In recent years, the phenomenon of global warming has been taken up as a serious environmental problem, and it is said to be owing to greenhouse gases, particularly $CO_2$. The one of many attempts to reduce the quantity of $CO_2$ emissions is the biological $CO_2$ fixation method using microalgae. Therefore, the purpose of this study was to develop a new biological $CO_2$ fixation process for a high $CO_2$ fixation rate.
The microalgae used in this study was Chlorella sp. HA-1 and M4N medium was used throughout the experiment. An internally illuminated photobioreactor was designed and the mixture of $CO_2$ and air was supplied from the bottom. The cell showed resistance to both 10% and 20% $CO_2$ concentration. By optimization of the major operating variables such as initial cell concentration, pH, VVM and light intensity, the $CO_2$ fixation rate could be raised to a reasonably high value, 372 g $CO_2$/㎡ day in a 3 L reactor.
In order to maintain the $CO_2$ fixation rate for a long time, the method of semi-continuous operation was employed, in which the dilution ratio was the controlling parameter. Starting with the dilution ratio of 0.5 with the increased increment of 0.1, the constant $CO_2$ fixation rate in each step was obtained. The series operation method was used to reduce the $CO_2$ concentration in the exit gas. As a result, it showed that the $CO_2$ fixation rate was proportion to the number of reactors. And a bench-scale 40 L reactor was operated to demonstrate the possibility of scale-up. So the maximum $CO_2$ fixation rate obtained in this study with a 40 L reactor was 786 g $CO_2$/㎡ day, which was the highest value among the results reported in the literature.