| Considering the energy security and the global environment, there is a pressing need to develop non-polluting and renewable energy sources. Alternatively, hydrogen is a clean energy carrier, producing water as its only by-product when it burns. Anaerobic bioconversion of organic wastes to hydrogen gas is an attractive option that not only stabilizes the waste/wastewater, but also generates a benign renewable energy carrier. The purposes of this study were to determine the kinetics of hydrogen production using different characteristics of substrates and to evaluate hydrogen production potential from different operating conditions in continuous operation.; The growth kinetics of hydrogen-producing bacteria using three different substrates including sucrose, non-fat dry milk (NFDM), and food waste were investigated through a series of batch experiments. The results demonstrated that hydrogen production potential and hydrogen production rate increased with an increasing substrate concentration. The maximum hydrogen yields from sucrose, NFDM, and food waste were 234, 119, and 101 mL/g COD, respectively. The low pH (pH < 4) inhibited hydrogen production and resulted in lower carbohydrate fermentation at high substrate concentrations. The Michaelis-Menten equation was employed to model the hydrogen production rate at different substrate concentrations. The equation gave a good approximation of the maximum hydrogen production rate and the half saturation constant (KS) with correlation coefficient (R2) over 0.85. The values of half saturation constant (KS) for sucrose, NFDM, and food waste were 1.4, 6.6, and 8.7 g COD/L, respectively. Based on the Ks values, the substrate affinity of the enriched hydrogen-producing culture was found to depend on the carbohydrate content of the substrate. The substrate containing high carbohydrates showed a lower KS value. The maximum hydrogen production rate was governed by the complexity of carbohydrates in the substrate. Biological hydrogen production from sucrose-rich substrate was investigated in an anaerobic sequential batch reactor (ASBR). The goal of this study was to investigate the effect of different hydraulic retention times (HRT) (8, 12, 16, 24, and 48 h), pHs (4.9, 5.5, 6.1, and 6.7), substrate concentrations (15, 25, and 35 g COD/L), and cyclic durations (4, 6, and 8 h) on biological hydrogen production. The maximum hydrogen yield of 2.53 mol H2/mol sucrose consumed and the maximum hydrogenic activity of 538 mL H2/g VSS-d were obtained at HRT of 16h, pH 4.9, sucrose concentration of 25 g COD/L, and feeding cycle of 4 h. Methane was detected in the biogas when solids retention time (SRT) exceeded 100 h at pH of 6.7. Based on the low ethanol concentration of nearly 300 mg/L, the metabolic pathway shift to solvent fermentation was not observed at pH of 4.9. The ratios of butyrate (HBu) to acetate (HAc) decreased from 1.25 to 0.54 mol/mol, when the sucrose concentration was increased from 15 to 35 g COD/L. This suggests that the metabolic pathway of acetate fermentation was predominant at higher sucrose concentrations. Hydrogen production was found to improve at a shorter feeding cycle of 4 h.; Fluorescent in situ hybridization (FISH) was applied for identifying and quantifying the specific microbial populations in the study. Most bacteria successfully identified by an EUB338 probe were counted and the percentages of 16S rDNA of EUB338 to DAPI at different reactor operating conditions were determined. Due to the false positive hybridization results, the ARC915 probe was found unsuitable for identifying cells belonging to the domain Archaea in this study. FISH results using the probe CLOST I were not fully determined because of the difficulty of recognizing the hybridized clostridia cluster I. Therefore, a correlation between hydrogen production and the number of Clostridium belonging to clostridia cluster I was not determined. |
