Improving light distribution and light/dark cycle of 900 L tangential spiral-flow column photobioreactors to promote CO2 fixation with Arthrospira sp. cells.

The light distribution and light/dark cycle were improved in 900 L tangential spiral-flow column photobioreactors (TSCP) to promote CO2 fixation with Arthrospira sp. cells. Solar irradiation model was employed in CFD simulation to investigate light distribution and light/dark cycle in flow field composed of culture medium, CO2 bubbles and Arthrospira sp. cells under actual sunlight irradiation considering geolocation and time. An accurate way to divide light/dark zone based on saturate light intensity and light intensity field was adopted for the first time. When Arthrospira sp. cell concentration increased from 0.1 to 0.9 g/L, light/dark cycle frequency of cells firstly increased from 0.650 Hz to 0.868 Hz and then decreased to 0.117 Hz. Intracellular chlorophyll a content and carotenoids content of Arthrospira sp. cells in TSCP were 6% and 41% higher than those in conventional bubble column photobioreactor. This promoted cellular photosynthesis and stress resistance, which contributed to increase CO2 fixation rate of Arthrospira sp. cells by 59%. When CO2 aeration rate, CO2 volume concentration, and circulating pump power were 0.210 L/min, 15%, and 30 W, chlorophyll a content, helix pitch, and CO2 fixation rate of Arthrospira sp. cells all reached peak values of 8.769 mg/L, 78.26 μm and 0.358 g/L/d, respectively.