Experimental validation of a nonequilibrium model of CO2 fluxes between gas, liquid medium, and algae in a flat-panel photobioreactor

Carbon dioxide (CO2) availability strongly affects the productivity of algal photobioreactors, where it is dynamically exchanged between different compartments, phases, and chemical forms. To understand the underlying processes, we constructed a nonequilibrium mathematical model of CO2 dynamics in a flat-panel algal photobioreactor. The model includes mass transfer to the algal suspension from a stream of bubbles of CO2-enriched air and from the photobioreactor headspace. Also included are the hydration of dissolved CO2 to bicarbonate ion (HCO (3) (-) ) as well as uptake and/or cycling of these two chemical forms by the cells. The model was validated in experiments using a laboratory-scale flat-panel photobioreactor that controls light, temperature, and pH and where the concentration of dissolved CO2, and partial pressure of CO2 in the photobioreactor exhaust are measured. First, the model prediction was compared with measured CO2 dynamics that occurred in response to a stepwise change in the CO2 partial pressure in the gas sparger. Furthermore, the model was used to predict CO2 dynamics in photobioreactors with unicellular, nitrogen-fixing cyanobacterium Cyanothece sp. The metabolism changes dramatically during a day, and the distribution of CO2 is expected to exhibit a pronounced diurnal modulation that significantly deviates from chemical equilibrium.