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Double paddlewheels were proposed to generate cycle flow for increasing horizontal fluid velocity between dark and light zones in a flat plate photo-bioreactor, which strengthened the mass transfer and the mixing effect to improve microalgal growth with 15% CO 2 . Numerical fluid dynamics were used to simulate the cycle flow field with double paddlewheels. The local flow field measured with particle image velocimetry fitted well with the numerical simulation results. The horizontal fluid velocity in the photo-bioreactor was markedly increased from 5.8 × 10 −5  m/s to 0.45 m/s with the rotation of double paddlewheels, resulting in a decreased dark/light cycle period. Therefore, bubble formation time and diameter reduced by 24.4% and 27.4%, respectively. Meanwhile, solution mixing time reduced by 31.3% and mass transfer coefficient increased by 41.2%. The biomass yield of microalgae Nannochloropsis Oceanic increased by 127.1% with double paddlewheels under 15% CO 2 condition. [ABSTRACT FROM AUTHOR]

• ZIF-8 nanoparticles promoted dissolving carbon in a photo-bioreactor. • ZIF-8 with small size was more effective at increasing mass transfer coefficient. • Biomass accumulation was increased by 25.5% with ZIF-8 nanoparticles. Low dissolution of CO 2 in closed tubular photobioreactors results in insufficient carbon source supply, and often limits microalgae growth. An optimized ZIF8-SE medium containing zeolitic imidazolate framework-8 (ZIF-8) nanoparticles was developed for strengthening CO 2 mass transfer in the photobioreactor, which improved CO 2 dissolution to increase microalgal biomass yield under 15% CO 2. The CO 2 mass transfer coefficient markedly increased from 1.82 to 3.85, leading to an increased dissolved inorganic carbon concentration. Meanwhile, microalgae biomass accumulation with 0.01 mmol/L ZIF-8 nanoparticles was 25.5% higher than that without ZIF-8 nanoparticles. Furthermore, the average cell size decreased from 2.61 to 2.38 μm and the cell fractal dimension increased from 1.38 to 1.56 when ZIF-8 nanoparticle concentration increased from 0 to 0.2 mmol/L, indicating that potential toxicity might occurred with excess ZIF-8 nanoparticles. Reutilization of ZIF-8 nanoparticles in the recycled culture medium improved microalgal growth by 23%, which was similar to the promotion of fresh ZIF8-SE medium. This new finding provides a new strategy to improve CO 2 mass transfer in the horizontal tubular photobioreactor, which makes it a feasible platform for CO 2 capture and utilization. [ABSTRACT FROM AUTHOR]

• A novel jet-aerated tangential swirling-flow plate photobioreactor was developed. • Average bubble diameter decreased by 80.2% and gas holdup increased by 3.7 times. • Mixing time decreased by 71.4% and mass transfer coefficient increased by 4.6 times. • Microalgal biomass dry weight and CO 2 fixation rate increased by 49.4% and 56.0%. To reduce bubble diameter and enhance mass transfer, a novel jet-aerated tangential swirling-flow plate photobioreactor was developed that improves the growth rate of microalgae. In this system, the circulating microalgal solution enters a jet aerator that takes up 15% CO 2 by vacuum suction and then injects into a plate photobioreactor through four centrally symmetric nozzles. Each jetflow is tangent to a tangential circle, driving vertical vortex movement of the surrounding microalgal solution, which markedly reduced the bubble diameter and enhanced mass transfer. The mass transfer coefficient was enhanced by decreasing the nozzle number (n) and increasing the ratio of tangential circle diameter to plate photobioreactor equivalent diameter (d/D). The average bubble diameter decreased by 80.2% to 0.37 mm and the mass transfer coefficient increased 4.6 times to 48.9 h−1 when n was 4 and d/D was 0.34. Finally, the optimized system increased the biomass dry weight of microalgae by 49.4%. [ABSTRACT FROM AUTHOR]