Cultivation Process Intensification And Magnetic Intensified Harvesting Of Botryococcus Braunii Cells

Botryococcus braunii is a promising algal species for biofuels production. Compared with other species, B. braunii has a higher CO2 fixation efficiency and hydrocarbon content. In addition, the produced hydrocarbon can be easily extracted and refined. However, the production of B. braunii is of high cost due to the slow speed of growth, which limits the industrialized production of B. braunii. In this study, the light attenuation principle was obtained in the cultivation of B. braunii, and its effection on the algal growth and hydrocarbon production was further investigated. A promising ultrsonic stimulation strategy was developed for enhancing the algal growth rate and hydrocarbon productivity in a plastic bag bioreactor under the outdoor conditions. In addition, an efficient magnetic flocculant together with a magnetic separator was developed for harvesting the algal cells from the outdoor cultivation system.(1) Light attenuation had obvious influence on the cultivation of B. braunii. The degree of light attenuation increased with the increase of the light path. Beer-Lambert model fitted well with the light attenuation in B. braunii broth when the algal biomass was less than 0.75 g/L. The hyperbolic model can estimate the light attenuation accurately at a wide range of biomass concentration. The growth and hydrocarbon accumulation of B. braunii cells were obviously affected by both the incident irradiance and the light path. Longer light path required higher incident light intensity in order to meet the requirement of the algal growth and hydrocarbon accumulation in the course of cultivation.(2) Ultrasonic stimulation was found to effectively accelerate the cell growth of B. braunii. The optimal sonication strategy was to subject the algal cells to 5-min ultrasonic treatment at a 4-day interval. The specific growth rate and biomass productivity increased from 0.077 day-1 and 38 mg/L/day to 0.089 day-1 and 43 mg/L/day, respectively, while the generation time decreased from 9.0 days to 7.8 days using this strategy. The ultrasonic treatment was proved to effectively enhance the membrane permeability of algal cells. In addition, the endogenous indole-3-acetic acid (IAA) biosynthesis was also obviously enhanced under the ultrasonic treatment conditions. The ultrasound stimulation strategy was proved to effectively improve the cell growth of B. braunii in the 200 L plastic bag bioreactor under outdoor culture conditions. The improvement of the specific growth rate and biomass productivity was 11.39% and 22.36%, respectively. While the hydrocarbon productivity increased from 14.48 mg/L/day to 17.63 mg/L/day using this strategy.(3) A superparamagnetic CPAM-Fe3O4 magnetic flocculant was developed for harvesting the algal cells. The harvesting efficiency was higher than 95% at the flocculant dosage of 25 mg/L at any pH value within 10 min. The flocculation process was multilayer adsorption onto heterogeneous sites, and the adsorption isotherm data fitted the Freundlich model better than the Langmuir model. The primary flocculation mechanism was bridging, which was assisted by the electrostatic interactions between the microalgal cells and the magnetic flocculant under acidic conditions. Using the CPAM-Fe3O4 magnetic flocculant and a magnetic separator, in situ microalgae harvesting from the bag bioreactors was carried out under the outdoor conditions. A recovery efficiency of more than 90% was achieved at a magnetic flocculant dosage of 120 mg/L while the cell-particle aggregates passed through the in situ magnetic separator at a flow rate of 100 mL/min. In addition, the magnetic separation system maintained high recovery efficiency over the entire separation process.