Anaerobic co-digestion of grease sludge and sewage sludge: The effect of organic loading and grease sludge content

► Grease sludge is a suitable co-substrate for sewage sludge anaerobic digestion. ► Co-digestion of GS and SS increases biogas yield up to 55%. ► Biogas yield of co-digestion systems depends on both GS content and OLR. ► The limit GS organic loading which provides for stable operation is 2.4 g VSGS/L/d. ► Anaerobic co-digestion of GS and SS may increase the energy recovery of an WWTP.