Bioethanol production from Cogongrass by sequential recycling of black liquor and wastewater in a mild-alkali pretreatment

Abstract

The present investigation evaluated Cogongrass as a prospective non-edible feedstock for second-generation bioethanol. An innovative recycling-based strategy was employed to minimize chemical consumption and wastewater generation during the mild-alkali pretreatment. The biomass was pretreated with 2% (W/V) sodium hydroxide (SH) solution at 85 °C for 90 min and subsequently subjected to separate saccharification and fermentation. Accordingly, the produced black liquor (BL) and waste wash-water (WWW) were sequentially withdrawn and partially fed back to the next pretreatment and washing processes, respectively. The results indicated that enzymatic hydrolysis of the biomass was substantially improved from 24.8% up to 90.8% following the alkali pretreatment; and afterward, it was progressively decreased to a minimum value of 66.4% upon recycling BL and WWW for ten times. Parallel hydrolysis experiments were conducted in the presence of three non-ionic environmentally-friendly surfactants at different concentrations (0.25–2% V/V). Notably, supplementation with 0.5% (V/V) Tween 80 significantly improved hydrolysis yield of recycled BL (RBL) pretreated substrate to a maximum value of ~88%. Fermentation of the RBL-pretreated Cogongrass did not reveal any cell inhibition and resulted in maximum ~76% ethanol production yield, while also saving 59% of the water consumption and over 45% of the alkali usage. Detailed analyses of the biomass using semi-quantitative techniques revealed that enhanced cellulose accessibility to the enzyme, decreased crystallinity, and extensive lignin and hemicellulose removal were the key factors contributing to the observed improvements.