CO₂ Fixation And Lipid Accumulation By Air-lift-type Microbial Carbon Capture Fuel Cell

As the problems of global warming and energy crisis are significantly serious, the solutions for CO2 capture and alternative sources of energy become an important development direction in the fields of environment and energy. Microalgae cultivation in municipal wastewater for photosynthetic CO2 fixation and biodiesel production is considered to be one of promising approaches. However, the nitrogen concentration in municipal wastewater is significantly lower than that in the medium of microalgae cultivation, which often leads to a relatively low biomass productivity, and thus low CO2 fixation efficiency and overall lipid productivity. Additionally, there were significant variations of cellular physiology, metabolic potential and genetics among strains that are morphologically similar. Herein, the first objective of the current study is to select the Chlorella strains that have fairly good tolerance for municipal wastewater influent (MWI), high CO2 fixation efficiency and lipid accumulation. Then an air-lift-type microbial carbon capture cell (ALMCC) was constructed for the first time by linking traditional microbial carbon capture cell (MCC) and air-lift-type photobioreactor (ALP) to address effectively the problem of the nitrogen insufficient in municipal wastewater. The established ALMCC system simultaneously achieved the high level of CO2 fixation, bioenergy recycle and municipal wastewater treatment effectively and efficiently. The dissertation avoides the use of the medium of microalgae, and it has important practical significance to reduce energy consumptions and greenhouse gas emissions in the processes of CO2 fixation and bioenergy production. The primary contents and results are as follows:(1) Ten strains of Chlorella were cultivated in municipal wastewater influent and their tolerance for MWI, CO2 fixation efficiency and lipid productivity were assessed. The results showed that the biomass concentrations of four strains were significantly higher than those of other strains, which were Chlorella vulgaris (ESP-6), Chlorella 64.01 (FACHB-752), Chlorella regularis var. minima (FACHB-729) and Chlorella sp. (FACHB-484), respectively. When the cultivation systems were aerated with 10%CO2, the CO2 fixation rate of FACHB-729 and FACHB-484 (123.15 and 124.73 mg·L-1·d-1, respectively) were higher than those of other strains (p?0.05), but there was no difference of CO2 fixation rate between FACHB-729 and FACHB-484 (p?0.05). The highest lipid accumulation (58.48%) was observed with ESP-6. Scanning electron microscopy images revealed that the cells of both FACHB-484 and ESP-6 kept their normal morphologies after 15 day batch culture. These findings indicated that FACHB-484 and ESP-6 have fairly good tolerance for MWI, and moreover, FACHB-484 and ESP-6 were appropriate for cultivating into MWI for CO2 fixation and biodiesel production.(2) For the problem of the nitrogen insufficient in municipal wastewater for microalgae cultivation, an air-lift-type microbial carbon capture cell (ALMCC) was constructed for the first time by linking MCC and ALP. The performances were investigated for ALMCCs with FACHB-484 and ESP-6 as cathodic microorganism. The two ALMCC systems showed differences in CO2 fixation, lipid production and power generation. The ALMCC system with ESP-6 produced a maximum power density of 558.22 mW·m-3, CO2 fixation rate of 223.68 mg·L-1·d·-1 and lipid productivity of 21.75 mg·L-1·d-1, indicating that ESP-6 performed better than FACHB-484. By conducting further experiments with ESP-6 under different light intensities (2.4,5.0,8.9 and 11.4 W·m-2), the ALMCC was found to be sensitive to light intensities. The maximum power outputs and CO2 fixation rate were observed at the light intensities of 8.9 W·m-2 (972.5 mW·m-3 and 887.8 mg·L-1·d-1, respectively). The lipid productivity was increased with the light intensity from 2.4 to 11.4 W·m-2. However, there was no difference of lipid productivities between light intensity of 8.9 and 11.4 W·m-2 (p? 0.05). These results suggested that 8.9 W·m-2 was the optimal light intensity for CO2 fixation, lipid production and power generation of the ALMCC with ESP-6. And the CO2 fixation rate at optimal light intensity was 7.12 folds that obtained in normal culture conditions.(3) Under the optimal light intensity (8.9 W·m-2), the performance of ALMCC with ESP-6 in fixing high concentration of CO2, producing energy (power and biodiesel), and removing COD together with nutrients, was investigated and compared with the traditional MCC and ALP. The ALMCC system produced a maximum power density of 972.5 mW·m-3 and removed 86.69% of COD,70.52% of NH4+-N and 69.24% of PO43--P, which indicate that ALMCC performed better than MCC in terms of power generation and wastewater treatment efficiency. Besides, the CO2 fixation rate of ALMCC was 9.98 and 1.88 folds that observed in ALP and MCC, respectively. Similarly, the ALMCC significantly presented a higher lipid productivity compared with MCC and ALP. More importantly, the preliminary analysis of energy balance suggested that the net energy of the ALMCC system was 2.06 and 4.48 folds of other systems and could theoretically produce enough energy to cover its consumption. It was confirmed that ALMCC performed better than MCC and ALP. In this work, the established ALMCC system simultaneously achieved the high level of CO2 fixation, energy recycle and municipal wastewater treatment effectively and efficiently.(4) A two-stage culture strategy was used to cultivate ESP-6 for increasing the lipid productivity. In the first stage, ESP-6 was grown in the cathode chamber of ALMCC to obtain maximum biomass productivity (nutrient-sufficient condition). In the second stage, after ESP-6 were accumulated a large amount of biomass productivity, the culture was transferred into the ALP to cultivate continuously in order to increase the lipid content of ESP-6 (nutrient-starvation condition). The maximum CO2 fixation rate (887.49 mg·L-1·d-1) was achieved on day 7, and the maximum lipid content (54.23%) was achieved on day 12. However, the maximum lipid productivity was 190.45 mg·L-1·d-1, which was 1.7 folds than cultivated in nutrient-sufficient condition and it was achieved on day 11. The results indicated that the maximum lipid productivity can be achieved on day 5 after ESP-6 was transferred to nutrient-starvation stage. In this study, the lipid productivity was improved through the two-stage culture strategy. In addition, it did not need centrifugation when the microalgae suspension was transferred to the second stage, thereby reducing the energy consumption and greenhouse gas emissions.