 mL CH4 g(-1) VS whereas WAS alone produced only 116 mL CH4 g(-1) VS.