Improvement in CH4/CO2 ratio and CH4 yield as related to biomass mix composition during anaerobic co-digestion

Sixteen data sets (two of which were measured in this study) with a combined total of 145 measurements of ultimate methane yield (UMY) during mono- and co-digestion of ternary biomass mixtures were used to assess impact of co-digestion on the relative change in UMY (ΔUMY) as a function of biomass mix composition. The data involved 9 biomass materials (brewery spent grains, chicken manure, cow manure, fresh grass clippings, pig manure, primary sewage sludge, vegetable food waste, wheat straw, and rice straw). Results of the assessment shows that co-digestion in 85% of yields positive values of ΔUMY regardless of the biomass materials used, however, a smaller fraction (15%) resulted in negative ΔUMY during co-digestion. The data further indicate that for each set of ternary biomass material mixtures there exists an optimal biomass mix composition at which ΔUMY is at a maximum. Statistical analyses based on the data used here indicate that the maximum value of ΔUMY (ΔUMYmax) is always positive regardless of biomass materials being co-digested.