Alkaline twin-screw extrusion pretreatment of Miscanthus with recycled black liquor at the pilot scale

Process wastewater (black liquor, BL) is one of the critical targets for reducing operational costs in alkaline pretreatment for bioethanol production. However, no previous study has evaluated the recycling or reuse of BL at the pilot scale. Thus, in this study, the feasibility of recycling pretreatment-generated BL for further pretreatment processes was investigated. The pretreatment was conducted with a twin-screw extrusion system under a barrel temperature of 100 °C, 0.6 M NaOH, biomass feeding of 5.5 kg/h, liquid feeding of 33 L/h, screw speed of 80 rpm, and residence time of 8 min. After enzymatic hydrolysis, fermentable glucose was produced at 313.6 g/kg biomass. In the subsequent pretreatment with the first- and second-generated BL, the converted glucose concentrations slightly decreased to 303.0 and 261.1 g /kg biomass, respectively. Notably, glucose yield from the biomass pretreated with the first BL was 96.7% of that treated with 0.6 M NaOH; thus, 69.7% of NaOH could be saved. In addition, the main BL factors contributing to lower pretreatment efficacy and saccharification yield were identified to be high viscosity and high lignin content. Thus, these results on BL recycling at the pilot scale might contribute to the development of new processes for bioethanol production from lignocellulosic biomass.