Strategic biorefinery approach for closing the loop in the sugar industry: Valorisation of exhausted pulp into PHB via integrated bioconversion and kinetic modelling

Abstract

The transition towards a circular bioeconomy requires the conversion of agro-industrial residues into valuable bioproducts. This study investigates an integrated approach to producing polyhydroxybutyrate (PHB) using exhausted sugar beet pulp (ESBP) as a sustainable source of material. Initially, the ESBP was processed via thermophilic dark fermentation to yield a volatile fatty acid (VFA) rich effluent (15.1 g/L) dominated by acetic and butyric acids. Subsequently, the effect of the C/N ratio (44, 35 and 20) on PHB synthesis by Cupriavidus necator was evaluated. The experimental results showed that the raw effluent (C/N 44) triggered the highest specific polymer accumulation (77.5% of cell dry weight). To gain insight into the bioconversion dynamics, an unlumped kinetic model was developed and validated (R² > 0.95), effectively decoupling residual biomass growth from PHB accumulation. The model successfully identified a critical harvest time by incorporating an intracellular degradation term, thereby preventing product loss under substrate-limiting conditions. For the C/N ratios of 44 and 35, the maximum volumetric productivity was in the range of 0.063–0.066 g·L⁻¹ ·h⁻¹ and the optimum harvesting time was in the range of 48.2–49.1 h, during which the PHB accumulation would be 65.3–65.5%. These findings suggest that non-supplemented fermentation of sugar beet pulp could be a cost-effective and efficient process for producing sustainable bioplastics.