Effect of substrate concentration on hydrogen production and 16S rDNA-based analysis of the microbial community in a continuous fermenter

The effect of substrate concentration on hydrogen production was investigated using a continuous-flow stirred-tank reactor (CSTR). Sucrose was used as a model substrate. The CSTR was started at a sucrose concentration of 30 g COD/L and exhibited stable H₂ production for 271 days at inlet sucrose concentrations of 10-60 g COD/L. Hydrogen production depended on the substrate concentration such that the highest values of 1.09 mol H ₂/mol hexose_added, 1.22 mol H₂/mol hexose _consumed, 7.65 L H₂/L/d, and 3.80 L H₂/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 H₂-producing Clostridium spp. At inlet sucrose concentrations below 20 g COD/L, the H₂ yield per hexose _consumed decreased along with a significant decrease in the n-butyrate/acetate ratio. At the same range of sucrose concentrations, Clostridium scatologenes (an H₂-consuming acetogen) was found in the sludge. At inlet sucrose concentrations over 35 g COD/L, substrate overload occurred and caused a decrease in the carbohydrate degradation efficiency and H₂ yield per hexose_added.