The aim of this paper was to assess the possibility of maximising methane production from waste activated sludge (WAS) by codigestion with fatty wastewaters (FW) collected from restaurants and to evaluate the impact of saponification/alkaline pretreatment of the mixed substrate. Batch anaerobic digestion (BMP) tests were carried out to determine the conditions of saponification/alkaline pretreatment (temperature and pH). Four substrates were used to feed semicontinuous anaerobic digesters during 4 months : WAS, two mixed substrates WAS/FW 90/10 and 60/40 vol/vol, and pretreated WAS/FW 90/10. As all the studied pretreatments led to almost the same BMP results, the softer one (80°C and pH=8, 0.14 gKOH gVS -1) was selected. It led to an enhancement of degradation kinetics, a very low impact on BMP values (+6%) and a significant increase of methane production in semicontinuous reactors (+58%). On the other hand, specific production of methane in semicontinuous ractors was lower than BMP values, particularly for waste activated sludge. The codigestion of waste activated sludge with fatty wastewaters from restaurants in high concentration (40% in volume) allowed to increase methane specific production to 362 mL CH4.g-1VS whereas WAS alone produced 116 mL CH4.g-1VS. This study showed feasibility of codigestion of waste activated sludge with fatty wastewaters in high concentration (49% in VS and 73% in COD, corresponding to a concentration of lipids of 7g L-1). The interest of alkaline pretreatment of continuous digestion of the mixed waste was also shown.