Comparison of the acidogenic and methanogenic potential of agroindustrial residues

The methanogenic and acidogenic potentials of six different agroindustrial residues, i.e. of fruit pulps and brewery residues, were determined. For all substrates, the methanogenic conversion yield was systematically higher than the acidogenic one in Chemical Oxygen Demand (COD) terms, ranging from 0.46 to 0.87 gCOD_CH4/gCOD_substrate_fed and from 0.24 to 0.56 gCOD_tVFA/gCOD_substrate_fed, respectively. During methanogenic conversion, brewery trub exhibited the highest methane potential (304 mlCH4/gCOD_substrate). Trub also exhibited the highest total volatile fatty acids (tVFA) concentrations in the mixed liquor (ML) during acidogenic conversion (29.7 gCOD_tVFA/kgML). Acetic, butyric and caproic acids were the main carboxylates produced by the different substrates. Despite the lower conversion yields, the economic value of the acidogenic product (carboxylate streams) is higher than that of methanogenic conversion (methane) due to the higher value of carboxylates and their potential use in finer applications (e.g. bio-based products) compared to energy production form methane.