(545aa) Assessment of Carbon BIO-Fixation By MIXED Indigenous Microalgae

It is only a matter of time before it becomes vital to further develop an effective Carbon Capture and Storage (CCS) processes. This has been made apparent because existing methods are no longer economically feasible (Leung et al. 2014, Zheng and Xu 2014). It is believed that CO₂ will evidently become accessible from a number of different sources (Kheshgi et al. 2012). With the GCC (Gulf Cooperation Council) this will include power plants, refineries, various petrochemical processes (methanol, ammonia), and will ultimately incorporate gas separation at processing plants and at LNG production facilities. Hence, CO₂ has to be removed from flue gas (CO₂-N₂) and from natural gas (CO₂-CH₄) mixtures.

Today, the average cost of capturing and storing of CO₂ is estimated to be at least 55 US$ per ton of CO₂ based on chemical absorption. These numbers are based off a coal-fired power plant which was used as a reference point (Hammond and Spargo 2014, Wang et al. 2017). The solvent regeneration process contributes to nearly 75% of the total energy consumption, which inevitably means that percentage is also going towards funding. This high energy penalty is one of the determining factors to expand the research of a more economical and sustainable approach for the CCS chain (Peeters et al. 2007).

So far, limited attention has been paid to the dual role of microalgae as advanced technology for treatment of different kinds of wastewater and as CO₂ bio-fixation technology. Moreover, there are knowledge gaps in understanding the performance of different microalgae strains in the treatment of wastewater and their capacity for CO₂ capturing. In particular, according to our literature review, no studies were carried out using mixed indigenous microalgae cultures as opposed to lab-grown pure culture strains. This knowledge is needed to select and match the right type of microalgae with the right type of wastewater, which is crucial in improving the operation and performance of microalgae-CO₂-biofixiation- wastewater treatment systems.

The objective this study, hence, was to investigate and compare the treatment efficiency and CO₂ bio-fixation capacity of lab-grown single strain and mixed indigenous cultures of microalgae in different wastewaters. Primary effluent, secondary influent and septic tank effluent were used in the study to determine possible applications and locations for the proposed treatment systems.

The effect of different concentration of CO₂ (2.5%, 5%, 10%, and 15% v/v) on the growth rate of two algae strains (Spirulina platensis (SP.PL) and mixed indigenous microalgae (M.I), biomass production, carbon dioxide bio-fixation rate, chemical oxygen demand and nutrient removals were assessed. The specific growth rate of algae increased by increasing the CO₂ concentration until the optimum value (10%), after that, increasing the CO₂ dose negatively affected the growth rate. The specific growth rate values ranged from 0.45-0.79 day^-1 for S. PL and from 0.48 to 0.86 day^-1 for M.I. Both algae strains have shown high biomass productivities (BP_max of 0.246 g_dw.L^−1.d^−1 for SP.PL and 0.384 g_dw.L^−1.d^−1 for MIMA) and maximum carbon bio-fixation rates (R_CO2 of 0.360 g_C.L^−1.d^−1 for SP.PL and 0.460 g_C.L^−1.d^−1 for MIMA). The M.I collected from a local wastewater treatment plant showed better performance than SP.PL in removing organic matter (% COD_removal ~ 97.2), total inorganic nitrogen (% TIN_removal ~ 99.6) and total phosphorus (TP_removal~ 99.41) under all studied conditions. Microalgae with fast a growth rate and naturally grown in the water system can be a sustainable and effective technology for CO₂ uptake and nutrients removals.