Conversion of food waste into hydrogen by thermophilic acidogenesis

Abstract

Conversion of food waste into hydrogen by thermophilic acidogenesis was investigated as a function of organic loading rate (OLR), hydraulic retention time (HRT) and pH in a continuous stirred tank reactor. In order to identify hydrogen-producing microorganisms, denaturing gradient gel electrophoresis (DGGE) of the polymerase chain reaction (PCR) - amplified V3 region of 16S rDNA analysis was conducted at each tested pH. The conversion of food waste into hydrogen was strongly influenced by the operational conditions. The hydrogen production was increased as OLR increased up to 8 gVSl(-1) d(-1), but drastically decreased at 10 gVSl(-1) d(-1). The yield of hydrogen was decreased from 2.2 to 1.0 mol-H-2/mol-hexose consumed as HRT decreased from 5 to 2 days. More carbohydrates in the food waste were decomposed at longer HRT, 76-90%, at HRT of 2-5 days. The hydrogen production peaked at pH 5.5 +/- 0.1 and significantly decreased at pH 5.0 +/- 0.1. The biogas produced was composed of hydrogen and carbon dioxide, but no methane was detected at all tested conditions. The hydrogen contents in the gas produced were more than 55% (v/v) and not sensitive to all tested conditions. The optimum operational condition for continuous hydrogen production from the food waste was obtained at 8 gVSl(-1) d(-1), 5 days HRT and pH 5.5 +/- 0.1 where the hydrogen production rate, content, yield and the efficiency of carbohydrate decomposition were 1.01 H-2/l-d, 60.5% (v/v), 2.2 mol-H-2/mol-hexose consumed and 90%, respectively. The hydrogen production was related with the concentration of total organic acids (TOA) which was strongly dependent on that of butyrate indicating that the reaction was mainly butyrate fermentation. The hydrogen-producing microorganism of Thermoanaerobacterium thermosaccharolyticum that involved in acetate/butyrate fermentation, was detected with strong intensity at all tested pHs by denaturing gradient gel electrophoresis (DGGE) of the polymerase chain reaction (PCR) - amplified V3 region of 16S rDNA analysis and sensitive to the tested pHs. The experimental results indicated that effective hydrogen production from the food waste could be obtained continuously by thermophilic acidogenesis at proper operational condition.