Enrichment of Low Carbon Dioxide Through Membrane-Based Gas Absorption Technology for Microalgae Cultivation

Traditionally, microalgae are cultivated in closed systems or open ponds, which are aerated or exposed to air to allow microalgae to capture CO2 from the atmosphere for cell growth. As the atmosphere contains only 0.03-0.06% CO2, it is expected that mass transfer limitation could slow down the cell growth of microalgae. Thus, microalgae combined with CO2 separation system could be avoided the limitation and made more-effective and environmentally sustainable for microalgae cultivation. In this study, 0.03-0.06% CO2 in the atmosphere could be concentrated to about 2% CO2 by means of membrane-based gas absorption technology which was built with two polypropylene hollow-fiber membrane modules and alkanolamines were chosen as the absorbent. And then, concentrated CO2 was used to Chlorella vulgaris culture in an air-lift photobioreactor. In the same microalgae density, the effect of CO2 concentration on the pH and DO of Chlorella vulgaris was investigated in detail. As increasing CO2 concentration, pH of algae solution was decreasing at an exponential rate. But DO values were increased first and then decreased. When CO2 concentration was about 1%, DO achieved 140%, the maximum. Moreover, CO2 fixation rate of Chlorella vulgaris achieved the maximum when CO2 concentration was 1%. Effects of CO2 and O2 concentrations in the inlet gas on the photosynthesis of Chlorella vulgaris were investigated, with the results showing that CO2 content in the normal air could not meet the needs of microalgae growth, while with pH oscillating at 8.5, the 1% CO2- rich gas could support the density higher above 1E8 cells ml-1.&'