In recent years, the phenomenon of global warming has been taken up as a serious environmental problem, and $CO_2$ is widely concerned as a major constituent of greenhouse gases. One of many attempts to reduce the amount of $CO_2$ emissions is the biological $CO_2$ fixation method using microalgae. In biological $CO_2$ fixation, the high cell growth rate and high cell density are required for effective $CO_2$ removal. But it is difficult to satisfy both aspects simultaneously in conventional cultivation methods. Therefore, this study was designed to develop a newly modified biological $CO_2$ fixation process which could remove $CO_2$ efficiently for long-term operation.
In this study, the microalgae, Chlorella sp. HA-1, was cultivated in an internally illuminated photobioreactor. The mixture of $CO_2$ and air was supplied from the bottom of reactor. In order to maintain a reasonable fixation rate for a long time, various cultivation methods were tested: fed-batch, medium replacement, semi-continuous, series and semi-continuous cultivation method combined with series operation. The high cell density and moderate growth rate were obtained by replacing old medium by new medium. The amount of $CO_2$ fixation in the fed-batch cultivation method was smaller than the amount in the medium replacement cultivation. The semi-continuous cultivation method showed constant cell growth rate for long-term periods. The series operation method was used to reduce the $CO_2$ concentration of the emitted gas. And the total amount of $CO_2$ fixation was proportional to the number of the connected reactors. The most effective increase of the $CO_2$ fixation rate in a long-term operation was achieved by combining semi-continuous mode with series operation.
