Photosynthetic productivity of conical helical tubular photobioreactors incorporating Chlorella sp. under various culture medium flow conditions

The characteristics of the flow of culture medium significantly affects the photosynthetic productivity of bioreactors incorporating microalgae. Therefore, in order to optimize the performance of a conical helical tubular photobioreactor (CHTP) designed to be useful in practical applications, we characterized the flow pattern of the culture medium through the reactor. The effects of medium flow conditions on the photosynthetic productivity of Chlorella sp. were investigated using several different CHTP units with 0.50-m2 installation areas which were designed to vary the direction and rate of flow driven by airlift. In addition, the performance of two- and four-unit systems constructed by combining individual CHTP units was evaluated. We found that when medium flowed from the bottom to the top of the photostage, it exhibited smoother flow of culture medium than when flowing from top to bottom, which led to higher photosynthetic productivity by the former. Consistent with theoretical calculations, varying the lengths of vertical flow passages caused flow rates to vary, and higher flow rates meant smoother circulation of medium and better photosynthetic performance. Flow of medium through a four-unit CHTP system was similar to that in single units, enabling a photosynthetic productivity of 31.0 g-dry biomass per m2-installation area per day to be achieved, which corresponded to a photosynthetic efficiency of 7.50% (photosynthetically active radiation (PAR; 400-700 nm)). This high photosynthetic performance was possible because smoother medium flow attained in single units was also attained in the four-unit system.