Conversion of Cotton Gin Waste to Bioethanol: Pretreatment, Hydrolysis and Fermentation

Abstract

The present research focuses on the conversion of cotton gin waste, a potential lignocellulosic biomass produced in cotton industry, to bioethanol. The major technological hurdle for utilizing this waste to bioethanol is the pretreatment process to release sugar components for ethanol fermentation. Even the most effective pretreatment method using dilute sulphuric acid suffers from several drawbacks such as the process is hazardous and produces toxic by-products which affect the growth of yeast during fermentation leading to lower bioethanol yield. Therefore, an alternative pretreatment strategy is essential for the removal of lignin, thereby releasing cellulose and hemicellulose as fermentable sugar components from cotton gin waste. In this context, pretreatment of biomass using organic acid might be attractive as it produces less toxic by-products and the method is environment-friendly. It is further reported that biological pretreatment is advantageous over chemical pretreatment methods because of the requirement of mild reaction conditions, low energy and formation of minimal toxic byproducts. Therefore, in the present research, pretreatment of cotton gin waste using both biological and organic acid treatment was performed and the results were compared with the most widely used dilute sulphuric acid pretreatment. Among the four organic acids, maleic acid pretreatment was found to be the most efficient yielding maximum pentosan sugar of 125.50±0.67 g/g (83% C5 sugar release) which was comparable to the most widely used sulfuric acid (132.08±1.06 g/g yield) pretreatment at optimum condition o130°C, 45 min and 500mM. However, the sulfuric acid pretreatment produced more toxic by-products in comparison to organic acids. The fermentation of 41.75 g/l mixed hydrolysate (C5 and C6) obtained from maleic acid pretreated biomass using sequential culture of Saccharomyces cerevisiae and Pichia stipitis yeast strains achieved maximum 18.74 g/l ethanol concentration, 0.48 g/g ethanol yield, 2.25 g/l/h ethanol productivity, 88% maximum theoretical yield and 0.30 g/g biomass yield at 30°C, 200 rpm and 5.5pH in a bench top bioreactor.