Bioethanol production from sugar beet pulp dark fermentation residues: A cascade biorefinery and kinetic modelling approach

The transition to a circular bioeconomy requires the complete valorisation of agro-industrial side-streams through zero-waste cascading biorefineries. This study investigates the conversion of the solid residue from dark fermentation (SRDF), a byproduct of biohydrogen and organic acid production from sugar beet pulp, into bioethanol. Compositional analysis revealed that dark fermentation acts as an effective biological pretreatment, significantly increasing the accessibility of the lignocellulosic matrix by removing amorphous fractions. Enzymatic saccharification of SRDF achieved a glucose yield of 83.33%, nearly doubling the 47.36% yield obtained from raw pulp. To enhance volumetric productivity, a co-fermentation strategy using sugar beet molasses was evaluated. While pure SRDF hydrolysates demonstrated high biocompatibility, supplementation with 30% w/w molasses triggered a stuck fermentation profile, leaving significant residual fructose. A hybrid logistic-Monod kinetic model was developed to elucidate these interactions. The model identified a 45% reduction in microbial carrying capacity (X max ) caused by the synergistic inhibitory effect of high mineral salt concentrations from molasses and residual organic acids from the dark fermentation stage. These findings establish critical operational limits for co-substrate utilization and provide a mechanistic framework for optimising second-generation sugar beet biorefineries.