The air-lift photobioreactors with flow patterning for high-density cultures of microalgae and carbon dioxide removal

A photobioreactor containing microalgae is a highly efficient system for converting carbon dioxide (CO₂) into biomass. Using a microalgal photobioreactor as a CO₂ mitigation system is a practical approach to the problem of CO₂ emission from waste gas. In this study, a marine microalga, Chlorella sp. NCTU-2, was applied to assess biomass production and CO₂ removal. Three types of photobioreactors were designed and used: (i) without inner column (i.e. a bubble column), (ii) with a centric-tube column and (iii) with a porous centric-tube column. The specific growth rates (μ) of the batch cultures in the bubble column, the centric-tube and the porous centric-tube photobioreactor were 0.180, 0.226 and 0.252 day¹, respectively. The porous centrictube photobioreactor, operated in semicontinuous culture mode with 10% CO₂ aeration, was evaluated. The results show that the maximum biomass productivity was 0.61 g/L when one fourth of the culture broth was recovered every 2 days. The CO₂ removal efficiency was also determined by measuring the influent and effluent loads at different aeration rates and cell densities of Chlorella sp. NCTU-2. The results show that the CO2 removal efficiency was related to biomass concentration and aeration rate. The maximum CO2 removal efficiency of the Chlorella sp. NCTU-2 culture was 63% when the biomass was maintained at 5.15 g/L concentration and 0.125vvm aeration (volume gas per volume broth per min; 10% CO₂ in the aeration gas) in the porous centric-tube photobioreactor.