Utilization of Anabaena sp. in CO2 removal processes: Modelling of biomass, exopolysaccharides productivities and CO2 fixation rate

This paper focuses on modelling the growth rate and exopolysaccharides production of Anabaena sp. ATCC 33047, to be used in carbon dioxide removal and biofuels production. For this, the influence of dilution rate, irradiance and aeration rate on the biomass and exopolysaccharides productivity, as well as on the CO2 fixation rate, have been studied. The productivity of the cultures was maximum at the highest irradiance and dilution rate assayed, resulting to 0.5 g(bio) l(-1) day(-1) and 0.2 g(eps) l(-1) day(-1), and the CO2 fixation rate measured was 1.0 gCO(2) l(-1) day(-1). The results showed that although Anabaena sp. was partially photo-inhibited at irradiances higher than 1,300 mu E m(-2) s(-1), its growth rate increases hyperbolically with the average irradiance inside the culture, and so does the specific exopolysaccharides production rate. The latter, on the other hand, decreases under high external irradiances, indicating that the exopolysaccharides metabolism hindered by photo-damage. Mathematical models that consider these phenomena have been proposed. Regarding aeration, the yield of the cultures decreased at rates over 0.5 v/v/min or when shear rates were higher than 60 s(-1), demonstrating the existence of thus existence of stress damage by aeration. The behaviour of the cultures has been verified outdoors in a pilot-scale airlift tubular photobioreactor. From this study it is concluded that Anabaena sp. is highly recommended to transform CO2 into valuable products as has been proved capable of metabolizing carbon dioxide at rates of 1.2 gCO(2) l(-1) day(-1) outdoors. The adequacy of the proposed equations is demonstrated, resulting to a useful tool in the design and operation of photobioreactors using this strain.