Enhanced Production of Biofuel from Sugar Industry Waste

Abstract

The continuous upturn in the cost of petroleum and increasing energy crises has directed the world’s interest to focus on alternative renewable energy resources. Recently, bioethanol is emerging as an alternative fuel to substitute gasoline, which is petroleum derived source of conventional energy. A significant variety of feedstocks can be used for the production of bioethanol; however, sugar industry waste is considered as the best option to evade food vs. fuel debate. In this study, two industrial wastes i.e. sugarcane molasses and bagasse were converted to bioethanol using different microbial strains and pretreatment strategies. To improve bioethanol production, different yeast strains were isolated from numerous sources, and MZ-4 labeled strain was selected on the basis of its maximum ethanol tolerance i.e. 15% (v/v). MZ-4 strain was then identified as Saccharomyces cerevisiae by 18SrRNA sequencing, and later compared with a comparatively better commercially available strain Lalvin EC-1118 strain, which was maximally tolerant to 18% (v/v) ethanol. The physicochemical parameters were optimized for both strains independently. During batch fermentation by strain MZ-4, the maximum ethanol yield was determined as 11.1% (v/v) with 69.3% fermentation efficiency, when pH 5 was adjusted for molasses dilution containing 25% (w/v) sugar concentration with 10% inoculum before incubation at 33°C for 72 h. However, Lalvin EC-1118 strain showed comparatively less ethanol yield of 10.9% (v/v) with fermentation efficiency of 68.1% under its optimal conditions i.e. pH 4.5; inoculum size of 7.5% and incubation at 30°C for 72 h. Additionally, the study on effect of various nitrogen sources showed that, MZ-4 produced more ethanol when 0.1% (w/v) NH 4 Cl was added; whereas, Lalvin EC-1118 demonstrated better production after the addition of 0.1% (w/v) (NH 4 ) 2 HPO 4 . Moreover, it was also observed that MZ-4 and Lalvin EC-1118 exhibited better yields when 0.01 and 0.04% (w/v) of K 4 Fe(CN) 6 was used respectively, as a chelating agent. During the fed batch fermentation, Lalvin EC-1118 produced a greater ethanol yield of 13.9% with fermentation efficiency of 81.1%, when 1.090 specific gravity of molasses dilution was adjusted and fed after every 12 h. However, the strain MZ-4 showed better fermentation efficiency of 83.2% with comparatively less Enhanced production of biofuel from sugar industry waste Page xviiiethanol yield i.e. 13.5% (v/v) by using molasses dilution of same specific gravity and 24 h feeding interval. Meanwhile, one of the main challenges for bioethanol production from lignocellulosic material such as sugarcane bagasse is the recalcitrance of the biomass. A second study evaluated the efficiency of an ionic liquid (IL) i.e. 1- butyl-3-methyl imidazolium acetate ([C 4 mim][OAc]) pretreatment at 110°C for 30 min, and compared it with high temperature autohydrolysis pretreatment (i.e. 110°C for 30 min, 190°C for 10 min and 205°C for 6 min). It was found that sugarcane bagasse exhibited a considerable decrease in lignin content, reduced cellulose crystallinity, and enhanced cellulose and xylan digestibility, when subjected to IL pretreatment. Pretreated samples were also characterized by Fourier transform infrared spectroscopy to verify these findings. Altogether, cellulose and xylan digestibility of IL pretreated bagasse was determined as 97.4 and 98.6% after 72 h of enzymatic hydrolysis, respectively. In the case of autohydrolysis, the maximum of cellulose and xylan digestibility was determined after 72 h as 62.1 and 5.7% from bagasse pretreated at 205°C for 6 min, respectively. X-ray diffraction analysis also showed a significant reduction in crystallinity of IL pretreated bagasse samples. During fermentation process, IL pretreated and autohydrolyzed bagasse (205°C for 6 min) exhibited maximum ethanol production of 78.8 and 70.9 mg/g substrate after 24 h of fermentation, respectively. Comparatively, the fermentation of bagasse autohydrolyzed at 190°C for 10 min and 110°C for 30 min yielded maximum ethanol of 66.0 and 28.4 mg/g substrate by using S. cerevisiae Lalvin EC-1118, respectively. Thus it can be concluded that, fed batch fermentation is employed for the maximum ethanol yield from sugarcane molasses using Lalvin EC-1118 strain, while IL pretreated bagasse gives maximum yield when fermented with strain MZ-4.