Energetic optimization of algal lipid production in bubble columns: Part II: Evaluation of CO2 enrichment

This study evaluated growth and lipid productivity of Nannochloropsis salina under sparging with carbon dioxide-enriched air. Carbon dioxide enrichments ranging from 0.5 to 9.5% and gas-to-culture volume ratios ranging from 0.02 to 1.0 min−1 were tested in 900 mL bubble column batch reactors. An energy-based approach is proposed to optimize CO2 enrichment. Cultures sparged with CO2-enrichments of 0.5–4% grew at nearly double the rate of those sparged with ambient air, accumulating 64% or more lipids. Based on energy efficiency, CO2-enrichment of 0.5% was found to be optimal while CO2-enrichments of 6.5% and greater were observed to cause inhibition. At this optimal CO2-enrichment of 0.5%, no significant changes were noted in the lipid content over the range of gas-to-culture volume ratios tested. These studies indicated that optimal energy recovery from N. salina can be achieved with CO2-enrichment of 0.5% sparged at a gas-to-culture volume ratio 0.18 min−1. Under this optimal condition, biomass growth rate was 0.161 g L−1 d−1, and the lipid content was 67.8%, yielding total lipid production of 0.771 g L−1 over 10 days at a net energy yield of 25 W m−3.