| Oleaginous microalgae can effectively utilize the solar energy to produce biodiesel thus fulfilling a green transforming from inorganic carbon into organic carbon. However, enhancing lipid productivity while reducing the energy consumption during the cultivation process is the bottleneck in industrial scaled production of algal biodiesel. The present work selected Chlorella sp. which could tolerate relatively high concentration of CO2 as a model microalgal species and explored the optimal microalgae cultivation condition and highest energetic efficiency in an airlift photobioreactor. In addition we took a preliminary research on the downstream technology of lipid extraction and fatty acid analysis. Unlike many previous related researches, this paper focused on energy utilization efficiency together with biomass productivity when considering the whole cultivation process, hoping to provide a valuable reference for the biodiesel production using microalgae as feedstock.By comparing the growth conditions of Chlorella sp. under different CO2 concentrations and illumination conditions, we found that relatively high CO2 concentration and light density could promote the growth rate to a great extent. Moreover the illumination has an impact on the utilization of carbon source, because of the difference in light intensity the optimal CO2 concentration in, external light source system and internal light source system were 10%(v/v) and 5%(v/v) respectively, and the corresponding optimal biomass concentration were 3.68 g/L and 0.47 g/L. When supplied with 5% CO2, Chlorella sp. obtained the optimal biomass concentration of 3.63 g/L and biomass productivity of 246.35 mg/(L·d) at the light intensity of 15.6 W/m2. Due to the relatively low light intensity, the internal optical fiber could only sustain a net biomass productivity of 28.30 mg/(L-d). However when compared with external light, the side-light optical fiber could distribute the light into medium more efficiently and the photosynthetic efficiency would be improved to great deal. When consuming same amount of energy, the internal light source system could enhance the biomass productivity to 8.6～25.6 folds and the maximal productivity per unit energy consumption of 4.40 g/(W·d) would be obtained at a proper flow rate(0.6 L/min).Nitrogen starvation was beneficial for the lipid accumulation but had an adverse effect on the enhancement of biomass productivity. When being in shortage of nitrogen source Chlorella sp. might use Chlorophyll as the internal nitrogen source, the Chlorophyll a concentration was decreasing as the nitrogen starvation period elongated. The highest lipid productivity could reach 5.66 mg/(L·d) when the NaNO3 and ferric ammonium citrate concentrations in the medium were 1.5 g/L and 6 mg/L respectively. Chlorella sp. was capable of using the mixotrophic metabolism with the coexistence of organic compounds and light source, in the synthetic wastewater a higher specific growth rate and lipid productivity of 0.033 d-1 and 6.96 mg/(L·d) could be obtained while the overall pollutant removal rate exceeded 95%.Grinding, sonication, freezing-thawing and acid heat methods all could accomplish the cell disruption to different extents and the best disruption rate of 65.25% was obtained with sonication disposal within 40 minutes. Among four organic solvents, the ethanol had the best extraction rate, and the highest lipid yield was 22.00% in 60℃. C16 and C18 were the predominant compositions in the fatty acid profile of Chlorella sp., the content of polyunsaturated fatty acid was relatively low which could satisfy some requirements of commercial biodiesel. On the other hand the solvents and cultivation conditions had an impact on the compositions and contents of fatty acid profile. From this perspective, we could select a specific solvent or cultivation method to promote the quality of biodiesel product according to our demands. |
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