| Summary: | 博士 === 逢甲大學 === 化學工程學所 === 92 === Abstract
Hydrogen has emerged as one of the most promising new energy carriers, because it is clean, recyclable, efficient, and can be used in fuel cells to generate electricity. Utilization of anaerobic microorganisms for hydrogen production from organic wastewater has become an innovative and promising environmental biotechnology. Recent advances in biohydrogen production processes suggest that the key to upgrade hydrogen production rate is to enhance the concentration of H2-producing bacterial population in the reactor. To achieve this goal, a novel carrier-induced granular sludge bed (CIGSB) bioreactor was developed in this study for efficient retention of biomass to produce H2 at an incredible rate via high-cell-density fermentation.
En route to the development of the CIGSB system, our first experimental approach focused on packed-bed systems, anticipating good retention of hydrogen-producing bacteria within the packed-bed bioreactors via cell attachment on the support matrix. Different carriers were assessed for biomass retention and hydrogen production in batch and continuous modes. The packed-bed bioreactors packed with cylindrical activated carbon exhibited a better hydrogen production rate of 1.32 L/h/L at 1 h of HRT. However, the packed-bed reactor had a limitation in reducing the effective volume in the reactor due to the presence of solid supports occupying a significant portion of working volume. Thus, the effect of void fraction of the bed (εb, 70-90%) on hydrogen fermentation in packed-bed systems was examined. The results show that higher εb favored hydrogen production, and the optimal hydrogen production rate (7.35 L/h/L) was obtained when the bed with εb = 90% was operated at HRT = 0.5 h with 20 g COD/L of sucrose in the feed. During the investigation of void fraction effect, we accidentally observed flocculation of cells to form granular sludge, which was more abundant when the void space increased. The behavior of sludge granulation markedly enhanced the retention of sludge and a novel carrier-induced granular sludge bed (CIGSB) bioreactor was thereby developed. A variety of carrier matrices were examined for their effectiveness in stimulating sludge granulation. Among the carriers examined, spherical activated carbon (SAC) and cylindrical activated carbon (CAC) was the more effective inducer for granular sludge formation. The SAC-CIGSB bioreactor achieved an optimal volumetric hydrogen production rate of 7.33 L/h/L and a maximal hydrogen yield of 3.03 mol H2/mol sucrose, when it was operated at a HRT of 0.5 h with an influent sucrose concentration of 20 g COD/L.
A series of operation strategies were performed to improve the efficiency and stability of hydrogen production using the CIGSB bioreactors. These strategies include supplementation of calcium ion, recirculation of liquid or gas streams, adjustment of height to diameter (H/D) ratio of the reactor, and finally optimizing the operation temperature. Supplementation of calcium ion was found to enhance mechanical strength of the granular sludge. Addition of 5.4-27.2 mg/L of Ca2+ also led to an over 3-fold increase in biomass concentration and a nearly 5-fold increase in the H2 production rate. Two reflux strategies were utilized to enhance the mass transfer efficiency of the CIGSB system. The liquid reflux (LR) strategy enhanced the H2 production rate by 2.2 fold at an optimal liquid upflow velocity of 1.09 m/h, which also gave a maximal biomass concentration of ca. 22 g VSS/L. Similar optimal H2 production rate was also obtained with the gas reflux (GR) strategy at a rate of 1.0-1.49 m/h, whereas the biomass concentration decreased to 2-7 g VSS/L and thereby the specific hydrogen production rate was higher than that with LR. Experiments were designed to adjust the height to diameter (H/D) ratios of the CIGSB bioreactor to improve mixing efficiency for better biomass-substrate contact. The results show that liquid upflow velocity (Vup,liq) is a key factor to influence the hydrogen production efficiency with different H/D ratio. Increasing H/D ratio gives higher Vup,liq, allowing better hydraulic mixing to enhance the biomass-substrate contact, but an excessively large H/D ratio may also cause sludge floatation to diminish the sludge retention. Reactors with a H/D ratio of 8 gave an optimal H2 production performance by attaining a maximal hydrogen production rate of 6.87 L/h/L along with the highest yield of 3.88 mol H2/mol sucrose. In addition, The CIGSB was operated at four different temperatures (30, 35, 40, and 45oC). Increasing temperature seems to inhibit formation of granular sludge, but specific hydrogen production rate tended to increase. This signifies that the cellular activity for H2 production was enhanced as the temperature increased. Combination of the two effects resulted in an optimal temperature of 40oC for the CIGSB system. The overall maximal hydrogen production rate and yield were 7.66 L/h/L and 3.88 mol H2/mol sucrose, respectively.
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