Biological Air Pollution Control Using Immobilized Microalgae for CO2 Removal from Gas Streams

Biological air pollution control is gaining attention as a sustainable alternative for treating CO₂-rich gas streams. This study evaluated a trickle-bed photobioreactor with immobilized Chlorella vulgaris UTEX 2714 for biological CO₂ removal. Microalgal biomass was encapsulated in sodium alginate capsules, providing approximately 100 g TS/dm³ of active biomass, and the reactor was supplied with ambient air enriched with technical-grade CO₂ to 25% v/v. The tests were conducted for 60 days in 20-day operating cycles, under controlled illumination and monitored pH and temperature conditions, and CO₂ concentration was determined by means of gas chromatography with a thermal conductivity detector. CO₂ removal depended strongly on gas volume flux. At 25 dm³/h, removal efficiency reached 39.9%, corresponding to 117.4 g CO₂/day, whereas at 200 dm³/h, it decreased to 5.9% because of the shorter gas–biomass contact time. The immobilized system enabled markedly higher biomass density than conventional suspended-growth photobioreactors, where typical C. vulgaris cultures reach substantially lower biomass concentrations; as a reference, suspended C. vulgaris supplied with desulphurized real exhaust gases produced 2120 ± 123 mg VS/L and achieved CO₂ utilization efficiency of 41.4%. A preliminary energy estimate indicated that artificial red/blue LED illumination would require approximately 100 W per module, giving a cost-magnitude energy demand of about 20 kWh/kg CO₂ removed under fully artificial 24 h lighting; this demand could be reduced by hybrid sunlight–LED operation. The results indicate that immobilized microalgae can enhance CO₂ biosequestration by increasing biomass density and improving gas–biomass contact. However, because effective treatment was achieved mainly at low gas flow rates, the technology should be regarded as suitable for small-to-medium, high-concentration CO₂ sources, such as biogas upgrading units or selected industrial kilns and furnaces, rather than the main flue gas stacks of large power plants.