Experimental Study for Construction of Microalgae Culturing System to Eliminate CO₂ and O₂ Regeneration in the Closed Air -Elimination of CO₂ and Regeneration of O₂ Using Microalgae Under Low CO₂ Concentration in Aeration Air

• Published in: SAE transactions 1996-01, Vol.105, p.906-910
• Main Authors: Adachi, Tadashi, Miya, Akiko, Taniguchi, Shin
• Format: Article
• Language: English
• ISSN: 0096-736X; 2577-1531

We have constructed a unit that uses microalgae to treat atmospheres containing low (i.e., (1000 ppm to 1%) concentrations of carbon dioxide. This experimental unit consists of a 20 I microalgae cultivation tank with eleven fluorescent lamps installed uniformly inside; and an analyzer with aeration/discharge, cultivation solutions supply/discharge function, and control capabilities. Using this unit, we conducted a series of experiments designed to reduce CO₂ concentration in air from about 1000 ppm to atmospheric levels, and to regenerate oxygen. We used Chlorella sorokiniana ATCC22521, a species of green algae, which is said to have a relatively high growth rate. We studied a number of necessary conditions. We performed a series of experiments under various cultivation temperature conditions. Although temperature had some influence on specific growth rate, its influence was negligible on the equipment's maximum CO₂ removal rate, as signified by represented an maximum cell growth or photosynthesis activity. We also performed a series of experiments under various pH conditions. We found that the CO₂ removal rate rose at a pH value which exceeded the optimum growth pH. However, we did not see any significant influence of pH on algae growth. These findings suggest that the CO₂ formation rate had a larger influence on the growth and photosynthetic activities of Chlorella sorokiniana ATCC22521 than other factors. Next, we conducted a series of experiments in which the oxygen concentration of the aeration gas was suppressed intermittently. When the oxygen concentration is suppressed, the equipment's CO₂ removal rate went up quickly. We observed this phenomenon in every step of cell growth. Moreover, even in conditions when we did not suppress oxygen concentration, CO₂ -removal performance was improved relative to when the oxygen concentration of the aeration gas was not suppressed at all. We believe several reasons explain this phenomenon: 1) Since oxygen, a product of photosynthesis, is effectively removed, photosynthesis activities rose. 2) Because cellular photo-respiration was controlled effectively, apparent photosynthetic activity rose. 3) Decreasing the cell toxicity inherent to oxygen improved cellular activity. Hence, in order to improve the performance of an atmospheric air-adjusting unit which uses microalgae in a low-CO₂ environment, it is important to control oxygen concentration. Moreover, control of oxygen concentration will likely enable the unit