Whereas biodigestion of crude glycerin as a mono-substrate is not viable as no nitrogen would be available for formation of bacterial mass, the addition of crude glycerin as a carbon source to substrates with sufficient nitrogen content, but lack in energy content, seems a promising approach. When supplementing 6% wt of glycerin to pig manure and maize silage, Amon et al. observed biogas production of 1.114 and 0.679 m3 kg−1 of volatile solids (VS, normal basis), respectively, as well as an increase in methane proportions of 5.6% and 9.8%, respectively, causing higher than the summarized CH4 yields of both substrates if digested separately, and contributed their findings to a synergistic “co-fermentation effect” which showed best results with glycerin added in the range of 3–6% wt