Maximization of H₂ yields by lowering H₂ inhibition

The idea of using low carbohydrate concentrations for maximizing H₂ production sounds counter intuitive since, for economic purposes, one would like to use a highly concentrated carbohydrates stream "as is" and recover as much as H₂ as possible. However, the yields, as demonstrated in this study, would decrease some 30% when the carbohydrate concentration decreases from 10 to 2.5 g/L. Biomass concentrations increased with decreasing HRT and increasing glucose concentration reaching a high concentration of 24.6 g/L (10 g/L, 1 hr HRT). However, the biomass concentration was relatively constant at the 2.5 g/L level (2.4 g/L) and the biomass washed out overnight as the HRT was decreased from 2.5 to 1 hr. Acetic and butyric acids were the dominant soluble products and increased as the initial glucose concentration and HRT increased. Acetone, ethanol, propanol, butanol and propionic acid were also detected but their concentrations were always < 20 mg/L (usually < 10 mg/L) each. The H₂ yields increased as the HRT increased and the substrate concentration decreased. The ability of the bacteria to flocculate and remain in the reactor allowed high conversion efficiencies at low HRT. All reactor runs except the 2.5 g/L run had bacteria flocculated. The tendency to flocculate increased as the glucose concentration increased and the HRT decreased. This is an abstract of a paper presented at the 228th ACS National Meeting (Philadelphia, PA 8/22-26/2004).