| Summary: | 碩士 === 國立成功大學 === 化學工程學系碩博士班 === 100 === Mitigation of CO2 emissions has become a globally hot issue due to its adverse effects on our environment and climate pattern. Microalga is an effective natural CO2 sink, converting CO2 to its biomass with a ratio of 1.6-2.0 (g CO2/g biomass). Some microalgae contain a large amount of carbohydrates (over 50% per dry weight of biomass), which can serve as carbon source for the fermentative production of biofuels, such as bioH2. In this study, the biomass of a green microalga, Chlorella vulgaris ESP-6, was used as the carbon source to produce hydrogen by dark fermentation with an isolated H2-producing strain Clostridium butyricum CGS5. The results show that C. vulgaris ESP-6 biomass was effectively hydrolyzed by 1.5% HCl (121oC, 20 min) to achieve sugar conversion of 99%. The microalgal hydrolysate was used to produce H2 by Cl. butyricum CGS5 under an optimal condition of 37oC, pH 7.0, and a microalgal biomass of 20 g/L (equivalent to 10 g/L of reducing sugar) with pH control at 5.5, giving a cumulative H2 production of 1475 ml/L, a maximum H2 production rate of 246 ml/L/h, and a H2 yield of 1.09 mol H2/mol reducing sugar, respectively.
Moreover, C. vulgaris ESP-6 was also used to assimilate the volatile fatty acids (mainly acetate, butyrate, and lactate) from dark hydrogen fermentation processes. The results show that the microalgal strain could grow on the 1/4x diluted supernatant of the dark fermentation broth. It was found that the microalgal growth was inhibited by lactate, butyrate, or HCO3- when their concentration was higher than 0.5, 0.1, or 2.72 g/L, respectively. To improve the consumption rate of those volatile fatty acids, the primary factors (e.g., light intensity and food to microorganism (F/M) ratio) affecting photoheterotrophic growth of the microalgal strain were investigated. The optimal condition was light intensity at 125 mol m-2s-1 and F/M ratio at 4.8. The results demonstrated that C. vulgaris ESP-6 can efficiently utilize the soluble metabolites of dark H2 fermentation for photoheterotrophic growth.
In addition, this work was also undertaken to evaluate the feasibility of using the microalgal strain to simultaneously assimilate the volatile fatty acids (soluble metabolites of dark fermentation) and fix the CO2 produced from dark fermentation under mixotrophic cultivation conditions. In these experiments, H2 was produced from continuous dark fermentation with a stable H2 production rate of 212.8±9.2 ml/h/L and a cell concentration of 1.86±0.1 g/L for 9 days. Then, the liquid the gas effluent of dark fermentation was connected with microalgae mixotrophic growth system, which was able to remove over 95% of CO2 and 97% of volatile fatty acids. The microalgal biomass produced from mixotrophic growth system was also used as feedstock to produce more H2 via dark fermentation and then the soluble metabolites and CO2 produced during dark fermentation were again assimilated by microalgae, resulting in a recycle system to produce hydrogen without CO2 emission and with a significant COD reduction of the dark fermentation effluent.
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