Transfer of carbon dioxide within cultures of microalgae: Plain bubbling versus hollow-fiber modules

In attempts to improve the metabolic efficiency in closed photosynthetic reactors, availability of light and CO₂ are often considered as limiting factors, as they are difficult to control in a culture. The carbon source is usually provided via bubbling of CO₂-enriched air into the culture medium; however, this procedure is not particularly effective in terms of mass transfer. Besides, it leads to considerable waste of that gas to the open atmosphere, which adds to operation costs. Increase in the interfacial area of contact available for gas exchange via use of membranes might be a useful alternative; microporous membranes, in hollow-fiber form, were tested accordingly. Two hollow-fiber modules, different in both hydrophilicity and outer surface area, were tested and duly compared, in terms of mass transfer, versus traditional plain bubbling. Overall volumetric coefficients (K_La) for CO₂ transfer were 1.48 × 10^-2 min^-1 for the hydrophobic membrane, 1.33 × 10^-2 min^-1 for the hydrophilic membrane, and 7.0 × 10^-3 min^-1 for plain bubbling. A model microalga, viz. Nannochloropsis sp., was cultivated using the two aforementioned membrane systems and plain bubbling. The produced data showed slight (but hardly significant) increases in biomass productivity when the hollow-fiber devices were used. However, hollow-fiber modules allow recirculation of unused CO₂, thus reducing feedstock costs. Furthermore, such indirect way of supplying CO₂ offers the additional possibility for use of lower gas pressures, as no need to counterbalance hydrostatic heads exists.