Insights of Raceway Bioreactor Scale-Up: Effect of Agitation on Microalgae Culture and Reduction of the Liquid Medium Speed

The scaling of Raceway bioreactors was studied by geometric and mechanical similarity, with an order of magnitude of 1:10. The hydrodynamic parameters involved (flow velocity, hydrodynamic stress or shear stress, dimensionless numbers of Re, Fr, and Euler Power) at different stirring speeds (30, 35, 40, and 45 rpm) were determined. The study, carried out using low-density particle imaging velocimetry (PIV), showed that the speed of the liquid medium remains above 30 cm/s from 30 rpm, which ensures turbulence in the system. The flow velocity suffers a decrease of approximately 18%, at different angular velocities, with similar biomass concentrations (3.24 × 105–3.72 × 105 cells/mL). This decrease in speed directly affects the values of all the parameters involved in the bioreactor. Furthermore, the measurement of hydrodynamic stress (τ) indicates that the microorganisms are exposed to a value of 0.299 Pa at 35 rpm and 0.370 Pa at 40 rpm. Due to mechanical agitation, hydrodynamic stress values in Raceway systems have not been previously reported. The studies were carried out in a 10 L Raceway bioreactor using a consortium of microalgae and cyanobacteria where Spirulina sp. and Pseudanabaena sp. predominate.