The aim of the present research work was to optimise biogas production from cattle manure by adding crude glycerin from the biodiesel industry. For this purpose, 6%v/v crude glycerin (the optimum amount according to previous research) was added to ground manure and the mixture was sonicated to enhance biodegradability prior to anaerobic co-digestion at 55°C. Two different reactors were used: continuously stirred (CSTR) and induced bed (IBR). The methanol and pure glycerin contents of the crude glycerin used in this study were 5.6% and 49.4% (w/w), respectively. The best results when operating in CSTR were obtained for an organic loading rate (OLR) of 5.4kg COD/m 3 day, obtaining 53.2m 3 biogas/t wet waste and 80.7% COD removal. When operating in IBR, the best results were obtained for an OLR of 6.44kg COD/m 3 day, obtaining 89.6% COD removal and a biogas production of 56.5m 3 /t wet waste.