Scaleup of an airlift bioreactor for cultivation of diatom, Chaetoceros calcitrans

Abstract

The optimal system configuration for the growth of Chaetoceros calcitrans in the 17 L airlift photobioreactor (ALPBR) was the system with the ratio between downcomer to riser cross sectional areas (A[subscript d] : A[subscript r]) of 2.62. The maximum specific growth rate and the maximum cell concentration were 7.41x10[superscript -2]h[superscript -1] and 8.88x10[superscript 6] cells mL[superscript -1] respectively for the cultivation with gas velocity (U[subscript sg]) of 3 cm S[superscript -1]. The growth of the diatom in the system did not depend on the level of gas-liquid mass transfer, but rather, it was controlled primarily by the light intensity. A few factors were found to have great influence on the growth characteristics. Firstly, the growth of the diatom was limited by the nutrient depletion. The cultivation of the diatom with two fold nutrient concentration increased the final cell concentration, and also the growth of the diatom in the refreshed spent medium resulted in almost the same growth rate and maximum cell concentration as obtained from the batch cultivation. However, the increase in the nutrient concentration could not enhance the growth rate of the diatom. Bubbles shading and self shading effects also affected the growth of the diatom by obstructing the light penetration into the system. The expanded top ALPBR was proposed to deal with the bubble shading effect as this system allowed an effective removal of small bubbles at the liquid surface. It was shown that no bubble shading effect could be seen in this expanded top ALPBR, but only a slight decrease in maximum cell concentration was observed at U[subscript sg] = 5 cm s[superscript -1] as this rather high U[subscript sg] could see an escape of small bubbles into the downcomer and decreasing the light intensity. Self shading effect could be dealt with using the ALPBRs-in-series setup when two or three culturing columns were connected in series. This setup allowed a better light penetration in the first column and confined the region of low light intensity to only the last column. The experiment demonstrated that the growth rate could be enhanced by 24.8% when compared with the conventional ALPBR. The diatom could be cultivated in both semi-continuous modes to ensure higher specific productivity. The analysis revealed that the continuous mode with medium feed rate of 3 mL min[superscript -1] was the most economical choice for the growth of this diatom. The diatom was also cultivated as a mass production in the large scale system. The maximum cell concentration and specific growth rate in the large scale bubble column, concentric ALPBR and multiple draft tubes ALPBR were not significantly different which implied that the effect of airlift inside such large system was only marginal.