Two-stage ultrasound-mediated enzymatic hydrolysis enhances dark fermentative hydrogen production from wheat straw through relay-type sugar utilization

The recalcitrant structure of lignocellulosic biomass (LB) limits hydrogen production during dark fermentation. To address this limitation, a two-stage ultrasound-mediated enzymatic hydrolysis strategy (EH-WSU) was developed for wheat straw, integrating the structural disruption effect of strong-power ultrasound with the enzyme-assisted conversion effect of weak-power ultrasound. Six pretreatment parameters were optimized using single-factor experiments and a Bellman extreme learning machine model. Under optimized conditions, the reducing sugar concentration reached 8.2 g/L and cumulative hydrogen yield increased to 160.9 mL/g TS, representing increases of 49.1% and 51.6%, respectively, compared with the enzymatic hydrolysis control. EH-WSU promoted relay-type sugar utilization, triggering secondary hydrogen production associated with microbial succession, butyrate-oriented metabolic shifts, and enhanced sugar metabolism functions. Techno-economic analysis based on scale-up projections indicated a 6.5-year payback period and 19.1% return on investment, suggesting that EH-WSU provides a promising strategy for improving hydrogen production from LB.