Comparison between airlift photobioreactor and bubble column for Skeletonema costatum cultivation

Abstract

The cultivation of diatom Skeletonema costatum was achieved in airlift photobioreactor and the system performance was compared to that of bubble column. The standard F/2 medium (Guillard's medium) for typical diatom cultivation could only yield the best growth character when the silicon concentration increased 4 times the normal value. In terms of cell growth, the airlift photobioreactor provided better performance than the bubble column where the maximum cell concentration, specific growth rate, and productivity in the airlift were 4.6 x 106 cells mL-1, 0.07 h-1, and 6.4 x 104 cells s-1 compared with 1.8 x 106 cells mL-1, 0.04 h-1, and 2.2 x 104 cells s-1 in the bubble column of the same size (3 L) and operated at the same aeration rate (superficial velocity = 1.5 cm s-1) and light intensity (34 μmol photons m-2 s-1). This was because the airlift photobioreactor allowed circulatory flow in the system which helps prevent cell precipitation and enhance light utilization efficiency. The optimal operating conditions in the airlift system which was found most optimal to cell growth were: the ratio between downcomer and riser cross sectional area (Ad:Ar) of 3.27, superficial gas velocity 1.5 cm s-1 and the light intensity 34 μmol photons m-2 s-1. Preliminary economical assessment on the cultivation of S. costatum in airlift system compared with that in bubble column was carried out, whereas the analysis for nutritional values of the obtained biomass indicated relatively high protein content.