Toward Waste-Minimising Biorefineries: Integrating Fermentation and Hydrothermal Fulvification of Grass

Biochemistry-based biorefineries frequently valorize agricultural residues but rarely achieve full carbon utilization, leaving substantial downstream solids and liquids underexploited. This study presents an integrated fermentation-hydrothermal fulvification (HTF) strategy that upgrades grass biomass into lactic acid and artificial humic substances. Acid pretreatment generated 45.1 g/L total sugars and up to 36 g/L lactic acid, while base-pretreated substrates yielded 37.1 g/L of total sugars and 29.7 g/L lactic acid, with lower inhibitor levels. Subsequent HTF under strongly alkaline conditions converted lignin-rich fermentation residues into hydrochars with yields ranging from 21.2 to 45.4 wt.% and produced artificial humic acids up to 10.8 wt.% with elevated carbon contents of 74.5 wt.%. A pronounced decrease in O/C ratio indicates progressive dehydration and condensation reactions. Process liquids reached up to 36.2 g/L total organic carbon and contained organic acids, including a chromatographic peak assigned to lactic acid up to 9.9 g/L, likely formed in situ under alkaline HTF conditions, alongside phenolic compounds up to 1121 mg/L. Spectroscopic and colloidal analyses confirmed partitioning into humic-rich solids and fulvic-like dissolved organics. Humic acid yields from post-fermentation residues remained comparable to standalone HTF of raw grass, indicating that upstream lactic acid production does not compromise downstream humification. By converting the remaining solid residues through HTF into artificial humic substances that do not require further separation, the integrated system supports a waste-minimising biorefinery approach, in which all major fractions are functionally utilised as products while reducing biological risks and potential greenhouse gas emissions associated with untreated residues.