| Summary: | 碩士 === 國立高雄第一科技大學 === 環境與安全衛生工程所 === 90 === ABSTRACT
Anaerobic digestion is a widely accepted process for high strength organic solid waste disposal. Organic waste could be converted to methane and carbon dioxide by different groups of microorganisms. These microorganisms are largely categorized to acidogenic and methanogenic bacteria. To enhance the performance of anaerobic digestion, optimal environments should be provided for major group of microorganisms. Phase separation in anaerobic digesters was developed and achieved good results. The two-phase system of acid- and methane-forming reactors are widely accepted. Hydrogen is evolved in anaerobic fermentation process. However, it was effectively converted to methane by methanogenic bacteria. Recently, hydrogen-fuel has been universally recognized as energy of the future. Biohydrogen production also draws great attention and has some achievement. Since the low conversion rate of hydrogen fermentation, the discharge still contains high concentration of organics which rich in energy. In order to promote total energy production, more studies are needed for the system of hydrogen production and the discharge utilization.
The purpose of this study was to investigate the energy production of hydrogenesis-methanogenesis process with different hydraulic retention times (HRT). A two-stage system (hydrogenesis-methanogenesis) and a single-stage system were set-up for the study, all reactors were complete stirred tank reactors and the effective volume were 4, 8 and 2 liter, respectively. Concentrate brewery wastewater from a nearby plant was collected as the substrate and the concentrate was adjusted to 110 g/L in COD for the feed. The two-stage system was operated in 9 consequent sets. In which, the hydrogenesis reactor was controlled at 3 different HRT, 24, 16 and 8 hours. The methanogenesis reactor was fed with hydrogenesis reactor effluent and controlled at 3 different HRT, 10, 15 and 20 days. The single-stage reactor was operated at HRT of 10, 15 and 20 days individually. Totally, there were 12 runs in the study.
In the hydrogenesis reactor tests, the optimal hydrogen concentration, hydrogen production rate and H2/VS conversion occurred at 8 hr HRT which were 32 %, 5.42 L-H2/ L-reactor/ day and 21 ml-H2/g VS added, respectively. These values decreased as HRT increased. Hydrogen production efficiency increased with ethanol, acetic acid decreased and butyric acid increased. The response of ethanol conc. was instantly with the increase of substrate conc., while it was 1 to 2 HRT later for acetic and butyric acids to increase. The optimal acidification efficiency occurred at HRT 24 hrs, and the major volatility fatty acid was acetic acid. This was no indication that high acidification efficiency resulted in a better hydrogen production reaction.
In the methanogenesis reactor tests, methane content increased with HRT. On the contrary, methane production and CH4/VS conversion decreased as HRT increased. It determined that energy production was affected by the VFAs of influent (effluent of hydrogenesis reactor). As the influent contented higher acetic acid (HRT 24 hours in hydrogenesis reactor), which could be direct utilized by methanogenic microorganism, the reactor presented a better methane production. On the other hand, as the methane production rate decreased, the major VFA of substrate was butyric acid (HRT 8 hours in hydrogenesis reactor). It believed that the reactor was in acidogenesis and acetogenesis stage. It also noted that propionic acid was accumulated as HRT increased which inhibited the methanogenic activities and caused the pathway toward short chain fatty acid formation instead of methane generation.
The energy production in the two-stage system was 2.5~14.2 kcal/L-reactor/day and 0.16~0.23 kcal/g VS removed in the hydrogenesis reactor and 3.1~25.6 kcal/L-reactor/day and 1.5~5.9 kcal/g VS removed in the methanogenesis reactor, respectively. The reactor with 8 hr HRT had the highest energy production in the hydrogen reaction, while the reactor with 10-day HRT fed with 24-hr HRT hydrogen reactor had the highest energy production in the methanogenesis reaction. However, energy production was as low as 1.3~2.9 kcal/ L-reactor/day and 0.8 kcal/g VS removed in the single-stage anaerobic fermentation. By using the methods of multiple regression and response surface design, the optimal energy production was the combination of 24- hr HRT hydrogenesis reactor and 10-day methanogenesis reactor, and then followed with 8-hr hydrogenesis reactor and 10-day methanogenesis reactor. The corresponding energy production were 25 and 20 kcal / L-reactor / day, respectively.
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