CFD–experimental assessment of open-tube airlift photobioreactors for CO₂ capture and nutrient removal

Abstract

This study presents a novel open-tube airlift photobioreactor (PBR) designed with the support of computational fluid dynamics (CFD) and experimentally compared with closed-tube and wall reactors. A validated Euler–Euler CFD model was ≤ 5 % error, indicating enhanced gas holdup, circulation uniformity, and mass transfer in the open-tube geometry, achieving a volumetric mass transfer coefficient (kLa) of 0.037 s⁻¹ at Usg = 0.05 m.s⁻¹ . Flow analysis confirmed the reduced vortex formation and more streamlined riser–downcomer transitions, which promoted longer bubble residence times. Cultivation with Chlorella vulgaris showed enhanced biological performance: biomass concentration reached 1.8 g.L⁻¹ , nitrate removal was 55 %, phosphate consumption was nearly complete after 10 days, and CO₂ capture efficiency peaked at 85 %, outcompeting conventional flat-panel and bubble column PBRs (60–75 %). These results highlight the open-tube airlift PBR a potential platform for simultaneous CO₂ bio fixation, nutrient removal, and biomass production with high potential for sustainable wastewater treatment.