Fractionation of Lignocellulosic Fibrous Straw Digestate by Combined Hydrothermal and Enzymatic Treatment

In industrial-scale biogas production from cereal straw, large quantities of solid fiber-rich digestate are produced as residual material. These residues usually contain high amounts of cellulose, hemicellulose and lignin and thus have potential for further utilization. However, they also contain impurities such as ammonia and minerals, which could negatively affect further utilization. Against this background, the present study investigates how this fibrous straw digestate can be fractionated by a combined hydrothermal and enzymatic treatment and what influence the impurities have in this process. Therefore, it is analyzed how the fractions cellulose, hemicellulose and lignin are modified by this two-stage treatment, using either raw digestate (including all impurities) or washed digestate (containing only purified fibers) as the substrate. For both substrates, around 50% of the hemicellulose is solubilized to xylans after 50 min of hydrothermal treatment using steam at 180 ∘ C. Furthermore, by subsequent enzymatic treatment, around 90% and 92% of the cellulose and hemicellulose still contained in the solids are hydrolyzed to glucose and xylose, respectively. Lignin accumulates in the remaining solid but structurally degrades during the hydrothermal treatment, which is indicated by decreasing ether and ester bond contents with increasing treatment times. Impurities contained within the raw digestate do not hinder this fractionation; they even seem to positively affect hemicellulose and cellulose valorization, but apparently lead to a slightly higher lignin degradation.