Optimization of bioethanol production from Thermochemically-pretreated water hyacinth using Thermophilic microbial consortium

Abstract

The rapid depletion of fossil fuels and their associated environmental consequences has sparked interest in the production of biofuel (bioethanol) from lignocellulosic biomass, such as water hyacinth (WH). However, the lignocellulose's refractory nature renders it difficult to convert to bioethanol, thus necessitating its pretreatment to increase enzymatic hydrolysis. Unlike yeast that utilizes only (C6) sugar, thermophilic bacteria can efficiently convert lignocellulosic hydrolysate (C5 and C6) sugars to bioethanol. The main objective of this study was to optimize bioethanol production from WH pretreated with NaClO2 in boiling water using thermophilic consortium. The specific objectives were; to isolate and characterize thermophilic bacteria using morphological and biochemical approaches, to develop microbial consortium, to pretreat and characterize WH , and to optimize the production of bioethanol with microbial consortium. The bacteria were isolated using serial dilution and plating technique on nutrient agar. Microbial consortium development was based on the degradation of filter paper and untreated WH. WH was treated in boiling water for 4hr with and without NaClO2 addition. Fourier transform infrared (FTIR) analyses were studied on NaClO2 with boiling water-treated and raw WH. The main factors that affect ethanol production such as temperature (40-60oC), time(48-96hr), and inoculum dosage(8-12%v/v) were chosen to be optimized by central composite design (CCD). The results of this research showed that nine thermophilic bacteria were identified and designated BO1, BO2, BOY, BOW, SO, OL, NW, YF, and CF. The bacteria isolates were bacillus, cocci, gram-positive, and gram-negative. Physiological characterizations indicate that all isolates could grow at temperatures 50-55oC, NaCl concentration of 2% (w/v), and a pH of 5.5-8.5. The biochemical features of the isolates showed that all of the isolates were positive in glucose fermentation, starch hydrolysis, and EMB agar fermentation, but the results of the other biochemical tests were different. Microbial consortium, developed from three isolates (BO1, BO2 & OL), were efficient at degrading filter paper and untreated WH as substrates. The time yielding maximum total reducing sugar (TRS) was 2nd hr resulting in 155 mg/g WH and 113 mg/g WH from, with and without NaClO2 addition pretreated samples respectively. FTIR characterizations of the pretreated sample revealed both breakdown and an increase in cellulose and hemicellulose content. The CCD indicated that the optimum conditions for fermentation were inoculum dose 8.1 %( v/v), temperature 48.8oC, and time 52.3hrs, which resulted in 7.2g/L predicted ethanol concentration. Meanwhile, 7.7g/L ethanol was produced during experimentation which is in close agreement to predicted value. Conclusively, utilizing NaClO2 and boiling water as pretreatment method and thermophic consortium as fermentation microbes is a good alternative for TRS and bioethanol production. This study suggests that more variables be tested in the pretreatment of WH to optimize TRS and reduce inhibitory byproducts.