Experimental Study for Construction of Microalgae Culturing System to Eliminate CO_2 and O_2 Regeneration in the Closed Air. -Elimination of CO_2 and Regeneration of O_2 Using Microalgae Under Low CO_2 Concentration in Aeration Air

摘要

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 CO_2 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 CO_2 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 CO_2 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 CO_2 formation rate had a larger influence on the growth and photo-synthetic 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 CO_2 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, CO_2-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-CO_2 environment, it is important to control oxygen concentration. Moreover, control of oxygen concentration will likely enable the unit to be better controlled as well.