Anaerobic digestion (AD) of wastewater sludge is an effective approach to produce biomethane and sub-sequently reduce the amount of sludge disposal to landfill generated from wastewater treatment plants (WWTPs). Recently, anaerobic co-digestion (ACD) of lipidic-waste such as fat, oil, and grease (FOG) has received much attention for the enhancement of biomethanation in WWTPs. However, there are several major challenges associated with ACD of FOG which includes inhibition by long chain fatty acids, sludge floatation and washout, and scum formation. This review summarizes the scientific and engineering aspects of the FOG supplementation as a co-substrate in AD as well provides appropriate solutions for challenges encountered during the anaerobic operation. The discussion on the existing pretreatment approaches (including mechanical, thermo-chemical, and biological) for rapid degradation of FOG was also highlighted. Metagenomic analysis elucidates the microbial community and the interspecies interactions which are responsible for the degradation of FOG and its biomethanation during ACD. Addition of FOG to AD was found be economically feasible in municipal WWTPs. (C) 2018 Published by Elsevier Ltd.