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Lignocellulosic biomass, comprising of cellulose, hemicellulose, and lignin, is a difficult-to-degrade substrate when subjected to anaerobic digestion. Hydrothermal pretreatment of lignocellulosic biomass could enhance the process performance by increasing the generation of methane, hydrogen, and bioethanol. The recalcitrants (furfurals, and 5-HMF) could be formed at high temperatures during hydrothermal pretreatment of lignocellulosic biomass, which may hinder the process performance. However, the detoxification process involving the use of genetically engineered microbes may be a promising option to reduce the toxic effects of inhibitors. The key challenge lies in the scaleup of the hydrothermal process, mainly due to necessity of upholding high temperature in sizeable reactors, which may demand high capital and operational costs. Thus, more efforts should be towards the techno-economic feasibility of hydrothermal pre-treatment at full scale.

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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. Codigestion 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 lagphase period for methane production.

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