A cost-effective room temperature alkali pretreatment method for enhanced bioethanol production from rice straw

The present study aimed to optimize NaOH pretreatment at room temperature (26°C), an economic approach not previously explored for rice straw (RS) pretreatment. Statistical optimization of RS pretreatment at room temperature gave an optimum pretreatment condition of 7.4% (w/v) biomass loading with 3.34% NaOH soaking for 16.04 h pretreatment time. The pretreatment of raw RS at optimum conditions resulted in 56.2%, w/w cellulose content and 67% delignification with an enhanced crystallinity index of 45.7% compared to raw RS, 33.2%. The effectiveness of the pretreatment method developed in this study was also assessed by the high cellulose conversion efficiency of 93% after enzymatic hydrolysis. Further, this study exploits the significance of pre-hydrolysis time in achieving a high cellulose conversion rate to bioethanol in the simultaneous saccharification and fermentation (SSF) process. A pre-hydrolysis time of 8 h at an enzyme dosage of 30 FPU/g pretreated RS followed by the SSF process of 15% (w/v) room temperature alkali pretreated RS produced a high ethanol titer of 24.5 g/L with a productivity of 1.02 g/L/h. Further increment of pretreated RS loading to 20% (w/v) resulted in an improved ethanol titer of 32.2 g/L with a productivity of 1.34 g/L/h (63% of maximum ethanol yield). Thus, the study assesses the potential of the developed pretreatment method operated with no external energy requirement, negligible cellulosic fraction loss after the pretreatment process, and a high overall ethanol yield of 205 L/tonne alkali pretreated RS.