Continuous biohydrogen production by mesophilic anaerobic fermentation of organic solid waste

Abstract

As a sustainable energy source with minimal or zero use of hydrocarbons and high energy yield (122 kJ/g), hydrogen is a promising alternative to fossil fuels. Conventional physicochemical methods for hydrogen production require electricity derived from fossil fuel combustion or nuclear fission; thus interest in biohydrogen production has increased substantially. Among biological processes, fermentative processes produce $H_2$ at a higher rate than photosynthetic processes. In addition, fermentative production can convert organic wastes to more valuable energy resources. This study was performed to evaluate continuous hydrogen production in mesophilic digestion of organic waste. Firstly, the effect of substrate concentration on continuous hydrogen production was investigated in a continuous-flow stirred-tank reactor using sucrose as a model substrate. The hydraulic retention time (HRT), pH, and temperature were maintained at 12 h, over 5.3±0.1, and 35±1℃, respectively. At the influent sucrose concentration of 5 g COD/L, start-up was not successful. When the reactor was started-up at 30 g COD/L, it exhibited stable H2 production for 271 days at influent sucrose concentrations of 10-60 g COD/L. Hydrogen production depended on substrate concentration such that the highest values of 1.09 mol $H_2$/mol $hexose_added$, 1.22 mol $H_2$/mol $hexose_consumed$, 7.65 L $H_2$/L/d, and 3.80 L $H_2$/g VSS/d were recorded at a sucrose concentration of 30 g COD/L. All bacterial species detected by polymerase chain reaction-denaturing gradient gel electrophoresis analysis were belonged to $H_2$-producing Clostridium spp. At influent sucrose below 20 g COD/L, the $H_2$ yield per $hexose_consumed$ decreased along with a significant decrease in soluble fermentation product distribution. Clostridium scatologenes, a $H_2$-consuming acetogen, was found in the range. At influent sucrose over 35 g COD/L, substrate overload occurred and caused a decrease in the carbohydrate degradatio...