Effects of fluid-flow regimes on Chlorella sorokiniana cultivation in cascade photobioreactors with either flat or wavy bottoms.

Computational fluid dynamics (CFD) was used to investigate cascade photobioreactors (cascade PBRs) with two different bottom configurations-flat and wavy-to establish the effect that fluid-flow regimes exert on the photosynthetic productivity of Chlorella sorokiniana. In the flat-bottom PBR, areal biomass productivities decreased from 6.8 to 4.2 g·m·d when the flow rate of a culture per unit of lane width was increased from 33 to 132 L·m·min. We found that this decrease in the areal productivity was the result of a decrease in the volumetric photon flux densities (volumetric PFDs), which was caused by an increase in the depth of the culture in the lane. Through CFD calculation and long-exposure photography, the flow of the culture in the wavy-bottom PBR was characterized in an upper straightforward section and underneath the swirling section. Under identical conditions of flow rate and volumetric PFD (66 L·m·min and 50 μmol·m·s, respectively), the cell growth accelerated in the wavy-bottom PBR with areal productivity that reached 6.5 g·m·d-productivity was 5.1 g·m·d in the flat-bottom PBR. The swirling flow in the wave troughs held the culture for longer periods in the illuminated lane, and the resultant extended period of mixing improved the photosynthetic productivity.