Technical insight on the requirements for CO2-saturated growth of microalgae in photobioreactors

Microalgal cultures are usually sparged with CO2-enriched air to preclude CO2 limitation during photoautotrophic growth. However, the CO2 vol% specifically required at operating conditions to meet the carbon requirement of algal cells in photobioreactor is never determined and 1-10% v/v CO2-enriched air is arbitrarily used. A scheme is proposed and experimentally validated for Chlorella vulgaris that allows computing CO2-saturated growth feasible at given CO2 vol% and volumetric O2 mass-transfer coefficient (k L a)O. CO2 sufficiency in an experiment can be theoretically established to adjust conditions for CO2-saturated growth. The methodology completely eliminates the requirement of CO2 electrode for online estimation of dissolved CO2 to determine critical CO2 concentration (Ccrit), specific CO2 uptake rate (SCUR), and volumetric CO2 mass-transfer coefficient (k L a)C required for the governing CO2 mass-transfer equation. Ccrit was estimated from specific O2 production rate (SOPR) measurements at different dissolved CO2 concentrations. SCUR was calculated from SOPR and photosynthetic quotient (PQ) determined from the balanced stoichiometric equation of growth. Effect of light attenuation and nutrient depletion on biomass estimate is also discussed. Furthermore, a simple design of photosynthetic activity measurement system was used, which minimizes light attenuation by hanging a low depth (ca. 10 mm) culture over the light source.