| Summary: | 博士 === 國立交通大學 === 生物科技學系 === 106 === Integration with wastewater and flue gas in microalgal cultivation can be used for CO2 reduction, wastewater purification and the resulting microalgal biomass can be a feedstock of biofuels. In this study, Chlorella sp. GD cultured in piggery wastewater (PW) and aquaculture wastewater (AW) aerated with boiler flue gas to efficiently produce biomass and lipid for biodiesel production was investigated. After that, to increase the CO2 utilization efficiency and total amount of CO2 fixation, the alkali-tolerant Chlorella sp. AT1 was screened by chemical mutagenesis and a Photobioreactors (PBRs)/Raceway circulating (PsRC) system was established. The stable growth performance of microalgae was achieved for long-term microalgal cultivation in a semi-continuous culture.
First, Chlorella sp. GD was cultivated with 0, 25, 50, 75 and 100% PW (diluted by medium) in batch cultures, the maximum specific growth rate and biomass productivity of the microalga obtained in 100% PW were 0.839 d−1 and 0.681 g/L/d, respectively. The highest lipid content and lipid productivity were 29% and 0.155 g/L/d at 25% PW, respectively. In semi-continuous cultures, the biomass and lipid productivities with 25−75% PW ratios were greater than 0.852 g/L/d and 0.128 g/L/d, respectively. These results show that Chlorella sp. GD grows efficiently in PW, and that a stable growth performance was achieved for long-term microalgal cultivation in a semi-continuous culture. Second, Chlorella sp. GD was also cultivated in AW aerated with boiler flue gas. When AW from a grouper fish farm was supplemented with additional nutrients, the microalgal biomass productivity was 0.794 g/L/d. When the microalgal strain was cultured with boiler flue gas in nutrient-added AW, biomass productivity increased to 0.892 g/L/d. In semi-continuous cultures, average biomass productivities and CO2 fixation efficiencies of the microalgal strain in 2-day, 3-day and 4-day replacement cultures were 1.296, 0.985 and 0.944 g/L/d, and 2.333, 1.773 and 1.699 g/L/d respectively. These results demonstrate the potential of using Chlorella sp. GD cultivations in AW aerated with boiler flue gas for reusing water resources, reducing CO2 emission, and producing microalgal biomass.
To enhance the CO2 utilization efficiency, an alkali-tolerant Chlorella sp. AT1 mutant strain was screened by N-methyl-N'-nitro-N-nitrosoguanidine (NTG) mutagenesis. The strain grew well in pH 6–11 media and the optimal pH for growth was 10. The CO2 utilization efficiencies of Chlorella sp. AT1 cultured with intermittent 10% CO2 aeration for 10, 20 and 30 min at 3-h interval in alkaline medium (pH = 11) were approximately 80, 42 and 30%, respectively. In pH 11 medium with intermittent 10% CO2 aeration for 30 min at 3-, 6- and 12-h intervals, the medium pH gradually changed to 10, and the biomass productivities of Chlorella sp. AT1 were 0.987, 0.848 and 0.711 g/L/d, respectively. When Chlorella sp. AT1 was aerated with 10% CO2 intermittently for 30 min at 3-h interval in semi-continuous cultivation for 21 days, the biomass concentration and biomass productivity were 4.35 g/L and 0.726 g/L/d, respectively. Our results show that CO2 utilization efficiency can be markedly increased by intermittent CO2 aeration and alkaline media as a CO2-capturing strategy for alkali-tolerant microalga cultivation.
Finally, a high efficiency of microalgal growth and CO2 fixation in a PsRC system combined with alkaline-CO2 capturing medium and operation was established and investigated. The PsRC system was composed of PBRs in series, a Raceway and circulation pump. In 1-ton indoor-simulated PsRC system, compared with a pH 6 medium, the average biomass productivity of Chlorella sp. AT1 cultured in a pH 11 medium at 2 L/min circulation rate between PBRs and Raceway was increased by about 2-fold to 0.346 g/L/d. The maximum amount of CO2 fixation and CO2 utilization efficiency of Chlorella sp. AT1 could be obtained at PBRs to Raceway ratio of 1:10 in an indoor PsRC system. An outdoor PsRC system with a 10-ton pilot scale for microalgal cultivation was also established. Under 20 L/min circulation rate and PBRs to Raceway ratio of 1:10 in an outdoor PsRC system, the stable growth performance of Chlorella sp. AT1 cultured by long-term semi-continuous operation was observed, and the total amount of CO2 fixation was around 1.2 kg/d with 50% CO2 utilization efficiency.
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