Dark fermentative hydrogen production from simple sugars and various wastewaters by a newly isolated Thermoanaerobacterium thermosaccharolyticum SP-H2

Abstract

The hydrogen-producing bacterium SP-H2 was isolated from a thermophilic acidogenic reactor inoculated with municipal sewage sludge and processing a carbohydrate-rich simulated food waste. Based on the 16S rRNA gene sequence, the bacterium was identified as Thermoanaerobacterium thermosaccharolyticum. The maximum growth rate was observed at 55–60 °C and pH 7.5. The H2-producing activity of the bacterium was studied using mono-, di- and tri-saccharides related to both hexoses (maltose, glucose, mannose, fructose, lactose, galactose, sucrose, raffinose, cellobiose) and pentoses (xylose and arabinose), as well as using real wastewaters (cheese whey, confectionery wastewater, sugar-beet processing wastewater). The highest H2 yield was observed during dark fermentation (DF) of maltose (1.91 mol H2/mol hexose or 77.8 mmol H2/L). The maximum H2 production rate was observed during DF of xylose (13.3 ml H2/g COD/h) and cellobiose (2.47 mmol H2/L/h). The main soluble metabolite products were acetate, ethanol and butyrate. The acetate concentration had a statistically significant positive correlation with the H2 content in biogas and the specific H2 yield. Based on the results of the correlation analysis, it was tentatively assumed that in the formic acid (mixed-acid) type fermentation, the rate of H2 production was higher than in the butyric acid type fermentation. With regard to real wastewater, cheese whey and confectionery wastewater were distinguished by a higher H2 yield (152 ml H2/g COD) and H2 production rate (0.57 mmol H2/L/h), respectively. The highest concentrations of confectionery wastewater and cheese whey, at which the DF process took place, were 5915 and 7311 mg COD/L, respectively. At the same time, SP-H2 dominated in the microbial community, despite the presence of indigenous microorganisms in wastewater. Thus, T. thermosaccharolyticum SP-H2 is a promising strain for DF of carbohydrate-rich unsterile wastewater under thermophilic conditions.