Aeration is a key parameter in mass transfer of CO2. Flue treatment productivity in particular will be decreased in low aeration rates, and adequate CO2 concentrations will be required. However, simply increasing CO2 aeration rates does not neces­sarily lead to a higher CO2 fixation efficiency (Li et al. 2011). For example, Fig. 7.5 shows that increasing aeration rates from 0.1 vvm (volume of gas per volume of culture per min) to 0.5 vvm in a S. obliquus WUST4 culture medium resulted in a decreasing CO2 removal efficiency from 67 to 20 % (Li et al. 2011). A comparable result was obtained for C. vulgaris (Fig. 7.6), with the capability of CO2 fixation and O2 evolution decreasing with increasing feed gas flow rates (Fan et al. 2007). Therefore, in low aeration rates, gas retention time increases leading to an increased interface between CO2 and microalgal cells (Fan et al. 2007). One factor may be the influence of bubble coalescence; as it increases with increased flow rates, larger bubbles rise to the surface at a faster rate than smaller bubbles and the bubble surface area per unit of gas volume declines. This leads to decrease in CO2 absorption (Chiu et al. 2009). However, this is far from consistent across the literature, as the opposite can result in which increasing aeration rates improves

CO2 removal rates (Ong et al. 2010). For example, the effect of aeration rates from 0.25 to 0.5 vvm on CO2 fixation rate of Chlorella sp. MT-7 and MT-15 is signif­icantly higher than CO2 fixation rate at 0.5 vvm as compared with 0.25 vvm (Table 7.6) (Ong et al. 2010). Furthermore, the effect of different aeration rates (0.001, 0.002, and 0.005 ms-1) and CO2 fixation rates (1.5 g d-1) on the dry weight of C. vulgaris and D. tertiolecta was studied (Hulatt and Thomas 2011). The maximum biomass concentration for D. tertiolecta was obtained at the 0.005 ms-1 gas flow rate and at 12 % CO2 (1.5 g d-1), and 0.005 ms-1 and at 12 % CO2 (1.12 g d-1) for C. vulgaris (Table 7.6) (Hulatt and Thomas 2011). This indicates that improved mass transfer occurs at higher gas flow rates. Therefore, coming to an overall conclusion on the effect of flow gas rates on CO2 fixation rates is compli­cated due to opposite results being reported (see Table 7.6). While it is true that normally decreasing aeration rates lead to CO2 fixation efficiency increase, the opposite results have been obtained. This might arise as a consequence of the various production parameters [biomass concentration, light regime, nutrients, and types of PBRs (Hulatt and Thomas 2011)], and how the individual microalgae species affect each system. Nonetheless, increasing or decreasing aeration rate effectively determines whether CO2 fixation rates will increase or decrease in a microalgal system.

7.3 Conclusion

To achieve economical bioremediation of CO2 emitted from power stations using microalgae requires much research in order to maximize its efficiency and at the same time improve the microalgal biomass productivity at larger scales. Further­more, various microalgae and cyanobacteria species exhibit very different CO2 bioremediation rates and potentials for large-scale production. Results presented in this chapter demonstrated that the most attractive species for environmental CO2 mitigation include S. obliquss, D. tertiolecta, C. vulgaris, Phormidium sp., A. microscopic negeli, and C. littorale. The CO2 removal rate by the aforementioned species will require customization and optimization to meet each system-specific requirements. This chapter has reported on the initial cell concentrations, initial input CO2 concentrations, and aeration rates impact on CO2 bioremediation. In general, increasing initial cell concentrations and decreasing aeration rates lead to increasing CO2 fixation efficiency. It is to be noted that lowering aeration rates lead to a higher CO2 biofixation efficiency because of improved CO2 mass transfer between microalgal cells and the culture medium. Moreover, the input CO2 con­centration influences removal efficiency of CO2, however, providing high levels of CO2 into culture mediums leads to acidification. In contrast, the consumption of CO2 by microalgae through photosynthesis results in pH increase that may impact growth rates of some microalgae species.