Improvement of Exhausted Sugar Beet Cossettes Anaerobic Digestion Process by Co-Digestion with Pig Manure

Exhausted sugar beet cossettes (ESBC) are one of the main byproducts generated in the sugar industries. The pellets of dried pulp of ESBC used in this study are composed of 85% beet pulp and 15% molasses and have a high lignocellulosic-type organic matter content (91% volatile solids) and a nitrogen deficiency. Mesophilic anaerobic co-digestion in batch of dried pellets of ESBC and pig manure (PM) was studied in this paper for five mixture ratios. The mixtures were selected on the basis of the carbon/nitrogen ratio, leading to ESBC:PM percentages of 0:100, 32:68, 48:52, 72:28, and 100:0. Co-digestion increased the methane yield in all cases relative to the digestion of ESBC alone. Acetic and propionic acids were the main volatile fatty acids observed in the tests. Initial accumulation of propionic acid in the reactors with higher ESBC content (and especially for the 100:0 reactor) indicated the difficulties associated with ESBC degradation. The best results were obtained for co-digestion of ESBC and PM in a 32:68 mixture, leading to 103% and 30% improvement in the specific methane production relative to the digestion of ESBC and PM, respectively. Furthermore, this mixture ratio was also optimal to minimize the lag-phase period for methane production.