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• Bicarbonate pool efficiently support CO 2 capture and microalgae cultivation. • Spirulina platensis with high pH tolerant up to 12.5 was screened. • Bicarbonate pool with bicarbonate concentration up to 0.5 M and pH up to12.5. • Direct capture CO 2 with microalgae cultivation may be driven by renewable energy. Direct air capture (DAC) of CO 2 from ambient air based on alkaline sorbents has great potential, but regenerating sorbents with physiochemical methods is energy-intensive. With free solar energy, however, sorbents can be regenerated with the consumption of CO 2 during photosynthesis by microalgae, but this regeneration is limited by the low efficiency of CO 2 capture from air, which is also the primary reason for the high cost of microalgae production. In this study, 100−500 mmol L−1 bicarbonate with a pH of 10.0–12.5 was used to efficiently capture CO 2 from the air, store it as a bicarbonate pool, and rapidly supply CO 2 during photosynthesis. The initial pH and bicarbonate concentration significantly affected biomass productivity and the carbon-capture rate, with maxima of 1.00 and 0.81 g L−1 d−1, respectively, indicating the high efficiency of this process. This process that can be completely driven by renewable energy in future, such as wave energy, will significantly reduce costs and facilitate both carbon capture and microalgae production in an environmental friendly manner. [ABSTRACT FROM AUTHOR]

Microalgae present great potential to replace land crops for the efficient production of large volumes of biomass for food, feed, fuels, and chemicals, as well as to treat wastewater and capture carbon. However, the commercialization of these technologies for bulk commodities requires a great reduction in the current microalgal biomass production cost. The bioreactor is the core of bioprocess engineering and is the premise for the commercial application of certain types of biotechnology. The challenges of phototrophic cultivation are completely different from those of heterotrophic processes because the efficiency of phototrophic cultivation is limited by the energy density of the input sunlight and the inorganic carbon supply. Thus, the development of microalgae cultivation technologies with low manufacturing and operating costs is key to addressing this problem, and floating photobioreactors (PBRs) are a promising solution. PBRs are deployed on the water surface without any land requirements, and wave energy provides free mixing energy. Additionally, the surrounding water can be used to control the culture temperature and to supply nutrients for microalgae growth. In this mini-review, the development of floating PBRs and their recent progress are presented. The effect of the carbon supply approach on the mixing and scaling-up of floating PBRs are critically discussed. The limitations and challenges in commercial applications of floating PBRs are analysed, and the need for future research is proposed. Finally, it is noted that microalgae farming on the ocean is a promising solution for human society to address the challenge of land space exhaustion due to the global population boom. [ABSTRACT FROM AUTHOR]

Background: Unlike conventional cultivation systems, liquid mixing in floating photobioreactors (PBRs) is solely induced by their hydrodynamic movement in response to waves, and this movement is affected by the wave conditions (wave height and wave period), the PBR configuration and the culture depth. However, to the best of our knowledge, a practical study of the hydrodynamic movements of PBRs has not been previously conducted. Results: This study aims to investigate the hydrodynamic performance of floating PBRs in response to wave conditions. First, the effects of the experimental wave height (2–10 cm) and wave period (0.8–1.8 s) on movement was investigated using two 1.0 m2 PBR models: a square PBR (1.0 m/1.0 m; length/width) and a rectangular PBR (1.7 m/0.6 m). The results indicated that wave movement became not only more intense with increasing wave height, but also less intense when the wave period decreased. However, the square PBR experienced more intense movement than the rectangular PBR, but also little mooring force. The effects of culture depth (0.5, 1.0 and 2.0 cm) were investigated and the results showed that the culture depth significantly affected the hydrodynamic movements of the PBRs; however, the mooring forces were unaffected. Finally, the movement and mooring-line forces of PBRs equipped with different mooring systems were investigated. The use of two different mooring systems had little effect on PBR movement; however, a mooring system with floaters was able to significantly reduce the mooring line forces compared to a system without floaters. During this study, the greatest force (10.5 N) was found for the rectangular PBR using a mooring system without floaters, whereas the lowest force (0.67 N) was observed for a rectangular PBR using a mooring system with floaters. Conclusions: These studies have provided basic data describing the fluid dynamics of floating PBRs; as well as their structural design and scale up. These results also provide guidance for the selection of ocean fields with suitable wave conditions; as well as a proper mooring methods to ensure safe operation. [ABSTRACT FROM AUTHOR]