Optimization Of Culture Conditions And Harvesting For Botryococcus Braunii

Microalgae are excellect sources for the production of biodiesel and bio-oil. To realize the commercialization of large-scale microalgal production, the concerned research is focused on the cultivation and harvesting of microalgae recently. Unlike other microalgae, Botryococcus braunii has higher oil content and is easier for oil extraction, which attracts researchers'interests. In this study, medium selection of B. braunii was carried out in the airlift photobioreactor and its metabolic mechanism was investigated. In addition, cryoprotectants were screened for the cryopreservation of B. braunii. A new fed-batch strategy was used for the cultivation of B. braunii and an optimization method for the inner structure of airlift photobioreactor was established by using Computational Fluid Dynamics (CFD). Two novel methods were applied for the harvesting of B. braunii. Main conclusions were listed as the follows:1) Chu13 was the optimal medium for the growth and hydrocarbon production of B. braunii (Race B), where biomass of 1.82 g/L and crude hydrocarbon content of 58.7% were achieved after 15-day cultivation in 2 L airlift photobioreactor. It was indicated that nitrate and phosphate were important nutrients for the growth of B. braunii. The hydrocarbon content increased with respect to biomass and the hydrocarbon composition kept consistent during the culture process. The hydrocarbon of B. braunii was consisted of two main botryococcenes: C₃₃H₅₆ and C₃₄H₅₈. There were at least three isomerides for C₃₄H₅₈. The main lipid compositions of B. braunii were palmitic acid, oleic acid and linolenic acid. Screening of cryoprotectants and cooling procedures was studied and the optimal condition was obtained for the cryopreservation of B. braunii. The maximal post-thaw viability of 98.9% was achieved at 6% of methanol with programmed cooling procedure. In addition, culture age of B. braunii exhibited significant effect on its post-thaw viability. It was suggested that the microalgae harvested from the late exponential phase would be favorable for the cryopreservation of B. braunii.2) In 2 L airlift photobioreactor, the optimal feeding strategy for the culture of B. braunii was two feedings with 2-day interval after 15 days of cultivation, where nitrate was fed to the initial normal level and phosphate only return to one quarter of its initial level. By using this strategy, the biomass production of B. braunii was enhanced to 2.87 g/L after 19 days of cultivation, where the hydrocarbon content kept at high level of 64.3% and the hydrocarbon yield was 40.3% higher than that in batch culture.3) A method based on CFD,was applied for the inner structure optimization of 2 L airlift photobioreactor. The main parameters affecting the growth of B. braunii included gas volume fraction, turbulent kinetic energy, period of light/dark cycle and ratio of light time. The optimal inner structure parameters for 2 L airlift photobioreactor were as the follows: A_d/A_r = 2.92,h₀ = 2 cm,h₁ = 2⁴ cm. The predicted maximal biomass production was 1.81 g/L. This method was used to optimize the inner structure of 40 L airlift photobioreactor and then the batch and fed-batch culture of B. braunii were investigated in this reactor. Biomass of 1.79g/L and hydrocarbon content of 58.8% were obtained in the batch culture. However, the cultivation period in 40 L airlift photobioreactor was proloned for 3 days. By using the optimal feeding strategy, microalgae biomass and hydrocarbon content reached 2.84 g/L and 63.8%, respectively. The production of biomass and hydrocarbon kept the same level as that before scaling up.4) An efficient electroflocculation method integrated with dispersed-air flotation has been developed for the harvesting B. braunii. When the voltage of 60V and aeration of 30 mL/min was used in this process, the proper aeration strategy was aeration for 10 min. The recovery efficiency of B. braunii reached 98.6% after 18 min by this strategy. However, by using the electroflocculation without dispersed-air flotation, the recovery efficiency of 92.5% was achieved after 28 min at the same voltage. Thus, 35.7% of operation time and 35.0% of energy consumption were reduced when the dispersed-air flotation was introduced to electroflocculation. The recovery efficiency was also improved. The naked Fe₃O₄ nanoparticles can be used for the harvesting of B. braunii by magnetic separation. The magnetic separation provides high recovery efficiency within much shorter time. Low pH of culture broth and high dose of Fe₃O₄ nanoparticles were beneficial for the harvesting process. The adsorption equilibrium data fitted very well to Langmuir adsorption isotherm models. It seemed that the magnetic separation cost less energy than that in the electroflocculation integrated with dispersed-air flotation.