Microbial dynamics in ethanol fermentation from glycerol

Bioethanol is a clean and renewable biofuel, which can be produced from low cost and renewable feedstocks via fermentation process. The microbes present in an upflow anaerobic sludge blanket (UASB) reactor for wastewater treatment were tested for application in batch fermentation of ethanol at varying glycerol concentrations, and pHs of culture medium. Ethanol was the main fermentative product in this work. Ethanol concentration between 3.9 and 11.1 g L−1; and ethanol yields between 0.66 and 0.81 mol ethanol mol−1 glycerol were obtained. The ethanol concentration and yield were highly dependent on the initial glycerol concentration. The highest ethanol concentration of 11.1 g L−1 was obtained after 72 h of fermentation at the initial glycerol concentration of 45 g L−1. Ethanol production and glycerol utilization were highly correlated. Denaturing gradient gel electrophoresis (DGGE) pattern revealed there were microbial selection and adaption during the fermentation. 16S rRNA gene sequences showed that the seed culture comprised largely of uncultured bacteria with various Proteobacteria, Bacteroidetes, and Firmicutes. Enrichment of Proteobacteria, particularly Enterobacter and Klebsiella bacteria occurred within 24 h of the fermentation and the ethanol producing microcosm at 72 h was dominated >95% by these 2 strains. The major ethanol producers were identified as Enterobacter and Klebsiella strains. This work demonstrated the potential use of biodiesel derived glycerol as a feedstock for the ethanol production, and revealed the major roles of Enterobacter and Klebsiella in the ethanol production from glycerol.