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. An effort was given to isolate fungi from the soil of dumping area of cotton gin waste generated in cotton mill. Among the isolated fungi, Aspergillus flavus (UNF1) was found to be most efficient fungal strain for the pretreatment of CGW achieving 67.04% lignin removal with the release of 66% and 74.5% of cellulose and hemicellulose at pH 4.5, 122
rpm and 35°C. Further, 34.83 g/l total sugar by enzymatic hydrolysis and 15.44 g/l ethanol concentration, 0.45 g/g yield, 1.74 g/l/h productivity, 0.35 g/g biomass yield were obtained by fermentation in the bioreactor.
Overall, it has been demonstrated that the pretreatment of cotton gin waste with maleic acid followed by delignification is comparatively more effective providing the maximum pretreatment efficiency with less time and finally bioethanol production than the fungal pretreatment method. A substantial bioethanol production was achieved by biological pretreatment using the Aspergilus flavus (UNF1) fungal strain isolated from the soil of the dumping area of cotton gin waste in the cotton industry as a new source. The biological pretreatment is favorable than the organic acid pretreatment from an economical point of view by avoiding an additional step of chemical delignification involved in organic acid pretreatment. Furthermore, the biological method may be a promising alternative to the widely used sulfuric acid pretreatment which requires additional delignification and detoxification steps. The higher pretreatment time required for biological pretreatment (24days) in comparison to the acid pretreatment (few hours) may be reduced by genetically modifying the isolated fungal strain thereby making the process more economically viable. Thus, it has been concluded that the delignification process using Aspergilus flavus UNF1 as pretreatment agent and the microbial system involving the sequential use of S. cerevisiae and P. stipitis yeast strains for fermentation may be an attractive option for large-scale bioethanol production from cotton gin waste in future.