Advanced control for photoautotrophic growth and CO₂-utilization efficiency using a membrane carbonation photobioreactor (MCPBR)

A membrane carbonation (MC) module uses bubbleless gas-transfer membranes to supply inorganic carbon (C_i) for photoautotrophic cyanobacterial growth in a photobioreactor (PBR); this creates the novel MCPBR system, which allows precise control of the CO₂-delivery rate and minimal loss of CO₂ to the atmosphere. Experiments controlled the supply rate of C_i to the main PBR by regulating the recirculation rate (Q _R) between the module of MC chamber and the main PBR. The experiments evaluated how Q_R controls the CO₂ mass transport in MC chamber and how it connects with the biomass production rate, C_i concentration, pH in the PBR, and CO₂-utilization efficiency. The biomass production rate and C_i concentration increased in response to the C_i supply rate (controlled by Q_R), but not in linear proportion. The biomass production rate increased less than C_i due to increased light limitation. Except for the highest Q_R, when the higher C_i concentration caused the pH to decrease, CO₂ loss to gas ventilation was negligible. The results demonstrate that this MCPBR offers independent control over the growth of photoautotrophic biomass, pH control, and minimal loss of CO₂ to the atmosphere.