Ethanol Production from Biodiesel-Derived Crude Glycerol by Enterobacter aerogenes with Tuna Condensate as a Nitrogen Source using Fed-batch Fermentation Process

Abstract

The ethanol production from crude glycerol by Enterobacter aeroegenes TISTR 1468 was investigated through the application of simple medium and fed-batch cultivation. Waste-based raw materials, crude glycerol from biodiesel plant and tuna condensate from tuna canning factory, were used to substitute the expensive complex medium. The optimum crude glycerol concentration was 21.43 g/L. Using only crude glycerol and tuna condensate (GT medium) as carbon and nitrogen source for studying the effect of initial C/N ratio (115 - 365 g/g) revealed the insignificant impact on ethanol production in every C/N ratio tested. Meanwhile, GT medium complemented with inorganic salts resulted in 2-folds ethanol production at C/N ratio of 115 g/g. Phosphate buffer were the most influential factors in inorganic salts with 77.6% contribution. However, phosphate buffer could be replaced by keeping the pH constant at 7.0. Under optimum operation (20 g/L glycerol, initial C/N of 115 g/g and the pH of 7.0), the glycerol to ethanol yield was 24% higher than the theoretical conversion of glycerol to ethanol or 12.33 g/L of ethanol due to the presence of carbon source in tuna condensate. The ethanol selectivity was also 3-fold higher than the complex medium. Hence, cultivation under microaerobic conditions was optimized where redox potential (ORP) sensor was employed as aeration controller to replace the conventional oxygen sensor. The performance using batch cultivation was evaluated on various aeration: continuous aeration at 0.5 vvm; controlled aeration at -350 mV and -400 mV; and no aeration. The 72-h batch fermentation showed that a more reductive environment (lower aeration) increased ethanol and yield, but decreased the productivity. No aeration system achieved the highest ethanol (18.78 g/L; 0.94 g-ethanol/g-glycerol). Meanwhile, a more oxidative environment (higher aeration) fastened the cell growth, but ethanol concentration and yield decreased. The highest specific ethanol production rate was achived at B400. Fed-batch with two-stage aeration strategy (-350 mV and - 400 mV) acquired higher ethanol (30.31 g/L; 0.86 g-ethanol/g-glycerol) than with single-stage aeration (25.95 g/L) and batch process (12.33 g/L). This study has revived the potential of crude glycerol biotransformation to ethanol and opened up opportunities for optimization of related microaerobic systems.